The development of cell junction during nephrogenesis

Neonatal Hyperoxia Downregulates Claudin-4, Occludin, and ZO-1 Expression in Rat Kidney Accompanied by Impaired Proximal Tubular Development

Hyperoxia is essential to manage in preterm infants but causes injury to immature kidney. Previous study indicates that hyperoxia causes oxidative damage to neonatal kidney and impairs renal development. However, the underlying mechanisms by which neonatal hyperoxia effects on immature kidney still need to be elucidated. Tight junction, among which the representative proteins are claudin-4, occludin, and ZO-1, plays a crucial role in nephrogenesis and maintaining renal function. Inflammatory cytokines are involved in the pleiotropic regulation of tight junction proteins. Here, we investigated how neonatal hyperoxia affected the expression of key tight junction proteins and inflammatory factors (IL-6 and TNF-α) in the developing rat kidneys and elucidated their correlation with renal injury. We found claudin-4, occludin, and zonula occludens-1 (ZO-1) expression in proximal tubules was significantly downregulated after neonatal hyperoxia. The expression of these tight junction proteins was positively correlated with that of IL-6 and TNF-α, while claudin-4 expression was positively correlated with injury score of proximal tubules in mature kidneys. These findings indicated that impaired expression of tight junction proteins in kidney might be a potential mechanism of hyperoxia-induced nephrogenic disorders. It provides new insights to further study oxidative renal injury and development disorders and will be helpful for seeking potential therapeutics for hyperoxia-induced renal injury in the future.

Renal tubular differentiation in mouse and mouse metanephric culture. I. Ultrastructural studies

We studied the morphologic features of renal tubular differentiation in transfilter metanephric culture. Differentiation of portions of the S-shaped loop into proximal convoluted tubule was detected shortly after 72 h of culture by the appearance of microvilli, coated membrane invaginations, and an apical vacuolar-microtubular network. These features developed synchronously, and the microvilli became progressively more numerous and more compact to form a brush border. Computer-assisted morphometric analysis showed only minor differences between proximal tubular cells from 18-day embryos and tubular cells from 7-day cultures of blastema taken from 11-day embryos. Segments of tubule corresponding to distal convoluted tubules were lined with relatively simple cells that contained few differentiating characteristics. Morphometric modeling of the tubular cells indicated a simple shape consistent with an inherent transport function.

Expression of the alpha-subunit of Na/K-ATPase in renal collecting duct epithelium during development

Aldosterone enhances synthesis and enzyme activity of Na/K-ATPase and has been found to stimulate sodium reabsorption by the renal cortical collecting duct. Moreover, chronic exposure to aldosterone is associated with a remarkable morphological-functional adaptation. This is seen as a magnification in basolateral membrane area of principal cells. In the present paper we investigated the acquistion of Na/K-ATPase alpha-subunit in renal tissue and examined whether aldosterone initiates functional and adaptive changes in cultured collecting duct cells similar to those observed in vivo. Using a monoclonal antibody, immunofluorescence microscopy demonstrated the acquisition of the alpha-subunit of Na/K-ATPase in the developing cortical collecting ducts of the kidney of neonatal rabbits. The mature collecting ducts in the medulla and papilla of the developing kidney were strongly labelled at the basolateral side, while in the cortical portion of the fetal collecting duct adjacent to the embryonic ampullae the immunolabel was found at the apical and the basolateral aspect of the epithelium. However, the embryonic collecting duct ampullae in the outer cortex did not show any reaction with the antibody. In the collecting duct cells cultured for 24 hours the alpha-subunit of Na/K-ATPase was found to be distributed at both the apical and basolateral aspect of the epithelium. After two to 16 days, the immunolabel was strictly found distributed at the basolateral side. Culturing collecting duct cells in the presence of aldosterone (10(-6) M), the hormone modulated the cellular shape of epithelia after five days by infolding the lateral plasma membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation properties of renal collecting duct cells in culture

