Physiology of the Skeletal System: Bone Growth and Ossification
Introduction to Bone Growth
Bone growth is a complex process, unlike simpler tissue growth that relies solely on mitosis.
It begins during the embryonic and fetal periods, continuing through infancy, childhood, and adolescence, and then ceases at adulthood with the closure of epiphyseal plates.
Humans, as mammals, exhibit a strategy of rapid bone lengthening and skeletal growth, which then stops in adulthood. This contrasts with animals like crocodiles, which grow slowly throughout their entire lives.
The complexity involves various processes of ossification, primarily intramembranous ossification (forming flat bones) and endochondral ossification (forming most other bones).
Review of Bone Tissue and Skeletal System Components
Bone is a specialized connective tissue, meaning it contains different cell types and an extracellular matrix.
Osteoblasts: Bone-forming cells that synthesize and secrete collagen fibers and other organic components to build bones.
Osteocytes: Mature bone cells, derived from osteoblasts, which are the main cells in bone tissue and maintain its daily metabolism through nutrient and waste exchange.
Osteoclasts: Large cells that resorb, or break down, bone matrix, playing a crucial role in bone remodeling.
The extracellular matrix of bone is unique, comprising both organic and inorganic components.
Organic components: Primarily collagen fibers (about ) which provide flexibility and tensile strength, and other organic molecules like proteoglycans.
Inorganic components: Primarily hydroxyapatite crystals (), a calcium phosphate salt that provides hardness and compressional strength.
Bone growth involves collaboration with other connective tissues, specifically cartilage (e.g., in epiphyseal plates responsible for longitudinal growth) and dense irregular connective tissue (e.g., periosteum covering the bone surface).
The skeletal system includes bones, joints, and associated supportive tissues like ligaments and tendons.
The complexity of bones involves bone tissue itself, cartilage, blood vessels (supplying nutrients and removing waste), nerves (providing sensory innervation), and other connective tissues.
Functions of the Skeletal System
Support: The skeleton holds up the body, bearing weight (e.g., vertebrae supporting the torso) and providing the structural framework for soft tissues and organs, as well as providing attachment points for muscles.
Protection: Bones enclose and protect vital organs. For example, the skull protects the brain, the rib cage protects the heart and lungs, and the vertebral column protects the spinal cord within its canal.
Movement: Primarily through the articulation of bones at joints, the skeletal system works as a system of levers with muscles to facilitate movement. Muscles attach to bones via tendons, and when muscles contract, they pull on bones, causing them to move.
Mineral Homeostasis: Bone tissue stores several essential minerals, especially calcium (about of the body's calcium is in bones) and phosphate, which are crucial for various physiological processes like nerve impulse transmission, muscle contraction, and blood clotting. Bones can release minerals into the blood to maintain appropriate levels in the body, regulated by hormones like parathyroid hormone and calcitonin.
Blood Cell Production (Hematopoiesis): Red bone marrow within certain bones (e.g., flat bones like the pelvis and sternum, and epiphyses of long bones in adults) is the site of hematopoiesis, the process by which red blood cells, white blood cells, and platelets are produced.
Triglyceride Storage: Yellow bone marrow, located primarily in the medullary cavity of long bones, stores triglycerides (fats), which serve as a potential long-term chemical energy reserve. This marrow is also known as fatty marrow.