Osseous Tissue and Bone Structure

Divisions and Functions of the Skeletal System

The skeletal system is divided into two major regions: the axial skeleton and the appendicular skeleton. Each division plays a distinct role in supporting and facilitating movement in the human body.

• Axial Skeleton: Composed of 80 bones along the longitudinal axis, including the skull, thorax, and vertebral column.

• Appendicular Skeleton: Consists of 126 bones of the limbs and the pectoral and pelvic girdles, which attach the limbs to the axial skeleton.

The skeletal system serves several essential functions:

• Support: Provides the framework for the body.

• Storage of Minerals: Stores calcium and phosphate, releasing them as needed to maintain homeostasis.

• Blood Cell Production: Red bone marrow produces red and white blood cells and platelets.

• Protection: Shields vital organs from injury (e.g., skull, ribs, vertebral column, pelvis).

• Leverage: Bones act as levers to change the magnitude and direction of muscle forces.

Classification of Bones by Shape and Structure

Bones are classified based on their shape and internal structure, which reflects their function and location in the body.

• Flat Bones: Two parallel surfaces, wide, protective (e.g., skull, ribs).

• Sutural (Wormian) Bones: Small bones within cranial sutures.

• Long Bones: Long and slender (e.g., humerus, femur).

• Irregular Bones: Complex shapes with projections (e.g., vertebrae).

• Sesamoid Bones: Small, embedded in tendons (e.g., patella).

• Short Bones: Cube-shaped (e.g., carpal bones).

Example: The patella is a sesamoid bone embedded in the quadriceps tendon.

Surface Features of Bones

Bones display various surface features that serve as sites for muscle attachment, passageways for nerves and blood vessels, and articulation points for joints.

• Canal or Meatus: Large passageway through bone.

• Process: Any projection or bump.

• Sinus: Chamber within bone, filled with air.

• Foramen: Small rounded hole for blood vessels and nerves.

• Fissure: Elongated cleft or gap.

Features of the humerus and femur include:

• Head: Expanded proximal end forming part of a joint.

• Tubercle: Small rounded projection.

• Sulcus: Deep narrow groove.

• Tuberosity: Low rough projection.

• Diaphysis: Shaft of a long bone.

• Trochlea: Smooth, grooved articular process shaped like a pulley.

• Condyle: Smooth rounded articular process.

• Trochanter: Very large, rough projection.

• Neck: Narrow connection between head and diaphysis.

• Facet: Small, flat articular surface.

Pelvic features include:

• Crest: Prominent ridge.

• Fossa: Shallow depression.

• Line: Low ridge, more delicate than a crest.

• Spine: Pointed or narrow process.

• Ramus: Extension making an angle with the rest of the structure.

Structure of a Typical Long Bone

Long bones have distinct regions and internal structures that facilitate growth, repair, and function.

• Epiphysis: End of the bone.

• Metaphysis: Junction between shaft and epiphysis; site of growth at the epiphyseal plate.

• Diaphysis: Shaft or long part.

• Articular Cartilage: Covers joint surfaces; limited repair capacity.

• Medullary (Marrow) Cavity: Contains yellow marrow (fat).

• Periosteum: Membrane covering bone surface.

Types of bone tissue:

• Compact Bone: External layer, strong and dense.

• Spongy Bone: Internal, lighter, contains red marrow.

Example: In the femur, spongy bone at the proximal epiphysis directs weight to compact bone in the diaphysis.

Blood Supply to Bone

Bones are highly vascularized, with several arteries and veins supplying nutrients and removing waste.

• Nutrient Artery and Vein: Enter through nutrient foramen, supply diaphysis.

• Epiphyseal Artery and Vein: Supply epiphysis.

• Metaphyseal Artery and Vein: Supply metaphysis and red marrow.

• Periosteal Arteries and Veins: Supply periosteum and outer compact bone.

• Sensory Nerves: Accompany blood vessels.

Cell Types in Bone and Bone Matrix

Bone tissue contains several specialized cell types, each with unique functions in bone formation, maintenance, and resorption.

