Special Morphological Features at the Interface of the Renal Stem/Progenitor Cell Niche Force to Reinvestigate Transport of Morphogens During Nephron Induction

Abstract

Formation of a nephron depends on reciprocal signaling of different morphogens between epithelial and mesenchymal cells within the renal stem/progenitor cell niche. Previously, it has been surmised that a close proximity exists between both involved cell types and that morphogens are transported between them by diffusion. However, actual morphological data illustrate that mesenchymal and epithelial stem/progenitor cell bodies are separated by a striking interface. Special fixation of specimens by glutaraldehyde (GA) solution including cupromeronic blue, ruthenium red, or tannic acid for electron microscopy depicts that the interface is not void but filled in extended areas by textured extracellular matrix. Surprisingly, projections of mesenchymal cells cross the interface to contact epithelial cells. At those sites the plasma membranes of a mesenchymal and an epithelial cell are connected via tunneling nanotubes. Regarding detected morphological features in combination with involved morphogens, their transport cannot longer be explained solely by diffusion. Instead, it has to be sorted according to biophysical properties of morphogens and to detected environment. Thus, the new working hypothesis is that morphogens with good solubility such as glial cell line-derived neurotrophic factor (GDNF) or fibroblast growth factors (FGFs) are transported by diffusion. Morphogens with minor solubility such as bone morphogenetic proteins (BMPs) are secreted and stored for delivery on demand in illustrated extracellular matrix. In contrast, morphogens with poor solubility such as Wnts are transported in mesenchymal cell projections along the plasma membrane or via illustrated tunneling nanotubes. However, the presence of an intercellular route between mesenchymal and epithelial stem/progenitor cells by tunneling nanotubes also makes it possible that all morphogens are transported this way.

Keywords: cell-to-cell connection; interface; kidney; stem/progenitor cell niche; transport of morphogens; tunneling nanotubes.

FIG. 1.

View to the renal stem/progenitor cell niche by transmission electron microscopy. For a reliable perspective the cortex of embryonic parenchyma must be exactly orientated in parallel to a lining collecting duct (CD) and perpendicular to the organ capsule (C). Epithelial (EPI) stem/progenitor cells are seen that are integrated in the tip of a CD ampulla (A), while one to two layers of mesenchymal (MES) stem/progenitor cells surround them. Further mesenchymal cells are separated from epithelial cells by an interface (asterisk). The basal aspect of epithelial stem/progenitor cells at a CD ampulla (A) tip is labeled by a cross (+). S marks a developing S-shaped body.

FIG. 2.

Transmission electron microscopy of the renal stem/progenitor cell niche. (a) Fixation of specimens in conventional glutaraldehyde (GA) solution elucidates that an interface (asterisk) is present between mesenchymal (MES) and epithelial (EPI) stem/progenitor cell bodies. Epithelial cells are covered by a basal lamina consisting of a lamina rara (L.r.), lamina densa (L.d.), and lamina fibroreticularis (L.f.). Projections (P) of mesenchymal cells cross the interface to touch the basal lamina of epithelial cells. Within the interface only few extracellular matrix is recognized. In contrast, samples fixed by GA solution including cupromeronic blue (CMB) show that numerous braces of proteoglycans are contained (b) on the surface of mesenchymal cell projections and (e) in the basal lamina. Specimens fixed by GA solution including (c, f) ruthenium red (RR) or (d, g) tannic acid (TA) illuminate earlier nonvisible extracellular matrix within the interface and on the basal lamina. (h, i) Tunneling nanotubes (arrow) establish a cell-to-cell connection between mesenchymal and epithelial cells. The basal plasma membrane of epithelial stem/progenitor cells is labeled by a cross (+).

FIG. 3.

Schematic illustration informs about the exchange of morphogens within the renal stem/progenitor cell niche in an earlier view. For nephron induction it was assumed that mesenchymal (MES) and epithelial (EPI) stem/progenitor cells have an intimate contact. Under such conditions all morphogens are transported by diffusion (arrows) through the interstitial space (asterisk). The basal lamina consisting of a lamina rara (L.r.), lamina densa (L.d.), and lamina fibroreticularis (L.f.) covers the basal aspect of epithelial cells. The basal plasma membrane of epithelial cells is marked by a cross (+).

FIG. 4.

Schematic illustration informs about the exchange of morphogens within the renal stem/progenitor cell niche in an actual view. Detected morphological features show that mesenchymal and epithelial cells are separated by an interface including a basal lamina and abundant extracellular matrix. Further mesenchymal cell projections cross the interface to establish a cell-to-cell communication with epithelial cells. On that special situation it is speculated that only one part of morphogens is transported by diffusion (dashed arrow) from (a) an epithelial to a mesenchymal cell or vice versa from (b) a mesenchymal to an epithelial cell. The second part of morphogens is secreted and then bound in extracellular matrix (xxx arrow). Here it is decided upon their free accessibility to the target cell or further binding, modification, storage, and delivery on demand. The third part of morphogens is transported by cell projections and tunneling nanotubes (solid arrow) from an epithelial to a mesenchymal cell or vice versa from a mesenchymal to an epithelial cell. The basal aspect of epithelial cells is marked by a cross (+).