Skeletal System: Structure, Function, and Development

Skeletal System Overview

Components and Functions

The skeletal system is a vital organ system composed of bones, cartilages, tendons, and ligaments. It provides the framework for the body, supports movement, and protects internal organs.

• Body Support: Bones are suited for bearing weight, holding the head up, and supporting the spinal cord.

• Organ Protection: The skull protects the brain; ribs protect the heart and lungs.

• Body Movement: Bones provide points of attachment for muscles. Contraction of skeletal muscles moves bones.

• Mineral Storage: Bones store calcium and phosphorus, essential for various body functions.

• Blood Cell Production: Cavities in bones contain red bone marrow, which produces blood cells.

• Triglyceride Storage: Adipose cells in yellow bone marrow store triglycerides.

Bones: Matrix and Cells

Bone Matrix Composition

Bones are connective tissues with cells separated by a matrix. The bone matrix is composed of 35% organic and 65% inorganic materials.

• Organic: Collagen, proteoglycans, glycosaminoglycans (GAGs), and glycoproteins provide flexible strength.

• Inorganic: Hydroxyapatite (calcium phosphate crystals) provides weight-bearing strength.

Bone Cell Types

There are four main types of bone cells, each with distinct functions:

• Osteoprogenitor Cells (Osteogenic Cells): Unspecialized bone stem cells that undergo cell division.

• Osteoblasts: Bone-building cells responsible for bone deposition. They secrete organic matrix (collagen, proteoglycans) and matrix vesicles containing Ca2+ and phosphate (PO43-), resulting in hydroxyapatite formation. Ossification: Formation of new bone by osteoblasts, primarily via appositional growth.

• Osteocytes: Mature bone cells that maintain the extracellular matrix (ECM). They reside in lacunae and extend processes through canaliculi, allowing nutrient exchange via gap junctions. Osteocytes are long-lived and comprise 90–95% of bone cells.

• Osteoclasts: Multinucleated cells derived from red bone marrow, responsible for bone resorption (breakdown). They mobilize calcium and phosphate ions for metabolic use.

Bone Resorption Process

1. Osteoclasts contact bone matrix with help from osteoblasts.

2. Osteoclasts attach to bone via podosomes and develop a ruffled border.

3. Release acidic vesicles (H+ ions) and protein-digesting enzymes (lysosomes).

4. Degradation products are transferred into osteoclasts, secreted into extracellular space, and absorbed into the blood.

Bone resorption is critical for bone remodeling and calcium homeostasis.

Bone Disorders

Osteopetrosis

Osteopetrosis is caused by defective osteoclasts that do not properly degrade bone, leading to overgrown and brittle bones ("marble bone disease"). Regular bone resorption is essential for calcium homeostasis, bone repair, and replacement of old bone.

Osteitis Deformans

Osteitis deformans (Paget's disease) results from overactive osteoclasts and excessive bone resorption. Osteoblasts increase bone formation, but the new bone is poorly formed and unstable, making it susceptible to deformation and fractures.

Bone Structure: Spongy and Compact Bone

Compact Bone

Compact bone has more bone matrix and less spacing. Its functional unit is the osteon, composed of concentric rings of matrix (concentric lamellae) surrounding a central canal containing blood vessels, nerves, and loose connective tissue.

• Lacunae: Spaces between lamellar rings housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae, allowing communication via gap junctions.

• Circumferential Lamellae: Outer surface of compact bone.

• Interstitial Lamellae: Remnants of resorbed osteons between osteons.

• Perforating Canals: Deliver blood to central canals of osteons.

Spongy Bone

Spongy bone is porous, with less bone matrix and more spacing. It consists of trabeculae (rods or plates of interconnecting bone), with spaces filled with bone marrow and blood vessels. Trabeculae are oriented along lines of mechanical stress to help bear weight and resist bending.

Classification of Bones

Type	Description	Example
Flat bones	Spongy bone sandwiched between two layers of compact bone	Skull, sternum
Long bones	Greater in length than width	Femur, humerus
Short bones	Length nearly equal to width	Carpals, tarsals
Irregular bones	Complex shapes not fitting other categories	Vertebrae, pelvis

Long Bone Anatomy

• Diaphysis: Central shaft of the bone (compact), surrounds the medullary cavity (marrow cavity).

• Epiphysis: Ends of the bone (mostly spongy), each with its own ossification center.

• Epiphyseal Plate: Growth plate between epiphysis and diaphysis, site of bone length growth; becomes the epiphyseal line when ossified.

• Articular Cartilage: Hyaline cartilage covering bone ends.

• Red Marrow: Site of blood cell formation.

• Yellow Marrow: Mostly adipose tissue; replaces red marrow in limbs during development.

• Periosteum: Connective tissue membrane covering the bone's outer surface, with an outer fibrous layer (dense irregular collagenous tissue) and an inner cellular layer (osteogenic cells).

• Perforating Fiber: Collagen fibers securing periosteum to bone.

• Endosteum: Single layer connective tissue lining internal bone cavities, containing osteoblasts, osteoclasts, and progenitor cells.

Cartilage

Types and Structure

• Hyaline Cartilage: Involved in bone growth and repair.

• Chondroblasts: Cells that create matrix for producing chondrocytes.

• Chondrocytes: Mature cartilage cells residing in lacunae.

• Perichondrium: Protective connective tissue covering cartilage, containing fibroblasts and chondroblasts. Nutrients are provided by diffusion from blood vessels and nerves.

• Articular Cartilage: Hyaline cartilage covering bone ends at joints; lacks perichondrium, blood vessels, or nerves.

Cartilage Growth

• Interstitial Growth: Chondrocytes in the center divide and add matrix between cells, resulting in growth in length.

• Appositional Growth: Chondroblasts in the perichondrium add new cartilage to the outside, resulting in growth in width.

Bone Development

Patterns of Bone Development

Bone development in the fetus occurs via two main processes: intramembranous ossification and endochondral ossification.

• Intramembranous Ossification: Begins within embryonic connective tissue membranes, forming many skull bones. Mesenchyme cells condense around the brain, forming a membrane of connective tissue with randomly oriented collagen fibers. Centers of ossification develop where bone formation begins.

• Endochondral Ossification: (Additional info: Not shown in the provided images, but typically involves bone formation from a cartilage model, essential for development of long bones.)