Lecture 15: Skeletal System

Introduction to the Skeletal System

The skeletal system is a fundamental organ system in the human body, composed primarily of bones. Bones serve as organs that perform a variety of essential functions necessary for survival and homeostasis.

• Bones are living organs made of connective tissue, mineral deposits, and specialized cells.

• The adult human skeleton contains 206 bones, classified by shape and function.

Skeletal System Functions

Overview of Functions

The skeletal system performs several critical functions that support and protect the body, maintain homeostasis, and enable movement.

• Protection

• Mineral storage

• Acid-base homeostasis

• Blood cell formation

• Fat storage

• Movement

• Support

1. Protection

Bones are among the hardest substances in the body and play a vital role in protecting internal organs from mechanical injury.

• Skull, sternum, ribs, and pelvis protect underlying organs such as the brain, heart, lungs, and reproductive organs.

• Orbital bones protect the eyes; temporal bones protect the ears.

2. Mineral Storage and Acid-Base Homeostasis

Bones act as reservoirs for essential minerals and help regulate the body's acid-base balance.

• Minerals stored: Calcium ions (Ca2+), phosphate ions (PO43-), and magnesium ions (Mg2+).

• These minerals are vital for electrolyte balance and acid-base homeostasis.

• Bone releases or absorbs minerals to maintain blood pH and electrolyte levels.

3. Blood Cell Formation

The skeletal system is the primary site of blood cell production, a process known as hematopoiesis.

• Red bone marrow produces red blood cells, white blood cells, and platelets.

• Hematopoiesis occurs mainly in the flat bones and the ends of long bones.

4. Fat Storage

Bones also serve as storage sites for energy in the form of fat.

• Yellow bone marrow contains adipocytes that store triglycerides.

• Fat stored in bone marrow can be mobilized during periods of energy deficit.

5. Movement

The skeletal system enables movement by providing attachment points for muscles.

• Muscles attach to bones via tendons and produce movement by contracting and pulling on bones across joints.

• Most skeletal muscles are attached to bones, allowing for voluntary movement of the body.

6. Support

The skeleton provides the structural framework that supports the body and maintains its shape.

• Bones support the weight of the body and anchor soft tissues.

• The skeleton is essential for posture and overall body stability.

Bone Structure and Classification

Classification by Shape

Bones are classified into five structural categories based on their shapes and functions.

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

• Short bones: As long as they are wide (e.g., carpals, tarsals).

• Flat bones: Thin and broad (e.g., skull, sternum, ribs, pelvis).

• Irregular bones: Complex shapes that do not fit other categories (e.g., vertebrae, certain skull bones).

• Sesamoid bones: Small, flat, oval-shaped bones found within tendons (e.g., patella).

Structure of Short, Flat, Irregular, and Sesamoid Bones

These bones typically consist of a sandwich-like structure with spongy bone (diploë) between layers of compact bone.

• No diaphysis, epiphyses, or medullary cavity.

• Periosteum is attached by perforating (Sharpey's) fibers.

Histology of Bone Tissue

Bone Tissue Composition

Bone is a specialized connective tissue with a unique extracellular matrix (ECM) that provides strength and flexibility.

• Inorganic matrix: ~65% of bone mass, mainly calcium and phosphate salts (hydroxyapatite crystals: ).

• Organic matrix: ~35% of bone mass, primarily collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins.

Inorganic Matrix

• Stores 85% of the body's calcium and most phosphate.

• Provides strength and resistance to compression.

Organic Matrix

• Collagen fibers resist torsion and tensile forces, increasing bone hardness.

• Proteoglycans and glycoproteins contribute to bone flexibility and resilience.

Bone Cells

Bone tissue contains three main types of cells, each with distinct functions.

• Osteoblasts: Derived from osteogenic cells; build bone by secreting bone ECM and mature into osteocytes.

• Osteocytes: Mature bone cells trapped in lacunae; maintain bone ECM and recruit osteoblasts for repair.

• Osteoclasts: Large, multinucleated cells that break down bone ECM (bone resorption) by secreting acids and enzymes.

Bone Tissue Types

Compact and Spongy Bone

Bone tissue is organized into two main types: compact bone and spongy (cancellous) bone.

• Compact bone: Dense, provides strength and structure; contains osteons with concentric lamellae, central canals, canaliculi, and lacunae with osteocytes.

• Spongy bone: Porous, resists forces in multiple directions; forms a protective framework for bone marrow and consists of trabeculae covered with endosteum.

Bone Formation: Ossification (Osteogenesis)

Overview of Ossification

Ossification is the process of bone formation, beginning in the embryo and continuing through childhood. There are two main types of ossification:

• Intramembranous ossification

• Endochondral ossification

Intramembranous Ossification

This process forms flat bones of the skull and clavicle, originating from mesenchymal (embryonic connective) tissue.

1. Mesenchymal cells differentiate into osteogenic cells, then osteoblasts, forming bone in the primary ossification center.

2. Osteoblasts secrete organic matrix, which calcifies and traps cells as osteocytes in lacunae.

3. Early spongy bone forms; trabeculae merge, and mesenchyme becomes periosteum.

4. Spongy bone calcifies to form immature compact bone.

Endochondral Ossification

This process forms most bones below the skull (except the clavicle), beginning with a hyaline cartilage model.

1. Hyaline cartilage forms the bone template.

2. Primary ossification center: Osteoblasts replace cartilage with bone in the diaphysis.

3. Medullary cavity forms as cartilage breaks down.

4. Secondary ossification centers develop in the epiphyses.

5. Cartilage remains at epiphyseal plates for bone growth.

Comparison of Ossification Types

Additional info:

• Images provided show bone cells and histological structure, supporting the discussion of bone tissue and cell types.

• Exam dates and lecture hours are organizational details and not part of the anatomical content.