Development of the human female reproductive tract

Abstract

Development of the human female reproductive tract is reviewed from the ambisexual stage to advanced development of the uterine tube, uterine corpus, uterine cervix and vagina at 22 weeks. Historically this topic has been under-represented in the literature, and for the most part is based upon hematoxylin and eosin stained sections. Recent immunohistochemical studies for PAX2 (reactive with Müllerian epithelium) and FOXA1 (reactive with urogenital sinus epithelium and its known pelvic derivatives) shed light on an age-old debate on the derivation of vaginal epithelium supporting the idea that human vaginal epithelium derives solely from urogenital sinus epithelium. Aside for the vagina, most of the female reproductive tract is derived from the Müllerian ducts, which fuse in the midline to form the uterovaginal canal, the precursor of uterine corpus and uterine cervix an important player in vaginal development as well. Epithelial and mesenchymal differentiation markers are described during human female reproductive tract development (keratins, homeobox proteins (HOXA11 and ISL1), steroid receptors (estrogen receptor alpha and progesterone receptor), transcription factors and signaling molecules (TP63 and RUNX1), which are expressed in a temporally and spatially dynamic fashion. The utility of xenografts and epithelial-mesenchymal tissue recombination studies are reviewed.

Keywords: Cervix; Human Müllerian duct; Urogenital sinus; Uterovaginal canal; Uterus; Vagina; Wolffian duct.

Figure 1.

Wholemount photos of developing human fetal female internal genitalia staged by heel-toe measurements. Note (a) increase in size and morphological complexity with time, and (b) that it is impossible to distinguish the uterine corpus, cervix and vagina. Specimens photographed with transmitted light (9, 10, 11, 13 and 15 weeks) permit visualization of internal (epithelial) organization in regions not too thick. The 10-week specimen is shown at both low and high magnifications. Red arrowheads demarcate the epithelium defining the lumen of the uterine (Fallopian) tube. Green arrowheads define the epithelium lining the uterus. Relative sizes of specimens are not exact, but increase with age. From Robboy et al (2017) with permission.

Figure 2.

(A) Formation of the Wolffian (mesonephric) duct, which by 24 days has grown caudally to join the cloaca. At 5 to 6 weeks the paramesonephric (Müllerian) ducts appear as invaginations of the coelomic epithelium. At 7 weeks (B) the MDs have grown caudally towards the urogenital sinus. Subsequently (C, 8 weeks), the opening of the MDs into the coelomic cavity is fimbriated, and with further growth the MDs reach the UGS, while the Wolffian ducts degenerate. Modified from (Park, 2016) with permission.

Figure 3.

Diagrammatic representation of the caudal growth of the Müllerian duct using the Wolffian duct as a “guide wire”. Note in (a) mesenchyme intervening between the Müllerian and Wolffian ducts, (b) contact of the basement membranes of Müllerian and Wolffian ducts, (c) direct contact of the epithelia of the Müllerian and Wolffian ducts. From Robboy et al (2017) with permission.

Figure 4.

Diagram of developing human female internal genitalia in the indifferent, bisexual stage (~54 days of gestation, Carnegie Stage 22). The Müllerian derivatives are red and Wolffian derivatives are purple. Note the changing anatomical relationships between the Müllerian and Wolffian ducts. From Robboy et al (2017) with permission.

Figure 5.

Early Müllerian duct growth and fusion to form the midline uterovaginal canal. Length of the uterovaginal canal increases with developmental age. In (A) the extent of MD caudal extension is depicted at Carnegie Stages 20 to 23 (50 to 56 days). (B–D) depict fusion of the right and left MDs to form the midline uterovaginal canal, formation of the septum and its subsequent disappearance. From Robboy et al (2017) with permission.

Figure 6.

Light sheet microscopy™of a human female reproductive tract at 9.5 weeks stained with an antibody to E-cadherin. The mesonephros and Wolffian ducts (WD) are present. Cranially, the unfused MDs are destined to form the uterine tubes. Midline fusion of the MDs has created the uterovaginal canal that terminates caudally by joining the urogenital sinus (UGS).

Figure 7.

Diagrams of human (A) and mouse (B) female urogenital tracts emphasizing the marked difference in the degree of MD fusion. From Robboy et al (2017) with permission.

Figure 8.

Wholemount images of human fetal female reproductive tracts (A–F) photographed with transmitted light, and H&E stained transverse sections of the uterovaginal canal (G–H). Dotted lines indicate the contours of the uterotubal junction and the uterine cavity. Green arrowheads indicate the epithelium of the uterine tubes. Note the changing shape of the uterine lumen. The cranial portion is laterally expanded (G) and narrow caudally (H). From Robboy et al (2017) with permission.

Figure 9.

α-Actin immunoreactivity in human female reproductive tracts at (A) 9, (B) 11, (C) 12, and (D) 18 weeks. Faint α-actin immunoreactivity first appears focally in mesenchyme of the 9-week uterovaginal canal, increasing in intensity by the 11 weeksBy 12 weeks α-actin immunoreactivity is strong in the middle 2/4^ths of the developing reproductive tract, and by 18 weeks, is strongly expressed throughout the developing female reproductive tract. From Robboy et al (2017) with permission.

Figure 10.

Cervical development. (A–D) are sagittal sections of the female reproductive tract of a 16 week fetus (A=H&E, B=ISL1 immunostain, C-D=keratin 19 immunostain). Boundaries between the vagina, cervix and uterus are nebulous up to 18 weeks, when vaginal fornices become apparent (F). (B) ISL1 immunostaining is strong in vaginal stroma, absent in uterine stroma, with a sharp fall off in staining intensity at the mid-point of the uterovaginal canal (red arrow in B). Keratin 19 immunostaining (C–D) may also be indicative of vaginal-exocervical boundary. Cervical glands are prominent at 21 weeks of gestation (E). Epi. = Epithelium, Mes. = mesenchyme, Strat. Sqm. = Stratified squamous, Vag. Plate = vaginal plate, Endo-Cx = endocervix. From Robboy et al (2017) with permission.

