[The sequence of nephron tubule differentiation in the definitive human kidney during the fetal period].

The regularities of the formation of the undifferentiated renal tubules in the definitive kidney were studied in 174 human embryos and fetuses at 4.5 to 40 gestational weeks. On the basis of the study of morphological and morphometrical parameters of these tubules, it was shown that the increase of the average tubular crossectional area was associated with the consecutive formation of the nephrons of the definitive kidney and the differentiation of the segments of their tubular portions. The increase of the proportion of the epithelium in the structure of undifferentiated tubule is determined by the segregation of the proximal and distal tubules within the forming nephrons. The sequence of the initial stages of the renal tubule differentiation is dictated by the ergontic correlations; it developes from the proximal tubule to the distal one and is different from the succession of their formation in the process of the early nephronogenesis.

[Provisionality phenomenon in the histo- organo- and systemogenesis].

This review summarizes the results of the study of the provisionality phenomenon in the development of the organs of the urinary system (mesonephros, metanephros) in viviparous (human, rat) and oviparous (bird) amniotes, human organs of mixed origin (pituitary, ovary, stomodeum) reparative regeneration of supporting tissues (in humans and Syrian hamster), morphogenesis of primary hepatic cancer developing against the background of superinvasive opisthorchiasis. It is shown that during the development of tissues and organs, the stages of provisional and definitive histo- and organogenesis could be determined; saltatory mechanism is typical for embryonic organogenesis (formation of cartilaginous skeletal primordia, meso- and metanephrons) and foci of oncogenesis. Transformation zones of the type of tissue organization in the epithelium are located in the areas of contacts of the derivatives of different embryonic primordia. Mechanism of transformation of the epithelial layer of stomodeum and Rathke's pouch is provided by local activation of apoptosis and by the formation of the epithelial cells of a qualitatively new generation.

[Human pituitary embryonic morphogenesis].

Basic stages of pituitary gland organogenesis were studied using the methods of light microscopy in human embryos at Carnegie stages (CS) 12-23 (25-57 postovulatory days). It is shown that Rathke's pouch (RP) is formed like a traction fold along the whole width of the stomodeum roof in embryos at CS 12 as a result of a neural tube bend, to which the epithelium is closely connected anatomically (attached part of RP front wall) in the middle plane of the embryo. At CS 15 the primordia of hypothalamic infundibulum and of neurohypophysis are formed as a thickening of diencephalic posterior wall. At CS 20-23 transorientation of brain components, including hypophyseal primordium, takes place. After that the attached part of RP front wall forms pars intermedia and pars tuberalis of the pituitary, and the epithelium of RP orifice and that of its posterior wall forms pars distalis. During CS 20-23, structural-functional units of adenohypophysis--epithelial cords--are formed due to invagination of RP epithelium.

[Role of provisionality principle in realization of phylembryogeneses].

The analysis of histogenesis of skeletogenic tissues and of organogenesis of the mesonephros and metanephros in higher mammalian animals and man, is presented. The new conception of provisionality is formulated, according to which embryonic primordium and/or its derivatives possess a determined capacity to form, on its course to definitive state, temporary structures (tissues or organs), which fulfil the vital functions in the developing organism and modeling the mechanisms of development and formation of structural and functional units or the whole organ at the level of definitive morphological substrate. Provisionality is regarded as the universal principle in realization of of phylembryogeneses.