1) General zoological data of species
Domestic cats are abundant and many genetic varieties ("breeds") exist, with color, hair and other differences. There are numerous breeding colonies as well as cat fancier clubs and Societies. The ultimate origin of cats has been studied extensively and is summarized by Nowak & Paradiso (1983). The domestic cat is believed to have derived from Felis (silvestris) lybica (Starck, 1995).
Detailed phylogenetic considerations have been published by Thenius (1967), and Collier & O'Brien considered the phylogeny of felidae from investigating their immunological distances. Robinson (1977) has reviewed the general genetics of domestic cats and their great variation in the standard text on cat genetics.
General gestational data
|Average female domestic "calico" or tortoiseshell cat.|
In the domestic cat, implantation occurs on days 12-14 post coitum and the trophoblast is superficially invasive (Leiser, 1979). Numerous publications have described early implantation. They were summarized by Denker et al. (1978). These authors suggested that, as in other species, there is dissolution (not rupture) of the zona pellucida and that proteinases are responsible for implantation. The blastocyst is much expanded before implantation and is oval. The embryonic disk attaches antimesometrially. There is an early yolk sac placentation that is soon replaced by the large allantoic placenta. The allantoic sac fuses with amnion and thus, this thin allantoic membrane possesses blood vessels. Vessels are classically lacking from the amnionic membrane. A girdle-like trophoblastic invasion occurs to produce the final placental shape. There is early invasion of the endometrial glandular mouths by trophoblast. This is particularly well illustrated by Kehrer (1973). He also illustrated the growth of the "lamellar" tongues of placental tissue in relation to the uterine axis. Later, the endometrial glands distend with secretion and contain degenerating epithelium. Adjacent to the placental girdle small hematomas of maternal blood develop that are, however, much smaller in cats than found in dog placentas. The portions of placenta lateral to the girdle are referred to as "paraplacenta". At this site, the trophoblast adheres to the uterine epithelium, thus making this area to have a truly epithelio-chorial relation. Rosettes, or chorionic vesicles, found here are numerous. Malassiné (1977) studied the erythrocytophagocytosis of maternal red blood cells by trophoblast at this site and suggested that this is an important site of iron absorption. This is, of course, the green/brown border of the zonary placental tissue. Whether much other transport occurs across this region is uncertain.
|General situs of the cat placenta: Maternal tissue - red; chorion - blue; amnion - purple; allantois - yellow; vitelline tissue - green; white triangle is exocoelom (adopted from Tiedemann, 1979).|
4) General characteristics of placenta
The cat has a zonary placenta without cotyledons and it has a relatively small marginal hematoma, more so than the tiger but less than the dog. The materno-fetal barrier is endothelial-chorial, and it is superficially invasive into the endometrium but not into the myometrium.
|Complete cross section of uterus with implanted cat placenta.|
5) Details of barrier structure
The cat has an endothelial-chorial placental barrier. This was confirmed by electronmicroscopy. A very detailed description comes from Malassiné (1974) that shows the diminution of maternal endothelium towards the end of gestation. The labyrinth has very straight trophoblastic lamellae that cover the fetal mesenchyme, which contains the fetal blood vessels and some connective tissue cells. The trophoblastic surface is syncytial, beneath which are some cytotrophoblastic cells. They rest on a basement membrane above the fetal connective tissue. The syncytium directly abuts the maternal capillary endothelium, making this an endothelial-chorial barrier. Only Zhemkova (1962) has considered this to be a hemochorial placenta by virtue of her cytologic investigations. It employed Barr body identification of cells at the border zone in male conceptuses. Wislocki (1920) injected colloidal dyes and found them to be taken up by the maternal endothelium; the dyes did not to cross the placenta. He also affirmed the endothelio-chorial relation. Moreover, electronmicroscopy by Wynn and Björkman, 1968, Malassiné, 1974, and others have shown there to be a maternal endothelial layer. The transport of nutrients and other substances through this seemingly simple barrier is controversial. It is especially unclear what the origin is of the basement membrane material is, and what the nature of the debris is that is found here. These unresolved aspects have been studied by Björkman (1973), and demonstrated in his electronmicrographs, but they have not found a final resolution.
The feline placental labryrinth has characteristic giant cells. These are not found in most other endotheliochorial placentas. Dempsy & Wislocki (1956) considered these cells to have derived from endometrial connective tissue cells, and Malassiné (1974) showed them to decrease towards term. They have a pale cytoplasm and large, single nuclei. Wynn & Björkman (1968) made the point that the labyrinth of the cat has also a substantial amount of acellular material, perhaps an "intermediate layer" resembling a basement membrane. They drew the analogy of this material to the "fibrinoid" in human placentas. The cells were investigated in some detail by Malassiné (174) and considered again to represent decidual giant cells. He did not ascribe great functional importance to these cells.
