Bones and Skeletal System

Skeletal Cartilage

The skeletal cartilage forms the initial framework of the skeleton in embryonic development and is gradually replaced by bone. Cartilage remains in adults in limited locations, serving essential functions.

• Composition: Made of cartilage tissue, primarily water, with no nerves or blood vessels.

• Perichondrium: Dense irregular connective tissue surrounding cartilage, resists expansion, and contains blood vessels for nourishment.

• Basic Components:

  • Cells called chondrocytes

  • Cavities in the extracellular matrix (ECM) called lacunae

• Types of Cartilage:

  • Hyaline cartilage: Provides support with flexibility and resilience; contains fine collagen fibers.

  • Elastic cartilage: Contains more elastic fibers, allowing it to "bounce back" after bending; found in the ear and epiglottis.

  • Fibrocartilage: Highly compressible with great tensile strength; found in menisci of the knee and intervertebral discs.

Cartilage Growth

Cartilage grows by two mechanisms, accommodating mitosis and expansion.

• Appositional Growth: Cells in the perichondrium secrete matrix, expanding cartilage externally.

• Interstitial Growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage internally.

• Growth typically stops during adolescence as the skeleton matures.

• Cartilage may calcify but calcified cartilage is not bone.

Functions of Bones

Bones perform several vital functions in the body.

• Support

• Protection

• Anchorage

• Mineral Storage (especially calcium and phosphate)

• Blood Cell Formation (hematopoiesis)

• Triglyceride Storage

• Hormone Production

Bone Classification and Shapes

Bone Classification

Bones are classified into two main groups based on location:

• Axial Skeleton:

  • Long axis of the body

  • Includes skull, vertebral column, and rib cage

  • Functions: protection, support, carrying other body parts

• Appendicular Skeleton:

  • Bones of upper and lower limbs, girdles

  • Attach limbs to axial skeleton

  • Functions: locomotion, manipulation of environment

Bone Shapes

Bones are categorized by shape, not size:

• Long Bones: Longer than wide, with a shaft and two ends (e.g., femur, phalanges)

• Short Bones: Cube-shaped (e.g., carpals, tarsals); includes sesamoid bones (e.g., patella)

• Flat Bones: Thin, flattened, slightly curved (e.g., sternum, scapulae, ribs, cranial bones)

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

Bone Structure Overview

Bone Tissue Types

Bones are organs containing several tissue types:

• Osseous tissue: Dominates bones

• Cartilage: Found in articular cartilages

• Dense connective tissue: External surface

• Muscle and epithelial tissue: Found in blood vessels

Levels of Bone Structure

• Gross

• Microscopic

• Chemical

Compact and Spongy Bone

• Compact Bone: Dense, solid, smooth outer layer

• Spongy Bone: Internal layer with small needlelike trabeculae; air spaces filled with bone marrow

Short, Irregular, and Flat Bones

• Thin plates of spongy bone covered by compact bone

• No shaft or expanded ends

• Compact bone covered outside and inside by periosteum and endosteum

Long Bones

• Diaphysis: Shaft, thick collar of compact bone surrounding medullary cavity (contains yellow marrow)

• Epiphyses: Bone ends, outer shell of compact bone, inner spongy bone, joint surfaces with hyaline cartilage

• Epiphyseal Line: Remnant of growth plate in adults

Membranes

• Periosteum: Covers bone except joint surfaces; outer fibrous layer (dense irregular connective tissue), inner osteogenic layer (contains osteoprogenitor cells)

• Endosteum: Covers internal bone surfaces, trabeculae of spongy bone, and canals passing through compact bone

Blood Vessels and Nerves

• Bones are highly vascularized; nerves accompany blood vessels

• Contrast: Cartilage is avascular

Hematopoietic Tissue

• Blood-forming tissue, also called red bone marrow

Bone Markings

• Explain how bones and ligaments work together

• Three categories:

  • Projections: Sites of muscle and ligament attachment

  • Surfaces: Form joints

  • Depressions/Openings: For blood vessels and nerves

Bone Cells

Types of Bone Cells

• Osteoprogenitor cells: Mitotically active stem cells, found in periosteum and endosteum, differentiate into osteoblasts

• Osteoblasts: Bone-forming cells, secrete unmineralized bone matrix (osteoid), actively mitotic

