Module 6.1: Introduction to Bones as Organs

The Skeletal System

The skeletal system is composed of bones, joints, and their associated supporting tissues. Bones are made up of osseous tissue as well as other connective tissues such as bone marrow.

• Bones: Rigid organs that form the skeleton.

• Joints: Structures where two or more bones meet.

Functions of the Skeletal System

• Protection: Shields vital organs (e.g., skull protects the brain).

• Mineral Storage and Acid-Base Homeostasis: Stores minerals like calcium and phosphorus; helps regulate blood pH.

• Blood Cell Formation: Occurs in red bone marrow (hematopoiesis).

• Fat Storage: Yellow bone marrow stores triglycerides.

• Movement: Bones act as levers for muscles.

• Support: Provides structural framework for the body.

Bone Structure and Classification

Bones are classified by shape and internal structure. There are 206 bones in the adult human skeleton, grouped into five classes:

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

• Short bones: Approximately equal in length and width (e.g., carpals).

• Flat bones: Thin and broad (e.g., sternum, skull bones).

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

• Sesamoid bones: Small, flat, and oval-shaped; develop within tendons (e.g., patella).

Structure of a Long Bone

• Periosteum: Dense connective tissue covering the bone's outer surface.

• Perforating fibers (Sharpey's fibers): Anchor periosteum to bone.

• Diaphysis: Shaft of the bone.

• Epiphyses: Ends of the bone.

• Marrow cavity: Central cavity containing bone marrow.

• Compact bone: Dense outer layer.

• Spongy bone: Porous inner layer.

• Endosteum: Thin membrane lining the marrow cavity.

• Epiphyseal lines/plates: Growth plates in children; become lines in adults.

Module 6.2: Microscopic Structure of Bone Tissue

Bone Matrix

The extracellular matrix of bone is unique, containing both inorganic and organic components:

• Inorganic matrix: Mainly hydroxyapatite crystals (), providing strength and resistance to compression.

• Organic matrix: Osteoid, composed of collagen fibers, proteoglycans, and glycoproteins, providing flexibility and tensile strength.

Bone Cells

• Osteoblasts: Build bone by secreting bone matrix (bone deposition).

• Osteocytes: Mature bone cells that maintain bone matrix; reside in lacunae.

• Osteoclasts: Break down bone matrix (bone resorption).

Histology of Bone Tissue

• Osteon (Haversian system): Structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Lamellae: Layers of bone matrix.

• Central canal: Contains blood vessels and nerves.

• Canaliculi: Small channels connecting osteocytes.

• Interstitial and circumferential lamellae: Fill spaces between osteons and around the bone's circumference.

• Perforating (Volkmann's) canals: Connect central canals of adjacent osteons.

• Spongy bone: Contains trabeculae, which provide structural support and house bone marrow.

Module 6.3: Bone Formation (Ossification)

Ossification Processes

Ossification (or osteogenesis) is the process of bone formation, beginning in the embryo and continuing through childhood.

• Intramembranous ossification: Forms flat bones (e.g., skull, clavicles) from mesenchymal tissue.

• Endochondral ossification: Forms most bones of the body from a hyaline cartilage model.

Module 6.4: Bone Growth in Length and Width

Growth in Length

Long bones grow in length via longitudinal growth at the epiphyseal plate, which consists of several zones:

• Zone of reserve cartilage

• Zone of proliferation

• Zone of hypertrophy and maturation

• Zone of calcification

• Zone of ossification

Growth continues as long as mitosis occurs in the zone of proliferation. The epiphyseal plate closes after adolescence, ending longitudinal growth.

Growth in Width

Appositional growth increases bone diameter as osteoblasts in the periosteum lay down new bone.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates chondrocyte and osteogenic cell division, and osteoblast activity.

• Testosterone: Increases appositional growth and bone thickness in males; stimulates growth spurts.

• Estrogen: Increases longitudinal growth and accelerates epiphyseal plate closure in females.

Module 6.5: Bone Remodeling and Repair

Bone Remodeling

Bone remodeling is a continuous process where old bone is replaced by new bone. It involves:

• Bone deposition: Addition of new bone by osteoblasts.

• Bone resorption: Removal of bone by osteoclasts.

Factors Influencing Bone Remodeling

• Hormones (e.g., parathyroid hormone, calcitonin)

• Age

• Calcium and vitamin D intake

• Protein intake

• Mechanical stress (compression, tension, pressure)

Calcium Ion Homeostasis

Bone stores most of the body's calcium. Blood calcium levels are regulated by a negative feedback loop involving:

• Stimulus and receptor: Low blood calcium detected by parathyroid glands.

• Control center: Parathyroid glands secrete parathyroid hormone (PTH).

• Effector/response: PTH increases bone resorption, releasing calcium into the blood.

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity.

• Vitamin D: Promotes calcium absorption from the gut.

Bone Repair

Bones can be injured, most commonly by fractures. The repair process involves several steps:

1. A hematoma fills the gap between bone fragments.

2. Fibroblasts and chondroblasts form a soft callus.

3. Osteoblasts build a bone callus.

4. The bone callus is remodeled and replaced with secondary bone.

5. Bone repair typically takes 6-8 weeks.

Classification of Fractures

Example: A femoral shaft fracture is a long bone injury that may require surgical intervention and a period of immobilization for proper healing.