Mossman (1987) asserted that the cytotrophoblastic cells are laden with debris, which originates from maternal erythrocytes, but Malassiné (1977) who studied the erythrocytophagocytosis in greater detail, found this to occur principally at the periphery of the paraplacenta.
|Floor of placenta attached to uterus showing giant cell border (top left), large maternal blood vessel and trabecular villous structure.|
|Higher power photograph of fetal surface of cat placenta showing the trabecular nature with dark trophoblastic cells lining maternal blood channels and delicate fetal connective tissue.|
|Lower power of the same cat's placental disk with uterus below. The endometrium has some cystic glands, next to the trophoblastic giant cells. Note the absence of trophoblast invasion in the endo/myometrium.|
|The dark nuclei are trophoblast; in between these two layers flows the maternal blood. The connective tissue (top left and in between the rows of trophoblast) carries fetal capillaries.|
|High power view of placental trabeculae. The delicate connective tissue is the fetal portion. The dark cells are the trophoblast with the light purple being the cytoplasm of trophoblast; in between flows the maternal blood. A mitosis is seen in the cytotrophoblast near the top middle of the picture.|
|"Floor" of the cat placenta with myometrium at right and very little endometrium. The border between villous tissue (left half) and uterus is a layer of giant cells, many with degenerative changes, which I believe to be trophoblast.|
|Another border between myometrium (bottom) and placenta (top). Distended endometrial glands are next to the "invading" trophoblast.|
|The dark streak in the center is where maternal blood is next to the trophoblastic surfaces. Fetal capillary and connective tissue above, with nucleated fetal red cell.|
|Another high power view of trophoblastic columns. The purple centers are maternal blood spaces. Fetal connective tissue with capillaries and a nucleated red cell are obvious.|
6) Umbilical cord
There are two pairs of vessels in the umbilical cord, two arteries and two veins. In addition there is an allantoic duct. The average length of the umbilical cord is 2-3 cm and 0.3-0.5 cm in diameter. It inserts at the margin of the zonary organ. There are no spirals, no vitelline duct, and no additional vessels or structures such as caruncles.
7) Uteroplacental circulation
No data are known to us.
8) Extraplacental membranes
The amnion has a very thin epithelium and no vasculature. It is attached to the chorion and, in some areas, to the large allantoic sac, which has an extensive vasculature, and a relatively thick epithelium that becomes thinner with advancing gestation. Tiedemann (1979) made the most detailed study of the epithelia of the three extraplacental saccular structures. He described, using fine-structural tools, that the amnionic epithelium degenerates completely by day 54 of gestation and thus the amnionic cavity becomes lined by connective tissue, an exceptional circumstance. The exocoelomic epithelium, he believed, to be responsible for the many macrophages in the membranes. The yolk sac epithelium was considered to be the most active epithelium of the membranes. In the same contribution he described the composition of the various fetal fluids. This author also showed that the various structures have often been mistaken one for another and discussed the difficulty one has with their individual identification. The schematic presentation above was drawn following the diagram he presented.
9) Trophoblast external to barrier
Since nidation is quite superficial, there are no invasive trophoblastic cells in the myometrium.
|Two-headed neonatal cat.|
In addition, numerous disorders of the reproductive system, such as hydrometra, endometrial hyperplasia, alterations due to hormones, etc. have been described in this well-studied species. They are ably summarized by McEntee (1990).
A litter of American Shorthairs that I observed delivered 4 days prematurely, with three pups, one of which was stillborn and meconium-stained. It was still enclosed in its sac when I received the specimen. The placenta of the stillborn was smaller (12 g) than that of the two survivors (18 and 21 g). It also had a zone that was much narrower than the usually quite uniform width of the ring-shaped placenta. Histologically, some areas of this placenta had massive distension of the maternal blood channels; making these areas appear to be hemorrhagic. Bordering these areas of "hemorrhage" was decidual necrosis with focal thrombosis of maternal blood vessels and infiltration with polymorphonuclear leukocytes. Organisms were absent and the fetal surface was not inflamed. The fetal ovaries were replete with oocytes, thus a chromosomal error appears to be an unlikely cause of fetal death. This is inferred, as most human neonates with trisomies or other major rearrangements have no mature oocytes. The dissection of the fetus also showed no abnormalities, other than growth retardation. At the peripheral end of this placenta the sac tapered off, suggesting that this was the peripheral-most implantation. Those placentas are shown next.
|This is the maternal aspect of the placenta from one of the normal fetuses.|
|Stillborn fetus "en caul", head is on the right.|
|A short cord attached the dead fetus to the placenta. The allantoic sac is visible beneath the amnion.|
|Fetal surface of stillborn's placenta with amnionic sac at left.|
|Maternal aspect of the stillborn fetus' placenta. Note the hemorrhagic-appearing villous tissue and the irregular width. The membrane that extends at the top is amnion, not the umbilical cord.|
|Zonary placenta of stillborn with former head region at right.|
|This is the maternal aspect of a normal cat placenta.|
|Histologic appearance of the stillborn fetus' placenta with massive distension of the maternal blood channels.|
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18) Other findings of interest
|Tortoiseshell female cat with midline division of orange and black due to "lyonization" of melanocytes that carry the color genes. (Courtesy Dr. T.C. Jones).|
|Tricolored male cat with 39, XXY chromosome constitution.|
|Tricolored male cat with XY/XX chimerism|
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