The Pronephros; a Fresh Perspective

Abstract

Contemporary papers and book chapters on nephrology open with the assumption that human kidney development passes through three morphological stages: pronephros, mesonephros, and metanephros. Current knowledge of the human pronephros, however, appears to be based on only a hand full of human specimens. The ongoing use of variations in the definition of a pronephros hampers the interpretation of study results. Because of the increased interest in the anamniote pronephros as a genetic model for kidney organogenesis we aimed to provide an overview of the literature concerning kidney development and to clarify the existence of a pronephros in human embryos. We performed an extensive literature survey regarding vertebrate renal morphology and we investigated histological sections of human embryos between 2 and 8 weeks of development. To facilitate better understanding of the literature about kidney development, a referenced glossary with short definitions was composed. The most striking difference between pronephros versus meso- and metanephros is found in nephron architecture. The pronephros consists exclusively of non-integrated nephrons with external glomeruli, whereas meso- and metanephros are composed of integrated nephrons with internal glomeruli. Animals whose embryos have comparatively little yolk at their disposal and hence have a free-swimming larval stage do develop a pronephros that is dedicated to survival in aquatic environments. Species in which embryos do not have a free-swimming larval stage have embryos that are supplied with a large amount of yolk or that develop within the body of the parent. In those species the pronephros is usually absent, incompletely developed, and apparently functionless. Non-integrated nephrons were not identified in histological sections of human embryos. Therefore, we conclude that a true pronephros is not detectable in human embryos although the most cranial part of the amniote excretory organ is often confusingly referred to as pronephros. The term pronephros should be avoided in amniotes unless all elements for a functional pronephros are undeniably present.

Fig. 1

Organization of kidney development. A) Diagram illustrating the three sets of excretory structures as supposed to be present in a human embryo during the fifth week (about 32 days, Carnegie stage 14) (after Moore 1988). Cr, cranial; Ca, caudal. B) Reconstruction of the urogenital system of a stage 16 human embryo specimen 6517 (37–42 days), including meso- and metanephros and their ducts (de Bakker et al. 2016). Cr, cranial; Ca, caudal; D, dorsal; V, ventral. C) Section through the mesonephric region of a stage 17 human embryo specimen 6521 (42–44 days). D, dorsal; V, ventral. D) Enlarged part of the section in C. A, arteriole; BC, Bowman’s capsule; C, coelom; D, mesonephric duct; G, gonad; GL, glomerulus; L, liver; MB, Malpighian body; PV, postcardinal vein; T, mesonephric tubules; V, vein.

Fig. 2

Schematic drawings of pro-, meso-, and metanephros. A) Schematic drawing of the non-integrated nephron of a pronephros in frogs. The glomus, a single glomerular unit that is formed over two to three body segments and later becomes more compact, is supplied by arterial sprouts from the dorsal aorta and filters wastes directly in the fluid of the coelomic cavity (Gerth et al. 2005). The glomus is not integrated in the tubules but it lies in the immediate vicinity of them, so there is almost no space between the glomus and nephrostomes. These ciliated nephrostomes collect the coelomic fluid and the primitive urine will be collected through the pronephric tubules via the pronephric duct into the cloaca. See also Fig. 3A. B) Schematic drawing of the integrated nephrons of a mesonephros. The glomeruli are supplied by arterial sprouts from the aorta that pass behind the coelom and filter wastes into Bowman’s cavities. The glomeruli are therefore integrated in the tubules. The collected wastes are transported through the mesonephric tubules toward the mesonephric duct and collected in the urogenital sinus. See also Fig. 3B. The ureteric bud which initiates metanephric development and will become the ureter is an outgrowth of the mesonephric duct. The cranial-most metanephric nephrons degenerate over time. Note that these degenerated nephrons have often been wrongly described as pronephric. C) Schematic drawing of a metanephros and its supplying renal artery are situated retro-peritoneal, so dorsal of the coelom. The metanephric nephron with its Bowman’s space and extensive tubule is firmly embedded within the renal cortex and renal pyramids (lighter triangles). Urine is collected in the renal pelvis and transported through the ureter toward the bladder. A, aorta; B, bladder; C, coelom; Ca, caudal; Cr, cranial; DA, dorsal aorta; PD, pronephric duct; US, urogenital sinus; U, ureter; UB, ureteric bud; MD, mesonephric duct.

