The practice of culturing organs dates back to the beginning of the 20th century1. Kidneys were among the first organs to be cultured in vitro when Rienhoff introduced the hanging drop method for embryonic chicken kidneys in 19222. In 1950's, embryonic mouse (Mus musculus) kidney cultures established the importance of mesenchyme-epithelium interactions in guiding developmental processes3,4. Since then, tissue and organ cultures have served as a method to study branching morphogenesis5,6,7,8,9,10,11,12, inductive signaling3,4,13,14, differentiation15,16,17,18,19, and their mechanisms. Still, highly relevant in vitro organ culture allows the addition of different signaling molecules, agonists, or antagonists/inhibitors to the cultures to monitor their effects on morphological events20,21,22.
Culturing genetically modified (GM) kidneys has provided valuable information on the renal differentiation regulation6,7,9,20,23,24,25,26,27. Genetic labeling methods and live imaging techniques enable visualization of specific cell populations or even individual cells, facilitating the analysis of their contribution to the morphological processes9,10,12,18,24,25,26,28,29,30.
At least four different types of kidney ex vivo culture systems exist: 1) the hanging drop method, where tissue is cultured in a small medium drop hanging from a coverslip or a petri dish lid2,31; 2) Trowell-type air liquid interphase culture, where explants are placed on a membrane on top of a metal grid or hanging from a plastic frame (commercial inserts) with media underneath the membrane32,33; 3) low-volume culture, where a very small amount (~85 µL) of culture medium and the tissue are placed inside a silicon ring on a glass slide34; and 4) Fixed Z-Direction (FiZD) culture optimized for confocal live imaging and where the kidney is compressed between a glass slide and a membrane separated by spacers35. The Trowell-type kidney culture is presented here for its simplicity, high cost-efficiency, ease of visualization of morphological processes, and since it is not linked to special microscopy6.
Kidney culture methods provide a means to understand organogenesis in a holistic manner. Primary cell cultures present a supplementary approach for studying biochemical and molecular mechanisms, especially those involved in cellular adhesion and its changes, delamination, and motility. For these, in vitro culture of epithelial cells is an excellent model36. The monolayer culture of primary ureteric bud (UB) is established in approximately 48 h, after which the cells do not divide but survive alive for approximately two weeks7. Such a non-proliferative phenotype can be effectively utilized to analyze, e.g., cell migration with scratch assay or inter-cellular and cell-to-matrix adhesion forces7,37,38. Additionally, cells derived from global GM mice may provide essential cell-type-specific information of renal differentiation6,7.
The permanent kidney (metanephros) development starts with the process known as kidney induction, where reciprocal inductive tissue interactions between the epithelial Wolffian duct (WD, also called nephric duct) and the metanephric mesenchyme (MM) induce the formation of the initial ureteric bud (UB)13,39,40,41. The MM secretes signals such as fibroblast growth factors (FGFs) and glial cell line-derived neurotrophic factor (GDNF), causing cells in the WD first to cluster and then to outgrow to form the initial primary UB, which invades into the MM5,40,42,43. In the mouse, the most common mammalian model used to study organogenesis, this happens at embryonic day 10.5 (E10.5) and in humans around gestational week 4-540,44,45,46.
The process of kidney induction is followed by the start of UB branching morphogenesis. Here, the bud tip first elongates, then balloons into an ampulla to finally bifurcate into a T-bud structure with two brand new tips (Figure 1)12,37,39,47,48. The newly formed UB tips not only receive regulatory cues from the MM but also actively influence the fate and behavior of the MM6,42,49. Accordingly, the differentiation of nephron epithelium is induced by each UB tip (Figure 1). In this way, the extent of UB branching determines the final nephron number in the kidney. The branching morphogenesis continues with iterative cycles until birth, but the majority of the branching happens during 10 to 13 rounds, taking place by E15.5 in mice, after which the rate of new tip formation decreases significantly30,49. Interestingly, the nephron formation continues longer, and the morphological and molecular basis for this is only beginning to be elucidated50,51,52,53,54.
