Chapter 6: Bones and Skeletal Tissues – Structured Study Notes

Bone and Skeletal Tissues

Overview

The skeletal system is composed of bones and cartilages, which together provide support, protection, movement, and other vital functions. This chapter focuses on the types of skeletal tissues, their growth, classification, structure, and the cells involved in bone maintenance and remodeling.

Skeletal Cartilages

Types of Cartilage

• Hyaline Cartilage: The most abundant type; provides support with flexibility and resilience. Found in articular surfaces, costal cartilages, respiratory structures, and nasal cartilage.

• Elastic Cartilage: Contains more elastic fibers, allowing for greater flexibility. Found in the external ear and epiglottis.

• Fibrocartilage: Highly compressible with great tensile strength. Found in intervertebral discs, pubic symphysis, and menisci of the knee.

Most abundant skeletal cartilage: Hyaline cartilage.

Growth of Cartilage

• Appositional Growth: Cartilage grows in width by the addition of new matrix on the external surface, secreted by chondroblasts in the perichondrium.

• Interstitial Growth: Cartilage grows from within as chondrocytes divide and secrete new matrix, expanding the cartilage from inside. This can increase both length and width.

Functions of Bone

• Support: Bones form the framework that supports the body and cradles soft organs.

• Protection: Bones protect vital organs (e.g., skull protects the brain, vertebrae protect the spinal cord, ribs protect the heart and lungs).

• Anchorage: Skeletal muscles attach to bones via tendons, enabling movement.

• Mineral Storage: Bones store minerals, primarily calcium and phosphate, which can be released into the bloodstream as needed.

• Blood Cell Formation (Hematopoiesis): Occurs in red bone marrow, producing red and white blood cells and platelets.

• Triglyceride (Fat) Storage: Yellow bone marrow stores fat, serving as an energy reserve.

• Hormone Production: Bones produce osteocalcin, a hormone that helps regulate insulin secretion and glucose homeostasis.

Classification of Bones

By Location

• Axial Skeleton: Includes bones of the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Includes bones of the upper and lower limbs, and the girdles (shoulder and pelvic/hip girdles).

By Shape

• Long Bones: Longer than they are wide; consist of a shaft and two ends (e.g., femur, humerus).

• Short Bones: Cube-shaped bones (e.g., carpals in the wrist, tarsals in the ankle).

• Sesamoid Bones: A subtype of short bones that form within tendons (e.g., patella, sesamoids in the thumb and big toe).

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, scapulae, ribs, most skull bones).

• Irregular Bones: Complicated shapes that do not fit other categories (e.g., vertebrae, sphenoid, ethmoid).

Bone Structure

Types of Bone Tissue

• Compact Bone: Dense outer layer that appears smooth and solid.

• Spongy Bone (Trabecular Bone): Internal layer with a honeycomb structure, filled with red or yellow marrow.

Structure of Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone (called diploe) covered by compact bone.

• No shaft or expanded ends; bone marrow is present between the trabeculae but no marrow cavity.

Definition of Diploe: Thin plates of spongy bone found between layers of compact bone in flat bones.

Anatomical Parts of a Typical Long Bone

• Diaphysis: The shaft or long axis of the bone; composed of compact bone surrounding the medullary (marrow) cavity.

• Medullary (Marrow) Cavity: Central cavity within the diaphysis; contains yellow marrow in adults.

• Epiphyses: The widened ends of the bone; proximal and distal epiphyses contain spongy bone and red marrow.

• Articular Cartilage: Hyaline cartilage covering the epiphyses at joint surfaces, reducing friction and absorbing shock.

• Epiphyseal Plate (Growth Plate): Hyaline cartilage plate between diaphysis and epiphysis in growing bones; site of bone lengthening.

• Epiphyseal Line: Remnant of the epiphyseal plate in adult bones, marking where growth has ceased.

• Metaphysis: Flared region where diaphysis and epiphysis meet; location of the epiphyseal plate/line.

Bone Membranes

• Periosteum: Double-layered membrane covering the external surface of bones (except at joints).

  • Outer Fibrous Layer: Dense irregular connective tissue.

  • Inner Osteogenic Layer: Contains osteoprogenitor (osteogenic) cells that can differentiate into bone-forming cells.

• Associated Cells: Osteoprogenitor cells (stem cells) that give rise to osteoblasts and other bone cells.

• Perforating (Sharpey's) Fibers: Bundles of collagen fibers that anchor the periosteum to the underlying bone.

• Endosteum: Thin membrane lining the internal surfaces of bone, including the medullary cavity; contains osteogenic cells.

Other Anatomical Features

• Nutrient Foramen: Small opening in the diaphysis through which blood vessels and nerves enter the bone.

Hemopoietic Tissue (Red Bone Marrow) in Adults

• Located in the epiphyses of long bones, bodies of vertebrae, ribs, sternum, and hip bones.

• Site of blood cell formation (hematopoiesis).

Bone Cells

Types of Bone Cells

• Osteoprogenitor (Osteogenic) Cells: Stem cells capable of mitosis; differentiate into osteoblasts.

• Osteoblasts: Immature bone-forming cells; secrete bone matrix (mainly collagen and calcium-binding proteins).

• Osteocytes: Mature bone cells residing in lacunae; maintain and monitor the bone matrix.

• Bone Lining Cells: Flat cells found on bone surfaces where bone remodeling is not occurring; help maintain bone matrix.

• Osteoclasts: Large, multinucleated cells that break down (resorb) bone matrix, releasing calcium into the blood; essential for bone remodeling and calcium homeostasis.

Summary Table: Types of Bone Cells

Cell Type	Function	Location
Osteoprogenitor (Osteogenic) Cells	Stem cells; differentiate into osteoblasts	Periosteum, endosteum
Osteoblasts	Secrete bone matrix; bone formation	Bone surfaces
Osteocytes	Maintain bone matrix	Lacunae within bone
Bone Lining Cells	Maintain bone matrix (inactive surfaces)	Bone surfaces
Osteoclasts	Bone resorption (breakdown)	Bone surfaces, resorption bays

Additional info: Bone remodeling is a dynamic process involving the coordinated actions of osteoblasts (formation) and osteoclasts (resorption), ensuring bone health and calcium balance.