Conjoined feline twins
Keywords: feline, conjoined, fetus, dystocia, radiographThis image is provide courtesy of Dr Ruth Massey (masseyruthellen@gmail.com, holder of copyright). It shows two conjoined feline fetuses and a radiograph of that specimen. Although remarkable, it is far from unique in the subject of fetal malformations. In fact, conjoined fetuses are quite common in humans; about 10 in every million births. Conjoined fetuses have been seen in all of the domestic species. Indeed, one of the references given here shows that it has been formally documented in cats as well. In birds, especially ducks, conjoined hatchlings are reported with some frequency; the same is true of fish.
In the case of the cat reported by Camon et al, the degree of fusion and exact levels of anatomical compromise were described accurately. In this case, the specimen was collected during an emergency cesarean section and there was little opportunity to examine it in detail.
The kittens died either during or shortly after delivery.
Although one of the twins were originally reported as being male and the other female, it should be appreciated that gender determination is often challenging in newborn kittens, even in normal individuals. The ano-genital distance is often used for this determination, that of the male being greater than the female. In a malformed fetus, this measurement may have been probably unreliable. Attention is drawn to this finding because a case of conjoined fetuses of different genders has never been reliably documented; indeed, it is highly unlikely to occur. This case then, is used as basis for a brief summary on the nomenclature and pathogenesis of conjoined twins.
First, it is common to use the “pagus” (fr. Gk pagos meaning “fixed”) together with a particular body part to describe where the fetuses are conjoined. In this case for example, both the thorax and heads are joined, therefore this specimen is a cranio-thoracopagus conjoined fetus. Interestingly, if there are two heads (often facing in the opposite direction) the specimen is referred as a “Janisceps” fetus. This term is more correctly spelled “Janusceps” because it refers to the Roman god Janus, who had two faces, one facing towards the future and the other backward to the past.
Most readers will be familiar with the fact that the incidence of fraternal (non-identical) twins is related to the incidence of multiple ovulations, not split embryos. Spontaneous multiple ovulations are well known to have a heritable basis but this is not generally the case for identical twins (or triplets), certainly not in mammals. In rare cases of inbreeding (in Burmese cats for example) it may have a genetic basis but generally speaking, it is not known why an early would split partially or completely into two or more identical embryos. However, using teratogens such as vincristine, urethane, nitrofurans, ethanol, thalidomide and DMSO, conjoined twins have been experimentally produced in small mammals. In fact, even variations in temperature and alterations in embryo culture have been used to induce conjoined twins. In spontaneous cases, the cause is seldom known.
It should be appreciated that conjoined twins only occur when splitting of the embryo occurs at a relatively late stage, i.e. when the primitive streak is forming. This too, is when the early amnion and chorion have already formed. Therefore, conjoined embryos usually have a shared single amnion and a shared single chorion. This situation only accounts for a very small proportion of twin embryos. The majority of twin embryos are formed when the embryo splits earlier than this. In humans for example, it is known that about 30 percent of identical twins are formed in the first few days of life. In that case, two completely separate entities develop, with separate amnions and separate chorions. These are known as dichorionic-diamnionic twins. However, close to 70 percent of identical twins form a little later, when a single chorion has already formed and the inner cell mass then splits into twin embryos, each forming its own amnion. These individuals become monochorionic- diamnionic-, identical twins. As mentioned, only the small remaining portion of identical twins are monochorionic-monoamnionic. If these proportions hold true for domestic animals it may help to explain why conjoined twins are such a rare occurrence.
Interestingly and intuitively, both normal and conjoined monoamnionic-monochorionic identical twins are mirror images of one another; in humans, their finger prints are mirror images of one another. As expected, situs inversus occurs in one of the twins as well. Situs inversus was not reported by Camon et al and was not examined in the case presented here. It would be interesting to investigate this in other cases of conjoined twins seen in domestic animals. In humans, it is also known that conjoined twins are significantly more common in female than male twins but this has not been documented in other animals, perhaps because of the rarity of the condition.
References:
M. H. Kaufman M. H. 2004The embryology of conjoined twins Childs Nerv Syst 20:508–525
Camon, J. et al 1992. Morphology of a dicephalic cat. Anat Embryol 185:45-55
