Interstitium as an integrating element
In the fetal human kidney during advanced pregnancy, the formation of new nephrons is restricted to the nephrogenic zone. Covered by the renal capsule (Fig. 1a–c), it consists of side-by-side aligned nephrogenic compartments (Table 1). In each of them, the initial development of a nephron can be observed. It starts in a determined sequence with the transient stages of nephron anlage and continues with the terminal differentiation of the maturing nephron in the subjacent maturation zone. When a transverse line is drawn at the section border between the head and conus of the neighboring collecting duct (CD) ampulla, a nephrogenic compartment is subdivided (Fig. 2a). In the upper district of progenitor cell recruitment, which is seen during the further development as a constant prone rectangle, the mesenchymal nephrogenic progenitor cells meet the epithelial progenitor cells, which are contained in the tip of the CD ampulla, at the nephrogenic niche. As a result of this interaction, first the pretubular aggregate arises, and then, the mesenchymal to epithelial transition takes place (Fig. 2b). This again announces the rising of the primitive renal vesicle (Fig. 2c and d). Due to the vertical elongation of the CD ampulla, the primitive renal vesicle remains in the area of nephron shaping, which is represented during the further development by an expanding quadrate. Here, the formation of the mature, extending, and extended renal vesicles (Fig. 3a–c) and the comma- (Fig. 3d) and S-shaped bodies (Fig. 4a–c) occurs. Then, in the subjacent maturation zone, the functional differentiation of the nephron proceeds (Fig. 4d).
When the arrangement of the interstitium is analyzed, beside the actual contours and the stage-specific location of the forming nephron, also the structural neighbors in a nephrogenic compartment have to be considered (Table 1). At the top, it is covered by the renal capsule (Fig. 1a–c), at the medial aspect by the ureteric bud derived CD ampulla, at the base by the connecting tubule of a previously developed nephron, and at the lateral aspect by a perforating radiate artery. Thus, the available space between the actual stage of nephron anlage and its structural neighbors reflects the current arrangement of the interstitium. However, during the running developmental process, not only the shape of the forming nephron but also the constellation with the structural neighbors is altering. For example, the forming nephron increases in size, takes progressive shapes, shifts between its structural partners, evolves proximal and distal poles, and shows asymmetrical forms at its medial and lateral aspects. In parallel, also the CD ampulla and the perforating radiate artery vertically elongate. Meanwhile, a network arises between the forming nephron and its structural neighbors, which contains, for example, collagen type 3 as a stabilizing compound of the extracellular matrix (Fig. 1b).
Interstitium privatum in the renal capsule
As it was mentioned, the top of a nephrogenic compartment is covered by the renal capsule (Table 1, Fig. 1). It represents an organ-specific kind of the connective tissue. The capsula fibrosa is externally situated, consists of several strata, and protects the growing kidney against inappropriate influences. On microscopic specimens, one can see that numerous fibroblasts occur between the transversely lining layers of collagen bundles (Fig. 1a). Inside the renal capsule, numerous arterial, venous, and lymphatic vessels are visible. The vertically ascending perforating radiate arteries reach the inner side of the renal capsule for joining the Rete capillare capsulare (Fig. 3b). From the neonatal rabbit kidney, it is further known that the collagen in the capsula fibrosa is specifically stacked. Histochemical label revealed, for example, that the external bundles are not stained by tannic acid, while the internal bundles are intensely labeled (Fig. 1c).
The subjacent tunica muscularis contacts the nephrogenic zone. Here, numerous myofibroblasts and atypical and typical smooth muscle cells are noticed (Fig. 1a). Electron microscopy revealed that a part of these cells is covered by a distinct basal lamina, while others show only a faint glycocalyx (Fig. 1c). Further on, at the inner side of the renal capsule and next to the external part of the nephrogenic zone, the neighboring cells establish numerous contacts via cell projections so that the intra- and extracellular space between the cell projections represents a complex tunnel system (Fig. 1d and e). It may serve the transport and distribution of interstitial fluid and to keep the balance of fluid pressure between the renal capsule and the nephrogenic zone.
Interstitial peculiarities between the renal capsule and the tip of a CD ampulla
Although it looks unspectacular in the optical microscope, a series of data indicates that the interstitial space between the inner side of the renal capsule and the tip of the CD ampulla is specially structured (Table 1; Fig. 1b and c).
