Cell is the smallest unit of live organisms. No smaller unit than a cell is capable to fulfill the basic functions of living organism: metabolism, motility, growth, reproduction, inheritance.
Growth, reproduction and inheritance, require mitosis, i.e. cell division.
Somatic cells divide into two daughter cells after duplication of the set of chromosomes, while germ cells during meiosis divide the set of chromosomes into two halves. Each cell capable of mitosis goes through the cell cycle, in which each mitosis of 30 - 120 minutes duration is separated from the next mitosis by an interphase of 6 - 36 hrs.
Certain ‘labile cells’ with short lifespan go continuously through the cell cycle, and thereby replace all deceased cells so that the cell count of each organ and tissue is constant. Epithelial cells of different tissues i.e. epidermis, mucosa of mouth, vagina and uterine cervix, of salivary glands, gastrointestinal tract, biliary passages, of uterus and lower urinary tract, and bone marrow cells, are ‘labile cells’. During mitosis one daughter cell remains usually undifferentiated, i.e. stem cell, and the other daughter cell gradually differentiates into no longer dividing cells, such as erythrocyte, granulocyte, spermatogonia.
Some ‘stable cells’, or ‘resting cells’, do not proliferate. After mitosis they enter resting state, known as phase Go. Parenchymal cells of liver, kidneys, pancreas, connective tissue and mesenchymal cells, i.e. fibroblasts, endothelial cells, chondrocytes, osteocytes, leiomyocytes, are ‘resting cells’. Only a tissue loss, or a major injury of tissue, reactivates the cells in the Go phase so that they re-enter mitosis. For example, normally only 1% of hepatocytes undergoes mitosis, but after injury as much as 10% of hepatocytes enters cell division. Fetal precursor cell transplantation increases the rate of mitosis in stable, but also labile, cells.
Transition from Go to G phase, as well as the stimulation of cell proliferation in general, requires binding of growth factors, or hormones, to the specific receptors, usually located on the cell surface. Receptors for growth factors are activated, with subsequent phosphorylation of various proteins of signaling chain until the cell nucleus is reached, DNA synthesis is stimulated, and cell division takes place.
Division of some resting cells is minimal without fetal precursor cell transplantation, i.e. neurons, peripheral and heart myocytes, etc.
Regulation of labile or resting cells cannot alone, i.e. by itself, reconstruct the original structure of the entire damaged tissue. For that an intact extracellular matrix is necessary, capable to direct shape, growth, migration and differentiation of cell. Extracellular matrix consists of fibrillar structural proteins, i.e. collagen I, II, V, elastin, and of adhesive glycoproteins of intercellular matrix, i.e. fibronectin, laminin, immersed in gel of proteoglycans and glucosaminoglycans. Integrins are cell membrane proteins that connect extracellular matrix with intracellular cytoskeleton and transmit signals for growth, migration and differentiation of cells.
In case of severe injury with extensive damage of extra-cellular matrix the repair is by scar tissue, unless proliferation of the resting connective tissue and mesenchymal cells is triggered, that possesses the ability of secretion of extra-cellular matrix and that can be directly stimulated only by fetal precursor cell transplantation.