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Bones: Structure and Types | Concise Medical Knowledge

Bone, as an organ, is made up of:

• Osseous tissue: a type of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology hardened by the deposition of minerals Minerals Electrolytes (primarily calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes and phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes )
• Blood
• Bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis
• Cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology and fibrous Fibrous Fibrocystic Change connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology
• Adipose tissue Adipose tissue Adipose tissue is a specialized type of connective tissue that has both structural and highly complex metabolic functions, including energy storage, glucose homeostasis, and a multitude of endocrine capabilities. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or “brite” adipose tissue, which is a transitional form. Adipose Tissue: Histology
• Nervous tissue

Bone can refer to:

• The entire organ
• The osseous tissue within the organ only

• Protection (e.g., the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull: Anatomy protects the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification , the ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy protect the heart and lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs: Anatomy )
• Support
• Movement
• Blood cell formation Blood cell formation The development and formation of various types of blood cells. Hematopoiesis can take place in the bone marrow (medullary) or outside the bone marrow (extramedullary hematopoiesis). Bone Marrow: Composition and Hematopoiesis
• Storage:
  • Minerals Minerals Electrolytes
  • Phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes

Axial Axial Computed Tomography (CT) bones:

• Skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull: Anatomy
• Vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column: Anatomy and sacrum Sacrum Five fused vertebrae forming a triangle-shaped structure at the back of the pelvis. It articulates superiorly with the lumbar vertebrae, inferiorly with the coccyx, and anteriorly with the ilium of the pelvis. The sacrum strengthens and stabilizes the pelvis. Vertebral Column: Anatomy
• Rib cage Rib cage The bony thoracic enclosure consisting of the vertebral column; the ribs; the sternum; and the costal cartilage. Chest Wall: Anatomy : ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy and sternum Sternum A long, narrow, and flat bone commonly known as breastbone occurring in the midsection of the anterior thoracic segment or chest region, which stabilizes the rib cage and serves as the point of origin for several muscles that move the arms, head, and neck. Chest Wall: Anatomy

Appendicular bones:

• Clavicle Clavicle A bone on the ventral side of the shoulder girdle, which in humans is commonly called the collar bone. Clavicle Fracture
• Scapula
• Arms and hands
• Pelvic girdle
• Legs and feet

Illustration representing the bones that form the axial skeleton

Illustration representing the bones that form the appendicular skeleton

• Long bones: length greater than width:
  • Upper extremity: humerus Humerus Bone in humans and primates extending from the shoulder joint to the elbow joint. Arm: Anatomy , radius Radius The outer shorter of the two bones of the forearm, lying parallel to the ulna and partially revolving around it. Forearm: Anatomy , ulna Ulna The inner and longer bone of the forearm. Forearm: Anatomy , metacarpals Metacarpals The five cylindrical bones of the metacarpus, articulating with the carpal bones proximally and the phalanges of fingers distally. Wrist Joint: Anatomy , and phalanges Phalanges Bones that make up the skeleton of the fingers, consisting of two for the thumb, and three for each of the other fingers. Hand: Anatomy
  • Lower extremity: femur, tibia Tibia The second longest bone of the skeleton. It is located on the medial side of the lower leg, articulating with the fibula laterally, the talus distally, and the femur proximally. Knee Joint: Anatomy , fibula Fibula The bone of the lower leg lateral to and smaller than the tibia. In proportion to its length, it is the most slender of the long bones. Leg: Anatomy , metatarsals, and phalanges Phalanges Bones that make up the skeleton of the fingers, consisting of two for the thumb, and three for each of the other fingers. Hand: Anatomy
• Short bones: length approximately equal to width:
  • Carpal bones Carpal bones The eight bones of the wrist: scaphoid bone; lunate bone; triquetrum bone; pisiform bone; trapezium bone; trapezoid bone; capitate bone; and hamate bone. Carpal Tunnel Syndrome (wrist)
  • Tarsal bones Tarsal Bones The seven bones which form the tarsus – namely, calcaneus; talus; cuboid, navicular, and the internal, middle, and external cuneiforms. Foot: Anatomy (ankle)
• Flat bones: enclose and protect soft organs:
  • Most skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull: Anatomy bones
  • Ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy
  • Sternum Sternum A long, narrow, and flat bone commonly known as breastbone occurring in the midsection of the anterior thoracic segment or chest region, which stabilizes the rib cage and serves as the point of origin for several muscles that move the arms, head, and neck. Chest Wall: Anatomy
  • Scapula
• Irregular bones: bones not fitting into other categories:
  • Vertebrae
  • Some skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull: Anatomy bones (e.g., sphenoid, facial bones)

