Bones and Skeletal Tissues: Structure, Function, and Development

Chapter 6: Bones and Skeletal Tissue

Skeletal Cartilages

Skeletal cartilages are specialized connective tissues that provide support, flexibility, and resilience to the skeleton. All types contain chondrocytes in lacunae surrounded by an extracellular matrix.

• Hyaline cartilage: Most abundant; provides support and flexibility. Found in articular, costal, respiratory, and nasal cartilages.

• Elastic cartilage: Similar to hyaline but contains elastic fibers. Located in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers for tensile strength. Found in menisci of the knee and intervertebral discs.

Classification of Bones

The human skeleton consists of 206 named bones, divided into two main groups:

• Axial skeleton: Long axis of the body (skull, vertebral column, rib cage).

• Appendicular skeleton: Bones of limbs and girdles attaching limbs to the axial skeleton.

Bones are also classified by shape:

• Long bones: Longer than wide (e.g., humerus).

• Short bones: Cube-shaped (e.g., wrist, ankle, sesamoid bones like the patella).

• Flat bones: Thin, flat, slightly curved (e.g., sternum, scapulae, ribs, skull bones).

• Irregular bones: Complicated shapes (e.g., vertebrae, coxal bones).

Functions of Bones

Bones perform several essential functions:

• Support: Framework for the body and soft organs.

• Protection: Shields brain, spinal cord, and vital organs.

• Movement: Acts as levers for muscle action.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities.

• Triglyceride storage: Energy stored in bone cavities.

• Hormone production: Osteocalcin regulates bone formation and metabolism.

Gross Anatomy of Bone

Bone Textures

Bones have two main textures:

• Compact bone: Dense outer layer, smooth and solid.

• Spongy bone (cancellous or trabecular): Honeycomb structure of trabeculae deep to compact bone.

Structure of Long Bones

Long bones have a distinct anatomy:

• Diaphysis: Tubular shaft forming the long axis, composed of compact bone surrounding the medullary cavity.

• Epiphyses: Bone ends with external compact bone and internal spongy bone; articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of childhood bone growth at the epiphyseal plate.

Membranes Covering Bone

• Periosteum: Double-layered membrane covering external surfaces except joints; contains osteogenic cells, nerve fibers, and blood vessels.

• Endosteum: Delicate membrane covering internal bone surfaces and trabeculae; contains osteogenic cells.

Hematopoietic Tissue

Red marrow is found in trabecular cavities of spongy bone and diploë of flat bones. In adults, active red marrow is limited to the heads of femur and humerus, diploë, and some irregular bones. Yellow marrow can convert to red if necessary.

Bone Markings

Bone markings are sites for muscle, ligament, and tendon attachment, joint surfaces, and conduits for blood vessels and nerves. They are classified as projections, depressions, and openings.

Microscopic Anatomy of Bone

Bone Cell Types

Five major cell types are found in bone tissue:

• Osteogenic cells: Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts or bone lining cells.

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• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid); actively mitotic.

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• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress sensors.

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• Bone lining cells: Flat cells on bone surfaces; help maintain matrix.

• Osteoclasts: Giant, multinucleate cells for bone resorption; derived from hematopoietic stem cells.

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Microscopic Anatomy – Compact Bone

Compact bone is organized into structural units called osteons (Haversian systems). Each osteon is an elongated cylinder parallel to the bone's long axis, composed of concentric lamellae with collagen fibers running in different directions to resist twisting forces.

• Central (Haversian) canal: Contains blood vessels and nerve fibers.

• Perforating (Volkmann's) canals: Connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae and central canal.

Microscopic Anatomy – Spongy Bone

Spongy bone consists of trabeculae aligned along lines of stress, containing irregularly arranged lamellae and osteocytes interconnected by canaliculi. Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Cells: Osteogenic cells, osteoblasts, osteocytes, bone-lining cells, and osteoclasts.

• Osteoid: Unmineralized bone matrix secreted by osteoblasts; contains ground substance (proteoglycans, glycoproteins) and collagen fibers.

• Collagen: Provides tensile strength and flexibility; sacrificial bonds between collagen molecules dissipate energy and prevent fracture.

Inorganic Components

• Hydroxyapatites (mineral salts): 65% of bone by mass; mainly tiny calcium phosphate crystals in and around collagen fibers; responsible for hardness and resistance to compression.

Bone Development and Growth

Ossification (Osteogenesis)

Ossification is the process of bone tissue formation, beginning in the second month of development and continuing throughout life.

• Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most of the skeleton.

• Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones like clavicles and cranial bones.

Postnatal Bone Growth

• Interstitial (longitudinal) growth: Increases length of long bones; requires epiphyseal cartilage.

• Appositional growth: Increases bone thickness; occurs throughout life.

Epiphyseal Plate Zones

Longitudinal bone growth involves five zones within the epiphyseal plate:

1. Resting (quiescent) zone: Relatively inactive cartilage.

2. Proliferation (growth) zone: Rapid cell division pushes epiphysis away from diaphysis.

3. Hypertrophic zone: Older chondrocytes enlarge and erode, creating spaces.

4. Calcification zone: Matrix calcifies, chondrocytes die, blood vessels invade.

5. Ossification zone: New bone forms as osteoblasts cover calcified cartilage spicules.

Appositional Growth

Osteoblasts beneath periosteum secrete bone matrix on external bone, while osteoclasts remove bone on endosteal surface, resulting in thicker, stronger bone.

Hormonal Regulation and Bone Homeostasis

Hormonal Regulation

• Growth hormone: Stimulates epiphyseal plate activity in childhood.

• Thyroid hormone: Modulates growth hormone activity for proper proportions.

• Sex hormones: Promote adolescent growth spurts and induce epiphyseal plate closure.

Bone Remodeling

Bone remodeling involves bone deposit and resorption, regulated by genetic factors and two control loops:

• Negative feedback hormonal loop for Ca2+ homeostasis: Controls blood calcium levels.

• Response to mechanical and gravitational forces: Determines where remodeling occurs.

Calcium Homeostasis

Calcium is essential for nerve transmission, muscle contraction, blood coagulation, secretion, and cell division. Blood calcium levels are tightly regulated (9–11 mg/dl).

• Parathyroid hormone (PTH): Removes calcium from bone, increasing blood calcium.

• Calcitonin: Lowers blood calcium levels temporarily in high doses.

Bone Repair and Fractures

Fracture Classification

Fractures are classified by:

• Position of bone ends (nondisplaced vs. displaced)

• Completeness of break (complete vs. incomplete)

• Whether skin is penetrated (open vs. closed)

Types of Fractures

Fracture Type	Description and Comments
Comminuted	Bone fragments into three or more pieces; common in aged, brittle bones.
Compression	Bone is crushed; common in porous bones subjected to trauma.
Spiral	Ragged break from excessive twisting forces; common sports fracture.
Epiphyseal	Epiphysis separates from diaphysis along epiphyseal plate; occurs where cartilage cells die and matrix calcifies.
Depressed	Broken bone portion pressed inward; typical of skull fracture.
Greenstick	Bone breaks incompletely; common in children with more organic matrix.

Fracture Repair

1. Hematoma formation: Torn blood vessels hemorrhage, clot forms, site is swollen and inflamed.

2. Fibrocartilaginous callus formation: Capillaries grow into hematoma, fibroblasts secrete collagen, repair tissue forms.

3. Bony callus formation: New trabeculae appear, callus converted to bony callus of spongy bone.

4. Bone remodeling: Excess material removed, compact bone reconstructs shaft walls.

Homeostatic Imbalances

Osteomalacia and Rickets

• Osteomalacia: Poorly mineralized bones, inadequate calcium salts, soft and weak bones, pain upon bearing weight.

• Rickets: Osteomalacia in children; bowed legs, bone deformities, enlarged bone ends; caused by vitamin D deficiency or insufficient calcium.

Osteoporosis

Bone resorption outpaces deposit, especially in spongy bone of spine and neck of femur. Vertebral and hip fractures are common.

• Risk factors: Aged, postmenopausal women, petite body form, insufficient exercise, poor diet, smoking, hormone-related conditions, immobility.

• Treatments: Calcium and vitamin D supplements, weight-bearing exercise, hormone replacement therapy (controversial), prevention by early calcium intake and exercise.

Paget's Disease

• Excessive and haphazard bone deposit and resorption; poorly formed bone, high ratio of spongy to compact bone, reduced mineralization.

• Usually affects spine, pelvis, femur, and skull; rare before age 40; cause unknown.

• Treatment includes calcitonin and biphosphonates.

Aging Bone

• Bone formation exceeds resorption in children/adolescents; balanced in young adults; bone mass, mineralization, and healing decrease with age.

• Bone loss greater in whites and females; genetics play a major role in bone density and osteoporosis risk.

Example: Electrical stimulation and daily ultrasound treatments can hasten bone repair.