The transverse mesenchymal cell layers: In the fetal human kidney, the distance between the tip of a CD ampulla and the inner side of the renal capsule is not accidental but has a vertical width of only 30 µm. Between them, the interstitial and nephrogenic progenitor cells are contained. It is striking that the innermost layer of the nephrogenic progenitor cells does not touch but is rather separated by a clear interface from the tip of the CD ampulla (Fig. 1a). At this specific site, the nephrogenic niche is visible.
The vertical microfibers: In the fetal human kidney and, for example, in the neonatal rabbit kidney, the distinct distance between the inner side of the renal capsule and the tip of a CD ampulla is caused by a special mounting. Immunohistochemical label for collagen type 3 in the neonatal rabbit kidney depicts extracellular microfibers. These originate at the inner side of the renal capsule and cross first the stromal and then the nephrogenic progenitor cell layers for the link with the basal lamina at the tip of a CD ampulla (Fig. 1b).
Interstitial fronts at the niche, pretubular aggregate, mesenchymal-to-epithelial transition, and primitive renal vesicle
The meeting between the innermost layer of the nephrogenic mesenchymal progenitor cells and the epithelial progenitor cells contained in the tip of a CD ampulla indicates the precise coordinates of a nephrogenic niche (Figs. 1a and 2a). However, a special feature is that the bodies of the two kinds of progenitor cells do not touch. Instead, these are separated by the clear interface (Fig. 1c). For the kidney of neonatal rabbit, it was further demonstrated that the tip of a CD ampulla is covered by a striking basal lamina. Improved contrasting of specimens by tannic acid (Fig. 1d) or cupromeronic blue (Fig. 1e), for the purpose of transmission electron microscopical analysis, additionally showed that the extracellular matrix within the interface is composed by textured properties. Finally, a concise feature is that single projections of the nephrogenic mesenchymal progenitor cells are enveloped in a specific sleeve of extracellular matrix. By this construction, they vertically cross the interstitial interface to penetrate the basal lamina and to contact via tunneling nanotubes the basal plasma membrane of the epithelial progenitor cells, which are integrated in the tip of a CD ampulla.
After induction by morphogenic molecules, the nephrogenic mesenchymal progenitor cells, which are located at the niche, become angular, migrate, and aggregate first along the tip and then the head of the CD ampulla. As a result, the tear drop-like pretubular aggregate arises (Fig. 2a). It represents a specialized interstitial cell construct, which separates only in part from the overlying nephrogenic mesenchymal progenitor cell layer. While its thin distal end remains in contact with the overlying nephrogenic progenitor cells, its broad end further expands towards the connecting tubule of a previously developed nephron. Due to the typical shape and the distinct position of the pretubular aggregate, its interstitial surrounding is built up asymmetrically. Most of its lateral part is exposed to the subcapsular interstitium, which is located at the inner side of the renal capsule. In contrast, the clear interface is visible between the medial part of the pretubular aggregate and along the tip and head of the CD ampulla. Strikingly, the lateral part at the proximal end of the pretubular aggregate is positioned opposite the perivascular interstitium of a vertically lining perforating radiate artery. The mid of its proximal end faces the peritubular interstitium at the connecting tubule of a previously developed nephron. Determining for the future development, the medial part of its proximal end shows an adhesion at the section border between the head and conus of the CD ampulla.
Typical signs for the mesenchymal-to-epithelial transition (MET) are noticed only at the proximal end of the pretubular aggregate (Fig. 2b). The first polar cells are seen opposite to the cone, which is formed between the peritubular interstitium at the connecting tubule of a previously developed nephron and the perivascular interstitium at the perforating radiate artery. At the same time, the medial part at the proximal end of the pretubular aggregate expands the adhesion at the CD ampulla. This site reflects the future physiological connection between the connecting tubule of the presently forming nephron and the prospective collecting duct tubule.
The morphogenesis of the primitive renal vesicle is first recognized as an open clamp at the proximal end of the pretubular aggregate (Fig. 2c). A circle of polarized cells becomes visible, when the primitive renal vesicle is established (Fig. 2d). Although a small lumen is yet recognized, its distal pole remains connected with the overlying pretubular aggregate. In this situation, the lateral part of the pretubular aggregate is positioned near the subcapsular interstitium, while the lateral aspect of the primitive renal vesicle encounters the perivascular interstitium of the perforating radiate artery. Its proximal pole stays near the peritubular interstitium at the connecting tubule of a previously developed nephron. However, at the medial aspect of the primitive renal vesicle, the adhesion expands at the section border between the head and conus of the CD ampulla. As a consequence, the clear interface lining between the CD ampulla and the medial part of the pretubular aggregate is interrupted at the site of adhesion.