In the present study, we were particularly interested in distinguishing specific patterns of structural and functional proteins in the collecting duct system of neonatal and adult kidneys and in cultured renal collecting duct epithelia in order to ascertain the degree of differentiation in the cultures. We studied the distribution of specific renal collecting duct cell markers using morphological, immunohistochemical and biochemical procedures. Cultured renal collecting duct epithelium undergoes maturation in vitro. Examples of morphological differentiation include the appearance of cilia and microvilli at the apical cell pole, and a basement membrane at the basal aspect of the epithelium. Tight junctions with five to seven strands separate the wide intercellular spaces from the apical cell surface. Physiological maturation from a 'leaky' to a 'tight' epithelium is evident from the acquisition of the alpha-subunit of Na/K-ATPase and the development of a high transepithelial potential difference and resistance. Biochemical differentiation is revealed by the expression of specific proteins. The simple-epithelium cytokeratins, PKK1 and PKK2, which are typical intracellular-matrix proteins of mature collecting duct epithelium, maintain the same distribution in cell culture as in neonatal and adult kidneys. An indicator of maturation in vitro is the expression of the collecting duct-specific proteins, PCD2 and PCD3. Newly developed monoclonal antibodies against these antigens reacted similarly with cultured cells and cells of the mature collecting duct system, but they did not label the embryonic ampullae in the cortex of neonatal rabbit kidneys. In contrast, a third collecting duct-specific protein, PCD1, is not expressed by the cultured cells, which indicates the retention of an embryonic characteristic in vitro. Embryonic collecting duct ampullae of the neonatal kidney in situ contain laminin during their development. Laminin is, however, absent in cultured collecting duct epithelium. Biochemical stimulation of the adenylate cyclase system by arginine vasopressin resulted in a twofold stimulation of the enzyme activity. This degree of stimulation is similar to that found in maturing kidneys of neonatal rabbits and indicates another embryonic feature of the cultures.

Differentiation of the segmented tubular nephron and excretory duct during lamprey metamorphosis

The tubular portion of the adult lamprey nephron differentiates into various morphologically distinct segments during the seven stages of metamorphosis. At stage 1, a rudimentary nephron unit (RNU) originates from a nephrogenic mass attached to the peritoneum and elongates to become associated at its distal end with the archinephric (excretory) duct. The rapidly dividing cells show little sign of differentiation. Separation of the RNU from the peritoneum in stage 2 is first accompanied by widening of lateral intercellular spaces and then eventually by the formation of a small lumen at the proximal end. Cells surrounding a portion of this lumen show early signs of ciliogenesis and hence are located in the position of the presumptive neck segment. The distal end of the stage 2 RNU contains an electron-dense material in an enlarged intercellular space and the cells make non-junctional contact with the archinephric duct. The larval cells of the latter structure are either lost during degeneration and desquamation or are transformed into adult cells. By the end of stage 3 replication of basal bodies and formation of cilia has resulted in the appearance of a ciliated neck segment while numerous microvilli extending into a narrow lumen mark the position of the proximal segment. Furthermore, most of the distal portion of the newly formed tubule possesses a lumen containing a compact mass of material likely secreted by the cells during lumen formation. This presumptive distal segment terminates in the transforming epithelium of the archinephric duct as a presumptive connecting or collecting segment. Fully differentiated neck and proximal segments are present in stage 4 with the latter having a complete apparatus for endocytosis but the distal segment shows only early signs of differentiation of the smooth tubular network. This network appears to arise as an expanded surface area of the plasma membrane. Stage 5 is characterized by elongation of the proximal portion of the tubule, expansion of the tubular network in distal cells, and the identification of a collecting segment. The last two stages show no further major differentiation of the segments. The early differentiation of tubular segments in the adult lamprey nephron likely reflects the immediate physiological needs of this organism as the larval kidney undergoes regression. The potential of the developing kidney of adult lampreys as a source of information on morphogenesis of microvilli, the endocytotic apparatus, mitochondria, and a smooth tubular network is discussed. It appears that the archinephric duct and non-nephrogenic mesenchyme may play some role in tubulogenesis in lampreys.

Intercellular junctions in the cells of the human enamel organ as revealed by freeze-fracture

Examined by thin sections and freeze-fracture replication techniques, secretory ameloblasts possessed two sets of the junctional complexes at both proximal and distal ends of the cell bodies, which consisted of tight junctions and occasional gap junctions and desmosomes. The proximal tight junction was fascia occludens, whereas the distal tight junction was zonula occludens. Between adjacent ameloblasts, mature gap junctions were frequent. The stratum-intermedium cells were connected to each other and to the stellate-reticulum cells and ameloblasts by well-developed desmosomes, gap junctions and fascia or macula-type tight junctions. Stellate-reticulum cells were inter-connected by many extensive cytoplasmic processes, in which well-developed desmosomes, small gap junctions and occasional macula-type tight junctions appeared. Thus fascia or macula-type tight junctions as well as many desmosomes seem to serve in mechanical, cell-to-cell adhesion during tooth formation. Frequent and large gap junctions between adjacent stratum-intermedium cells and between the stratum intermedium and the base of the ameloblast suggest that, in relation to enamel formation, these two cell layers form a functional unit.