Bones and Skeletal Tissue: Structure, Function, and Development

1. Functions of the Skeletal System

Overview

The skeletal system provides the framework for the body and performs several essential functions that support movement, protection, and homeostasis.

• Support and Framework: Bones create a basic structure and shape for the body, supporting soft organs.

• Protection: Bones shield vital organs; for example, the skull protects the brain, and the vertebral column surrounds the spinal cord.

• Anchorage and Movement: Muscles attach to bones, and bones act as levers to facilitate movement.

• 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 cavities 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 regulating insulin secretion, glucose homeostasis, and energy expenditure.

2. Blood and Nerve Supply of Long Bones

Vascularization and Innervation

Bones are highly vascularized, unlike cartilage, and receive 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 system (osteons) contains a central canal for blood vessels and nerves.

• Periosteal Vessels: The periosteum receives blood via vessels passing through the nutrient foramen.

• Nerve Supply: Nerves accompany blood vessels and are responsible for pain sensation in broken bones.

3. Histological Features of Osseous (Bone) Tissue

Microscopic Structure

Bone tissue is a dynamic, living tissue composed of several cell types and an extracellular matrix.

• Osteoblasts: Bone-forming cells that secrete the bone matrix; play a role in matrix calcification.

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

• Osteoclasts: Multinucleated cells responsible for bone resorption (breaking down bone).

• Bone Matrix: Composed of collagen fibers (organic) and hydroxyapatite crystals (inorganic).

• Canaliculi: Tiny canals that connect osteocytes for nutrient and waste exchange.

4. Cells

Specialized Bone Cells

• Osteoblasts: Secrete bone matrix and are active during bone growth and repair.

• Osteocytes: Mature cells that maintain bone tissue.

• Osteoclasts: Break down bone tissue during remodeling.

5. Organization of Compact and Spongy Bone

Bone Structure

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

• Compact Bone: Dense outer layer composed of osteons (Haversian systems).

• Osteon: Cylindrical structure with concentric lamellae around a central canal.

• Lamellae: Rings of bone matrix that resist torsion and stress.

• Central Canal: Contains blood vessels and nerves.

• Canaliculi: Radiate from the lacunae, connecting osteocytes.

• Spongy Bone: Honeycomb structure of trabeculae; spaces filled with marrow.

6. Cartilage Structure and Properties

Flexible Support Tissue

Cartilage provides flexible support and is composed of cells and an extracellular matrix.

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

• Chondrocytes: Cartilage cells located in lacunae.

• Vascularity: Cartilage is avascular; 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.

Cartilage Type	Structure	Function	Location
Hyaline Cartilage	Most abundant; matrix has fine (Type II) collagen fibers only. Looks like frosted glass.	Provides support with flexibility and resilience. Cushions the ends of bones and resists stress.	Articular cartilages (ends of long bones at joints), costal cartilages (ribs to sternum), respiratory cartilages (larynx, trachea), nasal cartilages (external nose).
Elastic Cartilage	Resembles hyaline cartilage but contains abundant stretchy elastic fibers. ECM is highly elastic.	Better able to stand up to repeated bending. Maintains shape and flexibility.	External ear, epiglottis (flap covering the larynx/esophagus).
Fibrocartilage	Consists of roughly parallel rows of chondrocytes alternating with thick collagen fibers.	Highly compressible with great tensile strength. Withstands pressure and tension.	Padlike cartilages (menisci of knee) and intervertebral discs between vertebrae.

8. Cartilage Growth

Appositional vs. Interstitial Growth

Growth Type	Mechanism	Process	Tissue Application
Appositional Growth	Growth "from the outside".	Cartilage-forming cells in the surrounding perichondrium secrete new matrix against the external face of the existing cartilage tissue.	Primary method for cartilage growth in childhood and adolescence.
Interstitial Growth	Growth "from within".	Chondrocytes inside the lacunae divide and secrete new matrix, thereby expanding the cartilage from the inside.	Crucial for the growth of epiphyseal plates (growth plates) of long bones.

9. Processes Involved in Bone Formation

Ossification and Calcification

• Ossification (Osteogenesis): The process of bone tissue formation, including growth, remodeling, and repair.

• Calcification: Hardening of bone matrix by deposition of calcium salts.

• Appositional Growth: Bone increases in diameter by adding new bone tissue to the surface.

• Interstitial Growth: Bone increases in length by expanding cartilage at the epiphyseal plate.

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. This process forms flat bones such as the skull and clavicle.

11. Growth in Length and Thickness of Bones

Epiphyseal Plate and Appositional Growth

• Epiphyseal Plate: Growth in length occurs at the cartilage plate between the epiphysis and diaphysis.

• Appositional Growth: Growth in thickness occurs by adding new bone tissue to the surface.

• Zones of Epiphyseal Plate: Includes proliferation, hypertrophy, calcification, and ossification zones.

12. Bone Remodelling and its Function

Continuous 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).

• Remodelling Cycle: Osteoclasts resorb bone, osteoblasts deposit new bone.

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.

Key Equation:

$\text{Blood Calcium Homeostasis:}~\text{PTH} \uparrow \Rightarrow \text{Ca}^{2+}~\text{release from bone}$ $\text{Calcitonin} \uparrow \Rightarrow \text{Ca}^{2+}~\text{deposition in bone}$

14. Classification of Bones Based on Shape

Bone Types

• Long Bones: Longer than they are wide; e.g., femur, humerus.

• Short Bones: Cube-shaped; e.g., wrist and ankle bones.

• Flat Bones: Thin, flattened, and usually curved; e.g., sternum, scapula, ribs.

• Irregular Bones: Complicated shapes; e.g., vertebrae, hip bones.

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

• Sutural Bones: Small bones found in the sutures of the skull.

15. Axial and Appendicular Divisions of the Skeleton

Skeleton Organization

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage (ribs and sternum); supports and protects organs.

• Appendicular Skeleton: Includes the limbs and girdles (shoulder and pelvic); 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.