Bones and Skeletal Tissue: Structure, Growth, and Remodeling

Bones and Skeletal Tissue

Bone Structure

The structure of bones is specialized to provide support, protection, and facilitate movement. Bones are classified by their shapes and internal organization, which determines their function and mechanical properties.

Regions of Long Bones

• Epiphysis: The ends of long bones, primarily composed of spongy bone and covered by articular cartilage at joint surfaces.

• Diaphysis: The shaft of a long bone, consisting of a thick layer of compact bone surrounding the medullary cavity.

• Metaphysis: The region between the diaphysis and epiphysis, containing the epiphyseal plate (growth plate) in children or the epiphyseal line in adults.

Superficial Structure: Periosteum

• Periosteum: A double-layered membrane covering the external surface of bones except at joint surfaces.

• Outer (fibrous) layer: Dense irregular connective tissue.

• Inner (cellular) layer: Contains osteoprogenitor cells, osteoblasts, and osteoclasts.

• Anchored to bone tissue via perforating (Sharpey’s) fibers (collagen fibers).

Deep Structure of Long Bones: Diaphysis

• Compact bone: Thick outer layer providing strength and resistance to stress.

• Spongy bone: Thin inner layer forming the medullary cavity, which contains bone marrow.

• Endosteum: Thin membrane lining the medullary cavity, composed of reticular connective tissue and bone cells.

Compact vs. Spongy Bone

Feature	Compact Bone	Spongy Bone
Location	Outer layer	Inside layer
Structure	Dense, solid	Honeycomb framework (trabeculae)
Function	Resists unidirectional stresses	Resists multidirectional stresses
Bone Marrow	Absent	Present (red/yellow)

Bone Marrow

• Red Bone Marrow: Hematopoietic tissue found in spongy bone and medullary cavity in children; in adults, mainly in the axial skeleton.

• Yellow Bone Marrow: Product of bone marrow degeneration, composed of triglycerides, adipocytes, and blood vessels. Can convert back to red bone marrow if needed.

Deep Structure of Long Bones: Epiphysis

• Outside layer: Compact bone.

• Inside layer: Spongy bone, lined by endosteum.

• Contains epiphyseal lines (adults) or epiphyseal plates (children), which separate epiphysis from diaphysis.

Structure of Short, Irregular, Sesamoid, and Flat Bones

• Do not have epiphyses, diaphyses, medullary cavities, or epiphyseal lines/plates.

• Do have periosteum, compact bone, spongy bone (diploë in flat bones), and sinuses (in certain skull bones).

Microscopic Anatomy of Bones

Bones are composed of specialized cells and an extracellular matrix that provides both strength and flexibility.

Bone Cells

• Osteoprogenitor cells: Mitotically active stem cells located in periosteum and endosteum; differentiate into osteoblasts.

• Osteoblasts: Active cells responsible for bone deposition; secrete organic bone matrix (osteoid) and become osteocytes.

• Osteocytes: Mature bone cells housed in lacunae; maintain bone matrix and communicate with osteoblasts and osteoclasts.

• Osteoclasts: Large, multinucleated cells responsible for bone resorption; secrete H+ ions and enzymes to break down bone matrix.

Extracellular Matrix

Component	Percentage	Main Function
Inorganic matrix	65%	Protection, resist compression
Organic matrix (osteoid)	35%	Strength, flexibility

Inorganic Matrix

• Hydroxyapatite crystals: Composed of calcium and phosphorus.

• Other ions: bicarbonate, potassium, magnesium.

• Functions: protection and resistance to compression.

Organic Matrix (Osteoid)

• Components: Collagen fibers and ground substance.

• Function: Provides strength and flexibility, resists stretching and twisting.

Importance of Matrix Components

• If bone lacked inorganic matrix: would be flexible but not protective.

• If bone lacked organic matrix: would be brittle and prone to fracture.

Formation and Dissolution of Matrix

Formation

1. Osteoblasts secrete osteoid.

2. Calcification: deposition of hydroxyapatite crystals.

• Requires calcium, phosphorus, vitamin D (for Ca2+ absorption), and vitamin C (for collagen fiber formation).

Dissolution

• Osteoclasts secrete HCl (dissolves inorganic matrix) and enzymes (digest organic matrix).

Bone Remodeling

Bone remodeling is a lifelong process of maintaining bone matrix, adapting to mechanical needs, repairing microdamage, and maintaining calcium homeostasis.

• Functions:

  • Meet changing mechanical needs

  • Repair microdamage

  • Maintain calcium homeostasis

Histology of Compact Bone

• Osteon (Haversian system): Structural unit of compact bone.

• Lamellae: Concentric, interstitial, and circumferential layers of bone matrix.

• Canals: Central (Haversian), Volkmann (perforating), and canaliculi (small channels connecting lacunae).

Histology of Spongy Bone

• Formed of trabeculae with concentric lamellae extending in multiple directions.

• Covered with endosteum; provides protective structure for bone marrow.

• Osteocytes within lacunae; canaliculi present.

• No central or perforating canals.

Ossification

Ossification is the process of initial bone formation, beginning in the second month of development and continuing into early adulthood.

Type	Forms	Begins As
Intramembranous Ossification	Clavicle, frontal, parietal, occipital, temporal bones	Mesenchymal membrane (embryonic connective tissue)
Endochondral Ossification	All bones inferior to head (except clavicles)	Cartilage

• Many bones ossified by age 7; others not until 20s.

Clinical Application: Achondroplasia

• Most common cause of dwarfism.

• Skeletal characteristics: shortened limbs, large skulls.

• Growth inhibited due to decreased endochondral ossification.

Bone Growth: Lengthwise and Widthwise

Type	Location	Mechanism
Longitudinal Growth	Epiphyseal plate (long bones)	Division of chondrocytes
Appositional Growth	All bones	Osteoblasts and osteoclasts

Zones of the Epiphyseal Plate

• Zone of reserve cartilage

• Zone of proliferation

• Zone of hypertrophy and maturation

• Zone of calcification

• Zone of ossification

Longitudinal Growth Steps

1. Chondrocytes divide in the zone of proliferation.

2. Chondrocytes enlarge and mature in the zone of hypertrophy.

3. Matrix calcifies in the zone of calcification.

4. Osteoblasts deposit bone in the zone of ossification.

Appositional Growth Steps

• Bone deposited by osteoblasts at the periosteum.

• Bone resorbed by osteoclasts at the endosteum.

Bone Remodeling

• Continuous process of bone formation (deposition) and loss (resorption).

• Osteoblasts deposit bone; osteoclasts break down bone.

Why Does Bone Remodeling Occur?

• Replacement of primary bone

• Bone repair

• Replace brittle bone

• Hormone influence

• Adaptation to tension and stress

• Calcium ion homeostasis

Parathyroid Hormone (PTH) and Calcium Homeostasis

• PTH regulates calcium levels in the blood by stimulating osteoclasts to break down bone and release Ca2+ into the blood.

• Calcium homeostasis is maintained within a narrow range (9–11 mg/100 ml).

• If calcium levels are too high, the body inhibits PTH release and stimulates bone deposition.

Additional info: Bone tissue is dynamic and constantly remodeled throughout life, adapting to mechanical demands and maintaining mineral balance essential for physiological processes.