Bones and Skeletal Tissues: Structure, Function, and Classification

Chapter 6: Bones and Skeletal Tissues

6.1 Skeletal Cartilages

The human skeleton initially consists of cartilage, which is gradually replaced by bone except in areas requiring flexibility. Cartilage is a specialized connective tissue that provides support and flexibility in the body.

• Skeletal cartilage: Made of highly resilient, molded cartilage tissue that consists primarily of water. It contains no blood vessels or nerves.

• Perichondrium: A layer of dense connective tissue surrounding cartilage like a girdle. It helps cartilage resist outward expansion and contains blood vessels for nutrient delivery to cartilage.

• Chondrocytes: Cartilage cells encased in small cavities called lacunae within a jelly-like extracellular matrix.

Basic Structure, Types, and Locations of Cartilage

There are three main types of cartilage, each with distinct properties and locations in the body.

• Hyaline cartilage:

  • Provides support, flexibility, and resilience.

  • Most abundant type; contains collagen fibers only.

  • Locations: Articular (joints), costal (ribs), respiratory (larynx), nasal cartilage (nose tip).

• Elastic cartilage:

  • Similar to hyaline cartilage but contains elastic fibers.

  • Locations: External ear and epiglottis.

• Fibrocartilage:

  • Thick collagen fibers; has great tensile strength.

  • Locations: Menisci of knee, vertebral discs.

Growth of Cartilage

Cartilage grows in two ways, allowing for expansion and maintenance throughout life.

• Appositional growth: Cartilage-forming cells in the perichondrium secrete matrix against the external face of existing cartilage. New matrix is laid down on the surface of cartilage.

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

• Calcification of cartilage occurs during normal bone growth in youth, but can also occur in old age. Hardened cartilage is not the same as bone.

6.2 Functions of Bones

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

• Support: Provides structural framework for the body and soft organs.

• Protection: Protects the brain, spinal cord, and vital organs.

• Movement: Acts as levers for muscle action.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities of certain bones.

• Triglyceride (fat) storage: Fat, used for energy, is stored in bone cavities.

• Hormone production: Osteocalcin secreted by bones helps regulate insulin secretion, glucose levels, and metabolism.

6.3 Classification of Bones

The human skeleton contains 206 named bones, classified by location and shape.

Classification by Location

• Axial skeleton: Long axis of the body (skull, vertebral column, rib cage).

• Appendicular skeleton: Bones of upper and lower limbs, and girdles attaching limbs to axial skeleton.

Classification by Shape

• Long bones: Longer than they are wide; includes limb bones.

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

• Flat bones: Thin, flat, slightly curved (sternum, scapulae, ribs, most skull bones).

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

6.4 Bone Structure

Bones are organs composed of several tissue types, including osseous tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels. Bone structure is studied at three levels: gross, microscopic, and chemical.

Gross Anatomy

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

• Spongy bone: Honeycomb of small, needle-like or flat pieces called trabeculae. Open spaces between trabeculae are filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

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

• Compact bone is sandwiched between connective tissue membranes: periosteum (outside) and endosteum (inside).

• Bone marrow is scattered throughout spongy bone; no defined marrow cavity.

• Hyaline cartilage covers areas of bone that are part of a movable joint.

Structure of Typical Long Bone

• Diaphysis: Tubular shaft forming the long axis of bone; surrounds the medullary cavity filled with yellow marrow in adults.

• Epiphyses: Ends of long bones; compact bone externally, spongy bone internally. Articular (hyaline) cartilage covers joint surfaces.

• Epiphyseal line: Remnant of childhood epiphyseal plate where bone growth occurs.

Membranes

• Periosteum: Double-layered membrane covering external surfaces except joints.

  • Fibrous layer: Dense irregular connective tissue with Sharpey's fibers securing bone matrix.

  • Osteogenic layer: Inner layer containing primitive osteogenic stem cells.

  • Contains nerve fibers and blood vessels entering the shaft through nutrient foramen openings.

