I am a reformed lover of mesoderm induction. Myassociation with the pronephros began for opportunisticreasons with the original plan being to exploit theexpression of pronephric genes as markers of the patterningand establishment of the intermediate mesoderm. However,after finding such markers and following their expression informing pronephroi, I became more interested in how thesegenes contributed to the regulation of kidney morphogenesisthan in simply using them as markers of earlier events. Uponexploring what was known about embryonic kidneydevelopment (very little) and what could be learned usingmodern molecular embryology (an enormous amount), myfuture research directions were established. The embryonickidneys are an ideal system in which to explore cellsignaling, specification, adhesion, shape change, morpho-genesis, and of course organogenesis. In addition to being awonderful intellectual problem, the analysis of embryonickidney development has many advantages in terms of theavailability of techniques with which to dissect the process.Some of the organisms with the most extensive and welldeveloped embryonic kidneys are also those with the mosthighly advanced genetic and embryological tools—a perfectmatch. Finally, similar genetic networks regulate thedevelopment of all nephric organs so data gleaned fromembryonic systems are as relevant to human congenitaldisease as they are to the understanding of a quaint model.

This first section of “The Kidney” covers thedevelopment of the embryonic kidneys, the pro- and meso-nephroi, in a depth never before attempted in a text onkidney development and function. For those who no longerrecall their undergraduate developmental biology course,

even the names of these organs may be unfamiliar. After all,some mammals (including humans) can survive until birthwithout any kidneys, so of what interest are transient organsthat some would posit are nothing more than evolutionaryartifacts? In this introduction some of the reasons forrefraining from such an opinion will be explored, as will therenaissance of research into the use of embryonic kidneys asmodel systems for the analysis of organogenesis. Thefollowing chapters provide a detailed description of theanatomy, development, function, and molecular biology ofthe transient embryonic kidneys as a resource for thosewilling to accept my arguments regarding relevancy.Similar arguments can be made supporting the relevance ofinvertebrate models of nephrogenesis, and Chapter 2 openswith a review of Malpighian tubule morphogenesis in thefruit fly, Drosophila melanogaster.

The embryonic kidney of amphibians and fish is knownas the pronephros, head kidney, or vorniere, while theiradult kidney is known as a mesonephros, Wolffian body, orurniere. In some instances, fish and frog permanent meso-nephroi are unnecessarily referred to as opisthonephroi, aterm used to distinguish them from the transient meso-nephroi of amniotes, but which results in more confusionthan clarification. To begin the description of the develop-ment of vertebrate embryonic kidneys, a brief description ofthe occurrence of pro- and mesonephroi is appropriate. Allvertebrates have distinct embryonic and adult kidneys(Goodrich, 1930; Burns, 1955; Saxén, 1987). Upondevelopment of the adult kidney, the embryonic kidneyusually either degenerates or becomes a part of the malereproductive system (Burns, 1955; Balinsky, 1970). In some

1Introduction: Embryonic Kidneysand Other Nephrogenic Models

Peter D. Vize

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The KidneyCopyright © 2002 Elsevier Science (USA).

All rights reserved.

instances the embryonic kidney switches to a new role as alymphoid organ (Balfour, 1882).

Well-developed, functional pronephroi are found in allfish, including dipnoids (e.g., lungfish), ganoids (e.g.,sturgeon), and teleosts (e.g., zebrafish), and in all amphi-bians. Reptiles vary in the degree to which pronephroi form,with the more primitive reptiles having the most advancedpronephroi (Chapter 3). Birds have only a poorly developedpronephros, as do most mammals. In organisms with aquaticlarvae, pronephroi are absolutely essential for survival. Thepronephroi excrete copious amounts of dilute urine thatallows such animals to maintain water balance. If thepronephroi are not functional, aquatic larvae die rapidlyfrom oedema (Chapter 3).

