Skeletal Cartilage and Bone Structure: Anatomy & Physiology Study Notes

Skeletal Cartilages

Basic Structure, Types, and Locations

Skeletal cartilage is a resilient connective tissue found in various locations throughout the human body, providing support, flexibility, and structure. It is primarily composed of chondrocytes within a jelly-like extracellular matrix.

• Hyaline cartilage: Most abundant type; contains collagen fibers. Provides support, flexibility, and resilience.

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

• Fibrocartilage: Contains thick collagen fibers; offers great tensile strength. Found in menisci of knee, vertebral discs, and pubic symphysis.

Perichondrium is a layer of dense connective tissue that surrounds cartilage, containing blood vessels for nutrient delivery.

Type of Cartilage	Main Locations	Main Fibers
Hyaline	Articular (joints), costal (ribs), respiratory (larynx, trachea), nasal (nose)	Collagen
Elastic	External ear, epiglottis	Elastic
Fibrocartilage	Menisci, vertebral discs, pubic symphysis	Thick collagen

Functions of Bone

Support, Protection, Anchorage, Mineral Storage, Blood Cell Formation, Fat Storage, Hormone Production

Bones serve multiple essential functions in the body:

• Support: Bones act as pillars and support soft organs.

• Protection: Cranial bones protect the brain; vertebral column protects the spinal cord; rib cage and pelvis protect vital organs.

• Anchorage: Tendons and ligaments attach muscles to bones, enabling movement.

• Mineral storage: Bones store minerals such as calcium and phosphorus, releasing them as needed.

• Blood cell formation (Hematopoiesis): Occurs in red bone marrow of spongy bone; produces RBCs, WBCs, and platelets.

• Fat storage: Triglycerides stored in medullary cavity as yellow marrow.

• Hormone production: Osteocalcin is secreted by bones, helping regulate insulin secretion, glucose levels, and metabolism.

Classification of Bones

By Location and Shape

The adult human skeleton consists of 206 bones, divided into two main groups:

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles (pectoral and pelvic).

Bones are also classified by shape:

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

• Short bones: Cube-shaped (e.g., wrist, ankle); includes sesamoid bones (e.g., patella).

• Flat bones: Thin, flat, slightly curved (e.g., sternum, scapulae, ribs, skull bones).

• Irregular bones: Complicated shapes (e.g., vertebrae, hip bones, facial bones).

Gross Structure of Bones

Compact Bone and Spongy Bone

Bones are organs composed of several tissue types. The two main types of bone tissue are:

• Compact bone: Dense outer layer; appears smooth and solid. Structural units are osteons.

• Spongy bone: Honeycomb of small, needle-like or flat pieces called trabeculae; spaces filled with red or yellow bone marrow.

Structure of Typical Long Bone

• Diaphysis: Tubular shaft forming the long axis; consists mostly of compact bone and surrounds the medullary cavity (filled with yellow marrow).

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

• Epiphyseal plate: Growth plate between diaphysis and epiphysis; site of bone growth in children.

Membranes Covering Bone

• Periosteum: Dense irregular connective tissue covering external bone surfaces (except joints).

• Endosteum: Delicate connective tissue membrane lining internal bone surfaces and trabeculae of spongy bone.

Microscopic Anatomy of Bone

Bone Cells

There are four main types of bone cells:

1. Osteogenic cells: Mitotically active stem cells in periosteum and endosteum.

2. Osteoblasts: Bone-forming cells; secrete bone matrix (collagen and calcium-binding proteins).

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

4. Osteoclasts: Multinucleated cells responsible for bone resorption (breakdown of bone matrix).

Compact Bone Structure

• Osteon: Structural unit; elongated cylinder parallel to bone axis.

• Lamellae: Rings of bone matrix; contain collagen fibers and resist twisting.

• Central canals: Run through osteons; contain blood vessels and nerves.

• Perforating canals: Connect central canals at right angles; link periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Hair-like canals connecting lacunae and central canal; allow communication and nutrient/waste exchange.

Bone Cell Type	Main Function
Osteogenic cell	Stem cell; differentiates into osteoblasts
Osteoblast	Bone formation; secretes matrix
Osteocyte	Maintains bone matrix; stress sensor
Osteoclast	Bone resorption

Spongy Bone Structure

• Trabeculae: Structural units; align along stress lines to resist forces.

Bone Markings

Types and Functions

Bones have markings that serve as sites for muscle, ligament, and tendon attachment, joint surfaces, and passages for blood vessels and nerves.

• Projection: Bulge outward due to muscle pull or joint formation (e.g., tuberosity, crest, trochanter).

• Depression: Bowl- or groove-like cutout (e.g., fossa, notch).

• Opening: Hole or canal for passage of blood vessels and nerves (e.g., foramen, canal, fissure).

Marking Type	Examples
Projection	Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process
Depression	Fossa, notch
Opening	Foramen, canal, fissure, meatus

Key Terms and Definitions

• Chondrocyte: Cartilage cell found in lacunae.

• Osteon: Structural unit of compact bone.

• Lamellae: Concentric rings of bone matrix.

• Trabeculae: Lattice-like structures in spongy bone.

• Periosteum: Outer membrane covering bone.

• Endosteum: Inner membrane lining bone cavities.

• Epiphyseal plate: Growth plate in children.

• Medullary cavity: Central cavity in long bones; contains marrow.

Additional Info

• Collagen makes up about 90% of the protein in bone matrix.

• Osteocalcin is a hormone secreted by bone that helps regulate metabolism.

• Bone remodeling is a continuous process involving osteoblasts and osteoclasts.

• Growth in length of long bones occurs at the epiphyseal plate; growth in thickness occurs by appositional growth.