Gubernaculum and testes descent
Keywords: testes, equine, cryptorchid, descent, inguinal canalThe composite image below shows the testis and adnexa of a 315 day (~10.4 months) fetus. If this fetus had survived to term at perhaps 330 to 355 days, it would have been born approximately 15 to 40 days after these images were taken. In other words, the fetus was close to the end of its gestation.
In the series of small images (captured from a video) at the base of the large image, note that the operator has grasped the gubernaculum and is using it to draw the testis through the inguinal opening. In the third and fourth images from the left, it is apparent that considerable tension was applied to the gubernaculum to draw the testis from the peritoneal cavity into, and through the inguinal canal. This is remarkable because of the relative maturity of this fetus. To the author, this demonstration shows a critical relationship between intra-abdominal pressure, contraction of the external abdominal muscles, gubernaculum swelling then gubernaculum contraction (there is no muscle in the gubernaculum) growth of the inguinal canal and relative elongation of the fetus; all of which act in sequence to allow normal descent of the testis into the scrotum.
In the last of the small images, the operator inserts a finger through the inguinal canal, distal-to-proximal.
Image size 1732 x 899px. Copyright: Dr Frederico Canisso. canisso@illinois.edu
In the main image i.e. the end point of the extraction exercise, the testis and gubernaculum are held aloft for inspection. Like the ductus deferens, the gubernaculum is a product of the embryonic mesonephric system. Although the gubernaculum is so obvious here, it is not visible in the adult animal. Its proximal portion becomes the proper ligament of the testis and the distal portion (distal to the tail of the epididymis) forms the ligament of the tail of the epididymis; both small and indistinct structures.
An item also worthy of note is the small, dark red structure (ringed) protruding from the proximal pole of the testis in this image. This was in fact, part of an ectopic adrenal gland, verified by histology. Although ectopic adrenal tissue in testes is common in humans it is comparatively rare in stallions. Interestingly, Dr M. Pozor (pozorm@ufl.edu) states that this structure also resembles an appendix testis (a remnant of the mullerian system sometimes referred to as a Hydatid of Morgagni). Various observers suggest that appendices testes may be present in most, if not all stallions.
Efferent ducts from the head of the epididymis have to form a close connection with the rete testes to allow sperm transport from the testis into the epididymis. The head of the epididymis is not yet well formed here and consequently, its connection with the rete is not substantial either. This author was not able to find data on the time span over which this connection is forged but certainly it begins in the early embryo, soon after the formation of the metanephros. However, it is not clear when the connection becomes fully functional in horses. This could be important because the connection between the efferent tubules in the head of the epididymis and the rete may be imperfect in both humans and domestic animals. In such cases,sperm granulomas form around the site where haploid sperm escape into the diploid environment.
To understand this entry more completely, it may be useful to see two other LORI entries; the position and anatomy of the testis in a 6 month old fetus and the descent of testis in stallions and bulls
The following image shows complete descent of a testis in a neonatal foal. The gubernaculum and testis can be seen through the parietal vaginal tunic. If that tunic were to be incised, the vaginal cavity would be opened and the shiny testis covered by its visceral vaginal tunic would be visible. A probe has been inserted into the internal inguinal opening.
Image size: 635 x 432px
Selected references:
1. Gier, H.T. and Marion, G.B. 1969 Development of Mammalian Testes and Genital Ducts. Biol. Reprod.1: Supplement 1 1-23
2. Heyns, C.F. et al 1985. The gubernaculum during testicular descent in the pig fetus. J Urology
133: 694-699
3. Marino, G. et al. 2012. Ectopic adrenal tissue in equine gonads: morphofunctional features. Turk. J. Vet. Anim. Sci. 36: 560-565