• Osteoprogenitor Cells: Stem cells that divide to produce osteoblasts; important for fracture repair.

• Osteoblasts: Bone-forming cells; produce osteoid and initiate calcification.

• Osteocytes: Mature bone cells trapped in lacunae; maintain bone matrix.

• Osteoclasts: Multinucleated cells that resorb bone tissue (osteolysis).

Bone Matrix: Composed of calcium salts (hardness) and collagen fibers (tensile strength).

Structure and Function of Compact and Spongy Bone

Compact and spongy bone differ in structure and function, providing strength and flexibility to the skeleton.

• Compact Bone: Composed of osteons (cylindrical units), central canals, concentric lamellae, lacunae, canaliculi, perforating canals, interstitial lamellae, and circumferential lamellae.

• Spongy Bone: Contains trabeculae (struts and plates), oriented along stress lines, lighter than compact bone, nutrients reach osteocytes by diffusion.

Appositional Bone Growth

Bone increases in diameter through appositional growth, involving the periosteum and endosteum.

• Periosteum: Contains osteoblasts that add circumferential lamellae to the outer surface.

• Endosteum: Contains osteoblasts and osteoclasts for bone remodeling.

• Osteoclasts: Enlarge the medullary cavity by resorbing bone.

Endochondral Ossification and Bone Growth in Length

Most bones develop through endochondral ossification, where bone forms inside hyaline cartilage.

• Primary Ossification Center: Osteoblasts produce spongy bone in the shaft.

• Secondary Ossification Centers: Form in the epiphyses.

• Epiphyseal Plate: Site of bone growth in length; cartilage is replaced by bone.

• Epiphyseal Closure: Occurs at puberty, leaving an epiphyseal line.

Intramembranous Ossification

Intramembranous ossification forms bone directly within fibrous connective tissue membranes, without a cartilage precursor.

• Occurs in: Clavicle, mandible, flat bones of the skull.

• Mesenchymal Cells: Differentiate into osteoblasts at ossification centers.

• Osteoblasts: Secrete matrix, become osteocytes.

• Bone Matrix: Forms spongy bone; superficial layers replaced by compact bone.

Abnormalities of Bone Formation and Growth

Various endocrine and metabolic disorders can affect bone growth and development.

• Pituitary Growth Failure: Reduced growth hormone leads to short bones.

• Achondroplasia: Early replacement of epiphyseal plates results in short, stocky limbs.

• Gigantism: Overproduction of growth hormone before puberty causes excessive bone length.

• Acromegaly: Overproduction of growth hormone after epiphyseal closure causes bone thickening.

Regulation of Blood Calcium Levels

Calcium is the most abundant mineral in the body, and its levels are tightly regulated by several mechanisms involving the bones, intestines, and kidneys.

• Absorption: Dietary calcium absorbed in the intestines (hormonally regulated).

• Loss: Calcium lost in urine (hormonally regulated).

• Bone Remodeling: Osteoblasts deposit calcium; osteoclasts resorb bone, releasing calcium.

Hormonal Regulation of Calcium Homeostasis

Blood calcium levels are maintained by the actions of parathyroid hormone (PTH) and calcitonin.

• Low Ca2+: PTH increases osteoclast activity, stimulates calcitriol production, increases intestinal absorption, and reduces urinary loss.

• High Ca2+: Calcitonin inhibits osteoclasts, increases calcium deposition in bone, and promotes urinary excretion.

Fractures and Bone Repair

Bone fractures are classified by their pattern and severity. Healing involves several stages, provided the blood supply and bone membranes survive.

• Fracture Hematoma: Blood clot forms at the site.

• Callus Formation: Internal and external calluses stabilize the fracture.

• Spongy Bone Formation: Cartilage is replaced by spongy bone.

• Remodeling: Osteoclasts and osteoblasts restore bone structure.

Types of fractures:

Medical Terminology

• Osteomyelitis: Infection of bone, often caused by Staphylococcal bacteria, characterized by fever, pain, and inflammation.

• Osteoporosis: Decrease in bone mass and strength due to increased bone resorption; bones are easily fractured.