Introduction to Skeletal Tissues

The skeletal system is composed of specialized connective tissues that provide structure, support, and protection to the body. The two primary supporting connective tissues are cartilage and bone, each with unique properties and functions.

Classification of Connective Tissues

Connective tissues are broadly classified into three categories: connective tissue proper, fluid connective tissues, and supporting connective tissues. Cartilage and bone are the main supporting connective tissues, providing the framework for the body.

Cartilage

General Features of Cartilage

Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints and is a structural component of the rib cage, ear, nose, bronchial tubes, and intervertebral discs.

• Prefix "chondro-" refers to cartilage.

• Cartilage contains few cells (chondrocytes) and an abundance of collagen fibers embedded in a ground substance called chondroitin sulfate, which may contain insoluble calcium salts.

• All cartilage is avascular (lacks blood vessels).

• Chondroblasts mature into chondrocytes, which reside in spaces called lacunae.

Suffixes in Skeletal Tissues

• -blast: Indicates a cell that is growing or actively secreting extracellular matrix (e.g., chondroblast, osteoblast).

• -clast: Indicates a cell that is breaking down matrix (e.g., osteoclast).

• -cyte: Indicates a mature cell that maintains the tissue (e.g., chondrocyte, osteocyte).

Types of Cartilage

There are three main types of cartilage, each with distinct locations and functions:

Hyaline Cartilage

• Function: Reduces friction at joints, maintains open respiratory passages.

• Location: Ends of long bones, ribs, larynx, trachea.

• Features: Chondrocytes often found in pairs within lacunae; smooth, glassy appearance.

Elastic Cartilage

• Function: Provides flexible support.

• Location: Auricle of the ear, tip of the nose, epiglottis.

• Features: Netlike pattern of dark elastic fibers; chondrocytes in large, closely packed lacunae.

Fibrocartilage

• Function: Shock absorption, friction reduction, protection of bones at joint surfaces.

• Location: Pubic symphysis, intervertebral discs, menisci of knee.

• Features: Numerous collagen fiber bundles; chondrocytes arranged in rows between fibers.

Bone (Osseous Tissue)

General Features of Bone

Bone is a rigid connective tissue that forms the skeleton. It provides support, protection, and enables movement. Bone tissue is highly vascularized and contains specialized cells and a mineralized matrix.

• Prefix "osteo-" refers to bone.

• Osteoblasts secrete the organic component of bone matrix (osteoid), which is later mineralized into hydroxyapatite crystals and calcium salts.

• Once surrounded by matrix, osteoblasts mature into osteocytes (reside in lacunae).

• Osteoclasts are responsible for bone resorption (breakdown).

• The periosteum is the outer covering; the endosteum lines the medullary cavity.

Types of Bone Tissue

• Spongy (cancellous/trabecular) bone: Composed of an open network of bony struts (trabeculae); directs forces and reduces weight.

• Compact (cortical) bone: Dense, solid layers; provides strength and support.

• Most bones contain both types.

Osteon: The Functional Unit of Compact Bone

The osteon (Haversian system) is the basic structural unit of mature compact bone.

• Osteocytes reside in lacunae between layers of matrix called lamellae (concentric and circumferential).

• Central (Haversian) canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) canals: Run perpendicular to central canals, connecting blood supply.

• Canaliculi: Tiny channels connecting lacunae, allowing communication and nutrient exchange.

Classification of Bones by Shape

Adult humans typically have 206 bones, classified by shape:

• Long bones (e.g., femur, humerus)

• Short bones (e.g., carpals, tarsals)

• Flat bones (e.g., skull, sternum)

• Irregular bones (e.g., vertebrae)

• Sesamoid bones (e.g., patella)

Gross Anatomy of a Long Bone

• Epiphyses: Ends of the bone, mostly spongy bone, covered with articular (hyaline) cartilage.

• Diaphysis: Shaft, mostly compact bone, directs forces through the body.

• Metaphysis: Region between diaphysis and epiphysis; in growing bones, contains the growth plate.

• Medullary cavity: Central cavity containing yellow marrow (fat storage); red marrow (hematopoiesis) found in spongy bone of epiphyses and flat bones.

Bone Development (Ossification)

Types of Ossification

• Intramembranous ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of skull).

• Endochondral ossification: Bone forms by replacing a hyaline cartilage model (e.g., long bones).

Zones of Ossification (Epiphyseal Plate)

The growth plate (epiphyseal plate) is responsible for longitudinal bone growth and consists of several zones:

• Resting (reserve) zone: Quiescent chondrocytes.

• Proliferation zone: Rapidly dividing chondrocytes.

• Hypertrophy zone: Chondrocytes enlarge and mature.

• Calcification zone: Chondrocytes die, matrix calcifies, and is replaced by bone tissue.

Mnemonic: Rats Prefer Houses with Cheese (Resting, Proliferation, Hypertrophy, Calcification).

Summary Table: Cartilage vs. Bone

Example: The femur is a long bone that develops via endochondral ossification, contains both compact and spongy bone, and has articular cartilage at its epiphyses.

Additional info: Bone remodeling is a lifelong process involving osteoblasts (formation) and osteoclasts (resorption), essential for calcium homeostasis and repair.