  • Anchoring points for tendons and ligaments.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces, including trabeculae of spongy bone and canals passing through compact bone. Contains osteogenic cells.

Hematopoietic Tissue in Bones

• Red marrow: Found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum).

• In newborns, medullary cavities and all spongy bone contain red marrow.

• In adults, red marrow is located in heads of femur and humerus, and most active areas are flat bone diploë and some irregular bones (e.g., hip bone).

• Yellow marrow can convert to red if a person becomes anemic.

Bone Markings

Bone markings are features on bones that serve as sites for muscle, ligament, and tendon attachment, or as conduits for blood vessels and nerves.

• Projection: Outward bulge of bone, often due to increased stress from muscle pull or as a modification for joints.

• Depression: Bowl- or groove-like cut-out that can serve as passageways for vessels and nerves, or play a role in joints.

• Opening: Hole or canal in bone that serves as passageways for blood vessels and nerves.

Table: Bone Markings

Name of Bone Marking	Description	Illustration
Tuberosity	Large rounded projection; may be roughened	Femur
Crest	Narrow ridge of bone; usually prominent	Hip bone
Head	Bony expansion carried on a narrow neck	Femur
Foramen	Round or oval opening through a bone	Skull
Fossa	Shallow, basin-like depression in a bone	Skull
Groove	Furrow	Mandible

Microscopic Anatomy of Bone

Bones are composed of several specialized cell types and structural units that contribute to their function and maintenance.

Cells of Bone Tissue

• Osteogenic cells: Also called osteoprogenitor cells; mitotically active stem cells in periosteum and endosteum. Differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix called osteoid. Osteoid is made up of collagen and calcium-binding proteins. Collagen makes up 90% of bone protein. Osteoblasts are actively mitotic.

• Osteocytes: Mature bone cells in lacunae that no longer divide. Maintain bone matrix and act as stress or strain sensors, communicating with osteoblasts and osteoclasts for bone remodeling.

• Bone-lining cells: Flat cells on bone surfaces believed to help maintain matrix. On external bone surface, called periosteal cells; on internal surfaces, called endosteal cells.

• Osteoclasts: Derived from hematopoietic stem cells that become macrophages. Giant, multinucleate cells that function in bone resorption (breakdown of bone). Located in depressions called resorption bays. Ruffled borders increase surface area for enzyme degradation of bone and help seal off area from surrounding matrix.

Compact Bone Structure

• Also called lamellar bone.

• Consists of:

  • Osteon (Haversian system): Structural unit of compact bone; elongated cylinder running parallel to long axis of bone. Consists of several rings of bone matrix called lamellae. Lamellae contain collagen fibers running in different directions in adjacent rings, which withstand stress and resist twisting. Bone salts are found between collagen fibers.

  • Canals and canaliculi: Central (Haversian) canal runs through the core of osteon, containing blood vessels and nerve fibers. Perforating (Volkmann's) canals occur at right angles to the central canal, connecting blood vessels and nerves of periosteum, medullary cavity, and central canal.

  • Lacunae: Small cavities that contain osteocytes.

  • Canaliculi: Hairlike canals that connect lacunae to each other and to the central canal, allowing communication between all osteocytes of osteon and permitting nutrients and wastes to be relayed from one cell to another.

  • Interstitial lamellae: Lamellae that are not part of osteon; fill gaps between forming osteons or are remnants of osteons cut by bone remodeling.

  • Circumferential lamellae: Layers just deep to periosteum, extending around entire surface of diaphysis; help long bone resist twisting.

Example: Osteon Structure

An osteon is a cylindrical structure consisting of concentric lamellae of bone matrix, with a central canal containing blood vessels and nerves. Collagen fibers in adjacent lamellae run in different directions, providing strength and resistance to twisting forces.

*Additional info: The above notes expand on the original slides by providing definitions, examples, and context for key terms and processes, ensuring a comprehensive understanding suitable for exam preparation.*