Histogenesis

HISTOGENESIS (from Greek histos-tissue and genesis-origin), the doctrine of the development and differentiation of cells and intercellular substances, as a result of which the tissues of the adult organism arise, specific to each organ. In the early stages of embryonic development, from the three germ layers, rudiments of organs arise through the formation of folds, indentations, outgrowths, condensation of cell masses in certain places, etc.; the mechanism of their origin comes down to cell multiplication and their movement; after this period of laying follows the period of H., when cells undergo metamorphosis (cytomorphosis), certain connections are established between them, intercellular substances are developed, and generally structures arise associated with the function of the given organ. The processes of H. are usually described in the appropriate chapters of histology. Despite the enormous number of works, many histogenetic problems remain controversial. This applies primarily to connective tissue, where such a fundamental question as the origin of fibers-directly from cells or through the intercellular substance, is still resolved differently; the question of the origin of blood elements from one primary cell or from two divides histologists into 2 camps (unitarists and dualists); in recent times the question of the common origin of blood cells and other elements of mesenchyme (which also includes endothelium) has been raised, also causing disagreement. In H. of muscles, the controversial question is the origin of the sarcolemma (cellular or connective tissue sheath); regarding H. of nerve fibers, there are two theories-outgrowth and cellular chain. Even the basic position on the specificity of germ layers, according to which from epithelium as a derivative of ectoderm, connective tissue formations cannot arise, is currently being criticized, and a number of scientists again derive the reticulum of the thyroid gland from epithelium. In pathology, the term histogenesis or histopathogenesis is used in relation to the development of various pathological proliferations (for example tumors) and the appearance of certain cellular elements under pathological conditions (for example wandering cells).