Feline embryo ~ 21-23 days

Key words: Embryo, feline, organogenesis, crown-rump, genital ridge.

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A feline embryo estimated to be approximately 21 to 23 days of age based on its (difficult to ascertain) crown-rump length and physical appearance in an image from Knospe, C. 2002.

Besides the intriguing organogenesis, of special interest to the author was the genital ridge. It is particularly obvious in this specimen. This structure is well known to reproductive biologists as the progenitor of the gonads; a ridge populated by primordial germ cells from the yolk sac. These cells  migrate to the ventral aspect of the mesonephros on either side of the spinal column. In this specimen it is not possible to determine if the gonads will develop into ovaries or testes.

The author extends thanks to Dr Glenda Wright (gwright@upei.ca) and Dr  Vanmathy Kasimanickam (vkasiman@wsu.edu) for their editorial comments on this image.

Selected references:

Davidson, A.P. et al. 1986. Pregnancy diagnosis with ultrasound in the domestic cat. Vet. Radiology and Ultrasound. 27:109-114

Knospe, C. 2002 Periods and Stages of the Prenatal Development of the Domestic Cat. Anat. Histol. Embryol. 31:37–51

Luvoni, G.C. 2013. Ultrasonographic Foetal Biometry in Dogs and Cats. WSAVA2013 - VIN

Nelson, N.S. and Cooper, J. 1975. The growing conceptus of the domestic cat. Growth 39:435-451

Topie, E. et al. 2015. Early pregnancy diagnosis and monitoring in the queen using ultrasonography with a 12.5 MHz probe. J.Feline Med.Surg. 17:87-93

Possible superfetation

Keywords: superfetation, feline, diapause, pregnancy.

This old but unique image provokes the question: Can superfetation occur in cats?

Figure 1. This uterus was extracted during routine ovariohysterectomy. The small embryos  seen in this pregnancy were examined histologically. This suggested that they were normal. The author suggests that this may be a case of superfetation. Image size: 1399 x 843 px

Queens can exhibit sexual behavior and breed during gestation. However, the fertility of  such breedings is not known. It is highly unlikely that additional conceptions could occur in one uterine horn after an earlier pregnancy had been established in the same horn. The intimate zonary structure of feline placentation forms a barrier to both oocytes and spermatozoa, preventing such conceptions. However, the two younger embryo/fetuses in this case were located in the horn contralateral to the older fetuses. Therefore superfetation may have occurred in this case. An immediate and obvious objection to this suggestion would be cervical integrity, preventing sperm from entering the uterus. This is a valid argument but may be based on simplistic dogma of progesterone and cervical closure. For example, in species that show embryonal diapause, the cervix remains open while embryos lie within the uterus in a state of suspended development. Indeed, in black bears, multiple conceptions from different sires can occur during pregnancy. This is not due to superfecundation (a well understood phenomenon in polytocous species) but breeding by different sires in separate periods of receptivity. In bears then, following variable periods of diapause (according to when matings took place) the cubs are born synchronously. Admittedly, progesterone does not dominate the endocrine environment in that case until activation of embryonal growth occurs.

The author does not suggest that the case entered here is one of embryonal diapause; in such a case, the development of the embryos is synchronous once re-activation of embryonic growth occurs. The conceptuses here are clearly not in synchronous development.

Superfetation has been suspected in cats because of the long breaks that occasionally occur during parturition. These may be several days in duration. However, the age discrepancy between the large and small fetuses in this case was estimated to be about 30 to 35 days i.e. extreme asynchrony, unlikely to result in normal birth of the younger fetuses.

This case remains an enigma but most closely resembles one of superfetation.

Selected references:

Chan, S.Y.W. 1982 Ovarian-endocrine-behavioural function in the domestic cat treated with exogenous gonadotrophins during mid-gestation. J. Reprod Fert. 65:395-399

Ptak, G.E. Embryonic Diapause is conserved across mammals. www.plosone.org Volume 7. Iss. 3  e33027

Roellig, K. et al. 2011 The concept of superfetation: a critical review on a ‘myth’ in mammalian reproduction. Biol. Rev. 86:77–95

Schmidt, P.M. et al 1983. Ovarian activity, circulating hormones and sexual behavior in the cat. II. Relationships during pregnancy, parturition, lactation and the postpartum estrus. Biol. Reprod. 28: 657-671