Structure of the avian kidney

The kidneys from 6 domestic fowl were fixed in situ by perfusion from the left ventricle. In the bird there are two types of nephrons. One reptilian-type without Henle's loop and medullary tissue, and one mammalian-type with Henle's loop lying in medullary tissue. Serial sections from kidney tissue embedded in plexiglass or in paraffin were used to study the architecture of eight reconstructed reptilian-type nephrons from different cortical levels. All reconstructed nephrons had four major bends, but particularly in the subcapsular nephrons additional bends parallel to the kidney surface were found. There was no loop of Henle, but before entering the collecting duct the distal tubule usually had a very thin-walled segment. No proximal convoluted part was found in the reptilian-type nephrons. The length of the tubules varied between 3,000 microns and 6,000 microns. In the distal tubule a macula densa segment was found in all nephrons of the reptilian and mammalian type. The capillary network between the inter- and intralobular veins was composed of increasingly larger capillaries towards the intralobular vein. Segments of the distal tubule were indented into these capillaries and completely surrounded by them. In the nephrons of the mammalian type the proximal tubule was found to be convoluted as is usual for mammalian species.     

Development of the metanephros in the chick: maturation of glomerular size and nephron length

Summary In the avian kidney three nephron types may be identified: mammalian-type nephrons with long (MTN-I) and short (MTN-II) loops of Henlé, and reptilian-type nephrons (RTN). By the method of microdissection the maturation of the nephrons of domestic fowl kidney has been studied. From the 14th day of incubation it is possible to isolate the MTN-I which appear first; all three nephron types may be isolated after 18 days of incubation. The thin limb of Henlé's loop in MTN-I appears after hatching, and the length of this segment in 1-day-old chicks is 0.1–0.13 mm. In 60-day-old chicks its length has approximately doubled. The transition of the thin segment to the thick segment is situated in the descending part of Henlé's loop. In the course of development, the relative length of all nephron segments in MTN-I increases uniformly, while in MTN-II and in RTN the relative length of the proximal and distal convoluted tubules increases. At all stages of development, MTN-I are the longest among the three groups of nephrons and have the largest glomeruli. The comparison between developing avian and mammalian kidneys shows great similarities in the process of maturation in analogous nephron types in these two classes of vertebrates.     

Tubule-tubule and tubule-arteriole contacts in rat kidney distal nephrons. A morphologic study based on computer-assisted three-dimensional reconstructions.

BACKGROUND: Functional investigations of the tubulo-glomerular feedback mechanism have indicated the existence of a contact between the distal nephron and the macula densa region. The structural justification of such a contact is investigated. EXPERIMENTAL DESIGN: Tubule-tubule and tubule-arteriole contacts were investigated in distal nephrons from normal rat kidneys. Computer-assisted three-dimensional reconstructions of distal nephrons were made from serial sections of renal cortical tissue and selected sections were examined by electron microscopy. RESULTS: In 14 of 15 reconstructed nephrons, the distal convoluted tubule or the connecting tubule approached the macula densa region. A wall-to-wall contact between two tubules corresponding to a three-dimensional distance below 28 microns between the axes of the two tubules was found in only five of the reconstructed tubules. The distal nephron contacts to afferent and efferent arterioles of the same nephron were also examined. The efferent arteriole revealed no consistent contacts but the afferent arteriole contacted the distal convoluted tubule/connecting tubules consistently in all 10 of the superficial nephrons and in 3 of 5 midcortical nephrons. Electron microscopy confirmed a close contact between the distal tubule and the afferent arteriole in superficial nephrons and small nerves were often found at or near the site of contact, but the morphology at the site of contact was not unique. The arteriole contacts were made with late distal convoluted tubules, connecting tubules, or cortical collecting ducts. CONCLUSIONS: In conclusion, the present study shows that tubule-tubule contacts are inconsistent between the macula densa region and the distal nephron but that the tubule-afferent arteriole contact is consistent and close in superficial nephrons. This morphology is compatible with the existence of a feedback mechanism between the superficial distal nephron and the afferent arteriole, apart from the one located at the juxtaglomerular apparatus.     

