The present study describes the development and maturation of the glomerular visceral epithelium and capillary endothelium in an attempt to clarify kidney function in the neonate. The puppy kidney undergoes nephrogenesis for at least two weeks after birth, and thus possesses nephrons of different ages and levels of maturation. Since nephron development varies considerably from inner to outer cortex, renal corpuscles were examined within three arbitrary zones (outer, middle and inner cortex). The visceral epithelium of the smallest glomeruli, i.e., those of the outer cortex, is composed of cuboidal cells which are densely clustered and rests directly upon the limited number of small capillaries. More deeply within the outer cortex, broad, flat epithelial processes extend around the capillary, but lack any visible pedicels. Mid cortical glomerular capillaries undergo an increase in diameter and length, and a t the same time the podocytes flatten and separate from one another. Pedicels begin to appear in this zone as thickened, disorganized processes. Juxtamedullary glomeruli are the largest, due primarily to their extensive capillary branching (looping). Podocytes are nearly adult-like in appearance, with many long primary processes and pedicels.The endothelial morphology of glomerular capillaries varies from one loop to another, as well as within a single loop. The immature endothelium of outer glomeruli is initially characterized by extensive expanses lacking pores, multiple cellular layers and small, scattered pinholes. Greater capillary maturity is apparent in the mid cortex, due to a n increase in pore number and diameter a s well as greater attenuation of endothelial cells. However, evidence of remodeling and a lack of pores still exists in some capillaries. The glomerular capillaries of the inner cortex appear the most mature, but possess regions that are still immature when compared to the adult. Larsson, '75). New nephrons are forming for a t least two weeks in the outer cortex while more adult-like tubules are seen in the inner cortex. Functionally, the puppy kidney is characterized by a low (1) glomerular filtration rate (GFR), (2) renal plasma flow (RPF), (3) filtra- tion fraction (F.F.), and (4) extraction ratio for p-aminohippurate (PAH).In order to correlate structural changes in the renal corpuscle with the rise in GFR after birth, the entire filtration barrier must be examined. Several morphological studies have described the development of the renal corpuscle (Clark, '57; Kurtz, '58; Suzuki, '59; Aoki, '67; Zamboni and de Martino, '68; Buss, '70; Kazimierczak, '71, '76; Miyoshi et al., '71; Spinelli, '74) with emphasis on the growth of the glomerulus and maturation of the visceral epithelium. Few reports, however, have de- scribed the maturation of the glomerular capillaries, which are also involved in the permselectivity of variously sized and charged molecules which may be filtered. The scanning electron microscope would appear to be well suited for the investigation of both glomerular v...

A B S T R A C T In uranyl nitrate (UN)-induced acute renal failure (ARF) glomerular ultrafiltration coefficient (Kf) decreases because of unknown reasons. Since transport of water across the glomerular capillary wall occurs predominantly extracellularly through the endothelial fenestrae (EF), a reduction in the diameter and/or the density of EF can reduce the extracellular filtration area and the glomerular Kf. To examine this possibility, ARF was induced in rats by intravenous administration of UN in low (15 mg/kg) and high doses (25 mg/kg). Fenestral density (i±SEM) per 5 cm2 from the scanning electron micrographs (x30,000) was 107+10, 103+9, and 101+11 at 2, 7, and 17h after the intravenous administration of bicarbonate saline to the control rats. In the low-dose UN group the EF density was 91±2, 52+8, and 45± 11 at 2, 7, and 17 h after the injection, whereas for the high-dose group at corresponding time intervals the EF density was 95±3, 54±9, and 44± 10. Fenestral diameters, in Angstrom units (i± SEM), were 751±53, 765+43, and 764±37 at 2, 7, and 17 h after the injection of bicarbonate saline to control rats. At corresponding intervals after the administration of UN, the fenestral diameters were 501±61,472+28, and 438±98 for the lowu-dose group and 525+43, 470+39, and 440± 56 for the high-dose group. 2,7, and 17 h after the injection of UN, fenestral area of the low-dose group decreased to 52.1,30.1, and 24.6% ofthe controls, whereas in the high-dose group, the fenestral area declined to 54.3, 30.2, and 23.6% of the controls. Administration of UN (15 mg/kg) to sodium-loaded rats did not alter renal function or endothelial cell morphology. It is suggested that in UN-induced ARF the morphological alterations in endothelial cells reduce the Kf of glomerular capillaries by reducing the filtration area.

