1) General Zoological Data
There are two species of beaver, the Canadian/American Castor canadensis and Castor fiber, the Eurasian species. "Castor" is the Greek word for beaver (Wilson & Reeder, 1992). Some authors have considered these species to be one and the same, but Lavrov & Orlov (1973) have drawn a distinction between them. They showed that C. fiber possesses 48 rather than the 40 chromosomes of the Canadian beaver. The Canadian/American beaver varies greatly in size from region to region, this being one reason why numerous subspecies have also been assigned. The same is true of the Eurasian species.
The rodent family Castoridae has an uncertain ancestry, a feature that is discussed in some detail by Romer (1966). Thenius & Hofer (1960) also supported the notion that the true ancestry of beavers is still debated and related in their book that beavers had once been of bear-like size, in the Pleistocene.
A detailed review of the animal's biology is found by Rue (1964). It states that the beavers live 12 years in the wild but may attain at least 19 years in captivity. The beavers are said to continue growing throughout their life. It is difficult to determine sex as, externally, the animals are very similar. Radiologically, the baculum can be visualized in adults (Puschmann, 1989).
Beavers weigh up to 45 kg; males are usually as big as females. Animals been found, however, that were much heavier. Additional and specialized anatomical data may be found in Hayssen et al. (1993). The most notable features, of course, are the webbed feet and the extraordinary tail of beavers, both adaptations to their aquatic environment. The aquatic adaptation also makes it more difficult to keep beavers in captivity and thus they are not often seen in zoological parks. Beavers are monogamous and the newborn Canadian beavers weigh between 360 and 700 g according to numerous studies listed by these authors. There are between 2 and 8 offspring, but 3-4 appears to be most common.
General Gestational Data
The length of gestation is estimated to be around 100 days, and was given as being between 90 and 128 days by some authors. Litter size is usually 3-4 pups. Newborns weigh around 500 g. There is one litter per year and the animals have a distinctly seasonal cycle.
General Characterization of the Placenta
Details of fetal/maternal barrier
Fischer's study of the subplacenta is really the most exhaustive and descriptive (1985) of this complex organ. He described it as comprising 1/3 of the total mass and having detached spontaneously from the underlying decidua basalis at birth, and as being traversed by two maternal arteries and a central vein. The arteries are markedly altered by trophoblastic destruction of their arterial walls, similar to what is observed in primates. The subplacenta, he suggested, contains villous structures that much resemble the villi of primate placentas as well. These villi of the subplacenta are depicted below. These villi contain loose connective tissue and thin-walled fetal capillaries, and they are covered by trophoblast. Fischer suggested that this is composed of an inner layer of cytotrophoblast and outer layer of syncytium. This was not always evident in my sections and the "barrier" may require future electronmicroscopy. Remarkably though, around the villi lie a profusion of ill-defined cells. There are maternal red blood cells (sometimes in larger pools), mononuclear cells (with much glycogen) and, Fischer stated, many pigmented cells that fail to give an iron staining reaction. (Note though that he did describe iron-positive reactions in the brown inverted yolk sac epithelium. See further discussion on the nature of the pigment in the chapters on Alpine Ibex and Nilgiri Tahr). Fischer also did not observe red cell phagocytosis. My placenta did not have any pigment at this location but the yolk sac epithelium had macroscopically the brown color of iron pigment. Apparently there is no explanation for the existence of the maternal red cells in the subplacenta and there is also no venous drainage of this region. Nevertheless, despite the lack of draining vessels, stagnation and fibrin deposits have not been seen here. Mossman (1987) urged that the blood supply of this placental region be studied by injection, as it may be partially supplied by the yolk sac vasculature. The "raison d'?tre" for this structure, the subplacenta, is ambiguous and, as Fischer (1985) pointed out, its construction is so different from that of the subplacenta of hystricomorpha, that only the name applied ("subplacenta") is synonymous. Mossman (1987) was also puzzled by this peculiarity and suggested that, otherwise, the placenta is more like that of sciuromorph rodents that the hystricomorphs.
|The labyrinth comprises 2/3 of the placental mass and it is peculiar (Mossman, 1987) in the absence of lobulation. He considered it to be "entirely zona intima", in that it has fetal capillaries in all regions, completely lacking a trophospongium. It is overwhelmingly made up of trophoblast and maternal blood channels with fetal connective tissue and capillaries being very small in quantity. In my placenta, most of the blood contained in this region was of maternal origin. The fetal capillaries contained very little blood and the amount of fetal connective tissue was quite small. Beneath the chorionic surface and labyrinth, as well as at the margin of the placenta, large trophoblast-lined channels exist, shown in several photographs next.|
There have been no studies to give information on the maternal vasculature and physiology of the placental circulation. Indeed, as Mossman (1987) has pointed out, these studies are badly needed to understand the complexity, especially that of the subplacenta. The placenta I was able to examine had no maternal endometrial tissue attached; nevertheless, the uterine arterial branches that run through the subplacenta were modified by invading trophoblast and one may expect it to travel more proximally as well.
Trophoblast external to barrier
No judgment can be made how deeply the trophoblast infiltrates the decidua basalis, as this has not been described in the very few studies of beaver placentas. It is clear, however, that the maternal arteries that course through the subplacenta are infiltrated and modified by trophoblast. Perhaps this extends further into the uterus.
I am unaware that any studies have been done.
Other remarks - What additional Information is needed?
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