Corticosterone binding sites along the rat nephron

Glucocorticoids influence numerous kidney functions but the precise location of glucocorticoid receptors in the nephron is not known. To identify the renal binding sites of corticosterone, the natural glucocorticoid in the rat, we measured the binding of [3H]corticosterone to discrete nephron segments microdissected from adrenalectomized rats. Highest specific binding capacity at 25 degrees C (expressed as fmol X cm-1 +/- SE) was found in the cortical collecting tubule (9.69 +/- 0.77) followed in decreasing order by the distal convoluted tubule (2.70 +/- 0.49), medullary collecting tubule (2.58 +/- 0.64), proximal convoluted tubule (1.09 +/- 0.10), and pars recta (0.57 +/- 0.08). Binding was lowest in the thick ascending limb of Henle's loop, with comparable values in the medullary (0.27 +/- 0.05) and cortical (0.26 +/- 0.05) portions of this segment. The apparent maximal binding capacity of the cortical collecting tubule for corticosterone exceeded by nearly two orders of magnitude that of aldosterone previously measured by us in this structure, which is in agreement with the observations of other investigators in kidney cytosol. Specific binding of corticosterone can be demonstrated along the entire rat nephron, but binding sites are concentrated in the cortical collecting tubule. This segment appears to be the main target site for corticosterone as it is for aldosterone.     

Structural evidence for counter-current flow in proximal tubules versus pertitubular capillaries in the rat kidney. Evaluation of the counter-current mechanism between the proximal convoluted tubules and the peritubular capillaries in the rat nephron

BACKGROUND: In spite of the very high exchange of water and solutes between the proximal tubules and the peritubular capillaries, very little is known about flow directions in these two interrelated structures. We therefore developed a morphological technique suitable for the quantitative evaluation of a counter-current system between the proximal convoluted tubules and the peritubular capillaries in rat renal cortex. METHODS: In male pentothal-anesthetized Wistar rats (body weight 200-250 g), India ink was injected into the aorta above the renal arteries, followed by instant freezing of the right kidney in isopentane at -165 degrees C, and subsequent freeze-substitution in alcohol. In microscopic slides from kidneys in which only 20-55% of the cortical peritubular capillary loops was filled with ink--representing the arterial end of the capillaries--and in which the proximal tubular segmentation could be identified in PAS-stained sections, the segments of the convoluted proximal tubules were quantitatively compared with regard to the presence of ink-stained and unstained peritubular capillaries in nephrons from the whole renal cortex. RESULTS: In the microscopic specimens of the five animals used both the loops from the first segment (P1) of the proximal convoluted tubule and those of the second segment (P2) were systematically packed closely together, the transitional segment (P1-2) being interposed between the groups. Around the loops of P1, 8%+/-2% of the capillaries was stained with India ink. In contrast, surrounding the P2 loops 67%+/-5% of the capillaries contained ink, significantly exceeding that for P1 (p<0.01). CONCLUSION: Throughout the rat renal cortex, the most proximal fraction of the peritubular capillaries surrounds the second segments of the proximal convoluted tubules, while the first tubular segments are surrounded by the more distal fraction of the peritubular capillaries. Consequently, the flows in the peritubular capillaries and in the proximal convoluted tubules in the rat renal cortex are systematically arranged as a counter-current system. This feature was previously identified only in superficial nephrons.     

Morphology of the kidney of the platypus (Ornithorhynchus anatinus: Monotremata)

The platypus kidney shows morphological similarities to those of other mammals. Macroscopically, the cortex is easily distinguishable from the fairly wide medulla. Within the medulla, no clear border is observed between the inner and outer zones. Light and transmission electron microscopically, the glomeruli show quite similar architecture to those of other mammals; however, the glomerular lobulation is very clear. The glomerular tufts are rather simple, but capillary lumen varies widely in size, which is one of the unique features of the platypus kidney. The urinary tubule is generally similar to that of human and other mammals in shape and segmentation; however, the staining specificities of histochemical reactions and the shape of epithelial cells of the Henle's loop differ from those ofother mammals. The most conspicuous features are: 1) although no protein casts are found in the tubular lumina, epithelial cells of the proximal convoluted tubule (PCT) have numerous electron-dense vesicles as in human nephrotic kidneys; and 2) the platypus Henle's loop consists of the thick epithelial cells similar to the mammalian type nephron of birds. As compared to those of other mammals such as humans and rats, our observations suggest that the platypus kidney is less developed, in terms of evolution.© Willey-Liss, Inc.     

