Skeletal System: Cartilage and Bone Structure, Development, and Function

Basic Structure, Types, and Locations of Cartilage

Overview of Skeletal Cartilage

Skeletal cartilage is a specialized connective tissue that provides support, flexibility, and resilience to various parts of the body. It is sculpted to fit specific body locations and functions.

• Skeletal cartilage: Made of some variety of cartilage tissue sculpted to fit its body location and function.

• Contains no nerves or blood cells.

• Surrounded by a layer of dense irregular connective tissue called the perichondrium.

• Acts as a reinforcement to resist outward expansion when the cartilage is compressed; the perichondrium contains blood vessels that nourish the cartilage cells.

• All types of cartilage tissues have the same basic components: cells called chondrocytes within an extracellular matrix.

Types of Cartilage

• Hyaline Cartilage: Most abundant type; provides support with flexibility and resilience. Contains spherical chondrocytes and only collagen fibers in its matrix.

  • Articular cartilage: Covers the ends of most bones at moveable joints.

  • Costal cartilage: Connects the ribs to the sternum.

  • Respiratory cartilage: Forms the skeleton of the larynx and reinforces other respiratory passages.

  • Nasals cartilage: Supports the external nose.

• Elastic Cartilage: Resembles hyaline cartilage but contains more stretchy elastic fibers. Found only in the external ear and epiglottis (flap that covers the opening of the larynx).

• Fibrocartilage: Highly compressible and has great tensile strength. Consists of roughly parallel rows of chondrocytes alternating with thick collagen fibers. Occurs in sites subjected to both pressure and stretch, such as the menisci of the knee and intervertebral discs.

Cartilage Growth and Development

Growth Mechanisms

Cartilage grows in two main ways:

• Appositional growth: Cartilage-forming cells in the surrounding perichondrium secrete new matrix against the external face of the existing cartilage tissue.

• Interstitial growth: Lacunae-bound chondrocytes divide and secrete new matrix, expanding the cartilage from within.

• Cartilage growth ends during adolescence, once the skeleton has stopped growing.

Functions of Bones

Seven Important Functions

Bones perform several vital functions in the body:

• Support: Provide a framework for the body.

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

• Anchorage: Skeletal muscles attach to bones by tendons, using bones as levers to move the body and its parts.

• Mineral storage: Bones act as reservoirs for minerals, especially calcium and phosphorus.

• Blood cell formation: Occurs in the red bone marrow of certain bones (hematopoiesis).

• Triglyceride (fat) storage: Source of energy for the body, stored in yellow bone marrow.

• Hormone production: Bones produce osteocalcin, a hormone that helps regulate insulin production.

Classification of Bones by Location and Shape

Axial and Appendicular Skeleton

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

• Appendicular skeleton: Consists of the bones of the upper and lower limbs and girdles that attach the limbs to the axial skeleton.

Bone Shapes

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

• Short bones: Roughly cube-shaped (e.g., bones of the wrist and ankle).

• Sesamoid bones: Special types of short bones that form in a tendon (e.g., patella).

• Flat bones: Thin, flattened, and usually curved (e.g., sternum, ribs, most cranial bones).

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

Bone Structure: Compact and Spongy Bone

Gross Anatomy

• Every bone has a dense outer layer that looks smooth and solid: compact bone.

• Internal to compact bone is spongy bone, which consists of a honeycomb of small needle-like or flat pieces called trabeculae.

Structure of Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone covered by compact bone.

• No shaft or expanded ends.

• Compact bone is covered outside and inside by connective tissue membranes.

Structure of a Typical Long Bone

• Diaphysis: Tubular shaft that forms the long axis of the bone; constructed of a thick collar of compact bone that surrounds a central medullary cavity (contains bone marrow).

• Epiphyses: Bone ends; absorb stress and cushion opposing bone ends during movement.

• Epiphyseal line: Between the diaphysis and each epiphysis of an adult long bone.

• Metaphysis: Flared portion of the bone where the diaphysis and epiphysis meet.

Membranes

• Periosteum: Double-layered membrane that covers the external surface of the entire bone except joint surfaces.

  • Outer fibrous layer: Dense irregular connective tissue.

  • Inner osteogenic layer: Contains osteoprogenitor cells.

  • Richly supplied with nerve fibers and blood vessels, anchoring for tendons and ligaments.

  • Perforating fibers: Bundles of collagen fibers that extend into the bone matrix.

• Endosteum: Delicate membrane covering internal bone surfaces.

Blood Vessels and Nerves

• Bones are well vascularized.

• Nutrient foramen: The nutrient artery and vein enter the bone through a hole in the wall of the diaphysis.

• Nerves accompany blood vessels through the nutrient foramen into the bone.

Hematopoietic Tissues in Bone

• Also called red bone marrow.

• Infants: Medullary cavity of the diaphysis and all areas of spongy bone contain red marrow.

• Adults: Red marrow has mostly been replaced by yellow marrow (fat storage).

• Red marrow in adults is found only in flat bones of the skull, sternum, ribs, clavicle, scapula, hip bones, vertebrae, thigh bone, and arm bone.

• Yellow marrow in the medullary cavity can revert to red marrow if a person becomes anemic.

Bone Markings

Bones have markings that provide information about how the bone and its attached muscles and ligaments work together. Markings fall into three categories:

• Projections: Sites of muscle and ligament attachment.

• Surfaces: Form joints.

• Depressions and openings: Allow passage of blood vessels and nerves.