Illustration representing the classification of bones by shape

Compact bone: 

• A smooth, solid outer layer of osseous tissue
• Locations:
  • Present in every bone in the body
  • In long bone: forms a cylinder, encases a medullary cavity Medullary Cavity Ewing Sarcoma
• Function: strength to withstand compressive forces

Spongy (cancellous) bone: 

• Loosely organized, inner-layer osseous tissue
• Consists of a lattice of small, thin pieces of osseous tissue called trabeculae or bony spicules:
  • Transfers force on the bone to the outer compact bone
  • Constantly reforming to meet the body’s needs (e.g., exercise increases the trabeculae; prolonged weightlessness in space reduces the trabeculae) 
• Locations: internal to compact bone:
  • At the ends of long bones 
  • In the middle of short, flat, and irregular bones

Image of the internal structure of a femur head:
Note the compact bone along the outside and spongy/cancellous bone in the center.

Two primary membranes surround bone tissue: the periosteum (externally) and the endosteum (internally).

Periosteum:

• Outer layer surrounding bone on the external surface (except at the joints, which are covered in articular cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology )
• Vascularized and innervated
• Consists of 2 layers:
  • Fibrous Fibrous Fibrocystic Change layer
  • Osteogenic layer
• Fibrous Fibrous Fibrocystic Change layer of periosteum: 
  • Tough outer layer of collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology
  • Sharpey fibers: collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology fibers from the fibrous Fibrous Fibrocystic Change layer of the periosteum:
    • Continuous with muscle tendons on top of the bone
    • Penetrate deep into the bone matrix to secure the periosteum and overlying muscle to the bone
• Osteogenic layer of periosteum: 
  • Contains bone-forming cells:
    • Osteoblasts Osteoblasts Bone-forming cells which secrete an extracellular matrix. Hydroxyapatite crystals are then deposited into the matrix to form bone. Bones: Development and Ossification
    • Osteoclasts Osteoclasts A large multinuclear cell associated with the bone resorption. An odontoclast, also called cementoclast, is cytomorphologically the same as an osteoclast and is involved in cementum resorption. Bones: Development and Ossification
    • Osteogenic cells Osteogenic cells Bones: Development and Ossification
  • Critical in bone growth and healing after injury

Endosteum:

• Lines internal surfaces of bone:
  • Lines the medullary cavity Medullary Cavity Ewing Sarcoma in long bones
  • Covers the trabeculae in spongy bone
• Contains the same bone-forming cells as the osteogenic layer of the periosteum

Membranes of the bone, periosteum, and endosteum:
The periosteum lines the external surface of the bone and the endosteum lines the internal surface of the bone.

The 3 primary anatomic regions of long bones:

• Diaphysis:
  • The shaft
  • Forms the long axis of long bones
  • Consists of a thick layer of compact bone, surrounding a central medullary cavity Medullary Cavity Ewing Sarcoma containing bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis
• Epiphysis:
  • Ends of the bones (at joints)
  • Wider than the diaphysis:
    • Strengthens the joint
    • ↑ Surface area for tendon and ligament attachment
  • Primarily composed of spongy bone
  • Outer layer of compact bone
  • Covered in articular cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology :
    • A type of hyaline cartilage Hyaline cartilage A type of cartilage characterized by a homogeneous amorphous matrix containing predominantly type II collagen and ground substance. Hyaline cartilage is found in articular cartilage; costal cartilage; laryngeal cartilages; and the nasal septum. Cartilage: Histology
    • Articular cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology + lubricating fluid → ↓ friction → significantly easier joint movement
    • Acts as a shock Shock Shock is a life-threatening condition associated with impaired circulation that results in tissue hypoxia. The different types of shock are based on the underlying cause: distributive (↑ cardiac output (CO), ↓ systemic vascular resistance (SVR)), cardiogenic (↓ CO, ↑ SVR), hypovolemic (↓ CO, ↑ SVR), obstructive (↓ CO), and mixed. Types of Shock absorber
• Metaphysis:
  • In between the epiphysis and diaphysis
  • Remnant of the epiphyseal plate Epiphyseal plate The area between the epiphysis and the diaphysis within which bone growth occurs. Cartilage: Histology or line: the hyaline cartilage Hyaline cartilage A type of cartilage characterized by a homogeneous amorphous matrix containing predominantly type II collagen and ground substance. Hyaline cartilage is found in articular cartilage; costal cartilage; laryngeal cartilages; and the nasal septum. Cartilage: Histology allowed for bone elongation Elongation Polymerase Chain Reaction (PCR) in childhood