Pronephric kidneys are very simple and form within aday or two of fertilization. They usually contain a single

nephron with an external glomerulus or glomus (Fig. 1.1).This glomus filters blood in an identical manner to standardglomeruli, except that the filtrate is deposited into a cavityrather than into Bowman’s space. In some instances, thiscavity is the coelom, in others, a dorsal subcompartment ofthe coelom known as the nephrocoel, and in yet others intothe pericardial cavity. The glomeral filtrate is collected fromthe receptive cavity by ciliated tubules known asnephrostomes. The nephrostomes in turn are linked to thepronephric tubules. These tubules have distinct proximaland distal segments. As with a classical mammaliannephron, the proximal segment functions in soluteresorption and waste excretion, whereas the distal segmentresorbs water. From the distal tubule urine passes down thepronephric duct to the cloaca. The entire pronephros is inessence a single large nephron. This section uses the term

2 Peter D. Vize

Figure 1.1 Embryonic kidney nephrons. (Left) Lateral and anterior views of a frog pronephric nephron at around the onset of function are illustrated.The anterior border of the distal tubule is marked in the lateral view. The posterior border of this segment has not yet been defined, but the transition regionis indicated. (Right) Two common forms of mesonephric nephron are illustrated. In each case, a glomerulus projects into the tip of the proximal segment.In the upper example the nephron branches into a peritoneal funnel that links the proximal tubule to the coelom. This type of nephron receives fluids fromtwo sources: the glomerulus via filtration and the coelom via ciliary action (Chapter 3). ns1, ns2, ns3, nephrostomes 1 through 3; db1, db2, db3; dorsalbranches 1 through 3; cmn, common (or broad) tubule; dstl, distal tubule; duct, nephric duct; p/d border, border between proximal and distal tubule zones;va, vas afferens; ve, vas efferens.

pronephros to describe an embryonic kidney that eitherutilizes an external glomus or is anatomically distinct fromthe mesonephric kidney in the same organism. Details ofpronephric anatomy and complete bibliographies areprovided by Chapters 3 through 5.

Mesonephric kidneys are more complex in organizationand consist of a linear sequence of nephrons (Fig. 1.2)linked to the nephric duct (Fig. 1.1). Mesonephric nephronscontain internal (or integrated) glomeruli, and in someinstances, particularly in anterior mesonephric tubules, alsolink to the coelom via ciliated tubules called peritonealfunnels. Such funnels are sometimes referred to asnephrostomes, which they resemble very closely, but thecorrect nomenclature of the two structures allows one tospecify whether the funnel links the coelom to theglomerulus or the glomerulus to the tubule. Nephrostomesare also sometimes present in mesonephroi so the distinctionis important.

The mesonephros is first functional at around 7.5 days ofdevelopment in the frog Xenopus (Nieuwkoop and Faber,1994) and continues to grow along with the animal. Inorganisms in which the mesonephros is transient, thecomplexity of this organ is extremely variable, ranging fromalmost no nephrons in rodents to 34 in humans and 80 inpigs (Felix, 1912; Bremer, 1916; Table 1.1). The anatomy ofa human mesonephros is illustrated in Fig. 1.3.

In animals in which the mesonephros is the terminalkidney, such as amphibians and fish, the final organ is verycomplex, containing a large number of nephrons, most ofwhich have an internal glomerulus. In the example of the frogRana, an adult mesonephros contains around 2000 nephrons(Richards, 1929) whereas an adult toad (Bufo) containsaround 3000 (Møbjerg et al., 1998). The general anatomy ofthe pro- and mesonephroi of the frog and the transitionbetween the two types of kidney are illustrated in Fig. 1.2.

In amniotes the degree of development of the meso-nephros, and even the presence of glomeruli, is linked to theform of placental development. Different amniotes have verydifferent placenta. In some organisms the fetal and maternaltissues are opposed epithelia (e.g., the pig), whereas in others

the maternal epithelium breaks down directly, bathing theintervillous spaces of the fetal epithelium directly, withblood, allowing for a more efficient supply of nutrients andremoval of wastes (e.g., rodents, primates). Animals with theformer type of placenta have large well-developedmesonephroi that remain until the adult kidney is functional,whereas those with the later have less complex embryonickidneys that often degenerate prior to the formation of themetanephroi (Bremer, 1916; Witschi, 1956).

