3) Implantation

Kirk's dik-dik placentation occurs usually in the right uterine horn. It develops a polycotyledonary, epitheliochorial placenta that is not invasive. Very few implanted placentas have been described, however, and more information on the site of attachment is needed. The best observations were made by Wislocki (1941). He had received an alcohol-fixed pregnant uterus from a hunter and described it in great detail. Unlike any other ruminant, his observations of a cotyledonary arrangement on "a stalk" are surely erroneous, however, despite the pictures that accompany the paper. It is my assumption that the stalks shown by him were produced by the alcohol fixation of the specimen. Alcohol tends to contract the muscle (myometrium) much more than the softer tissues and thus a stalk may have been produced artefactually. Besides, Strahl (1911) had earlier seen a complete, pregnant uterus and did not find this most unusual arrangement. I, therefore, believe that the stalks are a fixation artifact. Wislocki admits that his specimen suffered from some post mortem change. In addition, he counted "about 80 cotyledons", but depicts far less, more like the numbers here presented, around 40-50. The fixation must also be the reason why his cotyledons had such pronounced convex shape, when I and others found them to be flat in dik-diks.

4) General Characterization of the Placenta

The placenta of dik-diks is polycotyledonary and epithelio-chorial. I have been able to examine only this one term placenta in some detail. It was a delivered placenta and, thus, it did not have any maternal tissue in between the villi. The placenta had been peeled away from the caruncles at delivery in the usual manner. No maternal tissue accompanied it. It is here depicted and had 41 cotyledons, weighed 47 g, and measured 44x11x0.1 cm. The cotyledons varied from 2x0.1 to 1x0.1 cm in dimensions. The placenta mentioned in Wislocki's report was described to have "about 80 cotyledons" (1941). The cord inserts in the center.

7) Uteroplacental circulation

I know of no physiologic studies of the placentation, other than the microscopic observations of blood supply to the endometrial caruncles and the remarks by Wislocki (1941) that endometrial trabeculae and fetal villi had a rich, similar vascularization. He also mentioned that fetal villous capillaries often indent the trophoblast.

8) Extraplacental membranes

In Wislocki's description (1941) of a complete uterus, the allantoic sac nearly filled the left uterine horn, while the fetus was in the right side, within its amnion (1941). No metaplastic nodules or squamous metaplasia were present on this or in Wislocki's specimen. In the term placenta I examined, there had been meconium staining. As a result, the amnion is markedly degenerated, as is the case in human placentas following meconium discharge. No pigmented macrophages were present within the membranes, although meconium was present on the amnionic surface. The amnion is avascular; numerous blood vessels are in the allantoic membrane and allantochorion. The outer surface of membranes, the portion extending between the cotyledons, is covered with tall, columnar trophoblast. Beneath this, mostly over what must have been areolae in situ, is much cellular debris, as is also mentioned by earlier observers. Maternal blood extravasation was not present, a feature described by Strahl (1911) but not found by Wislocki (1941). There were no hippomanes.

10) Endometrium

The endometrium of ungulates undergoes considerable changes during gestation, although it does not acquire the decidual transformation of primates, not is the placenta detached in a decidual plane. Rather, the villi are peeled out of their caruncular crypts. It is true, however, as remarked by Wislocki (1941) that the endometrial glandular surface of the cisternae into which the villi project, is covered by a peculiar epithelium. While this could not be observed, of course, in this delivered placenta, it is the case in other ruminant specimens I have had (see slender-horned gazelle, for instance). The epithelium is flat, occasionally bunched, and Wislocki considered it to be possibly syncytial. Moreover, because of the apparent absence of some epithelium in the H&E preparations, he stated that one should perhaps consider this placenta as being syndesmochorial, rather than epithelio-chorial. Until electron microscopic study is undertaken, I would not favor such an assignment. At the base of the cotyledons, the endometrial stroma is dense, has large nuclei and occasional giant cells. But, these are not trophoblastic and their nature and origin are unknown. This is discussed extensively by Wislocki (1941). Despite the debris that also exists in this region, I hesitate to refer to this as a "subplacenta', as exists in carnivora.

11) Various features

I do not deem the changes just described that occur at the base of cotyledons sufficiently structured to speak of a "subplacenta".

12) Endocrinology

I know of no endocrine studies in this species. Because of the binucleate cells, one must assume that the placenta makes relaxin.

13) Genetics

The dik-diks have complex chromosomal arrangements. Animals with 2n=46 to 2n=48 seem to be the usual, but an animals with 2n=49 has existed as well. Some possess 47 chromosomes with X/A translocation (Benirschke & Kumamoto, 1987; Kumamoto et al., 1994; 1995). The two common "cytotypes" (46 and 48 chromosomes) are sufficiently different that hybrids are sterile (Kingswood & Kumamoto, 1997). Many zoos are now known to harbor hybrids between different cytotypes, with anomalous chromosome numbers and causing unexplained sterility. Moreover, examination of the testes of such hybrids has shown subfertility or lack of spermatogenesis. In addition, hybrids between Kirk's and Guenther's dik-dik exist and these are also infertile (Ryder et al., 1989). Albinism has been described in the past (Roosevelt & Heller (1915).

15) Pathological features

Numerous ectoparasites occur, and pneumonia has resulted from cold exposure. Cysticercosis infection was reported to occur in the Serengeti Park by Sachs & Sachs, 1968. Few other diseases are listed by Kingswood & Kumamoto (1997).

16) Physiologic data

Numerous studies have been done in dik-diks that concern their water metabolism, heat regulation and other aspects that explain their ability to withstand the severe climatic conditions to which they are exposed. These are accessible though the review by Maloy et al. (1988).

17) Other resources

Cell lines of many dik-dik species are available from CRES at the San Diego Zoo by contacting Dr. Oliver Ryder at: oryder@ucsd.edu.

18) Other remarks - What additional Information is needed?

More data on placental weight and implanted placentas are needed. Importantly, are the cotyledons on stalks or not?

Acknowledgement

I appreciate very much the help of the pathologists at the San Diego Zoo.

References

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