Bone tissue is composed of a mixture of living cells and minerals (mainly calcium and phosphorus). It is these minerals that make bones have solid physical properties. The bones have different shapes and sizes. For example, the arm bones are long bones, the wrist bones are short bones, the sternum and skull bones are flat bones, and the vertebrae are irregular bones. The bones of adults are mainly composed of two kinds of tissues: hard dense bone is outside, and porous cancellous bone is also called cavernous bone. The thigh bones, or femurs, in long bones are filled with fat called yellow bone marrow.
It is difficult to imagine that bones are living and growing tissues. How do bones grow?
Bone grows in a unique way: first, new cells are formed, then these cells produce a special organic substance that becomes the matrix surrounding itself, and finally, calcium salts are deposited in the matrix to gradually harden it. Human bones begin to form as early as two months of gestation. Since then, bones have continued to grow in women, around the age of sixteen, and in men, around eighteen. The sternum no longer hardens around the age of 25, but the strength and calcium of the other bones continue to increase until it stops around the age of 35. During most of a person's life, bones are constantly being reformed, and bone tissue is constantly worn out and replenished. During the fetal period, bones are formed in two ways. The bones at the top of the skull begin to grow in the connective tissue membranes, and most of the other bones begin with "chrysanthemum" cartilage. The embryonic cartilage is similar to real bone, but it is relatively soft and suitable for rapid growth, and is eventually replaced by real bone. Embryonic cartilage is gradually replaced by bone tissue. The replacement process of long bones begins with the center of the backbone and the ends of the bone; in the end, only a thin layer of cartilage is left between the backbone and the ends, called the growth plate. The growth plate continues to form new cartilage, which is then replaced by real bone, and the bone grows.
Bone growth is more complicated than simple elongation or simple enlargement. Most long bones increase their width externally through a process called subperiosteal juxtaposition (adding layers to existing layers), and are lost internally due to internal bone absorption (decomposition and absorption of matter at the center of mass). Up the bones. At the same time, long bones increase their length by adding a bone epi plate (the surface of the end of the bone). As they stretch, this type of bone undergoes a process called remodeling, during which their external shape also changes. In contrast, the individual bones of the skull are grown by juxtaposition (adding layers in the circumferential direction) and increasing thickness by adding layers (adhesion) on the surface and simultaneous absorption on the inner surface. Through this process, the skull expands and thickens, while allowing more brain space inside. The linear growth of long bones occurs through different processes. Long bones have more than one ossification center (the area where bones begin to grow) at birth. They grow during childhood until the end of the bone (bone s-plate) merges with the bone (backbone). This process is stimulated by hormones produced by the testes and ovaries, which provide developmental signals that the linear growth of long bones should reach full or full development. Both the round and flat bones of the bone can continue to grow throughout life.
Bone growth factors
Currently known bone growth factors include insulin-like growth factor-1 (IGF-1), insulin-like growth factor-2 (IGF-2), transforming growth factor beta (TGF-β), and Platelet-derived growth factor (PDGF).
Insulin-like growth factor 1 (IGF-1) is a hormone with a molecular structure similar to insulin. It plays an important role in childhood growth and has anabolic effects in adults. IGF-1 is produced mainly by the liver. The highest rate of IGF-1 production occurs during adolescent growth spurts. The lowest levels occur during infancy and old age.
Insulin-like growth factor 2 (IGF-2) is one of three protein hormones similar in structure to insulin. It has activities that regulate growth, insulin-like and mitogenic. Growth factors have a major but non-absolute effect. In contrast to insulin-like growth factor 1, which is the main growth factor for adults, it is considered the main fetal growth factor.
Transforming growth factor beta (TGF-β) is a multifunctional cytokine belonging to the transforming growth factor superfamily. Activated TGF-β complexes with other factors to form a serine / threonine kinase complex that binds to the TGF-β receptor. The TGF-β receptor consists of type 1 and type 2 receptor subunits. After TGF-β binds, the type 2 receptor kinase phosphorylates and activates the type 1 receptor kinase, thereby activating the signaling cascade. This leads to the activation of different downstream substrates and regulatory proteins, and induces the transcription of different target genes that play a role in the differentiation, chemotaxis, proliferation, and activation of many immune cells.
Platelet-derived growth factor (PDGF) is one of many growth factors that regulate cell growth and division. It can stimulate various cells such as fibroblasts, glial cells, and smooth muscle cells that are stagnated in the G0/G1 phase to enter the division and proliferation cycle. Platelet-derived growth factor PDGF was discovered in 1974 as a peptide regulator that stimulates the growth of connective tissue and other tissue cells. It is named because it is derived from platelets. It exists in the alpha particles of platelets under normal physiological conditions. Released and activated by disintegrating platelets, with biological activity that stimulates specific cells to chemo and promote specific cell growth.