Bone marrow inside the femur

• Outer layers: thin plates of periosteum-covered compact bone 
• Inner layer: endosteum-covered spongy bone
• In flat bones, the inner spongy bone is:
  • Known as diploë
  • Sandwiched between 2 layers of compact bone

Structure of a flat bone

Bone markings are areas of bone where tendons, ligaments, and fascia Fascia Layers of connective tissue of variable thickness. The superficial fascia is found immediately below the skin; the deep fascia invests muscles, nerves, and other organs. Cellulitis attach, including articulations, projections, and holes.

• Articular markings:
  • Condyle: rounded surface at an articular area
  • Epicondyle: eminence superior to a condyle
  • Facet: flat surface where bones articulate
• Projections:
  • Crest: ridge of a bone
  • Process: prominence feature
  • Protuberance: projection of bone
  • Spine Spine The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column: Anatomy : a sharp process
  • Tubercle: smaller rounded process
  • Tuberosity: rough surface
• Holes:
  • Canal: channel through bone
  • Foramen: passage through a bone 
  • Fossa: hollow or depressed area
  • Groove or sulcus: elongated depression or furrow
  • Sinus: irregularly shaped cavernous area

Illustration representing various types of bone markings on different bones in the body where tendons, ligaments, and fascia attach to the bones:
The green dots represent the bone markings present on the different bones in the body.

Illustration representing various types of bone markings on different bones in the body where tendons, ligaments, and fascia attach to the bones:
The green dots represent the bone markings present on the different bones in the body.

The 2 primary components of bone are cells and matrix.

Bone contains a relatively small number of cells compared to the amount of matrix. In addition to other functions, the cells synthesize and break down the bone. Four major types of osseous cells exist:

• Osteogenic cells Osteogenic cells Bones: Development and Ossification :
  • Stem cells arising from embryonic fibroblasts Fibroblasts Connective tissue cells which secrete an extracellular matrix rich in collagen and other macromolecules. Sarcoidosis
  • Can differentiate into osteoblasts Osteoblasts Bone-forming cells which secrete an extracellular matrix. Hydroxyapatite crystals are then deposited into the matrix to form bone. Bones: Development and Ossification → stimulated by stress (e.g., from exercise) and fractures
  • Found in the endosteum and osteogenic periosteum
• Osteoblasts Osteoblasts Bone-forming cells which secrete an extracellular matrix. Hydroxyapatite crystals are then deposited into the matrix to form bone. Bones: Development and Ossification :
  • Synthesize the collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology matrix (organic portion of bone)
  • Deposit calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes salts on the matrix (mineralization)
  • Unable to divide → all new osteoblasts Osteoblasts Bone-forming cells which secrete an extracellular matrix. Hydroxyapatite crystals are then deposited into the matrix to form bone. Bones: Development and Ossification must come from osteogenic cells Osteogenic cells Bones: Development and Ossification
  • Found in endosteum and osteogenic periosteum
• Osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification :
  • Osteoblasts Osteoblasts Bone-forming cells which secrete an extracellular matrix. Hydroxyapatite crystals are then deposited into the matrix to form bone. Bones: Development and Ossification trapped in the bone the osteoblasts Osteoblasts Bone-forming cells which secrete an extracellular matrix. Hydroxyapatite crystals are then deposited into the matrix to form bone. Bones: Development and Ossification created
  • Located in spaces within the mineralized matrix known as lacunae Lacunae Bones: Development and Ossification
  • Play no significant role in bone synthesis Synthesis Polymerase Chain Reaction (PCR) or resorption
  • Primary role: sense strain and communicate the message to surface osteoblasts Osteoblasts Bone-forming cells which secrete an extracellular matrix. Hydroxyapatite crystals are then deposited into the matrix to form bone. Bones: Development and Ossification
• Osteoclasts Osteoclasts A large multinuclear cell associated with the bone resorption. An odontoclast, also called cementoclast, is cytomorphologically the same as an osteoclast and is involved in cementum resorption. Bones: Development and Ossification :
  • Dissolve/resorb bone
  • Purpose of bone resorption Bone resorption Bone loss due to osteoclastic activity. Bones: Remodeling and Healing :
    • Removal of old, injured, or unnecessary bone
    • Release of stored calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes (to maintain tightly regulated calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes levels)
  • Develop from the fusion of monocytes Monocytes Large, phagocytic mononuclear leukocytes produced in the vertebrate bone marrow and released into the blood; contain a large, oval or somewhat indented nucleus surrounded by voluminous cytoplasm and numerous organelles. Innate Immunity: Phagocytes and Antigen Presentation in the bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis → results in large, multinucleated cells with a “ruffled border” (deep folds in the plasma membrane Plasma membrane A cell membrane (also known as the plasma membrane or plasmalemma) is a biological membrane that separates the cell contents from the outside environment. A cell membrane is composed of a phospholipid bilayer and proteins that function to protect cellular DNA and mediate the exchange of ions and molecules. The Cell: Cell Membrane to increase the surface area)