Fig. 3

Detailed architecture of a pronephric and a mesonephric nephron. A) Pronephric anatomy: the non-integrated nephron. A typical pronephric nephron, as can be found in amphibian larvae and some teleosts (Fraser 1950), consists of the following functional units; the coelom/nephrocoel with an external glomerulus or glomus, from which a ciliated nephrostome leads into the pronephric tubule that lastly drains into the pronephric duct (Fraser 1920, 1950; Dawson 1925; Lambert 1933; Davies 1950; Nieuwkoop and Faber 1994; Vize et al. 1997; Brandli 1999; Nishinakamura 2003; Raciti et al. 2008; Chimenti and Accordi 2011; Cho et al. 2011; Wessely and Tran 2011). The glomerulus or glomus is not integrated in the tubule. B) Mesonephric anatomy: the integrated nephron. The mesonephric tubules develop a Bowman’s capsule that encloses a vascularized internal glomerulus supplied by branches of the dorsal aorta (McCrory 1974). So the glomerulus is integrated in the mesonephric tubule. Bowman’s capsule, together with the internal glomerulus, constitutes a Malpighian body (Fraser 1950). The Malpighian body is regarded as a typical feature of the mesonephros (Wrobel and Suss 2000). The mesonephros differs from the pronephros by the absence of external glomeruli (Davies 1950, 1951; Nelson 1953; Hamilton et al. 1972; Vize et al. 1997; Wrobel and Suss 2000; Chimenti and Accordi 2011). The mesonephros is formed in all vertebrates, but while it degenerates and relinquishes its function to the metanephros in more advanced vertebrates, it serves as the adult kidney in fish and amphibians (Wessely and Tran 2011). DA, dorsal aorta; L, lateral; M, medial; VA, vas afferens; VE, vas efferens.

Fig. 4

Different manifestations of pronephric and mesonephric nephrons. The non-integrated nephrons in A and B are purely pronephric as the external glomerulus or glomus hangs freely in the coelom (A) or nephrocoel (B). The integrated nephrons in D and E are typical mesonephric because the connection with the coelom is lost and the internal glomerulus is enclosed by Bowman’s capule. The nephron in C, which includes both a peritoneal funnel (PF) and a nephrostome (NS), has been regarded by Vize as mesonephric (Vize et al. 2003). However, to avoid ambiguity we suggest to refer to these nephrons as intermediate nephrons. The region in which these nephrons are found has been called the evolutionary transition zone (ETZ) by multiple authors (Fraser 1920; Davies 1950; Sedgwick 1881; Renson 1883; Mihalkovics 1885; Hiruma and Nakamura 2003), since the glomerulus is still in contact with the coelom (a typical pronephric feature) and the PF has been regarded both pronephric (Fraser 1950; Davies 1951; Hamilton et al. 1972) as mesonephric (Vize et al. 2003). This intermediate type of nephron might actually represent the gradual evolutionary change from pronephros to mesonephros (Wiedersheim 1890; Davies 1950; Hiruma and Nakamura 2003). C, coelom; G, glomerulus; G/G, glomerulus/glomus; L, lateral; M, medial; NS, ciliated nephrostome, which links the coelom or nephrocoel with the proximal tubule; PF, ciliated peritoneal funnel, which links the coelom to the encapsulated glomerulus (primitive Bowman’s capsule); T, tubule.

