STUDIES OF THE HUMAN FETAL KIDNEY——Ⅱ. FREEZE-FRACTURE ELECTRON MICROSCOPIC OBSERVATION / 解剖学报

ABSTRACT

A 14-week human fetal kidney was studied by the method of freeze-fracture, the results were as follows1. All the nuclei of various cells at this stage of development possess distinct nuclear pores with the same diameter and they are distributed randomly on the nuclear membrane. The intramembraneous particles on the PF of both the inner and outer nuclear membranes are more numerous than that of the EF. The morphological features of the nuclear pores vary according to the plane of their fracture face; they appear as dimples on the EF and as valcano mouth on the PF. The interior of the nucleus usually contains homogeneously dispersed particles, but no such structures were seen in thin sections. In some nuclei a round vesicle was revealed.2. The cell membrane of various cells, at this stage, shows special structures to manifest their degree of differentiation. In the less differentiated epithelial cells of the renal tubules, the cell membrane is straight and the intramembraneous particles are randomly distributed, the intromembraneous particles on PF are more than on EF, comparetively well developed tight junctions are located at the latexal surface near the apical portion of the cell while in the differentiated proximal tubules, there are many microvilli on their luminal surface, but their lateral cell membrane is still straight without interdigitations. In some tubules small processes near the basal part may be seen, however, basal fold is still absent. The basal membrane of the renal tubules is very prominent. As to the epithelial cells of the viseral layer of the renal corpuscle, gap and tight junctions are revealed while they are columnar in shape, but as they differentiated into podocyte with primary and secondary processes, the structure of tight junctions become simple, less prominent and discontinuous.3. Cytoplasm In the process of cell differentiation the amount of cytoplasm increases and the ratio of nuclear end cytoplasmic volume decreases. The orgauelles are less in the undifferentiated cells, so the structure of the cytoplasm of them is much simpler than that of the more differentiated ones. The compartmentation phenomenon of the cytoplasm of differentiated cells become distince. In the freeze-fracture micrographs the membraneous structures, such as mitochondria, Golgi complex and secretion granules etc. are more stereoscopic than those in thin sections, but the opportunity to reveal the fine structures of various cells is minimized due to only one fracture face for each specimen and what structure will be fractured is beyond the control of the operator.