Morphological study of cell organelles during development II-The mitochondria of the renal and intestinal epithelium.

The ontogeny of the chondriome was studied in the developing kidney of fetuses and newborn rats and compared to its maturation in jejunum cells of adult rats. The tissues were immersed in a double lead and copper citrate solution which stains mitochondria selectively. Thick sections (0.3-0.5 micron) were observed with a standard transmission electron microscope (80-100 kV). Mitochondria have in developing cells a shape different from those in mature cells and seem to increase in number and volume during development. In the undifferentiated stages of both jejunum and nephron, the mitochondria are small, spherical and scattered throughout the cytoplasm with no particular orientation. As jejunal cells migrate towards the villus tip, the mitochondria orient themselves in the apical-basal direction; those of the proximal nephron become parallel to the lateral cell membranes. The morphological communications found in adult renal cells are much less prominent in the jejunum. With maturation, the close association between membranes, mitochondria and the endoplasmic reticulum takes place in both epithelia. Our studies suggest that the mitochondrial mass is increased by a continuous process of addition of sub-units and that the chondriome development appears to be more analogous to the growth, differentiation and modification of the endoplasmic reticulum system. The chondriome also seems to reach a certain stage of development before the onset of the organ function.