Metanephroi develop in all amniotes from reptilesthrough humans and are the most complex of kidneys(Chapter 10). Instead of the linear organization of nephronsfound in mesonephroi, metanephroi have a branchedarchitecture with arborized networks of nephrons. Thedevelopment of metanephroi is covered in detail elsewherein this book.

A final point worth discussing before presenting theembryonic kidneys and model systems in detail is thesimilarity between the genetic hierarchies that regulate thedevelopment of all three different forms of kidney. Evo-lution does not reinvent complex processes—it fine tunesexisting systems to variations in the environment. As mole-cular biology began to identify genes and determine theirfunction in embryonic development, it soon became clearthat the developmental roles of individual genes were highlyconserved between species. The human orthologue of a flygene often performs a closely related function even thoughthe development of these two organisms differs extra-ordinarily. In some instances, a mammalian gene can act asa functional substitute in an insect and rescue the animalfrom a loss of function phenotype (Leuzinger, 1998; Nagao,1998). Given such conservation of function between species,it was not surprising to find that the different kidney formsutilize similar sets of genes to regulate their development(Vize et al., 1997). Genes demonstrated to regulate keydevelopmental steps in mammalian metanephric kidneys intargeted ablation experiments are expressed in the embry-onic kidneys in patterns that imply similar activities. Also,in the few instances where gene function has been tested inembryonic kidneys, the results implied conserved activitiesin most instances. As the embryonic kidneys are obviouslyvery different than those of the adult kidney, there must bedifferences in gene expression—and some have been noted(Carroll and Vize, 1996; Chapter 3). Two of the modelsystems present in part 1, and hopefully three in the nearfuture (Bronchain, 1999) are amenable to genetic screens ona scale impossible in a mammalian system. The embryos ofsome of these models are also excellent for the experimentalembryology and microinjection approaches that haveprovided a wealth of information on the regulation ofdevelopmental processes in the recent past.

It is hoped that the following chapters provide a usefulresource for those willing to explore the many advantages ofthe simple kidneys as model organogenesis systems.

1 Introduction 3

Table 1.1 The Mesonephric Nephron Number

Embryo length

6–10 mm 11–16 mm 21–40 mm

Guinea pig 0 14 0Human 34 34 12Rabbit 40 42 34Cat 20 26 30Sheep 20 (+6) 20 (+50) 20 (+50)Pig 54 60 60

After Bremer (1916).

4 Peter D. Vize

A B

C D

AR

CA

CP

RT

AF.1

AF.2

AF.3

GEKS.1

KS.2

PND AGM

P

AP

EF.4

EF.3

AF.2

EF.2

AF.1

EF.1

A

EH

EM

LJ

AL

RT

OP

RVTO

KS.1

KS.3

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A

TC

TR

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A

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AF.3

EF.3

AF.1

EF.1

CG

LJ

O

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KF

GM

PND

A

F

OR

MS

KU

Figure 1.2 Transition between pro- and mesonephroi in the frog, Rana temporaria, ventral view (after Marshall, 1902). The arterial system is coloredred, the venous system blue, and the pronephric glomus (GM) purple. Pronephric (P) and mesonephric (MS) tubules are in green and the nephric duct (PND)is in yellow. Tadpoles of 6.5 mm (A), 12 mm (B), 40 mm (C), and a metamorph (D). Additional labeled structures correspond to A, dorsal aorta; AF, afferentbranchial vessels; AL, lingual artery; AP, pulmonary artery; AR, anterior cerebral artery, CA, anterior commissural artery; CG, carotid gland; CP, posteriorcommissural artery, EF, efferent branchial vessels; EH, efferent hyoidean vessel; EM, efferent mandibular vessel, GE, gill; GM, glomus; KS, nephrostome;KU, ureter; MS, mesonephros/mesonephric tubules; OR, genital ridge; PND, nephric duct; P, pronephros; KS, nephrostomes; RT, truncus arteriosus; RS,sinus venosus, RV, ventricle; TC, cloaca; TO, oesophagus, cut short; TR, rectal sprout.