Fig. 5

Histological features of human nephrogenic development. A) Transverse section through the caudal region of a stage 9 (26–30 days) human embryo specimen H712. The intermediate mesoderm (IM) is still hard to discern from somite (paraxial mesoderm) and lateral plate mesoderm (LM) B) Transverse section through the caudal region of a stage 10 (21–23 days) human embryo specimen 5074. The intermediate mesoderm is recognizable as a clump of undifferentiated mesenchymal cells. C) Transverse section through the caudal region of a stage 11 (23–26 days) human embryo specimen 6344. The intermediate mesoderm is still undifferentiated. D) Transverse section through the caudal region of a stage 12 (26–30 days) human embryo specimen 8943. The nephrogenic cord (NC) is now present. E) Transverse section through the mesonephric region of a stage 13 (28–32 days) human embryo specimen 836. Fig. 5 F) Transverse section through the mesonephric region of a stage 14 (31–35 days) human embryo specimen 6502. G) Enlarged part of the section in D. A very primitive glomerulus is recognizable, surrounded by a primitive Bowmans capsule. Bowmans capsule is not in contact with the coelom, also not on the adjacent and subsequent sections. The mesonephric duct is not yet lumenized at this stage. H) Enlarged part of the section in E. The glomerulus and Bowman’s capsule still appear in a primitive stage. The mesonephric duct becomes lumenized. I) Enlarged part of the section in F. The definitive morphology of the mesonephros is recognizable. The glomerulus and Bowman’s capsule together constitute the Malpighian body. The mesonephric duct and tubule are well defined and lumenized. At this stage the mesonephros can be assumed to be in function. Between stage 14 and stage 17 (L, O) the histological features of the mesonephros remain constant. J) Transverse section through the mesonephric region of a stage 15 (35–38 days) human embryo specimen 721. K) Transverse section through the mesonephric region of a stage 16 (37–42 days) human embryo specimen 6517. L) Transverse section through the mesonephric region of a stage 17 (42–44 days) human embryo specimen 6520. M) Enlarged part of the section in J. N) Enlarged part of the section in K. Two Bowman's capsules are present in this section. Also note the clear presence of a gonadal ridge. O) Enlarged part of the section in L. Two glomeruli can be appreciated in this section. A, arteriole; AO, aorta; BC, Bowman’s capsule; C, coelom; D, mesonephric duct; G, gonadal ridge; GL, glomerulus; IM, intermediate mesoderm; LM, lateral plate mesoderm; N, notochordal plate (CS 9, 10, 11) or notochord (CS 12, 13, 14); NC, nephrogenic cord; NG, neural groove; NT, neural tube; PV, postcardinal vein; S, somite; T, mesonephric tubule. We encourage the readers to study the histological sections of all presented stages. All stacks of sections can be downloaded from http://www.3datlasofhumanembryology.com.

Fig. 6

Overview of the appearance of a pronephros in animal species presented as evolutionary cladogram. For details and references, see Supplementary Table S1. (Cephalochordata and Cyclostomata): pronephros functioning as adult kidney. (Chondroichthyes, Actinopterygii, Sarcopterygii, and Amphibia): pronephros functioning in the larval stage. However, the pronephros seems to be absent in those Elasmobranchii that have no larval stage, and in the Amniota which develop within the body of the parent (Fraser 1950). (Squamata, Crocodylia, and Aves): Although a pronephros has been described in embryos of some of these animals (Supplementary Table S1), the term pronephros should be used with much restraint (Fraser 1950). Further research in these species is needed to clarify the contradictions that appeared in the literature as a result of the use of different definitions. (Mammals): no pronephros is present.

Fig. 7

General concept for renal evolution. Vertical columns: different kidney forms (i.e., pro-, meso-, and metanephros). Horizontal rows: different classes of species. In most jawless fish, like Amphioxus and hagfish, the pronephros remains functional through adulthood, often alongside a functional mesonephros. Larvae of teleosts and anamnia (e.g., tadpoles) generally pass a pronephric stage, while adult specimens (e.g., frogs) use a mesonephros for secretion. Embryos of amniotes (e.g., humans) do not pass a pronephric stage, but do use the mesonephros during the embryonic phase and the metanephros through fetal development, in childhood and in adulthood. The used kidney form thus gradually shifts from simple pronephric kidney as used by adult jawless fish, via the intermediate mesonephric kidney in more basal vertebrates toward the intricate metanephric kidney as used by more advanced vertebrates.