小鼠肾远曲小管三维可视化研究

目的 肾远曲小管（DCT）是肾单位最后一段小管,其与相邻小管的分界及走行的毗邻关系是理解肾形态发生中集合管与肾单位连接方式,以及该段小管参与水盐代谢调节机制的重要结构基础。本实验在肾组织连续切片基础上,采用微细结构三维可视化技术,建立小鼠肾远曲小管的空间走行。 方法 C57/BL/6 J小鼠灌流固定后取肾,垂直于肾长轴切取组织块,树脂812包埋,从肾被膜到肾外髓外带获得2.5 μm厚连续切片720张,获取数字化显微镜图像并通过计算机程序进行配准,追踪来自3只小鼠共90个肾单位远曲小管,观察其空间走行并测量其长度。 结果 肾远曲小管起始于远端小管致密斑后40～180 μm处,上皮由矮柱状或立方陡然变为高柱状,细胞核靠近腔面,走行在皮质迷路其自身的肾小球周围,区域相对独立,末端向被膜方向逐渐移行为连接小管;此处上皮再次变矮,细胞核排列不局限在近腔面。浅层皮质肾单位的远曲小管在回旋返折最高处接触肾被膜1次。远曲小管长度为500～900 μm。 结论 远曲小管较短且盘曲走行,较少与其他小管交错,所以区域较小而独立,可能有助于该节段的吸收功能受激素的精准调节。     

Distribution along the rat nephron of three enzymes of gluconeogenesis in acidosis and starvation.

Methods were devised or modified which made it possible to measure phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase in seven defined parts of single nephrons and in patches from thin limb and papilla areas dissected from freeze-dried microtome sections of rat kidney. All three enzymes were essentially confined to the proximal tubule. In normal kidneys, the levels were highest in the proximal convoluted tubule. Glucose-6-phosphatase was 20 times higher in the early part of the convoluted segment than in the late part of the straight segment. With one exception, in acidosis, only phosphoenolpyruvate carboxykinase increased (fourfold in the proximal convoluted segment but much less in the straight portion). In starvation, phosphoenolpyruvate carboxykinase increased about as much as in acidosis in the proximal straight tubule, but not as much in convoluted portions, whereas glucose-6-phosphatase rose modestly in both parts of the proximal tubule and fructose bisphosphatase rose only in the straight tubule, especially the early segment. It is suggested that ammoniagenesis can accompany gluconeogenesis in the proximal convoluted tubule but not in the straight segment.     

The anatomy and blood system of the kidney in the river lamprey, Lampetra fluviatilis

Summary The anatomy and blood system of the kidney in the river lamprey, Lampetra fluviatilis, was studied after injecting microfil into nephrons or the arterial system. Renal arteries arose at irregular intervals from the dorsal aorta and gave rise to a regular arrangement of afferent arterioles supplying the network of glomerular capillaries. Nephron units were arranged in two longitudinal series on each side of the glomerular capillaries, with the capsule of each nephron closely related to the capillary network. A short neck segment lead into the convoluted proximal segment, which accounted for over half the length of each nephron and was surrounded by a network of capillaries and sinusoids supplied by efferent glomerular arterioles. The end of each proximal segment formed the descending limb of a nephron loop, which lay parallel to the ascending distal limb and the end of each collecting duct. Peritubular blood flow in this region was generally opposite the flow of tubular fluid and blood eventually drained into large thin-walled sinuses connected to the post-cardinal vein.This work was supported by SRC research grant number GR/A 1820.7. We thank Miss V. Griffiths for technical assistance and Mr. P. Gaskins for the supply of lampreys     

Glucagon receptors along the nephron: [125I]glucagon binding in rat tubules

Binding of [¹²⁵I]glucagon was measured in microdissected pieces of tubules from the rat nephron. Specific glucagon binding sites were found only in nephron segments containing a glucagon-sensititive adenylate cyclase activity. At 7.5 nM labelled hormone, higher levels of specific binding (16–27×10^–18 mol mm^–1) were found in the thick ascending limb of the Henle's loop and in the distal convoluted tubule and lower binding levels (2–5×10^–18 mol mm^–) in the collecting tubule whereas specific binding could not be detected in the proximal tubule and in the thin segments of the Henle's loop. In the medullary thick ascending limb, Scatchard analysis of specific [¹²⁵I]glucagon binding indicated an apparent equilibrium dissociation constant of 2.4 nM. The stereospecificity of binding sites in medullary thick ascending limbs and medullary collecting tubules, was assessed by competition experiments using unlabelled glucagon, enteroglucagon and unrelated hormones (vasopressin, calcitonin, parathyroid hormone and insulin); in both segments, glucagon was more active than enteroglucagon in displacing labelled glucagon from its tubular binding sites, whereas all other hormones tested were inactive. These results indicate that tubule binding sites might be the physiological receptors for glucagon involved in adenylate cyclase activation.Abbreviations PCT proximal convoluted tubule - TDL thin descending limb - TAL thin ascending limb - MAL medullary thick ascending limb - CAL cortical ascending limb - DCT distal convoluted tubule - CCT cortical collecting tubule - MCT medullary collecting tubule     