1 Introduction 5

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5a567

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34nephric duct

mesonephric nephrons

Figure 1.3 Human mesonephros (9.5 mm). Anterior nephrons are undergoing degeneration. Each nephron has an S-shaped tubule linking theglomerulus to the nephric duct. There is some variation in the spacing of the mesonephric tubules, and some glomeruli share a common collecting duct (e.g.,glomeruli 15 and 16 and glomeruli 19 and 20 in the left mesonephros). After Felix (1912).

References

Balfour, F. M. (1882). On the nature of the organ in adult teleosteans andganoids, which is usually regarded as the head-kidney or pronephros.Quart. J. Micr. Sci. 22, Reprinted in “Works of Balfour”, Eds. M. Fosterand A. Sedgwick. Macmillan, London, 1885. pp. 848–853.

Balinsky, B. I. (1970). “An introduction to embryology.” Saunders,Philadelphia.

Bremer, J. L. (1916). The interrelations of the mesonephros, kidney andplacenta in different classes of animals. Am. J. Anat. 19, 179–209.

Bronchain, O. J., Hartley, K. O., and Amaya, E. (1999). A gene trapapproach in Xenopus. Curr. Biol. 21, 1195–1198.

Burns, R. (1955). Urogenital system. In “Analysis of development” (B.Willier, P. Weiss, and V. Hamburger, Eds.). W.B. Saunders, Philadephia.

Carroll, T. J., and Vize, P. D. (1996). Wilms’ tumor suppressor gene isinvolved in the development of disparate kidney forms: evidence fromexpression in the Xenopus pronephros. Dev. Dyn. 206, 131–138.

Felix, W. (1912). The development of the urogenital organs. In “Manual ofhuman embryology” (F. Kiebel and F. P. Mall, Eds.), pp. 752–979.J.B.Lippincott Co., Philadelphia.

Goodrich, E. S. (1930). “Studies on the structure and development ofvertebrates.” Macmillan and Co., London.

Leuzinger, S., Hirth, F., Gerlich, D., Acampora, D., Simeone, A., Gehring,W. J., Finkelstein, R., Furukubo-Tokunaga, K., and Reichert, H. (1998).

Equivalence of the fly orthodenticle gene and the human OTX genes inembryonic brain development of Drosophila. Development 125,1703–1710.

Marshall, A. M. (1902). “The frog: an introduction to anatomy, histology,and embryology.” Macmillan and Co., New York.

Møbjerg, N., Larsen, E. H., and Jespersen, Å. (2000). Morphology of thekidney in larvae of Bufo viridis (Amphibia, Anura, Bufonidae). J.Morph. 245, 177–195.

Nagao, T., Leuzinger, S., Acampora, D., Simeone, A., Finkelstein, R.,Reichert, H., and Furukubo-Tokunaga, K. (1998). Developmentalrescue of Drosophila cephalic defects by the human Otx genes. Proc.Natl. Acad. Sci. USA 95, 3737–3742.

Nieuwkoop, P. D., and Faber, J. (1994). “Normal table of Xenopus laevis(Daudin).” Garland, New York.

Richards, A. N. (1929). “Methods and results of direct investigations of thefunction of the kidney.” Williams and Wilkins Company, Baltimore.

Saxén, L. (1987). “Organogenesis of the kidney.” Cambridge UniversityPress, Cambridge.

Vize, P. D., Seufert, D. W., Carroll, T. J., and Wallingford, J. B. (1997).Model systems for the study of kidney development: use of thepronephros in the analysis of organ induction and patterning. Dev. Biol.188, 189–204.

Witschi, E. (1956). “Development of Vertebrates.” W.B. SaundersCompany, Philadelphia.

6 Peter D. Vize