The mature mesonephric nephron of the rabbit embryo. III. Freeze-fracture studies

Freeze-fracture observations on human fetal kidney in serum-free organ culture

The current study was undertaken to examine and characterize junctional complexes, through freeze-fracture, in developing human fetal kidney and in cultured renal explants maturing in vitro. Tissue specimens were cultured for 7 days in Leibovitz's L-15 medium in the absence of serum or hormones. In uncultured explants, cells in the different nephron segments were joined by zonulae occludentes which consisted of ridges on the P-face and grooves on the E-face of lateral membranes. Tight junction composition was heterogeneous and complexity increased from proximal to collecting tubules. Proximal tubule cells were also characterized by the presence of gap junctions and a brush border. Podocytes were joined by macular junctions, while zipper-like junctions were observed between collecting duct cells. Intercalated cells were decorated with rod-shaped intramembrane particles on lateral and apical membranes, instead of the usual spherical particles present in other cells. All these structures could be observed at various intervals during tissue culture, indicating the preservation of ultrastructural integrity of the explants. These observations extend and support previous studies made at the light and electron microscopic levels. Thence, the present culture model constitutes a valuable tool to study the direct effect of growth factors on nephrogenesis.

Structural and functional comparison of mesonephric and metanephric proximal tubules

Mesonephric kidneys of the mid-gestation fetus and metanephric kidneys of the adult are known to differ in many gross features but to be qualitatively similar in their cellular morphology. In the present studies, quantitative morphometric and in vitro microperfusion procedures were used to examine further the physical and transport properties of mesonephric and metanephric (S2) proximal tubule segments from rabbit. In perfused mesonephric proximal tubules the volume transport rate was 0.91 +/- 0.08 nl/min per mm tubule length, 2.7 +/- 0.2 nl/min per mm2 basement membrane (outer tubule) surface area, and 0.19 +/- 0.03 nl/min per mm2 basolateral cell membrane surface areas. For comparison, the S2 portion of metanephric proximal tubules is reported to transport 0.33 +/- 0.02 to 0.53 +/- 0.01 nl/min per mm length or, based on the present findings, 2.8 +/- 0.2 to 4.4 +/- 0.8 nl/min per mm2 basement membrane area and 0.16 +/- 0.02 to 0.25 +/- 0.07 nl/min per mm2 basolateral membrane area. Thus, proximal mesonephric and S2 metanephric segments exhibit approximately the same transport per unit area of basement membrane and per unit area of the basolateral cell membranes across which active transport presumably occurs.

The pig mesonephros. II. The proximal tubule: SEM, TEM and freeze-fracture images

The ultrastructure of the proximal tubule of mature mesonephric nephrons was studied in perfusion-fixed pig embryos of the 41st gestational day. Despite its 8-12 mm long course, the proximal tubule possesses no cytologically different subsegments except its very last cells at the abrupt transition into the distal tubule. The first brush-bordered proximal tubule cells stand considerably within Bowman's capsule, abuting its attenuated cells. In SEM specimens, the average luminal cell diameters are 8 X 13 micron. The cells are 6-11 micron in height with overlying microvilli 2-4 micron long. Lateral faces of perfectly disjoined cells exhibit plate-like interdigitating processes projecting more than 5 micron deep into the neighboring cells. The basal cell face is completely covered with microvilli. The TEM pictures reveal an endocytic apparatus largely matching its metanephric counterpart. Mitochondria account for 23% of the cytoplasm and together with the many basolateral cell membranes indicate a high capacity for energy-dependent transport processes. Small basal lipid droplets represent a species peculiarity. Freeze-fracture specimens show an electrocoupling of the cells by gap junctions. The tight junction, with only 1-2 strands, characterizes a "leaky" epithelium. In most features, this tissue is as mature as its metanephric counterpart.

The structure of the kidney of Japanese newts, Triturus (Cynops) pyrrhogaster

The construction of renal lobules in Triturus (Cynops) pyrrhogaster was studied by reconstruction from serial semithin sections, and the structure of nephrons, collecting ducts and ureters was investigated by means of light and electron microscopy. In T. pyrrhogaster the kidney was mesonephros in construction; renal lobules were arranged segmentally and each of them sent one ureter. Male ureters ran caudally and met together before joining the Wolffian duct. In renal lobules, long collecting ducts ran medio-laterally in the dorsal aspect of the kidney and sent several branches ventrally. Each branch duct or short collecting duct received one nephron. Each nephron had five segments; 1) renal corpuscle, 2) ciliated neck segment with or without a naphrostome, 3) proximal tubule, 4) ciliated intermediate segment and 5) distal tubule. Proximal and distal tubules were segregated spacially in renal lobules and occupied the peripheral and central zone respectively. The filtration barrier of the glomerulus consisted of both the basal lamina of podocytes and the subendothelial connective tissue, and was much thicker than the mammalian filtration barrier. Proximal tubule cells had a brush border, apical specialization for reabsorption of organic materials and well-developed smooth endoplasmic reticulum, but few baso-lateral interdigitations. In distal tubule cells, baso-lateral interdigitations and infoldings were well-developed. Collecting duct cells had a sparse cytoplasm. Ureter cells in males contained many secretory granules. On the basis of structural organization of the newt kidney as well as physiological data in literature, we suggest that in land vertebrates proximal tubules were primarily adapted to reabsorption of organic materials and distal tubules to reabsorption of electrolytes and water.