Bones and Skeletal Tissues: Structured Study Notes

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Ch. 6 Bones and Skeletal Tissues

Bone Growth: Appositional vs. Interstitial

Bone growth occurs through two primary mechanisms, each contributing to the development and maintenance of skeletal structure.

Appositional Growth: The process by which bones increase in diameter or thickness. New bone tissue is added to the surface of existing bones by osteoblasts in the periosteum.
Interstitial Growth: The process by which bones increase in length. This occurs at the epiphyseal plate (growth plate) through the division of chondrocytes and subsequent ossification.
Example: Appositional growth is prominent during childhood and adolescence, allowing bones to become thicker as they lengthen.

Anatomy and Physiology of the Periosteum and Endosteum

The periosteum and endosteum are connective tissue membranes essential for bone health and remodeling.

Periosteum: A dense, fibrous membrane covering the external surface of bones (except at joints). Contains osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells).
Endosteum: A thin membrane lining the internal surfaces of bones, including the medullary cavity. Also contains osteoblasts and osteoclasts.
Function: Both membranes are involved in bone growth, repair, and remodeling.

Hormonal Effects on Bone Physiology

Several hormones regulate bone growth, remodeling, and calcium homeostasis.

Parathyroid Hormone (PTH): Increases blood calcium levels by stimulating osteoclast activity.
Calcitonin: Lowers blood calcium levels by inhibiting osteoclasts and stimulating osteoblasts.
Growth Hormone: Stimulates bone growth, especially at the epiphyseal plate.
Sex Hormones (Estrogen and Testosterone): Promote bone growth and closure of the epiphyseal plate during puberty.
Example: Excess PTH can lead to bone resorption and osteoporosis.

Bone Markings

Bone markings are anatomical features that serve as sites for muscle attachment, passage of nerves and blood vessels, and articulation with other bones.

Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, trochanter).
Depressions: Allow passage of blood vessels and nerves (e.g., fossa, groove).
Openings: For passage of blood vessels and nerves (e.g., foramen, canal).
Example: The foramen magnum in the skull allows passage of the spinal cord.

Anatomy of the Long Bone

Long bones, such as the femur and humerus, have a distinct structure that supports their function.

Diaphysis: The shaft, composed of compact bone surrounding the medullary cavity.
Epiphyses: The ends of the bone, containing spongy bone and red marrow.
Epiphyseal Plate: The growth plate, site of lengthwise bone growth in children and adolescents.
Periosteum: Covers the external surface.
Endosteum: Lines the internal surfaces.

Function of the Epiphyseal Plate

The epiphyseal plate is a hyaline cartilage plate at the ends of long bones, responsible for longitudinal growth.

Growth: Chondrocytes divide and enlarge, pushing the epiphysis away from the diaphysis.
Closure: At maturity, the plate ossifies and becomes the epiphyseal line, ending bone growth in length.

Anatomy of an Osteon

The osteon (Haversian system) is the fundamental structural unit of compact bone.

Central Canal: Contains blood vessels and nerves.
Lamellae: Concentric rings of bone matrix.
Lacunae: Small spaces housing osteocytes.
Canaliculi: Tiny channels connecting lacunae for nutrient and waste exchange.

Red vs. Yellow Bone Marrow

Bone marrow exists in two forms, each with distinct functions.

Red Bone Marrow: Produces blood cells (hematopoiesis); found in spongy bone of flat bones and epiphyses of long bones.
Yellow Bone Marrow: Stores fat; found in the medullary cavity of long bones.
Example: In adults, most marrow is yellow, but red marrow persists in the axial skeleton.

Anatomy of a Flat Bone

Flat bones, such as the skull and sternum, have a unique structure suited for protection and muscle attachment.

Structure: Two layers of compact bone with a middle layer of spongy bone (diploë).
Function: Protect internal organs and provide broad surfaces for muscle attachment.

Types of Bone Cells and Their Functions

Bone tissue contains several specialized cell types, each with a specific role.

Osteoblasts: Bone-forming cells; synthesize bone matrix.
Osteocytes: Mature bone cells; maintain bone tissue.
Osteoclasts: Bone-resorbing cells; break down bone matrix.
Osteogenic Cells: Stem cells that differentiate into osteoblasts.

Ossification

Ossification is the process of bone formation, occurring during fetal development and throughout life.

Intramembranous Ossification: Forms flat bones directly from mesenchymal tissue.
Endochondral Ossification: Forms most bones by replacing hyaline cartilage with bone.

Function of Compact and Spongy Bone

Bone tissue is organized into two types, each with distinct functions.

Compact Bone: Dense and strong; provides structural support and protection.
Spongy Bone: Porous and lightweight; contains red marrow for hematopoiesis.

Steps in Endochondral Ossification

Endochondral ossification is the process by which most bones are formed from cartilage templates.

Formation of bone collar around hyaline cartilage model.
Cavitation of the cartilage within the model.
Invasion of internal cavities by periosteal bud and formation of spongy bone.
Formation of medullary cavity; secondary ossification centers appear in epiphyses.
Ossification of epiphyses; hyaline cartilage remains only at epiphyseal plates and articular surfaces.

Peak Bone Density

Bones reach their maximum density during early adulthood.

Typical Age: Peak bone density is usually achieved between ages 25 and 30.
Importance: Maintaining bone density is crucial for preventing osteoporosis later in life.

Wolff’s Law

Wolff’s law states that bone grows and remodels in response to the forces placed upon it.

Adaptation: Bones become stronger where stress is greatest.
Example: Weight-bearing exercise increases bone density.

Functions of the Skeletal System

The skeletal system performs several essential functions for the human body.

Support: Provides structural framework for the body.
Protection: Shields vital organs (e.g., skull protects brain).
Movement: Serves as levers for muscles to act upon.
Mineral Storage: Stores calcium and phosphorus.
Blood Cell Production: Houses red marrow for hematopoiesis.
Fat Storage: Yellow marrow stores triglycerides.