Microscopic Anatomy of Bone

Cells of Bone Tissue

• Osteoprogenitor cells (osteogenic): Mitotically active stem cells found in the periosteum and endosteum.

• Osteoblasts: Bone-forming cells that secrete the bone matrix; actively mitotic.

• Osteocytes: Mature bone cells that occupy spaces (lacunae) that conform to their shape.

• Bone lining cells: Flat cells found on bone surfaces where bone remodeling is not occurring.

• Osteoclasts: Multinucleate cells located at sites of bone resorption.

Microscopic Anatomy of Compact Bone

• Osteon: Structural unit of compact bone.

• Canals and canaliculi: Central canal runs through the core of each osteon; perforating canals lie at right angles to the long axis of the bone and connect the blood and nerve supply of the medullary cavity.

• Interstitial and circumferential lamellae:

  • Interstitial lamellae: Lying between intact osteons are incomplete lamellae.

  • Circumferential lamellae: Found just deep to the periosteum and just superficial to the endosteum.

Microscopic Anatomy of Spongy Bone

• Poorly organized compared to compact bone.

• Trabeculae: Present to help bones resist stress.

Chemical Composition of Bone

Organic Components

• Include bone cells and osteoid (unmineralized bone matrix).

• Osteoid includes ground substance and collagen fibers.

Inorganic Components

• Mineral salts, primarily calcium phosphates, which account for bone hardness.

Bone Development: Ossification and Osteogenesis

Formation of the Bony Skeleton

Ossification and osteogenesis refer to the process of bone tissue formation. The embryonic skeleton can accommodate mitosis.

• Endochondral ossification: Bone develops by replacing hyaline cartilage (resulting bone: endochondral bone).

• Intramembranous ossification: Bone develops from a fibrous membrane (resulting bone: membranous bone).

Endochondral Ossification

All bones below the skull (except clavicles) form by endochondral ossification. The formation of a large long bone involves several steps:

1. A bone collar forms around the diaphysis of the hyaline cartilage: primary ossification center.

2. Cartilage calcifies in the center of the diaphysis and then develops cavities.

3. The periosteal bud invades the internal cavities and spongy bone forms (secondary ossification centers).

4. The diaphysis elongates and a medullary cavity forms.

5. The epiphyses ossify.

Once secondary ossification is complete, hyaline cartilage remains only on the epiphyseal surfaces and at the junction of the diaphysis and epiphysis.

Intramembranous Ossification

• Forms most of the bones of the skull and the clavicles.

Bone Growth

Growth in Length of Long Bones

Long bones grow in length at the epiphyseal plate, which consists of five zones:

• Resting zone

• Proliferation zone

• Hypertrophic zone

• Calcification zone

• Ossification zone

As adolescence ends, the chondroblasts of the epiphyseal plates divide less often, and longitudinal bone growth ends when the bone of the epiphysis and diaphysis fuse (epiphyseal plate closure).

Growth in Width (Thickness)

• Growing bones widen as they lengthen.

Hormonal Regulation of Bone Growth

• Occurs until young adulthood.

• Controlled by a symphony of hormones.

• Most vital during infancy and childhood: epiphyseal plate activity, growth hormone.

• At puberty, sex hormones are released in increasing amounts; estrogen has a critical role in bone development.

• Excess or deficits in any of these hormones can result in abnormal skeletal growth.

Bone Remodeling

Bone Deposition and Removal

Bone remodeling involves the continuous deposition and removal of bone tissue, replacing about 5-10% of the skeleton annually.

• Bone deposition: Osteoblasts secrete osteoid, which binds calcium and triggers mineralization.

• Bone resorption: Osteoclasts secrete acid and lysosomal enzymes that dissolve bone minerals and digest organic matrix.

Control of Remodeling

• Regulated by hormonal feedback (e.g., parathyroid hormone maintains calcium homeostasis).

• Mechanical and gravitational forces also influence bone remodeling.

Calcium Homeostasis

• The body contains 1200-1400g of calcium.

• Calcium is absorbed from the intestine under the control of the active form of vitamin D.

• Parathyroid hormone (PTH) is the main hormone regulating calcium levels.

Bone Repair

Fracture Treatment and Repair

• Closed (external) reduction: Bone ends are coaxed into position.

• Open (internal) reduction: Bone ends are secured together surgically with pins or wires.

Stages of Bone Repair

1. Hematoma forms: Blood vessels in the bone and periosteum are torn, forming a hematoma.

2. Fibrocartilaginous callus forms: New blood vessels grow into the clot, and fibroblasts and chondroblasts begin to repair the bone.

3. Bony callus forms: Osteoblasts form spongy bone, replacing the fibrocartilaginous callus.

4. Bone remodeling occurs: The bony callus is remodeled to resemble the original bone.

Bone Disorders

Osteomalacia and Rickets

• Osteomalacia: Disorders in which bones are poorly mineralized; osteoid is produced, but calcium salts are not adequately deposited.

• Rickets: Analogous disorder in children; causes bowed legs and deformities of the pelvis, skull, and rib cage. Often caused by insufficient calcium or vitamin D deficiency.

Osteoporosis

• Bone resorption outpaces bone deposit; bones become fragile and porous.

• Risk factors include aging, postmenopausal women, genetics, smoking, and certain medications.

• Prevention includes adequate vitamin D and calcium intake, weight-bearing exercise.

Paget's Disease

• Excessive and haphazard bone deposit and resorption.

• High ratio of spongy bone to compact bone; bones are made quickly and are abnormal.