REVIEWS DEVELOPMENTAL GENETICS OF THE FEMALE REPRODUCTIVE TRACT IN MAMMALS Akio Kobayashi and Richard R. Behringer The female reproductive tract receives the oocytes for fertilization, supports the development of the fetus and provides the passage for birth. Although abnormalities of this organ system can result in infertility and even death, until recently relatively little was known about the genetic processes that underlie its development. By drawing primarily on mouse mutagenesis studies and the analysis of human mutations we review the emerging genetic pathways that regulate female reproductive-tract formation in mammals and that are implicated in congenital abnormalities of this organ system. We also show that these pathways might be conserved between invertebrates and mammals. AGENESIS The female reproductive tract is essential for the con- review examines our knowledge of the genetic pathways A condition in which a body tinuation of mammalian species: it provides the site that regulate the organogenesis of the female reproduc- part is absent or does not for the fertilization of oocytes by spermatozoa, for tive tract in mammals. Many of these studies show that develop completely. implantation and subsequent development of the interactions between the mesenchyme and epithelium of ATRESIA embryo, and for delivery of the fetus. Abnormalities in the developing female reproductive tract are important A condition in which an opening female reproductive-tract formation — which are esti- for its formation and differentiation. Interestingly, or passage for the tracts of the mated to occur in 0.1–3.0% of live births in humans1 — although mammals and invertebrates differ markedly body is absent or closed. can lead to infertility and even death during pregnancy both in the morphology of their reproductive organs and or childbirth. The range of documented defects includes in their mode of reproduction, some genetic pathways for SEPTATION Refers to the state of being AGENESIS, ATRESIA and SEPTATION of the female reproductive female reproductive-tract organ development seem to be divided internally by a partition tract1,which are thought to result from abnormalities conserved between them. Other important related issues, or partitions. In the female that occur during embryonic development. such as embryo implantation and postnatal hormone- reproductive tract, septation is The female reproductive tract, which in mammals regulated differentiation, have been reviewed elsewhere3,4. observed longitudinally or transversely. includes the oviducts (fallopian tubes), uterus, cervix and vagina (FIG. 1a), is also a prominent organ site for dis- Embryology of the female reproductive tract ease. Malignancies of the cervix and uterus accounted The development of the vertebrate urogenital system — Program in Developmental for 14.8% of all cancers among women in the United which comprises the kidneys, gonads, and urinary and Biology, Baylor College of States in the year 2000 (REF.2).Surprisingly, despite the reproductive tracts — begins soon after gastrulation, Medicine and Department importance of this organ system for the fertility and through the differentiation of the INTERMEDIATE MESODERM. of Molecular Genetics, University of Texas M. D. health of women, relatively little is known about the This embryonic tissue subsequently proliferates and Anderson Cancer Center, molecular and cellular mechanisms that regulate its some cells undergo the transition from the mesenchy- 1515 Holcombe Boulevard, development during embryogenesis. Recent findings, mal to the epithelial cell type to generate the tubules that Houston, Texas 77030, USA. predominantly from mouse knockout studies, have compose the male and female reproductive tracts, as well Correspondence to R.R.B. e-mail: identified a set of genes that are essential for the devel- as the kidneys and testes. Before sexual differentiation, [email protected] opment of this organ system (TABLE 1).By primarily mammalian embryos have two pairs of genital ducts: the doi:10.1038/nrg1225 drawing on mouse and human genetic studies, this Wolffian ducts (MESONEPHRIC DUCTS) and the Müllerian NATURE REVIEWS | GENETICS VOLUME 4 | DECEMBER 2003 | 969 REVIEWS a Ovaries ducts (PARAMESONEPHRIC DUCTS) (BOX 1).The Wolffian ducts Oviduct differentiate into structures of the male reproductive tract, such as the epididymides, vas deferentia and semi- A nal vesicles. By contrast, the Müllerian ducts, which sub- sequently form adjacent to the Wolffian ducts (FIG. 2a), differentiate into the oviducts, uterus, cervix and upper portion of the vagina of the female reproductive tract. P Uterus The expression of a lin-11, Isl1 and mec-3 homologue (Lim1, also known as Lhx1), which encodes a LIM class Myometrium Endometrium