Comprehensive Classification and Structure of the Human Skeletal System
Components and General Organizational Principles of the Skeletal System
The skeletal system is composed of modified connective tissue (CT) designed to provide rigid support throughout the human body. The system consists of three primary elements: bones, cartilage, and ligaments. Bones serve as the main component, forming a complex, dynamic living framework for the rest of the body's tissues; they are characterized by a constant cycle of being broken down and replaced. Cartilage is less strong but more elastic than bone, providing semi-rigid support and acting as a protective layer at many joint surfaces. Ligaments are cords or bands of connective tissue that unite two structures, typically connecting bone to bone at joints to provide stability and store energy due to their elastic properties. The adult skeleton contains named bones (including the patellae but excluding smaller sesamoid bones), and was documented by Mrs. M. Smith and Mr. C. Mupfiga of the University of the Western Cape.
Primary Functions of the Skeletal System
The skeletal system performs five primary functions essential for bodily maintenance and movement. First is blood cell production, where red blood cells, white blood cells, and other elements are synthesized within the red bone marrow. Second is movement; skeletal muscles attach to bones via tendons, allowing bones to function as levers to move the body and its various parts. Third is protection, as the skeleton surrounds soft tissues and vital organs such as the brain, spinal cord, heart, and both abdominal and pelvic viscera. Fourth is storage, specifically acting as a reservoir for minerals such as calcium and phosphate, as well as lipids. Finally, the system provides structural support for the entire body.
Divisions of the Adult Skeleton
The adult skeleton is divided into two primary parts: the axial skeleton and the appendicular skeleton. The axial skeleton forms the longitudinal axis (trunk) of the body and consists of bones. This division includes the skull and its associated bones, the vertebral column, and the thoracic cage. Its functions involve protecting and supporting internal organs, participating in vital functions like respiration, and providing an extensive surface area for muscle attachment. The appendicular skeleton consists of bones, comprising the limbs and the supporting elements known as girdles which connect them to the trunk. This includes the pectoral (shoulder) girdle, upper limb bones, the pelvic girdle, and lower limb bones. The primary function of the appendicular skeleton is to allow for the manipulation of the environment and movement from place to place, with long bones dominating this division to support the limbs.
Classification of Bones by Shape
Bones are living tissues hardened by mineral salts like calcium and are classified by shape, internal tissue organization, bone markings, and type of development. There are six primary shapes. Long bones are long and thin, with a length greater than their width, consisting of a shaft (diaphysis) and variable extremities (epiphyses); examples include the humerus, radius, ulna, metacarpals, phalanges, femur, tibia, fibula, and metatarsals. Short bones are small, thick, and cube-shaped, with nearly equal length and width, such as the carpal and tarsal bones. Flat bones have thin, flat, parallel surfaces and are usually curved; examples include cranial bones, the sternum, scapulae, and ribs. Sutural bones, also known as Wormian bones, are small and irregular, found between the flat bones of the skull, varying in size and shape. Irregular bones have complex shapes with short, flat, notched, or ridged surfaces, including the sphenoid, ethmoid, hyoid, sacrum, hip bone, and vertebrae. Sesamoid bones are small, flat, oval bones shaped like sesame seeds that develop inside tendons passing over joints in the knees, wrists, hands, and feet, with the patella (kneecap) being the largest example.
Internal Tissue Organization and Bone Structure
Bone tissue is categorized into two types based on the amount of solid matter and the size of interior spaces. Compact (cortex or cortical) bone is dense and forms the superficial thin layer around all bones. Spongy (trabecular or cancellous) bone consists of a lattice-work of trabeculae and forms the central mass of bones, except where it is replaced by a medullary cavity. In long bones, the structure includes the epiphysis (an expanded area at each end covered with articular cartilage), the diaphysis (the shaft containing a medullary cavity), and the metaphysis (the region where the diaphysis and epiphysis meet). The medullary cavity is filled with red bone marrow, involved in blood cell production, and yellow bone marrow, which serves as an energy reserve. Flat bones resemble a sandwich, with two layers of compact bone surrounding a central layer of spongy bone, which in the cranium is specifically referred to as the diplo ".
Classification of Bone Markings
Bone markings are landmarks found on the surface of bones, formed where tendons, ligaments, blood vessels, or nerves interact with the tissue. Elevations and projections include the Process (any projection or bump) and Ramus (an extension forming an angle). Projections where muscles, tendons, or ligaments attach include the Trochanter (large, rough projection), Tuberosity (smaller, rough projection), Tubercle (small, rounded projection), Crest (prominent ridge), Line (low ridge), and Spine (pointed/narrow process). For articulation, markings include the Head (expanded articular end separated by a neck), Neck (narrow connection between epiphysis and diaphysis), Condyle (smooth, rounded articular process), Trochlea (smooth, grooved articular process shaped like a pulley), and Facet (small, flat articular surface). Depressions include the Fossa (shallow depression) and Sulcus (narrow groove). Openings include the Foramen (rounded passageway for blood vessels/nerves), Canal (duct or channel), Meatus (passageway through a bone), Fissure (elongated cleft or slit), and Sinus (a chamber within a bone normally filled with air).
Bone Development and Ossification Processes
Ossification is the physical formation of bone, starting from embryonic mesenchyme. Some bones, like the humerus, begin ossifying at weeks in utero and do not complete the process until age . Intramembranous ossification occurs directly from mesenchyme and forms flat bones of the skull, the mandible, and the clavicle. In this process, mesenchymal cells differentiate into osteoblasts to form an ossification center; bone expands as spicules, eventually trapping blood vessels and assuming a spongy bone structure. Endochondral ossification occurs when bone replaces a hyaline cartilage model. Chondrocytes enlarge and die as the matrix calcifies, blood vessels grow around the perichondrium converting cells to osteoblasts, and a primary ossification center forms in the diaphysis. Secondary ossification centers later form in the epiphyses. Growth continues at the epiphyseal cartilage (plate), where osteoblasts replace cartilage with bone on the shaft side while new cartilage is added on the epiphyseal side.
Characteristics and Types of Cartilage
Cartilage is a resilient, semi-rigid connective tissue found where flexibility is required, such as where costal cartilages attach ribs to the sternum or at the articulating surfaces of synovial joints. It functions to give shape and support, cushion joints, smoothen bone surfaces to reduce friction, and is essential for the growth of long bones. There are three types: Hyaline cartilage (found in the larynx, trachea, nose, and costal cartilages), Elastic cartilage (found in the external ear and epiglottis), and Fibrocartilage (found in intervertebral discs, the pubic symphysis, and the meniscus of the knee).
Ligaments, Joints, and Functional Classification
Ligaments are short, tough, flexible bands of connective tissue that hold bones together at joints, guiding and limiting movement. They are named by position (collateral), attachment (coracoclavicular), or shape (deltoid). A joint is the site where two or more bones meet, providing flexibility to the skeleton. Joints are classified into three types. Fibrous joints are joined by fibrous tissue and allow little to no movement (e.g., sutures of the skull, gomphosis of teeth, or syndesmosis between the radius and ulna). Cartilaginous joints are united by cartilage and allow minimal movement (e.g., synchondrosis at growth plates or symphysis in the pubic region and intervertebral discs). Synovial joints feature an articular cavity containing fluid and allow for a significant range of movement; they include a joint capsule, synovial membrane, and hyaline cartilage.