The present study utilizes the scanning electron microscope (SEMI to reveal the surface morphology of proximal tubular cells and the parietal cells of Bowman's capsule of the adult rabbit nephron. To facilitate the examination of the basal surface of these cells, proximal tubules were dissected free and treated with collagenase t o remove the basememt membrane. Other blocks of tissue were cryofractured to expose the lateral cell surfaces of the proximal tubules. Our investigation has shown that the lateral and basal surfaces of both the convoluted and straight segments of the proximal tubule have numerous processes. However, the arrangement and degree of branching is distinctly different in the two segments. The convoluted segment has large lateral ridges which form a t the base of the microvilli and fan out to divide into lateral-basal processes. Many of the lateral-basal processes reach the basement membrane intact, interdigitating with complementary processes from adjacent cells. However, some of the lateral-basal processes branch into short, knobby projections (basal villi) which may also reach the basement membrane. Patches of basal villi are interspersed between broad regions of interdigitating lateralbasal processes. Therefore, in the convoluted segment, the lateral-basal processes form the major part of the basal cell surface. In tubular cells of the pars recta, unlike convoluted tubular cells, the majority of the ridges remain unbranched and pass directly to the basal surface where they divide into elaborate basal villi. Thus the basal surface of the pars recta cells is highly complex, appearing leaf-like, being a composite of numerous basal villi with a few lateral ridges.The basal surface of some parietal cells of Bowman's capsule have parallel ridges, which results in patches of striations.The complex morphology of cells of the proximal tubule (PT) of the kidney has interested investigators for many years (Wedl, 1850; Heidenhain, 1874; Schachawa, 1876; Landauer, 1895; Zimmermann, 1898; Zimmermann, '11). Heidenhain (1874) was one of the early light microscopists who noted extensive lateral projections on the proximal tubule cells. Subsequent workers have studied kidneys from a variety of animals and corroborated the data of the earlier studies (von Mollendorff, '30; Grafflin and Foote, '39). A renewed interest in the shape of the PT cells has developed with the advent of the scanning and transmission electron microscopes and functional data that suggests a direct relationship of cell shape to function (Rhodin, '58; Bulger, '65; Waugh et al., '67; Andrews and Porter, '74; Bulger et al., '74; Curran and MacIntosh, '62; Diamond and Bossert, '67). Several models of proximal tubule cell shape have appeared in the literature. Welling and Welling ('76) have recently published an elegant quantitative three-dimensional model of the shape of the epithelial cells and intercellular channels in the rabbit proximal tubule. However, the lack of data on the precise ultrastructure of the basal surface has limit...

Fusion of atrioventricular endocardial cushions begins during Hamburger‐Hamilton stage 27 and is completed during stage 28 (5 to 6 days incubation age). Connective tissue cells of these cushions are derived from the proliferating endocardial cell layer. The character of the organelles in cells of these cushions changes progressively as the cushions approach, meet and fuse: granular endoplasmic reticulum accumulates; Golgi complexes become extensive, and nuclei develop irregular shapes. These occur first in the connective tissue cells of the cardiac jelly and later in the endocardial cells. Fusion apparently is accomplished by liberation of the endocardial endothelial cells into the cardiac jelly, where, along with pre‐existing connective tissue cells, they become cells of the membranous interventricular septum. Gradually the cells appear to merge into one morphological population and thus become indistinguishable from each other. Both endocardial cells and connective tissue cells exhibit uptake of tritiated thymidine and undergo mitosis before, during and after fusion of the cushions. An outstanding characteristic of these cells is their continuing vitality throughout the entire process of fusion.

The fine structure of mitosis in myocardial cells possessing myofibrils from embryonic chick hearts (4.5-6 days incubation age) is described. In these cells during prophase, chromosomes appear, but the nuclear membrane, Golgi complexes, and Z bands mostly disappear. In metaphase, lipid droplets and large quantities of membranous cisternae localize around the centrioles, while sarcomeric lengths of myofibrils (largely without Z bands) are found disoriented in the periphery, mainly in the polar zones. In anaphase the cisternae collect near the chromosomes, as reconstruction of the nuclear membrane commences. An intact nuclear envelope and well-formed Golgi complexes characterize telophase, but Z bands still are not present. During all of the mitotic stages, junctional complexes between dividing cells and adjacent resting cells persist.