Bone has a mineralized matrix (as opposed to an aqueous matrix in most other tissues, through which nutrients can easily diffuse). Bone has both organic and inorganic components:

• Organic component (⅓ of the matrix by weight):
  • Collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology fibers
  • Proteoglycans Proteoglycans Glycoproteins which have a very high polysaccharide content. Basics of Carbohydrates
  • Glycoproteins Glycoproteins Conjugated protein-carbohydrate compounds including mucins, mucoid, and amyloid glycoproteins. Basics of Carbohydrates
• Inorganic component (⅔ of the matrix by weight):
  • Hydroxyapatite crystals Hydroxyapatite crystals A group of compounds with the general formula m10(PO4)6(OH)2, where m is barium, strontium, or calcium. The compounds are the principal mineral in phosphorite deposits, biological tissue, human bones, and teeth. They are also used as an anticaking agent and polymer catalysts. Calcium Hemostasis and Bone Metabolism : calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes salts (85% of inorganic component by weight)
  • Calcium carbonate Calcium carbonate Carbonic acid calcium salt. An odorless, tasteless powder or crystal that occurs in nature. It is used therapeutically as a phosphate buffer in hemodialysis patients and as a calcium supplement. Hypocalcemia (10% of inorganic component by weight)
  • Other ions: magnesium Magnesium A metallic element that has the atomic symbol mg, atomic number 12, and atomic weight 24. 31. It is important for the activity of many enzymes, especially those involved in oxidative phosphorylation. Electrolytes , sodium Sodium A member of the alkali group of metals. It has the atomic symbol na, atomic number 11, and atomic weight 23. Hyponatremia , potassium Potassium An element in the alkali group of metals with an atomic symbol k, atomic number 19, and atomic weight 39. 10. It is the chief cation in the intracellular fluid of muscle and other cells. Potassium ion is a strong electrolyte that plays a significant role in the regulation of fluid volume and maintenance of the water-electrolyte balance. Hyperkalemia , fluoride Fluoride Inorganic salts of hydrofluoric acid, hf, in which the fluorine atom is in the -1 oxidation state. Sodium and stannous salts are commonly used in dentifrices. Trace Elements , sulfate, and hydroxide (5% of inorganic component by weight)
• The combination of organic (protein) and inorganic (mineral) components allow the bones to be strong and solid, but not too brittle.

In long bones, the majority of cells and matrix are arranged in functional units known as osteons.

Osteons:

An osteon (also known as a haversian system) is a cylinder of cells and matrix running longitudinally within compact bone. An osteon is made up of a longitudinal central canal, which is surrounded by concentric rings of osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification , and bone matrix known as lamellae.