Figure 11.

Sagittal sections of a 12-week human female fetal reproductive tract immunostained with PAX2 (A–B) and FOXA1 (C–D). PAX2-reactive epithelial cells extend to near the junction with urethral epithelium (A–B). FOXA1-reactive epithelial cells extend only a short distance into the solid vaginal plate (C–D). Scale bar in A also refers to D. Scale bar in D also refers to C. From Robboy et al (2017) with permission.

Figure 12.

Sagittal sections of a 16-week human female fetal reproductive tract immunostained with PAX2 (A–B) and FOXA1 (C–D). FOXA1-reactive epithelial cells form all of the solid vaginal plate (C–D). PAX2-reactive epithelial cells constitute a epithelium lining the lumen of the uterovaginal canal (A–B). Scale bar in A also refers to D. Scale bar in D also refers to C. From Robboy et al (2017) with permission.

Figure 13.

Sagittal sections of an 18-week human female fetal reproductive tract immunostained with PAX2 (A–B) and FOXA1 (C–D). PAX2-reactive epithelial cells line the lumen of the female reproductive tract from the uterine tube to the cranial aspect of the vagina (A–B). PAX2 staining is present, but weak in vaginal epithelium (A–B). FOXA1-reactive epithelial cells line the lower (caudal) vagina (C–D). Note abrupt fall off in KRT19 staining in the lower vagina (arrowheads). Scale bar in A also refers to D. Scale bar in D also refers to C. (E) KRT19 mirrors PAX2 immunostaining. From Robboy et al (2017) with permission.

Figure 14.

Sagittal sections of a 21-week female reproductive tract. Section (A) depicts the lower portion of the vagina (introitus to upper vagina). Section (B) depicts the upper portion of the specimen (vagina, cervix and uterine corpus) (both (A & B) are H&E stain. The vaginal epithelium is many layers thick due to estrogenic stimulation (A–B). Sections (C, E & F) are adjacent sections. In (B, C & E) note the abrupt transition in epithelial differentiation at the vaginal/cervical border and the change in PAX2/FOXA1 staining (indicative of a MD/UGE boundary). The vaginal epithelium is PAX2-negative (C), while epithelium of the uterine corpus (D) is strongly PAX2-reactive. FOXA1 immunostaining was seen uniformly throughout the entire vagina, and FOXA1 immunostaining abruptly stopped at the vaginal/cervical border (E–F). Asterisks in (B, C and E) are at the vaginal exocervical boundary. From Robboy et al (2017) with permission.

Figure 15.

Summary of the changing patterns of PAX2 epithelial expression (green) indicative of Müllerian epithelium and FOXA1 epithelial expression (red) indicative of endodermal urogenital sinus epithelium over the time frame of 12 to 21 weeks.

Figure 16.

Xenografts of twin 13-week human female reproductive tracts grown for 4 weeks in untreated ovariectomized (A, B & D) and ovariectomized DES-treated athymic mice (C & E). Note in (A–B) uniform strong ESR1 immunostaining of the tubal epithelium in the untreated ovariectomized host, and (C) the mixture of weakly ESR1positive and ESR1-negative tubal epithelial cells in the DES-treated specimen. Sections (B & D) depict the uterine corpus of a xenograft grown in an untreated ovariectomized host. The epithelium of the uterine corpus is ESR1-negative, while the uterine mesenchyme is ESR1-reactive. Section (E) depicts the uterine corpus of the twin 13week specimen grown in a DES-treated ovariectomized host. Note the strong uniform expression of ESR1 in the uterine epithelium. Note also the substantial DES-induced plication of the uterine tube (compare B & C), and the profound DES-induced uterine gland formation (compare (B & E).

Figure 17.

Sections of a 13-week vaginal specimen grown for 1 month in a DES-treated host via a 20mg subcutaneous pellet and immune stained as indicated. Mature stratified squamous vaginal epithelium and simple columnar adenotic epithelium exhibit remarkably different marker profiles (see Table 6).

Figure 18.

Tissue recombinants composed of neonatal mouse vaginal mesenchyme plus 13-week human fetal uterine tube epithelium (mVgM+hTubE) grown for 4 weeks in DES-treated hosts and immune stained for various vaginal epithelial markers as indicated. Human uterine tube (A, D, G, J) and vagina (B, E, H, K) at 16 to 18 weeks of gestation serve as controls. Note induction of KRT6, TP63 and RUNX1 and down regulation of AR in epithelium of the mVgM+hTubE recombinants, indicative of an effect of mouse vaginal mesenchyme on expression of differentiation markers in human tubal epithelium. (+) and (−) indicate epithelial marker expression. From Cunha et al (2018) with permission.

Figure 19.

Tissue recombinants composed of neonatal mouse wild-type uterine mesenchyme plus human fetal uterine tube epithelium (wt UtM+hUtE) (A &B) and αERKO uterine mesenchyme plus human uterine tube epithelium Esr1KO UtM+ h UtE) (C & D) grown in DES-treated hosts and immune stained for ESR1 (A & C) and PGR (B & D). DES induced ESR1 and PGR even when the mesenchyme was genetically devoid of ESR1. Sections (C) and (D) are adjacent sections stained for ESR1 (C) and PGR (D). From Cunha et al (2018) with permission.