1. Functions of the Skeletal System

Overview

The skeletal system provides the structural framework for the body and supports vital physiological functions. It is essential for movement, protection, mineral storage, and blood cell production.

• Support and Framework: Bones provide a rigid structure, supporting the body and protecting soft organs.

• Protection: Bones shield vital organs, such as the brain (skull) and spinal cord (vertebral column).

• Anchorage and Movement: Muscles attach to bones, enabling movement at joints.

• Mineral Storage: Bones store minerals, especially calcium and phosphate, which are released into the bloodstream as needed.

• Blood Cell Formation (Hematopoiesis): Bone marrow produces blood cells, primarily in the red marrow of certain bones.

• Fat Storage (Triglyceride Storage): Yellow marrow stores fat in the cavities of long bones.

• Hormone Production: Bones produce osteocalcin, a hormone involved in insulin secretion and energy regulation.

2. Blood and Nerve Supply of Long Bones

Vascularization and Innervation

Bones are highly vascularized, receiving blood and nerve supply through specialized structures.

• Vessels of the Diaphysis: The nutrient artery and vein enter the shaft through the nutrient foramen, supplying the bone marrow and compact bone.

• Vessels of the Epiphyses: Each epiphysis has its own arterial and venous supply.

• Vessels of Compact Bone: The Haversian and Volkmann's canals contain blood vessels and nerves, connecting the bone's interior to the periosteum and central canal.

• Nerves: Nerves accompany blood vessels, entering through the nutrient foramen and innervating the periosteum.

3. Histological Features of Osseous (Bone) Tissue

Microscopic Structure

Bone tissue consists of specialized cells and an extracellular matrix that provides strength and flexibility.

• Osteocytes: Mature bone cells located in lacunae, responsible for maintaining bone tissue.

• Osteoblasts: Bone-forming cells that secrete the bone matrix.

• Osteoclasts: Large, multinucleated cells involved in bone resorption (breakdown).

• Extracellular Matrix: Composed of collagen fibers (providing tensile strength) and hydroxyapatite crystals (providing hardness).

4. Cells of Bone Tissue

Specialized Bone Cells

• Osteoblasts: Actively produce bone matrix and play a role in matrix calcification.

• Osteocytes: Mature bone cells that maintain bone tissue and communicate via canaliculi.

• Osteoclasts: Responsible for bone resorption, breaking down bone matrix and releasing minerals.

5. Organization of Compact and Spongy Bone

Structural Differences

Bones are organized into compact (cortical) and spongy (trabecular) bone, each with distinct features.

• Compact Bone: Dense outer layer, organized into osteons (Haversian systems) with concentric lamellae and central canals.

• Spongy Bone: Honeycomb-like structure of trabeculae, providing strength with minimal weight.

6. Cartilage: Structure and Function

Flexible Support Tissue

Cartilage provides flexible support and cushioning in joints and other structures.

• Extracellular Matrix (ECM): Rich in water, collagen, and elastic fibers.

• Cells: Chondrocytes reside in lacunae and maintain the matrix.

• Vascularity/Nerves: Cartilage is avascular and lacks nerves; nutrients diffuse from the perichondrium.

7. Distinction Between the Three Types of Cartilage

Comparison Table

The three types of cartilage—hyaline, elastic, and fibrocartilage—differ in structure, function, and location.

8. Growth of Cartilage

Mechanisms of Cartilage Growth

9. Processes Involved in Bone Formation

Ossification and Calcification

• Ossification (Osteogenesis): Formation of bone tissue from osteoblasts.

• Calcification: Deposition of calcium salts, hardening the bone matrix.

10. Definitions of Intramembranous Ossification and Endochondral Ossification

Bone Development Pathways

• Endochondral Ossification: Bone forms by replacing a hyaline cartilage model. Most bones below the skull are formed this way.

• Intramembranous Ossification: Bone develops from a fibrous membrane, forming flat bones like the skull and clavicle.

11. Growth in Length and Thickness of Bones

Epiphyseal Plate and Appositional Growth

• Length: Growth occurs at the epiphyseal plate through zones of proliferation, hypertrophy, calcification, and ossification.

• Thickness: Appositional growth adds new bone matrix to the periosteal surface.

12. Bone Remodelling and its Function

Continuous Bone Renewal

Bone remodelling is the process of bone deposition and resorption, maintaining bone strength and mineral homeostasis.

• Calcium Homeostasis: Regulates blood calcium levels.

• Response to Stress: Bones remodel in response to mechanical stress (Wolff's Law).

13. Role of Hormones in Blood Calcium Balance

Hormonal Regulation

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts.

• Vitamin D: Increases calcium absorption in the intestine.

14. Classification of Bones Based on Shape

Bone Types

• Long Bones: Longer than wide; shaft and two ends (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., wrist, ankle).

• Flat Bones: Thin, flattened, often curved (e.g., sternum, scapula, ribs).

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

• Sesamoid Bones: Embedded in tendons (e.g., patella).

• Sutural Bones: Small bones within cranial sutures.

15. Axial and Appendicular Divisions of the Skeleton

Organization of the Human Skeleton

• Axial Skeleton: Skull, vertebral column, and thoracic cage (80 bones). Provides support and protection for the brain, spinal cord, and thoracic organs.

• Appendicular Skeleton: Limbs and girdles (126 bones). Enables movement and manipulation of the environment.

Fontanelles

• Location: Membrane coverings connecting incomplete skull bones at birth.

• Functions:

  • Allow slight skull compression during birth.

  • Permit brain growth after birth.