RC

Structure and Functions of Bone

Structure of Bone

  • A long bone consists of several essential parts:

    • Diaphysis (Bone Shaft): The long, central part of the bone.

    • Epiphyses: The two ends of the bone at the joints, subdivided into:

    • Proximal Epiphysis: The end nearest to the body.

    • Distal Epiphysis: The end farthest from the body.

    • Metaphysis: The region between diaphysis and epiphysis, including two metaphyses in conjunction with the epiphyses.

    • Articular Cartilage: Cartilage covering both epiphyses for joint cushioning.

    • Spongy Bone: Contains red or yellow bone marrow, contributing to hematopoiesis.

    • Red Bone Marrow: Involved in blood cell formation.

    • Yellow Bone Marrow: Primarily composed of adipose tissue.

    • Epiphyseal Line: A remnant of the growth plate (epiphyseal plate), indicating where growth occurred during adolescence.

    • Periosteum: The connective tissue surrounding the diaphysis, rich in blood vessels and nerve fibers.

    • Medullary Cavity: A hollow space within the diaphysis containing bone marrow.

    • Endosteum: A thin membrane lining the medullary cavity, essential for bone growth and repair.

  • Nutrient Artery: Supplies blood to the bone.

  • Compact Bone: Dense bone tissue that forms the outer layer.

Functions of Bone and the Skeletal System

  • Bone is classified as an organ made up of various tissue types, including:

    • Bone itself

    • Cartilage

    • Dense connective tissue

    • Adipose tissue

    • Nervous tissue

  • The entire anatomical framework consisting of bones and their cartilages forms the Skeletal System.

  • Bones fulfill seven key functions:

    1. Support: Provides structure and support for the body and soft organs.

    2. Protection: Safeguards critical components such as the brain, spinal cord, and vital organs.

    3. Movement: Serves as levers for muscle actions, facilitating coordinated movement.

    4. Mineral Homeostasis and Growth Factor Storage: Acts as a reservoir for minerals (calcium and phosphorus) and growth factors.

    5. Blood Cell Formation: Hematopoiesis takes place in the red marrow cavities of specific bones.

    6. Triglyceride Storage: Stores fat in bone cavities, primarily in yellow marrow, which serves as a potential energy source.

    7. Hormone Production: Secretes osteocalcin, which regulates insulin secretion, glucose levels, and energy metabolism.

Types of Bone Cells

  • Four primary cell types exist within bone tissue:

    1. Osteoprogenitor Cells:

    • Stem cells capable of differentiating into other bone cell types.

    • Serve as precursors to osteoblasts.

    1. Osteoblasts:

    • Bone-building cells responsible for secreting the bone matrix and initiating calcification.

    1. Osteocytes:

    • Mature bone cells that maintain bone tissue health and homeostasis.

    1. Osteoclasts:

    • Cells involved in bone resorption, breaking down the bone matrix to release calcium.

    • Feature a ruffled border that enhances bone resorption.

  • The cellular architecture of spongy bone includes structures such as:

    • Bone Lacunae: Spaces housing osteocytes within the bone.

    • Concentric Bone Lamellae: Layers of bone matrix around a central canal.

    • Bone Canaliculi: Small channels that allow nutrient and waste exchange between osteocytes.

Bone Growth: Endochondral Ossification

  • Endochondral Ossification: A key process where bone develops from hyaline cartilage, essential for lengthening long bones.

    • Occurs in the epiphyseal plates of long bones as they grow.

    • Interstitial Growth: Involves the growth of cartilage within the epiphyseal plate.

    • The epiphyseal plate maintains constant thickness as:

    • The rate of cartilage growth on one side is balanced by bone replacement on the opposite side.

    • The epiphyseal plate consists of five zones:

    1. Resting (Quiescent) Zone: A zone of inactive chondrocytes.

    2. Proliferation (Growth) Zone: Active chondrocytes undergoing mitosis, producing new cells.

    3. Hypertrophic Zone: Chondrocytes enlarge and prepare for calcification.

    4. Calcification Zone: Chondrocytes undergo apoptosis and the matrix calcifies.

    5. Ossification (Osteogenic) Zone: New bone is formed from the calcified cartilage.

Types of Bone: Compact vs. Spongy Bone

  • The skeleton comprises various bone types, characterized by varying amounts of compact and spongy bone.

    • Compact Bone:

    • Dense, strong tissue providing protection and support; forms the outer layer of bones.

    • Spongy Bone:

    • Lighter and less dense; composed of interconnected trabecular structures, also called cancellous bone.

    • Provides support but with less weight.

  • Visual descriptions of bone architecture:

    • Medullary Cavity: Found in the diaphysis, housing bone marrow.

    • Internal Circumferential Bone Lamellae: Layers of bone matrix surrounding the medullary cavity.

    • External Circumferential Bone Lamellae: Layers on the surface of the bone.

    • Osteons: The functional unit in compact bone consisting of bone lamellae surrounding a central canal that contains blood vessels.

  • The organization of bone includes:

    • Perforating Canals: Pathways that allow blood vessels to reach the inner bone structure.

    • Periosteum: A double-layered connective tissue that surrounds the outer surface of all bones, consisting of an outer fibrous layer and an inner osteogenic layer.