Effect of water intake on Na−K-ATPase in nephron segments of the desert rodent,Jaculus orientalis

Na–K-ATPase activity was measured in individual pieces of nephron microdissected from collagenase-treated kidneys of jerboas,Jaculus orientalis. Na–K-ATPase activity was high in the distal convoluted tubule, intermediate in the thick ascending limb of the loop of Henle and low in the proximal and collecting tubule. When jerboas were adapted for several weeks to a hydrated diet and excreted a more diluted urine, Na–K-ATPase activity was altered in specific segments of the nephron: 1. In the proximal convoluted tubule, Na–K-ATPase activity decreased, especially in the juxtamedullary nephrons, suggesting that internephron heterogeneity was diminished; 2. In the medullary thick ascending limb, but not in the cortical portion, Na–K-ATPase activity decreased by 30%; 3. Na–K-ATPase was also diminished in the cortical collecting tubules (by 20%) but not in the medullary collecting tubule. Morphometric measurements also indicate that changes in Na–K-ATPase activity observed in the thick ascending limb are correlated to a cell atrophy, whereas in the collecting tubule, they occur independently of any visible morphological alteration. These differences in Na–K-ATPase activity are likely to be secondary to the changes in the plasma concentration of vasopressin previously described during such adaptation and to be involved in the control of water and sodium handling.     

Sites of thyroid hormone action on Na−K-ATPase along the rabbit nephron

Although Ismail-Beigi and Edelman demonstrated in 1971 that thyroid hormones control the activity of Na–K-ATPase in the mammalian kidney, the actual site of this regulation inside the organ was not located. We therefore decided to study the relationship between thyroid hormones and Na–K-ATPase activity in individual nephron segments obtained by microdissection of collagenase-treated rabbit kidneys. For this purpose, the changes in the activity and number of catalytic sites of Na–K-ATPase in response to thyroidectomy or triiodothyronine administration were examined. Eight to 12 days after thyroidectomy, Na–K-ATPase activity had dropped by 40 to 80% in the convoluted and straight portions of the proximal tubules, and in the cortical and outer medullary collecting tubules, but not in the thick ascending limbs of Henle's loops or distal convoluted tubules. The apparent number of catalytic sites for Na–K-ATPase, as measured by specific binding of³H-ouabain, decreased in parallel with Na–K-ATPase activity, and therefore this enzyme's specific activity was not altered. Fourty eight hours after injection of thyroidectomized animals with a single dose of either 100 or 500 g/kg triiodothyronine, Na–K-ATPase activity in target segments was restored to the level measured in control animals. These effects of thyroid hormone were specific for Na–K-ATPase, since the activity of adenylate cyclase, another marker of the basolateral membrane, was not altered by thyroidectomy. The results obtained indicate that triiodothyronine controls Na–K-ATPase activity in specific nephron segments, by altering the number of this enzyme's catalytic sites.Abbreviations PCT proximal convoluted tubule - PR pars recta - MAL medullary thick ascending limb of Henle's loop - CAL cortical thick ascending limb - DCT_b initial bright portion of the distal convoluted tubule - DCT_g granular portion of the distal convoluted tubule - CCT cortical collecting tubule - MCT outer medullary collecting tubule - TX thyroidectomized - T₃ triiodothyronine - AVP arginine vasopressin - PTH parathyroid hormone - ISO isoproterenol     

硒酸锌金属自显影技术检测游离锌离子在小鼠肾脏的分布

目的研究游离锌离子在小鼠肾脏的定位分布。方法应用硒酸锌金属自显影技术(ZnSeAMG)检测小鼠肾脏内的游离锌离子分布。结果游离锌离子在肾脏内分布广泛,皮质中有大量AMG反应阳性颗粒,髓质中的AMG阳性颗粒较少。其中,近曲小管、远曲小管、近直小管和远直小管上皮细胞近腔侧均分布有大量的棕黑色AMG阳性颗粒,肾小体、细段和集合管上皮细胞中AMG阳性颗粒较少。结论小鼠肾脏内含有丰富的游离锌离子,锌离子可能参与肾脏的功能。     

