Bones and Skeletal Tissues: Structure, Function, and Development

Chapter 6: Bones and Skeletal Tissues

6.1 Cartilages Help Form the Skeleton

Cartilage is a resilient connective tissue that plays a crucial role in the early development and structure of the human skeleton. It provides flexibility, support, and a template for bone formation.

• Human skeleton starts as cartilage: Most of the fetal skeleton is cartilage, which is replaced by bone around 8 weeks of development.

• Skeletal cartilage: Highly resilient, mostly water, no blood vessels or nerves.

• Perichondrium: Dense connective tissue membrane surrounding cartilage; highly vascularized and resists outward expansion.

• Cartilage cells: Chondrocytes reside in lacunae within the extracellular matrix (ECM).

Cartilage Structure, Types, and Location

• Hyaline cartilage:

  • Most abundant type; provides support, flexibility, and resilience.

  • Contains only collagen fibers; glassy appearance.

  • Locations: ends of long bones, ribs, costal connections, larynx, respiratory passages, nasal cartilage.

• Elastic cartilage:

  • Contains elastic fibers; most similar to hyaline but more flexible.

  • Locations: external ear, epiglottis.

• Fibrocartilage:

  • Thick collagen fibers; great tensile strength and compressive resistance.

  • Locations: menisci of knee, intervertebral discs, pubic symphysis.

Growth of Cartilage

• Ends during adolescence; cannot usually be repaired in adults.

• Grows by two mechanisms:

  • Appositional growth: New matrix added to the external face of existing cartilage.

  • Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

6.2 Functions of Bones

Bones serve multiple vital functions in the human body, contributing to structure, protection, movement, and metabolic processes.

• Support: Framework for body and soft organs.

• Protection: Shields brain, spinal cord, and vital organs.

• Anchorage: Levers for muscle action.

• Mineral storage: Reservoir for calcium and phosphorus.

• Blood cell formation (hematopoiesis): Occurs in red bone marrow.

• Triglyceride storage: Energy source in yellow marrow.

• Hormone production: Osteocalcin regulates insulin and energy expenditure.

6.3 Classification of Bones

Bones are classified by location and shape, reflecting their diverse roles in the body.

• By location:

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

  • Appendicular skeleton: Limbs and girdles.

• By shape:

  • Long bones: Longer than wide, with a medullary cavity (e.g., femur, humerus).

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

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

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

6.4 Bone Structure

Bones are organs composed of several tissue types, with a complex structure that supports their functions.

• Bone (osseous) tissue: Most abundant; also contains nervous tissue, cartilage, dense CT, muscle cells, and epithelial cells in blood vessels.

• Three levels of structure:

  • Gross anatomy: Visible to naked eye.

  • Microscopic anatomy: Cellular and tissue-level details.

  • Chemical composition: Organic and inorganic components.

Gross Anatomy of Compact and Spongy Bone

• Compact bone: Dense outer layer; appears smooth and solid.

• Spongy bone: Honeycomb of trabeculae (needle-like or flat pieces); open spaces contain red or yellow marrow.

• Bone membranes:

  • Periosteum: Covers outside of compact bone; double-layered (fibrous and osteogenic layers).

  • Endosteum: Covers internal bone surfaces and trabeculae of spongy bone.

• Bone marrow: Scattered throughout spongy bone; no defined cavity except in long bones.

• Hyaline cartilage: Covers areas of bone that are part of movable joints.

Structure of a Typical Long Bone

• Shaft (diaphysis): Tubular shaft; compact bone surrounds medullary cavity filled with yellow marrow.

• Bone ends (epiphyses): Compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal line: Between diaphysis and epiphysis; remnant of growth plate.

Hematopoietic Tissue in Bones

• Red bone marrow: In newborns, found in medullary cavities and all spongy bone; in adults, mainly in flat bones and heads of femur/humerus.

• Yellow bone marrow: Can convert to red marrow if needed (e.g., anemia).

Bone Markings

• Sites of muscle, ligament, and tendon attachment; areas involved in joint formation or conduits for blood vessels and nerves.

• Three categories:

  • Projections

  • Surfaces

  • Depressions and openings

Microscopic Anatomy of Bone Tissue

• Cells of bone tissue:

  • Osteoprogenitor/osteogenic cells: Stem cells; mitotically active, especially in periosteum and endosteum.