The UB is molecularly and functionally divided into two subpopulations, the tip and the stalk, which differ in their gene expression profiles and proliferation capacity12,55,56,57,58,59. Most of the bifurcations that generate new branches occur in the tips, while differentiation takes place in the trunk region where the two types of collecting ductal cells, principal and intercalated cells, are born11,12,60,61,62. Studies heavily dependent on the in vitro kidney cultures have identified that the UB tips host a progenitor population capable of giving rise to new tips, the trunk, the ureter, and the entire collecting duct system5,25. Despite the known tip-trunk regionalization, the mechanisms controlling the differentiation versus self-renewal decisions within collecting duct progenitors are poorly known. Importantly, the in-situ identity of collecting duct progenitors and the means to identify them are only emerging37.
The UB tip forms a fundamental niche for the establishment of the cap mesenchyme, which constitutes the nephron progenitor (NP) pool13,63. On one side, the cap mesenchyme is surrounded by the UB, and on the other side by the stromal progenitors giving rise to interstitial cells, such as pericytes and mesangial cells (Figure 1)27,64,65,66,67,68. During each UB branch cycle, a subset of NP cells self-renew to keep up the NP population, and another portion is induced to differentiate into functional nephrons66. Much less is known about the behavior of stromal progenitors, which though persist around the NP cells in the periphery of the growing embryonic kidney65,69.
The cap mesenchyme hosting the NP cells is highly organized and dynamic in nature28,66. The cells closest to the tips are the true progenitors, while those localizing a bit further away, in the armpits of T-bud, are pre-committed to differentiation18,28,67. Thus, the position of the NP cells within the cap clearly affects their fate, while their dynamic behavior, as evidenced by their capability to migrate in and out of the cap compartment, suggests flexibility in fate decisions at the individual cell level. Recent comparative studies have established that the overarching regulatory themes of renal progenitors are well conserved between mouse and us humans18,46,70,71,72 supporting the use of organ culture in the future as well.
Nephron differentiation occurs in the close vicinity of UB epithelium and starts when signals from the UB, and possibly the surrounding stroma, induce the committed nephron progenitors to form a cell condensation known as the pretubular aggregate (Figure 1)13,47. This cellular cluster then undergoes a mesenchymal-to-epithelial transition to form the renal vesicle, an axially polarized structure with fates of proximal and distal nephron segments established. This means that the cell's position in the vesicle dictates whether it will become part of the renal corpuscle containing podocytes, glomerular tuft, and mesangial cells, or incorporate into the more distal segments of proximal convoluted tubule, descending loop of Henle, ascending loop of Henle, macula densa, and distal convoluted tubule. The eventual identity is determined by the intricate signals emanating from the UB, stroma, and the vesicle itself, and includes, e.g., NOTCH, RHO, and WNT pathways19,21,73,74. The fate determination in the renal vesicle represents a classic example of inductive tissue interactions taking place in an intact organ, which cannot be studied without 3D structural organization.
From the renal vesicle, the nephrogenesis progresses through well-described morphological steps to form comma- and S-shaped bodies, where the regional identities are further strengthened and additional cell types, like endothelia, invade into developing glomeruli48,75. Finally, at the cessation of organogenesis, the undifferentiated progenitor pool is used up to provide the renal filtration capacity for the life18,67. The anatomical and morphological steps of nephrogenesis, including innervation, are well represented in the in vitro nephrogenesis76,77. However, the vascularization of the kidney does not occur in the classic kidney culture systems. This aspect can be studied with certain limitations by kidney grafting onto avian chorioallantontoic membrane78,79.
In addition to whole organ culture, there are methods to generate kidney-resembling organoids from pluripotent cells. Different protocols with specific emphasis on creating nephrons80,81,82,83, ureteric bud84,85,86, and organoids with higher-ordered structures87, as well as tubuloids, the 3D cultures of nephron segment and collecting duct epithelium88,89, exist. These are valuable tools for studying renal morphogenesis in, e.g., humans, where a limited amount of fetal material is available. However, organoid methods cannot recapitulate the intact organogenesis similarly as in vitro organ culture. In this article, we provide detailed instructions for establishing Trowell-type kidney cultures and primary epithelial cell colonies from the UB. These protocols have primarily been used for mouse embryonic kidneys, but also work for developing rat kidneys. We demonstrate that Trowell-type culture is also effective for the embryonic kidneys of spiny mouse (Acomys dimidiatus) - a novel rodent model with exceptional regeneration capacity90,91,92,93,94. The advantage of embryonic kidney culture is its ease of setup and cost-efficient use.