Bone Tissue & Skeletal System

Bone Tissue

  • Connective tissue with a matrix hardened by calcium phosphate and other minerals (mineralization/calcification).

  • Functions:

    • Support, protection, movement, electrolyte balance, acid-base balance, blood formation, hormone secretion.

  • Types of Bones:

    • Short, irregular, flat, and long bones.

Bone Structure

  • Compact Bone: Dense outer layer.

  • Spongy Bone: Loosely organized inner layer.

  • Diaphysis: Shaft of a long bone.

  • Epiphysis: Expanded head of a long bone.

  • Epiphyseal Plate/Line: Growth zone in children/remnant in adults.

  • Articular Cartilage: Hyaline cartilage covering joint surfaces.

  • Periosteum: Sheath covering bone (outer fibrous and inner osteogenic layers).

  • Endosteum: Lines internal marrow cavity and spongy bone.

Bone Cells

  • Osteogenic cells: Stem cells that create other bone cell types.

  • Osteoblasts: Bone-forming cells (osteogenesis).

  • Osteocytes: Mature osteoblasts in lacunae; maintain bone density and blood concentrations of calcium and phosphate.

  • Osteoclasts: Bone-dissolving cells (osteolysis).

  • Osteocalcin: Secreted by osteoblasts and osteocytes; important to the endocrine system.

Bone Matrix

  • Inorganic matter (hydroxyapatite, calcium carbonate, ions).

  • Organic matter (collagen, protein-carbohydrate complexes).

Compact vs. Spongy Bone

  • Compact: Concentric lamellae around a central canal (osteon/haversian system).

  • Spongy: Spicules and trabeculae; arranged along stress lines, covered with endosteum.

Bone Marrow

  • Red Marrow: Produces blood cells; found in children and limited locations in adults.

  • Yellow Marrow: Replaces red marrow in adults; can convert back in severe anemia.

Bone Development (Ossification/Osteogenesis)

  • Intramembranous: From mesenchyme; forms flat bones.

    1. Mesenchyme condenses into a soft sheet of tissue permeated with blood vessels.

    2. Osteoblasts sprout dendrites and deposit calcium phosphate and other minerals on the collagen fibers of the tissue, which hardens the matrix.

    3. More bone-adjacent mesenchyme condenses and form fibrous periosteum on each surface.

    4. Osteoblast beneath the periosteum deposit layers of bone, fills in the spaces between trabeculae, and create compact bone on each side.

  • Endochondral: From hyaline cartilage model; forms most bones.

    1. Mesenchyme developing into hyaline cartilage covered in perichondrium, where the future bone will be.

    2. Chondrocytes will inflate and die while the thin walls between them calcify and the perichondrium stops producing chondrocytes.

    3. Blood vessels invade the primary ossification center creating a hollow center and osteoblasts deposit layers of bone, lining the cavity that thickens the shaft.

    4. The secondary ossification center hollows out creating a secondary marrow cavity and expands outward in all directions.

    5. During infancy and childhood, the epiphyses fill with spongy bone.

    6. By late teens to early twenties, all cartilage in the epiphyseal plate is consumed, and the gap between the epiphysis and diaphysis closes.

Bone Growth

  • Elongation: Occurs at epiphyseal plate (zones of reserve cartilage, cell proliferation, hypertrophy, calcification, bone deposition).

  • Widening/Thickening: Appositional growth (new tissue at the surface).

Bone Remodeling

  • Wolff's Law: Bone adapts to mechanical stress.

  • Involves osteoclasts (resorption) and osteoblasts (deposition).

Physiology of Osseous Tissue

  • Mineral Deposition: Calcium, phosphate, and ions are deposited into bone tissue as hydroxyapatite crystals.

  • Mineral Resorption: Osteoclasts dissolve bone, releasing minerals into the blood.

Calcium Homeostasis

  • Hypocalcemia: Calcium deficiency.

  • Hypercalcemia: Calcium excess.

Hormonal Regulation:

  • Calcitriol (Vitamin D): Raises blood calcium levels by increasing absorption in the small intestine and calcium resorption from the skeleton.

  • Calcitonin: Lowers blood calcium levels by inhibiting osteoclasts and stimulating osteoblasts.

  • Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, promoting calcium resorption by the kidneys, and promoting calcitriol synthesis.

Phosphate Homeostasis

  • Calcitriol raises phosphate levels.

  • Parathyroid hormone lowers phosphate levels.

Factors A ecting Bone Growth

  • Hormones (growth hormone, estrogen, testosterone).

Bone Disorders

  • Fractures: Stress or pathological.

  • Healing: Hematoma formation, soft callus formation, hard callus formation, remodeling.

Skeletal System

Axial Skeleton

  • Skull, auditory ossicles, hyoid bone, vertebral column, thoracic cage.

Appendicular Skeleton

  • Bones of the upper and lower limbs, pectoral and pelvic girdles.

Anatomical Features of Bones

  • Articulations (Joint Surfaces): Condyle, facet, head.

  • Extensions and Projections: Crest, epicondyle, line, process, protuberance, ramus, spine, trochanter, tubercle, tuberosity.

  • Depressions: Alveolus, fossa, fovea, sulcus.

  • Passages and Cavities: Canal, fissure, foramen, sinus.

Skull

  • 22 bones connected by sutures.

  • Foramina allow passage of nerves and blood vessels.

Cavities:

  • Middle-ear, inner-ear, oral cavity.

Bones:

  • Maxillae, palatine, zygomatic, lacrimal, mandible, nasal.

Nasal Cavity

  • Paranasal sinuses (frontal, sphenoidal, ethmoidal, maxillary).

  • Nasal bones, inferior nasal conchae, vomer.

Cranial Cavity

  • Cranial bones enclose the brain.

  • Meninges (dura mater) separate brain from cranium.

  • Calvaria (dome) and base (floor).

  • Cranial fossae (anterior, middle, posterior).

Cranial Bones:

  • Frontal, parietal, temporal, occipital, sphenoid, ethmoid.

Bones Associated with the Skull

  • Auditory ossicles (malleus, incus, stapes).

  • Hyoid bone.

Infancy and Childhood

  • Fontanelles (spaces between unfused bones) that will ossify.