• Osteon central canal (haversian canal):
  • Longitudinal canal at the core of each osteon
  • Contains:
    • Blood vessels
    • Lymphatic vessels Lymphatic Vessels Tubular vessels that are involved in the transport of lymph and lymphocytes. Lymphatic Drainage System: Anatomy
    • Nerves
  • Lined with endosteum 
  • Provides nutrients and removes waste from osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification immediately adjacent to the central canal
• Perforating canals:
  • Contain neurovasculature
  • Run perpendicular to the central canals, connecting the canals to:
    • External nerves and vessels
    • One another
• Lamellae:
  • Concentric rings of calcified matrix
  • Osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification are located between lamellae in spaces known as lacunae Lacunae Bones: Development and Ossification .
  • Canaliculi (tiny canals in the matrix) allow thin, finger-like projections of the osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification to connect to one another via gap junctions Gap Junctions Connections between cells which allow passage of small molecules and electric current. Gap junctions were first described anatomically as regions of close apposition between cells with a narrow (1-2 nm) gap between cell membranes. The variety in the properties of gap junctions is reflected in the number of connexins, the family of proteins which form the junctions. The Cell: Cell Junctions :
    • Allows for nutrient delivery and waste removal from osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification in outer rings, without being immediately adjacent to vasculature
    • Allows for communication Communication The exchange or transmission of ideas, attitudes, or beliefs between individuals or groups. Decision-making Capacity and Legal Competence of strain signals
• Collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology :
  • Fibers “corkscrew” down the matrix in a given lamella
  • Different helical Helical Computed tomography where there is continuous x-ray exposure to the patient while being transported in a spiral or helical pattern through the beam of irradiation. This provides improved three-dimensional contrast and spatial resolution compared to conventional computed tomography, where data is obtained and computed from individual sequential exposures. Computed Tomography (CT) arrangements are present within adjacent lamellae:
    • Right-handed coils vs. left-handed coils
    • Varying tightness of coils
  • Creates a “crisscrossing” network of collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology → significantly ↑ strength to resist bending and compression Compression Blunt Chest Trauma

A microscopic image of an osteon

Cross section of bone demonstrating the structure of an osteon:
Concentric lamellae form a ring around the central osteonal canals and contain osteocytes. Within the central canal, blood vessels deliver nutrients to the neighboring osteocytes.
The osteocytes have long, thin, “finger-like” projections branching into canaliculi (channels within the matrix). The projections allow the osteocytes to be connected to one another via gap junctions, providing a mechanism for nutrient delivery and waste removal from osteocytes beyond the central canal.

Circumferential lamellae: 

• Lamellae running parallel to the bone surface around the entire circumference of the bone
• Not part of an osteon functional unit
• Locations:
  • Immediately within the osteogenic layer of periosteum
  • Lining the inner medullary cavity Medullary Cavity Ewing Sarcoma

Interstitial lamellae: 

• Irregular regions of bone tissue between osteons
• Remnants of old osteons partially broken down during bone remodeling Bone remodeling The continuous turnover of bone matrix and mineral that involves first an increase in bone resorption (osteoclastic activity) and later, reactive bone formation (osteoblastic activity). The process of bone remodeling takes place in the adult skeleton at discrete foci. The process ensures the mechanical integrity of the skeleton throughout life and plays an important role in calcium homeostasis. An imbalance in the regulation of bone remodeling’s two contrasting events, bone resorption and bone formation, results in many of the metabolic bone diseases, such as osteoporosis. Bones: Remodeling and Healing

Microscopic structure of compact bone

In spongy bone, rather than forming concentric rings within osteons, lamellae form concentric rings, which create the trabeculae.

Similarities between spongy and compact bone:

• Lamellae form concentric rings
• Osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification live within lacuna between lamellae
• Osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification are connected to each other via canaliculi

Differences between spongy and compact bone:

• Trabeculae are arranged along lines of force (osteons are parallel to one another in compact bone).
• Trabeculae form a lattice-like network, creating space within the bone filled with bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis .
• No central canals → not needed because no osteocytes Osteocytes Mature osteoblasts that have become embedded in the bone matrix. They occupy a small cavity, called lacuna, in the matrix and are connected to adjacent osteocytes via protoplasmic projections called canaliculi. Bones: Development and Ossification are far from the blood supply (surrounding marrow)

Microscopic structure of spongy bone

Bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis : a general term for hematopoietic soft tissue Soft Tissue Soft Tissue Abscess occupying the spaces within bone.

