An appendix testes (Hydatid of Morgagni)
Keywords: Morgagni, hydatid, cyst, appendix, testis,
The image below shows the lateral aspect of a left testis, one of two removed from the inguinal canals of a two year old stallion. The appendix testes (AT) is the small, corrugated, rounded structure adjacent to the caput epididymis (see arrow). A similar structure was present on the other testes.
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This fascinating vestige of the müllerian system reminds one of the bi-potential nature of early embryos. Were it not for the presence of the testis against which it rests, this small vestige would have developed into a substantial portion of the female reproductive tract. Within the seminiferous tubules of the testis, there are of course millions of Sertoli cells. It is Anti Müllerian Hormone (AMH) produced by those cells that has inhibited the development of the rest of this female tract.
Müllerian tubes are clearly visible in early mammalian fetuses, lying against the testes. Then, as the fetus ages, the distal parts of the tubes undergo apoptosis, gradually disappearing towards the head of the epididymis. By the time the male is born, the AT is quite small in most species and it becomes even less distinct as the animal ages. AMH production continues into adulthood, therefore the appendix testis become progressively smaller with time.
On the testicle of this four year old Quarter horse (lateral aspect, right testicle) the appendix is still obvious.
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There is obviously variation in the rate of apoptosis of the appendix testis but on the testes of the 6 year old Welsh stallion seen below, the AT is scarcely visible.
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In general, the AT is particularly obvious in stallions but is also described in boars, small ruminants and dogs. In adult bulls it is usually not present.
Interestingly, the AT is larger in humans than in other mammals and occasionally become pedunculated and able to twist on that axis. Torsion of the peduncle of an AE causes an acute, painful condition requiring surgery. A similar condition has not been described in animals.
This structure is also known as an Hydatic of Morgagni named for the 17th century Italian anatomist Giovanni Morgagni; a man who has cataracts, caruncles, cysts, conchas and columns all bearing his name. However, this structure should not be one of them, for the term hydatid is derived from from the Greek word hudatid, referring to a water filled vesicle. In no species is the AT a vesicle or cyst-like structure.
| Some clarification is required here: A hydatid of Morgagni is present in females where it is also known as a cyst of Kobelt (especially by gynecologists). This is indeed a cystic structure so the term may be justified. According to Monika Jacob and Klaus Barteczkoas (2005) the müllerian vs. wolffian origin of paraovarian cysts is still not clear in females. In males it now appears certain that the AT (Hydatid of Morgani) arises from the müllerian system and the appendix epididymis (AE) arises from the wolffian system. Compared to AT, an AE is a rare finding . |
The image below shows a low magnification view of the AE seen in the image at the top of this page. Note the semi-solid appearance of the structure. The asterisk refers to high magnification views of the structure in question. Please see those below this image.
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High magnification views of the lumen indicated by an asterisk in the image above. In view of the fact that the AT is essentially derived from a female structure, it is interesting to see ciliated and non ciliated columnar tubular epithelium in the AT, similar to that seen in uterine tubes (oviducts).
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Selected references:
Aguilar, J.J. et al, 2012. Histological characteristics of the equine oviductal mucosa at different reproductive stages. J.Equine Vet Sci 32:99–105
Gelberg, H. B. et al 1986 Pathology of the Canine and Feline Uterine Tube.Vet. Pathol. 23:770-775
Griffiths, J. 1894 Observations on the appendix of the testicle, and on the cysts of the epididymis, the vasa efferentia, and the rete testis. J Anat Physiol 28:107–124
Jacob, M. et al. 2005, Contribution to the origin and development of the appendices of the testis and epididymis in humans. Anat Embryol 209:287–302
Murhaghan, C.F. et al. 1959. The appendages of the testis and epididymis: A short review with case reports. Brit. J. Urology 31: 190–195
Sahni, D. et al. 1996. Incidence and structure of the appendices of the testis and epididymis J. Anat. 189: 341-348
Vlahakis-Miliaras, E. et al. 1998. Paratubal cysts in young females as an incidental finding in laparotomies performed for right lower quadrant abdominal pain. Pediatric Surg. International. 13: 141-142
Watson, J.H. 1902. Some observations on the origin and nature of the so-called Hydatids of Morgagni found in men and women, with especial reference to the fate of the Müllerian duct in the epididymis. J.Anat.Physiolo.36:147-161