  • Osteoblasts: Bone-forming cells; secrete unmineralized bone matrix (osteoid).

  • Osteocytes: Mature bone cells in lacunae; maintain bone matrix, respond to mechanical stimuli.

  • Bone-lining cells: Flat cells on bone surfaces; help maintain matrix.

  • Osteoclasts: Multinucleated cells; break down bone, located in resorption bays.

Microscopic Anatomy of Compact Bone

• Osteon (Haversian system): Structural unit; cylinder with rings of bone matrix (lamellae).

• Lamellae: Collagen fibers run in different directions; resist twisting.

• Canals and canaliculi:

  • Central (Haversian) canal: Core of osteon; contains blood vessels and nerves.

  • Perforating (Volkmann's) canal: Perpendicular to central canals; connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

  • Canaliculi: Hair-like canals between lamellae; allow osteocytes to communicate and exchange nutrients/waste.

• Interstitial and circumferential lamellae:

  • Interstitial: Remnants of osteons cut by bone remodeling.

  • Circumferential: Deep to periosteum, extend around diaphysis.

Microscopic Anatomy of Spongy Bone

• Appears poorly organized but is arranged along lines of stress.

• Trabeculae: High tensile strength cables; no osteons, but irregularly arranged lamellae and osteocytes connected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

• Organic components:

  • Cells: osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts.

  • Osteoid: unmineralized bone matrix; ground substance and collagen fibers (1/3 of bone matrix).

  • Sacrificial bonds: Provide resilience and flexibility.

• Inorganic components:

  • Hydroxyapatites (mineral salts): 65% of bone mass; mainly calcium phosphate crystals.

  • Responsible for hardness and resistance to compression.

  • Bones remain after death due to mineral composition.

6.5 Bone Development

Bone development, or ossification, is the process by which bone tissue forms. It begins in the embryo and continues throughout life as bones grow and remodel.

• Ossification (osteogenesis): Formation of bony skeleton begins in month 2 of development; postnatal growth continues until early adulthood; bone remodeling and repair are lifelong.

• Formation of the body skeleton:

  • Up to week 8, fibrous membranes and hyaline cartilage of fetal skeleton are replaced with bone tissue.

  • Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most of skeleton; begins in month 2 of development.

    • Requires breakdown of hyaline cartilage before ossification.

    • Primary ossification center in center of shaft; blood vessels infiltrate perichondrium, converting it to periosteum.

    • Five steps:

      1. Bone collar forms around diaphysis of cartilage model.

      2. Central cartilage calcifies, develops cavities.

      3. Periosteal bud invades cavities, brings blood vessels, nerves, bone marrow, osteoblasts/osteoclasts; leads to spongy bone formation.

      4. Diaphysis elongates, medullary cavity forms; secondary ossification centers appear in epiphyses.

      5. Epiphyses ossify; hyaline cartilage remains only in epiphyseal plates and articular cartilages.

  • Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones (skull, clavicles).

    • Four steps:

      1. Ossification centers form within mesenchymal cells.

      2. Osteoblasts secrete osteoid, which is calcified.

      3. Woven bone and periosteum form.

      4. Lamellar bone replaces woven bone; red bone marrow appears.

• Postnatal bone growth:

  • Long bones lengthen by interstitial growth of epiphyseal plate cartilage.

  • Bones increase thickness by appositional growth.

  • Bone growth during adolescence; bone lengthening ends at 18 for females, 21 for males.

• Growth in width (thickness):

  • Occurs throughout life; bones thicken in response to mechanical stress.

  • Osteoblasts beneath periosteum secrete bone matrix; osteoclasts remove bone on endosteal surface.

Hormonal Regulation of Bone Growth

• Growth hormone, thyroid hormone, and sex hormones regulate bone growth and development.

Key Equations

• Bone Mass Equation: $\text{Bone Mass} = \text{Organic Matrix} + \text{Inorganic Salts}$

Example: Endochondral Ossification

• Formation of long bones such as the femur and humerus involves the replacement of hyaline cartilage with bone tissue through the five-step process described above.

Additional info: Some details have been expanded for clarity and completeness, including definitions and examples of bone types, and the steps of ossification.