• Red marrow: myeloid tissue (can produce blood cells)
• Yellow marrow: fatty marrow (no longer produces blood cells)

• The medullary cavities of long bones
• Diploë of flat bones
• Trabecular cavities of spongy bone

• In infants and children: Nearly all bone cavities contain red marrow.
• In young to middle-aged adults:
  • Most red marrow has become yellow marrow
  • Red marrow exists in:
    • Vertebrae
    • Ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy
    • Sternum Sternum A long, narrow, and flat bone commonly known as breastbone occurring in the midsection of the anterior thoracic segment or chest region, which stabilizes the rib cage and serves as the point of origin for several muscles that move the arms, head, and neck. Chest Wall: Anatomy
    • Parts of the pelvic girdle
    • Proximal head of the humerus Head of The Humerus The upper rounded extremity of the humerus fitting into the glenoid cavity of the scapula. Arm: Anatomy and femur
  • Yellow marrow can revert to red marrow in severe or chronic anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview and Types .

• Osteoporosis Osteoporosis Osteoporosis refers to a decrease in bone mass and density leading to an increased number of fractures. There are 2 forms of osteoporosis: primary, which is commonly postmenopausal or senile; and secondary, which is a manifestation of immobilization, underlying medical disorders, or long-term use of certain medications. Osteoporosis : a decrease in bone mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast and density leading to an increased number of fractures. Osteoporosis Osteoporosis Osteoporosis refers to a decrease in bone mass and density leading to an increased number of fractures. There are 2 forms of osteoporosis: primary, which is commonly postmenopausal or senile; and secondary, which is a manifestation of immobilization, underlying medical disorders, or long-term use of certain medications. Osteoporosis is most commonly caused by a loss of protective estrogen Estrogen Compounds that interact with estrogen receptors in target tissues to bring about the effects similar to those of estradiol. Estrogens stimulate the female reproductive organs, and the development of secondary female sex characteristics. Estrogenic chemicals include natural, synthetic, steroidal, or non-steroidal compounds. Ovaries: Anatomy and/or testosterone Testosterone A potent androgenic steroid and major product secreted by the leydig cells of the testis. Its production is stimulated by luteinizing hormone from the pituitary gland. In turn, testosterone exerts feedback control of the pituitary LH and FSH secretion. Depending on the tissues, testosterone can be further converted to dihydrotestosterone or estradiol. Androgens and Antiandrogens later in life, immobilization Immobilization Delirium , underlying medical disorders, or long-term use of certain medications. Osteoporosis Osteoporosis Osteoporosis refers to a decrease in bone mass and density leading to an increased number of fractures. There are 2 forms of osteoporosis: primary, which is commonly postmenopausal or senile; and secondary, which is a manifestation of immobilization, underlying medical disorders, or long-term use of certain medications. Osteoporosis most often presents clinically with frequent fractures and loss of vertebral height. Diagnosis is established by measuring bone mineral density. Management includes lifestyle modifications, maintaining adequate levels of calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes and vitamin D Vitamin D A vitamin that includes both cholecalciferols and ergocalciferols, which have the common effect of preventing or curing rickets in animals. It can also be viewed as a hormone since it can be formed in skin by action of ultraviolet rays upon the precursors, 7-dehydrocholesterol and ergosterol, and acts on vitamin D receptors to regulate calcium in opposition to parathyroid hormone. Fat-soluble Vitamins and their Deficiencies , and the use of bisphosphonates Bisphosphonates Bisphosphonates are pyrophosphate analogs most well-known for treating osteoporosis by preventing bone loss. Bisphosphonates end in the suffix “-dronate” or “-dronic acid” (e.g., alendronate, risedronate, pamidronate) and bind to hydroxyapatite crystals in bone, inhibiting osteoclast-induced bone resorption. Bisphosphonates .