Interstitial features at the mature, extending, and extended renal vesicles
Regarding the microanatomical situation, the subcapsular interstitium accompanies very early steps of nephron formation such as the recruitment of progenitor cells, the process of nephron induction, the formation of the pretubular aggregate, the mesenchymal to epithelial transition, and the arise of the primitive renal vesicle (Fig. 2c–d). However, while the CD ampulla elongates vertically, the primitive renal vesicle starts to separate at its distal pole from the overlying pretubular aggregate. At the same time, it is translocated from the district of progenitor cell recruitment to the subjacent area of nephron shaping. Henceforth, the installation at the medial aspect of the developing renal vesicle is bordered not anymore by the head but by the upper conus of the CD ampulla. At its lateral aspect, the renal vesicle faces yet the radially lining perforating radiate artery. Its distal pole is fixed near the proximal end of the overlying pretubular aggregate, while its proximal pole stays next to the connecting tubule of a previously developed nephron.
The development from the primitive to the mature renal vesicle coincides with a transverse but only partial separation from the pretubular aggregate. In parallel, the adhesion at the section border between the head and conus of the CD ampulla proceeds (Fig. 3a). The separation is first recognized as a narrow transverse cleft. It starts at the end of the clear interface and the initial site of adhesion, and it continues up to the center of the pretubular aggregate. At a later time, it can be observed that the arising cleft is invaded by interstitial cells. At the adhesion site, the connection between the future connecting tubule and the CD ampulla is under work. Surprisingly, the laterally located cells at the distal pole of the mature renal vesicle stay, for a moment, connected with the pretubular aggregate via a two-layered progenitor cell strand serving the progressive recruitment with progenitor cells. Initially, only a close adhesion is noticed between the medial aspect of the mature renal vesicle and the upper conus of the CD ampulla. Then, it is replaced by a narrow cleft, which expands vertically. This is formed due to a cone-shaped interstitial process, which originates in part from the perivascular interstitium of the perforating artery and in part from the peritubular interstitium at the connecting tubule of a previously formed nephron.
At the extending renal vesicle, one can observe that the future connecting tubule is perforating the epithelium of the CD ampulla at the border between its head and conus (Fig. 3b). Furthermore, it is registered that the cleft between its distal pole and the proximal end of the pretubular aggregate extends transversely. Especially remarkable, the lateral part of the distal pole at the extending renal vesicle is still connected with the overlying pretubular aggregate via the two-layered progenitor cell strand. The medial cell strand lines to the mid at the proximal end, while the lateral strand extends to the lateral part of the pretubular aggregate. It is noticed that the lateral cell strand is located at the perivascular interstitium of the perforating radiate artery. Further on, it crosses the subcapsular interstitium including the transverse progenitor cell layers to reach the inner side of the renal capsule. In parallel, between the upper conus of the CD ampulla and the medial aspect of the extending renal vesicle, an elongation of the narrow vertical interstitial cleft is recognized.
While the extended renal vesicle establishes, the tubule anlage expands vertically and laterally (Fig. 3c). At its distal end, the epithelium of the future connecting tubule is connected with the CD ampulla. In the lumen near the proximal pole, the inner epithelial fold becomes visible. This causes the interstitial cleft, which is seen between the tubule anlage and the laterally located progenitor cell strand, to elongate first in an oblique and then in a vertical direction. The entire lateral aspect of the extended renal vesicle is exposed to the perivascular interstitium of the perforating radiate artery. The proximal pole of the extended renal vesicle faces the peritubular interstitium at the connecting tubule of a previously developed nephron. From here, the interstitium invades as a narrow vertical cleft between the conus of the CD ampulla and the medial aspect of the extended renal vesicle.