闰细胞在大鼠远端肾单位的分布及超微结构

本文通过光镜、电镜技术对大鼠远端肾单位中闰细胞(intercalated cell,简称I细胞)分布及超微结构进行了研究。结果表明,I细胞分布在远端小管曲部末段、连接小管、皮质内集合小管,髓质外带集合小管和髓质内带集合小管始段。同时描述了I细胞超微结构的特点,并对皮质内的I细胞某些结构进行了立体计量学分析。     

Effects of human calcitonin on water and electrolyte movements in rat juxtamedullary nephrons: inhibition of medullary K recycling

The effects of synthetic human calcitonin (HCT) on water and electrolyte deliveries to the thin limbs of Henle's loop of juxtamedullary nephrons were investigated by micropuncture in the rat. To avoid undesirable interference with exogenous calcitonin, experiments were performed in hormone-deprived rats with reduced circulating calcitonin, antidiuretic hormone, parathyroid hormone and glucagon, all four of which stimulate the adenylate-cyclase activity in the thick ascending limb and the distal tubule. Administration of HCT (1.0 mU/min·100 g body wt) to such rats significantly reduced the urinary fractional excretion rate of water, Mg, Ca and K. At the tip of the long-looped nephrons, the fractional delivery of water diminished in the presence of HCT, although the glomerular filtration rate of these nephrons was unaltered. Simultaneously, the loop fluid osmolality rose significantly. HCT, however, did not alter the fraction of total filtered solutes remaining in the thin limbs, nor the NaCl fractional delivery. As previously observed in this laboratory with dDAVP, the reduced fractional delivery of water at the hairpin turn was accompanied by a decrease in Mg and Ca deliveries in rats given HCT, indicating that the handling of these two ions along the descending limb may be linked in part to the water movements in this nephron segment. The fractional deliveries of K at the hairpin turn and in urine were significantly correlated, and both decreased in the presence of HCT. Since, as shown previously, HCT reduces the net addition of K along the superfical distal tubule, it is concluded that calcitonin inhibits the medullary recycling of K between the nephron terminal segments and the loop of Henle of juxtamedullary nephrons.     

Morphological and morphometrical study of the kidney of the male tropical lizard Tropidurus torquatus

A morphological and morphometrical study of the adult male Tropidurus torquatus kidney was undertaken. The nephron is composed of the following segments: renal corpuscle, neck segment, proximal convoluted tubule, intermediate segment, and distal tubule. The nephron is continued into the collecting duct and sexual segment. A large number of ciliated cells in the intermediate segment, the presence of 2 kinds of cells in the collecting ducts and a well developed permanently retained sexual segment were recorded as special features of this organ. The components of the renal parenchyma had the following relative volumes: proximal convoluted tubule = 56.4%, intermediate segment = 5.1%, distal tubule = 13.0%, collecting duct = 5.2%, and sexual segment = 11.6%.     

The ultrastructure of the nephrons of the desert rodent (Psammomys obesus) kidney II. Thin limbs of henle of long-looped nephrons

Long nephrons are derived from juxtamedullary glomeruli. In their descent through the outer medulla toward the inner medulla, the thin descending limbs (TDLs) of long loops of Henle are consistently excluded from the vascular bundles and occupy the interbundle region. The outer medullary segment of long TDLs (Type II epithelium) is elaborately developed with numerous cellular interdigitations. microvilli, and a cytoplasm well equipped with numerous organelles. The inner medullary segment of these long TDLs is characterized by yet another epithelium that is markedly reduced from its Type II predecessor and is designated as Type III epithelium. It is a very low, flattened epithelium with few cytoplasmic details. In a cross section of the inner medullary TDL, the Type III epithelium appears poorly interdigitated, with only two to four cell processes and their junctional complexes. Shortly before the bend of a long loop of Henle, the epithelium is again altered to one that is well-interdigitated and better equipped with cytoplasmic organelles (Type IV epithelium). It persists through the actual bend of the loop and throughout the entire thin ascending limb (TAL), until the latter's transition to the distal tubule at the level of the inner/outer medullary border. We conclude that the Type II epithelium of the outer medullary segments of long TDLs in Psammomys is suitably constructed for involvement in energized transport of solutes. The possibility for a similar role in the Type IV epithelium of the bends and TALs is not excluded by our data. However, the Type III epithelium of the inner medullary segments of long TDLs is most suitably constructed for the concentration of its luminal fluids via water extraction. A comparison between these epithelial types and their permeabilities in various species is presented.     

