Bones and Skeletal Tissue: Structure, Function, and Development

Bones and Skeletal Tissue

Skeletal Cartilage and Its Types

The human skeleton is initially formed from hyaline cartilage, which is gradually replaced by bone except in areas requiring flexibility. Cartilage is a resilient tissue composed mainly of water, with chondrocytes housed in lacunae within a jelly-like matrix. The perichondrium surrounds cartilage, providing nutrients and resisting expansion.

• Hyaline cartilage: Provides support, flexibility, and resilience; found in articular surfaces, costal cartilage, respiratory structures, and nasal cartilage.

• Elastic cartilage: Contains elastic fibers; found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers for tensile strength; found in menisci and intervertebral discs.

Growth of Cartilage

Cartilage grows by two mechanisms:

• Appositional growth: New matrix is added to the surface by cells in the perichondrium, increasing width.

• Interstitial growth: Chondrocytes divide within the cartilage, expanding it from within and increasing length.

Functions of Bones

General Functions

Bones serve multiple essential functions in the body:

• Support: Provide structural framework for the body and soft organs.

• Protection: Shield vital organs such as the brain, spinal cord, and thoracic organs.

• Movement: Act as levers for muscle action.

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

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

• Triglyceride storage: Fat stored in bone cavities serves as an energy reserve.

• Hormone production: Osteocalcin regulates insulin secretion and metabolism.

Classification of Bones

Axial and Appendicular Skeleton

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

• Axial skeleton: Skull, vertebral column, and rib cage (long axis of the body).

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

Classification by Shape

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

• Short bones: Cube-shaped, nearly equal in length and width (e.g., carpals, tarsals).

• Flat bones: Thin, flattened, and usually curved (e.g., sternum, skull).

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

Bone Structure

Gross Anatomy: Compact and Spongy Bone

Bones are organs containing various tissues. The two main types of bone tissue are:

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

• Spongy bone: Honeycomb structure of trabeculae, with spaces filled by bone marrow.

Gross Anatomy of Long Bones

Long bones have distinct regions:

• Diaphysis: Shaft providing leverage and support; contains medullary cavity with marrow.

• Epiphysis: Ends of the bone; composed of compact and spongy bone, covered by articular cartilage.

• Metaphysis: Region between diaphysis and epiphysis; contains the epiphyseal plate (growth plate).

• Medullary cavity: Central cavity containing red marrow in children and yellow marrow in adults.

Periosteum and Endosteum

• Periosteum: Double-layered membrane covering external bone surfaces (except joints); fibrous layer secures bone, osteogenic layer contains stem cells.

• Endosteum: Delicate membrane lining internal bone surfaces and canals; contains osteogenic cells.

Bone Marrow

• Red marrow: Site of hematopoiesis; found in trabecular cavities of spongy bone and diploë of flat bones.

• Yellow marrow: Fat storage; can convert to red marrow if needed.

Bone Markings

Classification and Examples

Bone markings are features that serve as sites for muscle attachment, joint formation, or passage of blood vessels and nerves.

Name of Bone Marking	Description	Illustrations
Projection (e.g., tuberosity, crest, trochanter)	Sites of muscle and ligament attachment	Examples: femur, pelvis
Surface (e.g., facet, condyle)	Help to form joints	Examples: mandible, skull
Depression or Opening (e.g., foramen, fossa)	Passage for blood vessels and nerves	Examples: skull, vertebrae

Microscopic Anatomy of Bone

Bone Cells

Five major bone cell types, all derived from a common lineage:

• Osteogenic cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete osteoid (collagen and proteins).

• Osteocytes: Mature bone cells in lacunae; maintain matrix and sense mechanical stress.

• Bone-lining cells: Flat cells maintaining bone matrix; called periosteal or endosteal cells depending on location.

• Osteoclasts: Multinucleate cells responsible for bone resorption; derived from hematopoietic stem cells.

Compact Bone Structure

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

• Lamellae: Rings of bone matrix; collagen fibers run in different directions for strength.

• Central canal: Contains blood vessels and nerves.

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

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae and central canal.

• Interstitial and circumferential lamellae: Fill gaps and encircle diaphysis.

Spongy Bone Structure

• Trabeculae: Irregularly arranged lamellae and osteocytes; organized along lines of stress.

• Capillaries: Supply nutrients via endosteum.

Chemical Composition of Bone

Organic and Inorganic Components

• Organic: Cells and osteoid (collagen, ground substance); provides tensile strength and flexibility.

• Inorganic: Hydroxyapatites (calcium phosphate crystals); responsible for hardness and compression resistance.

Bone Development and Growth

Ossification (Osteogenesis)

Bone tissue formation begins in the second month of development and continues throughout life.

• Endochondral ossification: Bone replaces hyaline cartilage; forms most bones, especially long bones.

• Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones of the skull, some facial bones, and part of the clavicle.

Postnatal Bone Growth

• Interstitial growth: Lengthwise growth at the epiphyseal plate.

• Appositional growth: Increase in bone thickness.

Epiphyseal Plate Zones

• Resting zone: Inactive cartilage.

• Proliferation zone: Rapid cell division.

• Hypertrophic zone: Older chondrocytes enlarge.

• Calcification zone: Matrix calcifies, chondrocytes die.

• Ossification zone: New bone forms.

Appositional Growth

Bones thicken in response to mechanical stress; osteoblasts deposit new matrix, osteoclasts remove bone from internal surfaces.

Hormonal Regulation and Bone Remodeling

Hormones Influencing Bone Growth

• Growth hormone: Stimulates epiphyseal plate activity.

• Thyroid hormone: Modulates growth hormone activity.

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

• Calcitonin and parathyroid hormone (PTH): Regulate blood calcium levels.

Bone Remodeling

• Bone deposit: Osteoblasts lay down new matrix.

• Bone resorption: Osteoclasts break down bone matrix, releasing minerals.

Calcium Homeostasis

Blood calcium levels are tightly regulated by PTH and calcitonin. PTH increases blood calcium by stimulating osteoclasts; calcitonin lowers blood calcium by inhibiting osteoclasts.

Fractures and Bone Disorders

Types of Fractures

• Complete: Bone broken through.

• Compound: Bone breaks through skin.

• Simple: Bone does not break skin.

• Greenstick, hairline, compression, stress: Various patterns and severities.

Fracture Healing

1. Hematoma formation: Blood clot forms at fracture site.

2. Fibrocartilaginous callus formation: Repair tissue bridges the break.

3. Bony callus formation: New trabeculae form, converting callus to bone.

4. Bone remodeling: Bone is reshaped and restored.

Bone Disorders

• Osteomalacia: Poor mineralization; soft, weak bones.

• Rickets: Osteomalacia in children; bone deformities due to vitamin D deficiency.

• Osteoporosis: Bone resorption exceeds deposit; decreased bone mass and increased fracture risk.

• Paget's disease: Excessive, disorganized bone remodeling; abnormal bone structure.

Timeline of Bone Development

Most long bones begin ossifying by 8 weeks of development, with primary ossification centers by week 12. At birth, most long bones are ossified except at epiphyses. Epiphyseal plates persist through childhood and adolescence, and complete ossification occurs by age 25.