Interstitial constellation at the comma-shaped body
A special but to date unexplored meaning has the interstitium at the developing comma-shaped body (Fig. 3d). In this phase, beside the elongation of the tubule anlage and the extension of the epithelial folds, the proximal–distal positioning and the defoliation of the arising glomerulus can be observed. Due to the appearance of new contours, parts of the surrounding interstitium disappear, while others retreat or are rebuilt. Interestingly, the progenitor cell strand at the distal pole dissolves, so that the connection between the comma-shaped body and the pretubular aggregate is lost. As a result, the interstitial cleft, located between the tubule anlage and the progenitor cell strand, is fused with the perivascular interstitium of the neighboring perforating radiate artery. At the lateral aspect of the comma-shaped body, precisely between the lateral fold and the tubule anlage, a vertical interstitial cleft develops. It ends at the turn up of the inner fold. This is a clear morphological sign that the glomerulus is starting to defoliate. The future Bowman’s capsule (lateral leg of the lateral fold) faces the perivascular interstitium of the perforating radiate artery. The proximal pole is now located opposite the peritubular interstitium at the connecting tubule of a previously developed nephron. Between the conus of the CD ampulla and the medial aspect of the comma-shaped body, the narrow vertical interstitial cleft further expands. At the connection site between the future connecting tubule and the conus of the CD ampulla, it changes direction to follow the peritubular interstitium at the developing tubule portions towards the geometric center of the comma-shaped body.
Interstitial characteristics at the S-shaped body
During the formation of the early S-shaped body, the physiological link between the connecting tubule and the CD ampulla is completed (Fig. 4a). At this site, an interstitial cone becomes visible. It is bordered at the top by the proximal end of the overlying pretubular and the subcapsular interstitium and by the laterally situated perivascular interstitium of the perforating radiate artery and the peritubular interstitium of the presently forming connecting tubule. Between the conus of the CD ampulla and the medial aspect of the S-shaped body, the narrow vertical interstitial cleft is extended. It lines up to the peritubular interstitium, which surrounds inside the S-shaped body the elongating connecting, distal, intermediate, and proximal tubule portions. At its lateral aspect, the peritubular interstitium of the extending distal and intermediate tubule portions is fused with the perivascular interstitium of the perforating radiate artery. Importantly, the transverse cleft further prolongs at the deep lateral aspect. It opens up between the overlying proximal tubule portion and the subjacent visceral podocyte cell layer of the arising glomerulus. Moreover, one can observe that the cells, which originate from the perivascular interstitium of the perforating radiate artery, invade the transverse cleft. This is a morphological sign that the capillary loops and the intra- and the extraglomerular mesangium are starting to develop. The proximal pole of the S-shaped body is represented by the developing Bowman’s capsule. It is facing at its medial part the peritubular interstitium of a previously developed connecting tubule and at its lateral part the perivascular interstitium of the perforating radiate artery.
During development of the mid S-shaped body, at its proximal pole, the Bowman’s capsule and the Bowman’s space expand. Furthermore, the podocytes develop a typical cobblestone-like appearance (Fig. 4b). In the overlying transverse cleft, beside the numerous capillaries, the formation of the intraglomerular mesangium and the arise of the afferent and efferent arterioles are recognized. At the opening of the transverse cleft, the close topological relation between the perivascular interstitium of the perforating radiate artery and the glomerular tuft is obvious. At the medial aspect of the S-shaped body, it is noticed that a certain distance is formed between the conus of the CD ampulla and the tubule portions. The interjacent space is filled by a loose peritubular interstitium.
Interstitial transition from the nephrogenic zone to the maturation zone
The late S-shaped body as the last transient stage of nephron anlage stays positioned perpendicular to the renal capsule. It demonstrates a morphological segmentation along its proximal–distal axis (Fig. 4c). At the proximal pole, the typical morphological features of the glomerulus appear. For example, the Bowman’s capsule is becoming flat, the Bowman’s space is clearing, and the podocytes show now their typical shape. Further numerous glomerular capillaries become visible, while a broadening of the intraglomerular mesangium occurs. At the medial aspect, the Bowman’s capsule is located near the conus and neck of the CD ampulla and also along the differentiating collecting duct tubule. However, the elongating tubule portions are separated from the neck and conus of the CD ampulla by a newly formed but faint peritubular interstitium. At the lateral aspect, the situation is different, since the connecting and distal tubule portions face the perivascular interstitium of the perforating radiate artery.
Due to the ongoing radial extension of the nephrogenic zone, the late S-shaped body is leaving the area of nephron shaping for the further development in the maturation zone (Fig. 4d). Its distal pole, which is seen at the tip head of the CD ampulla, is positioned near the subcapsular interstitium. At the proximal pole and in its interior, the arising glomerulus and the meandering tubule portions are covered by a faint peritubular interstitium. In this region, single strands of extracellular matrix are noticed, which line in vertical direction. At the meandering tubule portions, one can recognize that within their coves, a filigree interstitium arises. Occasionally, long thin cells with an extended but flat nucleus are here noticed. These are similar to the telocytes.
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