Bones and Skeletal Tissues: Structure, Function, and Development

Bones and Skeletal Tissues

Cartilages Help Form the Skeleton

The human skeleton initially consists of cartilage, which is gradually replaced by bone except in areas requiring flexibility. Skeletal cartilage is a resilient tissue primarily composed of water, lacking blood vessels and nerves. The perichondrium is a dense irregular connective tissue surrounding cartilage, providing nutrient delivery and resisting outward expansion. Cartilage is made up of chondrocytes housed in lacunae within a jelly-like extracellular matrix.

Cartilage Structure, Types, and Locations

There are three main types of cartilage, each with distinct structural and functional properties:

• Hyaline cartilage: Provides support, flexibility, and resilience; contains only collagen fibers. Found in articular (joints), costal (ribs), respiratory (larynx and passageways), and nasal cartilage (external nose).

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

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

The Bones and Cartilages of the Human Skeleton

The skeleton is composed of both bones and cartilages, with cartilage persisting in areas requiring flexibility and support.

Growth of Cartilage

Cartilage grows by two mechanisms:

• Appositional growth: New matrix is laid down on the surface by cells in the perichondrium.

• Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Cartilage growth ends during adolescence.

Functions of Bones

Bones serve seven essential functions:

• Support: Framework for the body and soft organs.

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

• Anchorage: Acts as levers for muscle action.

• Mineral storage: Stores calcium and phosphorus.

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

• Triglyceride storage: Fat stored in bone cavities for energy.

• Hormone production: Osteocalcin regulates insulin, glucose, and energy expenditure.

Classification of Bones

The human skeleton contains 206 named bones, divided into:

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles attaching limbs to axial skeleton.

Classification of Bones By Shape

Bones are classified by shape:

• Long bones: Longer than wide, with a medullary cavity (e.g., limb bones).

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

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

• Irregular bones: Complex shapes (e.g., vertebrae, hip bones).

Bone Structure

Bones are organs containing various tissues: bone (osseous), nervous, cartilage, dense connective, muscle, and epithelial cells. Structure is studied at three levels: gross, microscopic, and chemical.

Gross Anatomy of Compact and Spongy Bone

Compact bone is the dense outer layer, appearing smooth and solid. Spongy bone consists of a honeycomb of trabeculae, with spaces filled by red or yellow bone marrow.

Structure of a Typical Long Bone

Long bones have a shaft (diaphysis), ends (epiphyses), and membranes:

• Diaphysis: Tubular shaft, compact bone surrounding medullary cavity (yellow marrow in adults).

• Epiphyses: Ends with compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of childhood epiphyseal plate.

Structure of Short, Irregular, and Flat Bones

These bones consist of thin plates of spongy bone (diploë) covered by compact bone. Periosteum covers the outside, endosteum covers the inside. Bone marrow is scattered throughout spongy bone, with no defined cavity. Hyaline cartilage covers areas involved in movable joints.

Hematopoietic Tissue in Bones

Red bone marrow is found in trabecular cavities of spongy bone and diploë of flat bones. In newborns, all spongy bone and medullary cavities contain red marrow; in adults, it is mainly in flat bones and some irregular bones. Yellow bone marrow can convert to red if needed.

Bone Markings

Bone markings are sites for muscle, ligament, and tendon attachment, joint formation, or passage for blood vessels and nerves. They are categorized as:

• Projections: Attachment sites.

• Surfaces: Form joints.

• Depressions and openings: Passageways for vessels and nerves.

Category	Description	Examples
Projections	Sites of muscle and ligament attachment	Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process
Surfaces	Form joints	Head, facet, condyle, ramus
Depressions and Openings	Passageways for blood vessels and nerves	Groove, fissure, foramen, notch, meatus, sinus, fossa

Microscopic Anatomy of Bone

Bone tissue contains five major cell types:

• Osteoprogenitor (osteogenic) cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete osteoid (collagen and calcium-binding proteins).

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

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

• Osteoclasts: Multinucleate cells responsible for bone resorption.

Microscopic Anatomy of Compact Bone

Compact bone (lamellar bone) consists of:

• Osteon (Haversian system): Structural unit; elongated cylinder with concentric lamellae.

• Canals and canaliculi: Central canal contains vessels and nerves; canaliculi connect lacunae.

• Interstitial and circumferential lamellae: Fill gaps and extend around diaphysis.

Microscopic Anatomy of Spongy Bone

Spongy bone is organized along lines of stress, with trabeculae conferring strength. It lacks osteons but contains lamellae and osteocytes interconnected by canaliculi. Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Bones are composed of organic and inorganic components:

• Organic: Cells and osteoid (ground substance and collagen fibers) provide tensile strength and flexibility.

• Inorganic: Hydroxyapatites (calcium phosphate crystals) provide hardness and resistance to compression.

Bone Development (Ossification)

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

• Endochondral ossification: Bone forms by replacing hyaline cartilage; most bones below the skull.

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

Endochondral Ossification

Five main steps:

1. Bone collar forms around diaphysis.

2. Central cartilage calcifies and forms cavities.

3. Periosteal bud invades, forming spongy bone.

4. Diaphysis elongates, medullary cavity forms.

5. Epiphyses ossify; hyaline cartilage remains at plates and articular surfaces.

Intramembranous Ossification

Four steps:

1. Ossification centers form as mesenchymal cells become osteoblasts.

2. Osteoid is secreted and calcified.

3. Woven bone forms around vessels; periosteum develops.

4. Lamellar bone replaces woven bone; red marrow appears.

Postnatal Bone Growth

Long bones grow in length via interstitial growth at the epiphyseal plate and in thickness via appositional growth. Growth ceases during adolescence.

Growth in Length of Long Bones

The epiphyseal plate consists of five zones:

• Resting zone

• Proliferation (growth) zone

• Hypertrophic zone

• Calcification zone

• Ossification zone

Growth in Width (Thickness)

Bones widen through appositional growth, which can occur throughout life. Osteoblasts build bone externally, while osteoclasts remove bone internally.

Hormonal Regulation of Bone Growth

Growth hormone stimulates epiphyseal plate activity; thyroid hormone modulates growth hormone. Sex hormones (testosterone, estrogens) promote growth spurts and induce plate closure.

Bone Remodeling

Bone remodeling replaces 5–10% of bone annually, involving both deposit (by osteoblasts) and resorption (by osteoclasts). Remodeling is regulated by hormonal and mechanical factors.

Control of Remodeling

Remodeling is controlled by:

• Hormonal feedback: Parathyroid hormone (PTH) increases blood calcium by stimulating osteoclasts; calcitonin lowers calcium levels.

• Mechanical stress: Wolff’s law states bones remodel in response to stress; thicker where stresses are greatest.

Bone Repair

Fractures are classified by position, completeness, and skin penetration. Repair involves four stages:

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Bone Disorders

Imbalances in bone deposit and resorption cause diseases:

• Osteomalacia and rickets: Poor mineralization; soft, weak bones due to vitamin D or calcium deficiency.

• Osteoporosis: Bone resorption exceeds deposit; common in aged, postmenopausal females.

• Paget’s disease: Excessive, disorganized bone remodeling; affects spine, pelvis, femur, skull.

Developmental Aspects of Bone

Embryonic skeleton ossifies predictably; most long bones begin ossifying by 8 weeks. At birth, most long bones are ossified except at epiphyses. Bone mass and healing decrease with age, especially in females and whites.