Skeletal System

The skeletal system provides the structural framework and support for the body also storing essential ions. Key components include bones, joints, cartilages, and ligaments. It is divided into two main parts: the axial skeleton (head, vertebral column, rib cage) and the appendicular skeleton (limbs, shoulder, and hip).

Beyond support, bones enable movement, protect internal organs, store minerals and fats, and are sites for hematopoiesis (blood cell formation). An adult typically has 206 bones, compared to a fetus's 302, as fetal bones primarily consist of cartilage that ossifies over time. Bones are categorized by porosity into compact (dense, homogeneous) and spongy (porous, heterogeneous) types. Based on shape, they are classified as long, short, flat, and irregular.

The gross anatomy of a long bone includes the diaphysis (shaft), composed mainly of compact bone for rigidity, and the epiphyses (ends), consisting primarily of spongy bone. The epiphyses are growth centers where new cell addition occurs in the epiphyseal plate, allowing bones to lengthen.

Microscopically, long bones feature the periosteum (outer fibrous membrane connecting to muscles), Sharpey's fibers (securing periosteum to bone), arteries (supplying nutrients to bone cells, osteocytes), articular cartilage (hyaline cartilage reducing friction at joints), and the medullary cavity (containing yellow marrow, fat in adults, and red marrow, blood cell formation in infants).

Bone markings are surface features that serve as attachment points for muscles, tendons, and ligaments, and passages for nerves and blood vessels. These can be projections/processes (grow out from surface) or depressions/cavities (indentations). The human skeleton begins as hyaline cartilage, especially in the fetus to facilitate birth. This cartilage gradually ossifies into bone through endochondral ossification, where cartilage is replaced by bone tissue. Intramembranous ossification describes bone growth directly from connective tissue. Bones are constantly remodeled, lengthening during growth and changing shape.

Bone tissue contains three main cell types: osteocytes (maintain bone matrix), osteoblasts (build new bone tissue), and osteoclasts (resorb bone matrix, releasing calcium ions crucial for bodily functions). Bone fractures are treated by reduction (realignment) and immobilization. Common fracture types include comminuted, compression, depressed, impacted, spiral, and greenstick (common in children).

Bone fracture repair occurs in several stages: hematoma formation (blood accumulation brings necessary cells), fibrocartilage callus formation (temporary protection and framework), bony callus formation (replacement of fibrocartilage with spongy bone), and bone remodeling (reshaping the bony callus to a permanent patch).

The axial skeleton forms the body's central axis, protecting vital organs. It comprises the skull, vertebral column, and bony thorax. The skull has two sets of bones: the cranium (flat bones joined by sutures) and facial bones. The mandible is the only movable skull bone. Paranasal sinuses are hollow portions in the skull that lighten it and provide voice resonance. The hyoid bone is unique, not articulating with any other bone, supporting the tongue. Fetal skulls have fontanels, fibrous membranes that allow brain growth during infancy.

The vertebral column protects the spinal cord and provides structural support, exhibiting normal curvatures (cervical, thoracic, lumbar). It consists of 24 movable vertebrae, the fused sacrum (5 vertebrae), and the fused coccyx (4 vertebrae). The bony thorax (rib cage, sternum, thoracic vertebrae) protects organs in the mediastinum and abdominopelvic cavity. Ribs are classified as true (directly attached to sternum), false (indirectly attached), or floating (no sternal attachment).

The appendicular skeleton consists of the limbs, pectoral (shoulder) girdle, and pelvic girdle. The shoulder girdle, made of the clavicle and scapula, allows for free upper limb movement. The upper limb includes the humerus (upper arm), ulna and radius (forearm), carpals (wrist, short bones), metacarpals (palm), and phalanges (fingers).

The pelvic girdle, consisting of hip bones (ilium, ischium, pubis), supports upper body weight and protects reproductive, excretory, and lower digestive organs. Sexual dimorphism in humans is observed in pelvic width, with females having a wider pelvis to facilitate childbirth. The lower limb comprises the femur (thigh), tibia and fibula (leg), tarsals (ankle), metatarsals (foot arch), and phalanges (toes).

Joints are articulations where bones meet, providing mobility while holding bones together. They are functionally classified as synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable). Structurally, they are fibrous (immovable, e.g., skull sutures), cartilaginous (slightly movable, e.g., pubic symphysis, intervertebral joints), or synovial (freely movable, with a joint cavity filled with synovial fluid). Synovial joints are reinforced by ligaments and supported by bursae and tendon sheaths. Different types of synovial joints allow various movements (plane, hinge, pivot, condyloid, saddle, ball-and-socket). Common joint diseases include bursitis (inflammation of bursae), tendonitis (inflammation of tendons), and arthritis (inflammatory or degenerative, including osteoarthritis and rheumatoid arthritis).