Summary
Highlights
Bones are not completely solid and have spaces. Based on the size and distribution of these spaces, bone regions are classified as compact or spongy. Compact bone makes up 80% of the skeleton, forming the outer layer of all bones, providing protection and support. It consists of units called osteons or Haversian systems. Spongy bone makes up the remaining 20% and consists of lamellae arranged in an irregular network called trabeculae. Some bones have these spaces filled with red bone marrow.
Bone tissue is a special living tissue that is constantly changing. It contains special cells called osteocytes and osteoblasts that build, destroy, and regenerate bone. The bone tissue is like a never-ending construction site.
Five keywords are essential to understanding this topic: bone, cartilage, medulla, skeleton, and mineralization.
Bones are made of connective tissue and have a high mineral content, giving them hardness, elasticity, growth mechanisms, and regenerative capacity. The process of bone formation is called ossification, which begins in the embryonic stage and continues until adolescence. The basic functions of the skeletal system include support for soft tissues and muscles, protection of internal organs (e.g., skull protects the brain, rib cage protects the heart and lungs), movement in conjunction with muscles and joints, mineral homeostasis by storing and releasing calcium and phosphorus, production of blood cells (red blood cells, white blood cells, and platelets) in the red bone marrow, and storage of triglycerides in the yellow bone marrow.
Bones consist of several parts: the diaphysis (cylindrical body), epiphysis (proximal and distal ends), metaphysis (junction between diaphysis and epiphysis), articular cartilage (hyaline cartilage covering epiphysis), periosteum (tough connective tissue layer protecting the bone, aiding fracture repair, and serving as attachment for tendons and ligaments), medullary cavity (internal space of diaphysis containing yellow bone marrow), and endosteum (layer lining the medullary cavity and containing bone-forming cells).
Bone tissue has an abundant matrix and widely separated cells. The matrix is composed of 25% water, 25% protein fibers, and 50% crystallized mineral salts. Different types of cells are found in bone tissue: osteogenic cells (unspecialized stem cells that divide to form osteoblasts), osteoblasts (bone-building cells that synthesize the bone matrix and initiate calcification), osteocytes (mature bone cells derived from osteoblasts, responsible for nutrient exchange with blood), and osteoclasts (large cells that destroy bone tissue through lysosomal enzymes, facilitating bone development, growth, maintenance, and repair). Bone hardness depends on crystallized mineral salts, while flexibility relies on collagen fibers.
The embryo initially contains cartilage structures that gradually undergo ossification and osteogenesis. Osteoblasts, specialized bone-forming cells, transform the cartilage into bone. Ossification involves the synthesis of the organic bone matrix by osteoblasts and the calcification of the matrix with inorganic compounds like calcium and phosphorus salts. Bone growth in length occurs from the epiphyseal cartilage, which disappears when maximum length is reached. Bones increase in diameter through the combined action of osteoclasts (resorbing bone from the medullary cavity walls) and osteoblasts (producing new bone on the exterior).
Bone remodeling is a continuous process where old bone tissue is replaced by new bone tissue, maintaining skeletal integrity and function. Osteoclasts and osteoblasts work together to balance bone formation and resorption. This process helps repair microfractures, replaces aging bone, and adjusts the skeleton to changing demands. Bone remodeling also regulates calcium and other minerals like phosphorus in the body. Hormones like Parathyroid Hormone (PTH) and calcitonin regulate blood calcium levels. PTH, secreted by parathyroid glands in response to low blood calcium, stimulates osteoclasts to release calcium into the blood. Calcitonin, secreted by the thyroid gland in response to high blood calcium, reduces osteoclast activity and stimulates osteoblasts to form bone, thereby lowering blood calcium levels. The balance of these hormones maintains stable blood calcium levels and ensures strong, healthy bones.
Bones are living tissues in constant change and growth, formed from early life by specialized cells. The skeletal system provides shape, support, protects vital organs, enables movement, produces blood cells, and regulates essential minerals like calcium and phosphorus. It is an intelligent, active, and fundamental system for life.