Cervix homeodomain protein, in the epithelium of the Vagina Wolffian and Müllerian ducts highlights the initial sex- ual duality of the forming reproductive systems (see b Duplex uterus without vagina c Duplex uterus with two lateral vaginae below; FIGS 2b,c, 3). (monotremes) and a median birth canal (marsupials) The morphology of the female reproductive tract can differ markedly among mammalian species (FIG. 1b–g). Müllerian duct formation is similar between species and the morphological diversity mainly results from differences in the extent of fusion of the two Müllerian ducts anteriorly5.At one extreme are monotremes and marsupials in which Müllerian fusion is absent or lim- ited, which leads to the formation of two uteri (‘duplex’ US US uteri in FIG. 1b,c). At the other extreme, the Müllerian ducts of higher primates (including humans) fuse more anteriorly, which results in the formation of a sin- d Duplex uterus with a single vagina e Bipartite uterus seen in pigs, seen in rodents and rabbits marine mammals and mice gle (‘simplex’) uterus with a single cervix and vagina (FIG. 1g).Anatomical variation of the female reproduc- tive tract can even be observed within a species; for example, subspecies of bats can have different types of uterus6. Molecular genetics in the mouse Targeted mutagenesis in the mouse has identified several genes that are essential for female reproductive-tract development. On the basis of their mutant phenotypes, the genes that are knocked out in these mice can be cate- gorized into those that are required for initial Müllerian duct formation in both sexes, for its regression in males f Bicornuate uterus seen in g Simplex uterus seen in most higher most bats, cows and horses primates, including humans or for its differentiation in females. Each of these three main developmental stages are discussed below. Molecular expression analyses in wild type and knockout mice have also contributed to understanding the relation- ships between genes and have been used to build a mole- cular genetic pathway for female reproductive-tract development. Müllerian duct formation. A small set of homeodomain- containing transcription factors and signalling molecules Figure 1 | Female reproductive-tract variation in mammals. a | Basic anatomical features of are required for female reproductive-tract formation in the female reproductive tract. Oocytes leave the ovaries and move into the oviduct, where mice. One of these, paired-box gene 2 (Pax2), encodes a fertilization occurs. The cervix is the boundary between the uterus and the vagina or urogential homeodomain transcription factor that is homologous sinus. With the exception of in the egg-laying mammals (monotremes), embryos implant in the to the Drosophila PAIR-RULE GENE paired (prd) (REF. 7). uterus and are delivered through the vagina. The developing female reproductive tract has two layers, the epithelium and the surrounding mesenchyme, which differentiate into the endometrium Pax2-null mutant mice die soon after birth, have no and the myometrium, respectively, in the uterus. b,c | Absent (or limited) fusion of the Müllerian kidneys and lack a reproductive tract owing to the ducts leads to the presence of two uteri. The urogenital sinus (US) is connected to the female degeneration of the Wolffian and Müllerian ducts dur- reproductive tract (b). Müllerian duct fusion is physically blocked by the presence of the ureters, ing embryogenesis — a phenotype that is consistent which leads to the formation of three vaginae (c). d | The duplex uterus shown here has a pair of with the expression of this gene in the kidney and in the cervices. e | In the duplex bipartite uterus seen in many mammalian species, Müllerian fusion in epithelium of the Wolffian and Müllerian ducts. the uterine region does not occur, or is limited, which leads to the formation of a pair of uterine horns that can support the development of many fetuses. f | A larger portion of the uterus However, the anterior portion of both tracts initially 8 forms the uterine body. g | Müllerian ducts fuse anteriorly to generate a single uterine body that forms in Pax2-null mutants .A closely related gene, supports a single fetus or a small number of fetuses per pregnancy. A, anterior (cranial); Pax8, is co-expressed with Pax2 in the developing P, posterior (caudal). Panels b–g adapted with permission from REF. 5 © (2003) McGraw-Hill. Wolffian and Müllerian ducts and kidney, although 970 | DECEMBER 2003 | VOLUME 4 www.nature.com/reviews/genetics REVIEWS Table 1 | Mouse genes that are required for female reproductive tract development Gene name Genetic map Molecule encoded Tissue of Female reproductive-tract References position expression phenotype abnormality (mode of inheritance) Formation Pax2