• Osteomalacia Osteomalacia Disorder caused by an interruption of the mineralization of organic bone matrix leading to bone softening, bone pain, and weakness. It is the adult form of rickets resulting from disruption of vitamin d; phosphorus; or calcium homeostasis. Osteomalacia and Rickets and rickets Rickets Disorders caused by interruption of bone mineralization manifesting as osteomalacia in adults and characteristic deformities in infancy and childhood due to disturbances in normal bone formation. The mineralization process may be interrupted by disruption of vitamin d; phosphorus; or calcium homeostasis, resulting from dietary deficiencies, or acquired, or inherited metabolic, or hormonal disturbances. Osteomalacia and Rickets : disorders of decreased bone mineralization Bone mineralization Calcium (Ca2+) and phosphate (PO43–) combine to form hydroxyapatite crystals on the bone matrix. Bones: Development and Ossification . Rickets Rickets Disorders caused by interruption of bone mineralization manifesting as osteomalacia in adults and characteristic deformities in infancy and childhood due to disturbances in normal bone formation. The mineralization process may be interrupted by disruption of vitamin d; phosphorus; or calcium homeostasis, resulting from dietary deficiencies, or acquired, or inherited metabolic, or hormonal disturbances. Osteomalacia and Rickets affects the cartilage Cartilage Cartilage is a type of connective tissue derived from embryonic mesenchyme that is responsible for structural support, resilience, and the smoothness of physical actions. Perichondrium (connective tissue membrane surrounding cartilage) compensates for the absence of vasculature in cartilage by providing nutrition and support. Cartilage: Histology of the epiphyseal growth plates Growth Plates The area between the epiphysis and the diaphysis within which bone growth occurs. Osteosarcoma in children. Osteomalacia Osteomalacia Disorder caused by an interruption of the mineralization of organic bone matrix leading to bone softening, bone pain, and weakness. It is the adult form of rickets resulting from disruption of vitamin d; phosphorus; or calcium homeostasis. Osteomalacia and Rickets affects the sites of bone turnover in children and adults. Both disorders are most commonly caused by vitamin D deficiency Vitamin D Deficiency A nutritional condition produced by a deficiency of vitamin D in the diet, insufficient production of vitamin D in the skin, inadequate absorption of vitamin D from the diet, or abnormal conversion of vitamin D to its bioactive metabolites. It is manifested clinically as rickets in children and osteomalacia in adults. Fat-soluble Vitamins and their Deficiencies . Rickets Rickets Disorders caused by interruption of bone mineralization manifesting as osteomalacia in adults and characteristic deformities in infancy and childhood due to disturbances in normal bone formation. The mineralization process may be interrupted by disruption of vitamin d; phosphorus; or calcium homeostasis, resulting from dietary deficiencies, or acquired, or inherited metabolic, or hormonal disturbances. Osteomalacia and Rickets commonly presents with skeletal deformities and growth abnormalities. Osteomalacia Osteomalacia Disorder caused by an interruption of the mineralization of organic bone matrix leading to bone softening, bone pain, and weakness. It is the adult form of rickets resulting from disruption of vitamin d; phosphorus; or calcium homeostasis. Osteomalacia and Rickets can present with bone pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways , difficulty with ambulation, and pathologic fractures. Treatment includes vitamin D Vitamin D A vitamin that includes both cholecalciferols and ergocalciferols, which have the common effect of preventing or curing rickets in animals. It can also be viewed as a hormone since it can be formed in skin by action of ultraviolet rays upon the precursors, 7-dehydrocholesterol and ergosterol, and acts on vitamin D receptors to regulate calcium in opposition to parathyroid hormone. Fat-soluble Vitamins and their Deficiencies , calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes , and phosphorus supplementation.
• Paget disease of bone: a focal disorder of bone metabolism Bone metabolism Bone is the primary storage site of calcium in the body; thus, bone metabolism plays a critical role in maintaining normal calcium levels. Bone metabolism (and thus calcium levels) are primarily regulated by 3 hormones, namely, calcitonin, parathyroid hormone (PTH), and vitamin D. Calcium Hemostasis and Bone Metabolism commonly affecting the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull: Anatomy , spine Spine The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column: Anatomy , pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 “hip” bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis: Anatomy , and long bones of the lower extremity. The main clinical manifestations of Paget disease are bone pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways and the consequences of bone deformities (e.g., fractures, osteoarthritis Osteoarthritis Osteoarthritis (OA) is the most