• Osteocytes: Mature bone cells, occupy lacunae, monitor and maintain bone matrix

• Bone lining cells: Flat cells where bone remodeling is not occurring, help maintain matrix

• Osteoclasts: Multinucleate, bone-resorbing cells, derived from macrophages, ingest bone material by phagocytosis

Microscopic Anatomy of Compact Bone

Osteon (Haversian System)

• Weight-bearing pillars, group of hollow tubes of bone matrix (lamellae)

• Collagen fibers run parallel in one lamella, opposite in adjacent lamellae

• Alternating pattern resists torsion and twisting

Canals and Canaliculi

• Central Canal: Vascularized and innervated

• Perforating Canals: Connect blood and nerve supply of medullary cavity to central canals

• Lacunae: Located at lamella junctions, contain osteocytes

• Canaliculi: Connect lacunae to each other and to central canal

Interstitial and Circumferential Lamellae

• Not part of complete osteons; fill gaps or surround osteons

Microscopic Anatomy of Spongy Bone

• Contains trabeculae, resists stress

• No osteons present

Chemical Bone Anatomy

Organic Components

• Cells

• Osteoid: Ground substance and collagen fibers

Inorganic Components

• Mineral Salts: Mostly calcium phosphates

Formation of the Bony Skeleton

Ossification (Osteogenesis)

Bone tissue formation begins just before week 8-9 of embryonic life. The process is called ossification or osteogenesis.

• Embryonic skeleton: hyaline cartilage and fibrous membranes

• Cartilage is replaced by bone, not converted into bone

• Two main processes: endochondral ossification and intramembranous ossification

Endochondral Ossification

• Bones develop by replacing hyaline cartilage

• Forms most bones below the skull (except clavicles)

• Uses hyaline cartilage models as patterns

Steps of Endochondral Ossification

1. Step 1: Formation of bone collar around diaphysis of hyaline cartilage model; perichondrium converts into periosteum

2. Step 2: Cartilage calcifies and develops cavities

3. Step 3: Formation of spongy bone by invasion of periosteal bud into internal cavities (contains artery, vein, nerve fibers, red marrow, osteoprogenitor cells, osteoclasts)

4. Step 4: Elongation of diaphysis and formation of medullary cavity; secondary ossification centers form in epiphyses

5. Step 5: Ossification of epiphyses; spongy bone is retained, no medullary cavity forms

Intramembranous Ossification

• Bones develop from a fibrous membrane

• Forms cranial bones and clavicle (majority: flat bones)

• Begins around week 8-9

• Four main steps:

  1. Development of ossification center in fibrous connective tissue

  2. Secretion and calcification of osteoid

  3. Formation of periosteum and immature spongy bone

  4. Replacement of immature spongy bone with compact bone and development of red bone marrow

Bone Growth and Remodeling

Bone Growth

• Long bone growth: interstitial growth of epiphyseal plate cartilage

• Zones:

  • Proliferation zone: Chondrocytes divide, lengthen bone

  • Hypertrophic zone: Chondrocytes enlarge, lacunae erode

  • Calcification zone: Matrix calcifies, chondrocytes die, blood vessels invade

  • Ossification zone: Calcified spicules invaded by marrow elements, replaced by bone

• Bone shape maintained by both resorption and deposition

• All bone thickness growth by appositional growth (periosteum: osteoblasts, endosteum: osteoclasts)

Bone Remodeling

• Bone cells continuously break down (resorb) and deposit bone

• Occurs every year, replaces about 5-10% of skeleton

• Occurs primarily at endosteal surface

• Consists of both resorption and deposition

• Key players: osteoblasts and osteoclasts

Bone Resorption

• Performed by osteoclasts

• Attach to bone via ruffled border, seal off area

• Secrete acids (H+) and lysosomal enzymes to digest matrix

• Osteoclasts phagocytose dead osteocytes and undigested matrix

• Osteoclasts undergo apoptosis after resorption

Bone Deposition

• Follows resorption

• Osteoblasts deposit new bone matrix (osteoid)

• Osteoid calcification: proteins bind calcium ions, raising local concentration

Key Equations and Terms

• Ossification:

• Bone Matrix:

Summary Table: Types of Cartilage

Summary Table: Bone Cell Types

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.