Microvascular anatomy of ovary and oviduct in the adult African Clawed Toad (Xenopus laevis DAUDIN, 1802)–Histomorphology and scanning electron microscopy of vascular corrosion casts

Abstract Ovaries and oviducts of the adult African Clawed Toad (Xenopus laevis DAUDIN, 1802) were studied by light microscopy (LM) of paraplast embedded tissue sections and scanning electron microscopy (SEM) of vascular corrosion casts (VCCs). Histomorphology revealed that ovarian vessels located in the thecal layers. Ovarian and interlobar arteries displayed a horse‐shoe shaped longitudinally running bundle of vascular smooth muscle cells. Follicular blood vessels showed flattened profiles, which were confirmed by scanning electron microscopy in vascular corrosion casts. The flattened profiles obviously led to high intravasal pressures, which locally prevented filling of the follicular capillary bed. Oviduct arteries pierced the fibrous stroma surrounding the oviduct mucosa. In the pars convoluta, the mucosa consisted of a ciliated simple columnar epithelium and tubular oviduct glands that opened between ciliated epithelial cells into the oviduct lumen. Oviduct arteries branched at the basolateral surfaces of tubular glands. After a short tangential course, arterioles branched into capillaries which ran radially between oviduct glands towards the subepithelium. Anastomoses at different heights connected capillaries of neighbouring glands. Subepithelially, capillaries ran longitudinally and undulated. Postcapillary venules radiated centrifugally towards the stroma to finally drain into oviduct veins located in the stroma. Oviduct vascular densities clearly reflected non‐ovulatory and ovulatory states.


who investigated oogenesis in
Xenopus laevis (Daudin) by light and electron microscopy described the structure of blood vessels within the theca of ovarian follicles and referred to them as "… small capillaries with thin endothelium" (Dumont & Brummett, 1978). Yoshizaki (1985) studied the fine structure of the oviduct epithelium in Xenopus laevis but referred only to the pars recta as"…well vascularized….". Xiang, Burnett, Rawls, Bieber, and Chandler (2004) who focused upon the expression of the sperm chemoattractant "allurin" in Xenopus oviducts only mentioned the presence of a central capillary beneath the mucosal epithelial folds (domes) of the oviduct and of capillaries in between the tubular glands. Due to techniques used, none of these studies presented vascular patterns and spatial relations of ovarian and oviduct vessels.

| Animals
Eight adult females of Xenopus laevis were studied. Animals (body weight: 18 g-48 g; body lengths: 69 mm-80 mm) were housed in aquaria (tap water depth: 15 cm) equipped with aquarium filters and fed twice a week with either dried Gammarus pulex or grinded beef heart.

| Ovary
Xenopus ovaries consisted of many lobes that contained follicles with pre-vitellogenic and vitellogenic oocytes ( Figure 1a). The ovarian sac, which contained the follicles, consisted of peritoneal epithelium

| Oviduct
The oviduct pars convoluta consisted (from luminal to abluminal) of a ciliated epithelium, tubular glands below, and a stroma with connective tissue fibres, intermingled fibrocytes, smooth muscle cells and

| Ovary vasculature
Urogenital arteries splitted at the ventral surface of the kidneys into renal, ovarian and oviduct arteries (Figure 3a). Ovarian arteries ran within the mesovarium towards the ovary (Figure 3b). In the ovary, they branched into interlobar arteries that ran along the lobar margins ( Figure 3c). Interlobar arteries gave off follicular arterioles, which finally fed the capillary network ensheathing ovarian follicles

| Oviduct vasculature
The The density of the oviduct vascular bed differed greatly (compare Figure 4d with Figure 5a,b). While in less densely vascularized specimens, intramural spacing of vessels was wide, that in highly vascularized specimens was very narrow resulting in a close apposition of highly undulating arteries, veins and capillaries (Figure 5a,b).
Slightly anterior to the rostral margin of the urinary bladder, the convoluted portions of oviducts became wide, flat and formed uteri (Figure 6a,b). Uteri displayed prominent venous plexuses at their ventral peritoneal surface and drained into the posterior caval vein (Figure 6a,b). Closely attached to the plexuses a network of capillaries was found which originated from and drained into the plexuses (Figure 6b, inset). Luminally, the dorsal walls of uteri displayed capillary rows that extended obliquely from anterior-lateral to posterior-medial (Figure 6b,c). At the midline, capillary rows changed directions and ran longitudinally (Figure 6c). Small oviduct arterioles fed the row capillaries (Figure 6d).

| D ISCUSS I ON
In her study on the vascular anatomy of adult Xenopus laevis, Millard (1940) performed detailed binocular dissections on gross arterial supply and venous drainage of adult gonads. Due to the limited depth of focus and spatial resolution of the dissecting microscope, she was not able to visualize the microvasculature in detail, but she carefully described and illustrated gross gonadal blood supply and drainage by coloured illustrations Millard (1940). Our results gained by scanning electron microscopy of vascular corrosion casts, a method which also allowed to study the microvascular bed of well circumscribed areas confirmed her binocular findings (Millard, 1940), but allowed also a highly detailed insight into the microvascular anatomy.
In respect to the vasculature of ovarian follicles, we found that vascular profiles embedded in the theca were flat in perfused tissues sections ( Figure 3). This finding was confirmed by the flat profiles of cast follicular vessels embracing pre-vitellogenic and vitellogenic oocytes. These flat vascular profiles most likely reflected the high intravasal resistance that locally resulted in incomplete fillings of the ovarian vascular bed which we found in many of our cast preparations (see Figure 3d, asterisk). Incomplete fillings by their rounded endings were easy to differentiate from broken vessels, which dis- The microvasculature of the convolute portions of the oviduct clearly indicated the ovulatory status of the animal. While the rather loose vascular network most likely reflected the non-ovulating status, oviducts with dense vascular networks represented the ovulating status. In the latter, intervascular distances down to the capillary bed were extremely small and feeding arterioles often coiled enormously.
These findings reflect the intense vascular growth, which ensures an appropriate supply of the highly secretory oviduct glands. These glands deliver the jelly coat to the oocytes when they pass the oviduct.

ACK N OWLED G EM ENTS
We thank Dr. Heidi Bartel and Christine Radner, BSc. for specimen preparation for light microscopy and processing as well as recording of SEM specimens and Dr. Wolf-Dietrich Krautgartner for his excellent technical advice in the SEM facility.

CO N FLI C T O F I NTE R E S T
The authors have no conflict of interests.

DATA AVA I L A B I L I T Y S TAT E M E N T
Further micrographs (LM, SEM) that support the findings of this study are available from the corresponding author upon request.