common form of arthritis, and is due to cartilage destruction and changes of the subchondral bone. The risk of developing this disorder increases with age, obesity, and repetitive joint use or trauma. Patients develop gradual joint pain, stiffness lasting < 30 minutes, and decreased range of motion. Osteoarthritis , nerve impingement). Management of Paget disease includes bisphosphonates Bisphosphonates Bisphosphonates are pyrophosphate analogs most well-known for treating osteoporosis by preventing bone loss. Bisphosphonates end in the suffix “-dronate” or “-dronic acid” (e.g., alendronate, risedronate, pamidronate) and bind to hydroxyapatite crystals in bone, inhibiting osteoclast-induced bone resorption. Bisphosphonates , calcitonin Calcitonin A peptide hormone that lowers calcium concentration in the blood. In humans, it is released by thyroid cells and acts to decrease the formation and absorptive activity of osteoclasts. Its role in regulating plasma calcium is much greater in children and in certain diseases than in normal adults. Other Antiresorptive Drugs , and surgery to manage fractures, deformities, or complications. 
• Hyperparathyroidism Hyperparathyroidism Hyperparathyroidism is a condition associated with elevated blood levels of parathyroid hormone (PTH). Depending on the pathogenesis of this condition, hyperparathyroidism can be defined as primary, secondary or tertiary. Hyperparathyroidism : a condition associated with elevated blood levels of parathyroid Parathyroid The parathyroid glands are 2 pairs of small endocrine glands found in close proximity to the thyroid gland. The superior parathyroid glands are lodged within the parenchyma of the upper poles of the right and left thyroid lobes; the inferior parathyroid glands are close to the inferior tips or poles of the lobes. Parathyroid Glands: Anatomy hormone (PTH). Hyperparathyroidism Hyperparathyroidism Hyperparathyroidism is a condition associated with elevated blood levels of parathyroid hormone (PTH). Depending on the pathogenesis of this condition, hyperparathyroidism can be defined as primary, secondary or tertiary. Hyperparathyroidism may be due to an inherent disease within the parathyroid Parathyroid The parathyroid glands are 2 pairs of small endocrine glands found in close proximity to the thyroid gland. The superior parathyroid glands are lodged within the parenchyma of the upper poles of the right and left thyroid lobes; the inferior parathyroid glands are close to the inferior tips or poles of the lobes. Parathyroid Glands: Anatomy gland or abnormalities of calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes metabolism. Individuals classically present with “stones ( nephrolithiasis Nephrolithiasis Nephrolithiasis is the formation of a stone, or calculus, anywhere along the urinary tract caused by precipitations of solutes in the urine. The most common type of kidney stone is the calcium oxalate stone, but other types include calcium phosphate, struvite (ammonium magnesium phosphate), uric acid, and cystine stones. Nephrolithiasis ), bones (↓ bone mineral density), abdominal groans (nonspecific abdominal pain Abdominal Pain Acute Abdomen ), and psychiatric overtones (neuropsychiatric symptoms).” Diagnosis is based on laboratory assessment of serum PTH, calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes and phosphate Phosphate Inorganic salts of phosphoric acid. Electrolytes levels, and urinary calcium Calcium A basic element found in nearly all tissues. It is a member of the alkaline earth family of metals with the atomic symbol ca, atomic number 20, and atomic weight 40. Calcium is the most abundant mineral in the body and combines with phosphorus to form calcium phosphate in the bones and teeth. It is essential for the normal functioning of nerves and muscles and plays a role in blood coagulation (as factor IV) and in many enzymatic processes. Electrolytes . Management is typically surgical and the treatment of any underlying conditions.
• Bone fractures Bone fractures Breaks in bones. Bones: Remodeling and Healing : a partial or complete interruption in the continuity of a bone (periosteum and/or cortex) resulting from mechanical stress (typically injuries or metabolic disorders of the bone). Clinical presentation varies depending on the cause and location of the injury. Presentation generally includes deformity Deformity Examination of the Upper Limbs , pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways , edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema , and inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation . Diagnosis is made clinically and confirmed with imaging. Management may be splinting or surgery.

Osteomalacia

Rickets

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