Bones and Skeletal Tissues

Skeletal Cartilages

The human skeleton initially consists of cartilage, which is gradually replaced by bone except in areas requiring flexibility. Skeletal cartilage is composed of highly resilient, molded tissue that is primarily water, providing flexibility and cushioning. It contains no blood vessels or nerves and is made up of chondrocytes, cells encased in lacunae within a jelly-like extracellular matrix.

• Hyaline cartilage: Provides support, flexibility, and resilience; most abundant; contains only collagen fibers. Found in articular (joints), costal (ribs), respiratory (larynx), and nasal (nose tip) cartilages.

• Elastic cartilage: Similar to hyaline but contains elastic fibers. Located in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers for great tensile strength. Found in menisci of the knee and intervertebral discs.

Growth of Cartilage

Cartilage grows by two mechanisms:

• Appositional growth: Cartilage-forming cells in the perichondrium secrete matrix on the external face of existing cartilage.

• Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Functions of Bones

Bones serve seven essential functions in the human body:

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

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

• Movement: Act as levers for muscle action.

• Mineral and growth factor storage: Reservoir for 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 source.

• Hormone production: Osteocalcin regulates insulin secretion, glucose levels, and metabolism.

Classification of Bones

Axial and Appendicular Skeleton

The 206 named bones in the human skeleton are divided into two groups:

• Axial skeleton: Long axis of the body; includes the skull, vertebral column, and rib cage.

• Appendicular skeleton: Bones of the upper and lower limbs and girdles attaching limbs to the axial skeleton.

Classification by Shape

Bones are classified into four shapes:

• Long bones: Longer than they are wide; found in limbs.

• Short bones: Cube-shaped; found in wrist and ankle. Sesamoid bones form within tendons (e.g., patella).

• Flat bones: Thin, flat, and slightly curved; includes sternum, scapulae, ribs, and most skull bones.

• Irregular bones: Complicated shapes; includes vertebrae and 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 that appears smooth and solid.

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

Structure of Short, Irregular, and Flat Bones

These bones consist of thin plates of spongy bone (diploë) covered by compact bone. The periosteum covers the outside, and the endosteum covers the inside. Bone marrow is scattered throughout spongy bone, with no defined marrow cavity. Hyaline cartilage covers areas involved in movable joints.

Structure of Typical Long Bone

Long bones have a shaft (diaphysis), bone ends (epiphyses), and membranes:

• Diaphysis: Tubular shaft forming the long axis, consisting of compact bone surrounding the medullary cavity (filled with yellow marrow in adults).

• Epiphyses: Ends of long bones, with compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of the childhood epiphyseal plate where bone growth occurs.

Membranes: Periosteum and Endosteum

• Periosteum: White, double-layered membrane covering external surfaces except joints. The fibrous layer consists of dense irregular connective tissue (Sharpey's fibers), and the osteogenic layer contains stem cells.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces, including trabeculae and canals. Contains osteogenic cells.

Hematopoietic Tissue in Bones

Red marrow is found within trabecular cavities of spongy bone and diploë of flat bones. In newborns, all spongy bone and medullary cavities contain red marrow. In adults, red marrow is mainly in flat bone diploë and some irregular bones. Yellow marrow can convert to red if needed.

Bone Markings

Bone markings are sites for muscle, ligament, and tendon attachment, joint formation, or passageways for blood vessels and nerves. They are classified as:

• Projection: Outward bulge of bone, often due to muscle pull or joint modification.

• Depression: Bowl- or groove-like cut-out serving as passageways or joint surfaces.

• Opening: Hole or canal for passageways of blood vessels and nerves.

Microscopic Anatomy of Bone

Cells of Bone Tissue

Five major cell types are present in bone tissue:

• Osteogenic cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), composed of collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress sensors.

• Bone-lining cells: Flat cells on bone surfaces, maintaining matrix.

• Osteoclasts: Giant, multinucleate cells responsible for bone resorption; derived from hematopoietic stem cells.

Compact Bone (Lamellar Bone)

Compact bone consists of:

• Osteon (Haversian system): Structural unit; elongated cylinder with rings of bone matrix (lamellae).

• Canals and canaliculi: Central canal contains blood vessels and nerves; canaliculi connect lacunae and allow communication.

• Interstitial and circumferential lamellae: Fill gaps between osteons and extend around the diaphysis.

Spongy Bone

Spongy bone is organized along lines of stress, with trabeculae conferring strength. It lacks osteons but contains irregularly arranged lamellae and osteocytes interconnected by canaliculi. Capillaries in the endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

Organic components include osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid. Osteoid, secreted by osteoblasts, consists of ground substance and collagen fibers, contributing to tensile strength and flexibility. Sacrificial bonds between collagen molecules provide resilience.

Inorganic Components

Inorganic components are mainly hydroxyapatites (mineral salts), making up 65% of bone mass. These are tiny calcium phosphate crystals in and around collagen fibers, responsible for hardness and resistance to compression. Bone is half as strong as steel in resisting compression and as strong as steel in tension.

Summary Table: Organic vs. Inorganic Components

End of Lecture