Mineralcorticoid receptors along the nephron: [3H]aldosterone binding in rabbit tubules

To identify the site of mineralocorticoid action along the nephron, we measured the specific binding of [3H]aldosterone to nephron segments microdissected from aldosterone-deficient rabbits. Specific binding was defined as the difference between binding measured in the absence or in the presence of 2,000-fold excess of unlabeled hormone (in 10(-18) mol X cm tubule length-1 +/- SE). High specific binding capacity was found in the branched collecting tubule (108 +/- 4), the cortical collecting tubule (119 +/- 9), and the outer medullary collecting tubule (115 +/- 16), whereas specific binding was negligible in the proximal convoluted tubule (8 +/- 9), pars recta (2 +/- 6), medullary thick ascending limb (4 +/- 6), cortical thick ascending limb (6 +/- 2), and distal convoluted tubule (6 +/- 6). In cortical collecting tubules, Scatchard analysis of the specific [3H]aldosterone binding indicated a dissociation constant (KD) of 2.2 X 10(-9) M and a maximum number of binding sites of 157 X 10(-18) mol X cm tubule length-1. The steroid specificity was assessed from the competition of various steroids for [3H]aldosterone binding sites. Receptors from the cortical collecting tubule revealed the following sequence of affinities: aldosterone greater than DOCA greater than spironolactone greater than dexamethasone greater than 5 alpha-dihydrotestosterone = progesterone = 17 beta-estradiol, indicating that the binding sites in the collecting tubule are mineralocorticoid receptors. These results demonstrate significant [3H]aldosterone binding to receptors of high affinity and mineralocorticoid specificity only in the collecting tubule and suggest that this nephron segment is the target site of mineralocorticoid action in the rabbit kidney.     

Calcium transport in the nephron.

Ionized and complexed calcium are filtered at the glomerulus and more than 95% of the filtered load is reabsorbed along the length of the nephron. In the proximal convoluted tubule calcium is absorbed in proportion to sodium and water, suggesting a passive mechanism. The high permeability of this segment is compatible with passive transport, but evidence for active transport has been advanced. A role for Ca2+-ATPase and/or for a Ca2+/Na+ antiport has also been proposed. The straight portion of the proximal tubule appears to transport calcium actively but little is known about the mechanism and regulation of calcium absorption in this segment. Both passive and active transport of calcium in the thick ascending limb have been demonstrated, and heterogeneity in the function of medullary and cortical segments has been proposed. Definite evidence has been advanced for avid active calcium absorption in the distal convoluted tubule. Both chlorothiazide and parathyroid hormone enhance the transport of calcium in this segment. The granular portion of the collecting tubule resembles in its properties and function the distal convoluted tubule. The light portion, however, is incapable of transporting calcium. The distal tubule and collecting tubule may be the final regulators of urinary excretion of calcium but much more data are required before this view can be adopted.     

The structural organization of the kidney of Typhlonectes compressicaudus (Amphibia, Gymnophiona)

Summary The structural organization of the kidney ofTyphlonectes compressicaudus (Amphibia, Gymnophiona) was studied by light microscopic (LM) examination of serial paraffin and semithin Epon sections. The kidney is slender and quite long and has a mesonephric segmental construction; the excretory duct (Wolffian duct), running along the lateral side of the kidney, segmentally receives the terminal trunks of the collecting duct system. The nephron has the following parts: renal corpuscle, neck segment, proximal tubule, intermediate segment, distal tubule and connecting tubule. The distal tubule is located in a ventromedial (central) zone of the kidney; all other tubular segments lie in a dorsolateral (peripheral) zone. The renal corpuscles are found at the border between these two zones.The renal corpuscle is very large; its urinary pole faces the peripheral zone. A small proportion of neck segments receive either a nephrostomal duct or a blind branch. The proximal tubule is a thick, highly convoluted tubule. The intermediate segment is ciliated and makes a few coils. The distal tubule is composed of three portions: a highly convoluted part in the central zone, subsequently an attachment site with the renal corpuscle and a short postattachment-part. The connecting tubule and the collecting duct have a heterogeneous epithelium consisting of light and dark cells. The collecting duct is distinguished by dilated intercellular spaces. The Wolffian duct has a pseudostratified epithelium.The present study correlates the course and segmentation of the renal tubule ofTyphlonectes. The tubule has three major convolutions. The first occurs in the proximal tubule in the peripheral zone; the second is established by the distal tubule and occurs in the central zone; the third is formed by the connecting tubule and is found in the peripheral zone.Research fellow of the Alexander von Humboldt foundation: home address: Department of Anatomy, Faculty of Medicine, University of Tokyo, Tokyo 113, Japan