Sus scrofa domestica
1) General Zoological Data
Suidae had their origin in Eurasia and distributed widely while speciating into significantly different phenotypes and behavioral repertoires. The Suiformes are presumed to be the most primitive of the artiodactyls. The Artiodactyla are considered to have formed three major groups: the Suiformes, Tylopoda and Pecora (Groves, 1981). Warthogs are believed to be perhaps the most recent derivatives of suidae. Bosma (1978) reviewed the reasons for placing Phacochoerus away from the other suidae because of their differential dental structures. Nevertheless, the similarity of chromosomes (v.i.) argues against this notion. She suggested a recent origin of this species. On the other hand, in his searching discussion of suid development, Thenius (1970) found justification of separating this most recent suid species; he placed the first ancestor into the middle of the Oligocene (+/- 28 MYA).
There are a very large number of special strains of domestic pigs. Some of the better-known varieties being the pot-bellied pig, miniature pig, etc. Males are generally larger. The longevity of various Sus species in captivity was given by Jones (1993). Maximal life span for a European wild pig was 21 years. Domestic pigs were not included in that survey. It is known, however, that external influences (leg weakness Jorgensen, 2000), age of first conception (Koketsu et al., 1999) and other factors affect longevity and productivity of swine and these may have important commercial ramifications (D'Allaire et al., 1992).
General Gestational Data
Hayssen et al. (1993) stated that "..the reproductive biology of domestic pigs is reviewed in supplements 33 and 40 of the Journal of Reproduction and Fertility published in 1985 and 1990 respectively". Pigs have a 21 day cycle with a period of heat 2-3 days long. Ovulation is spontaneous and pregnancy lasts 112 - 115 days (average 114 days). Domestic pigs have 3-16 young, much depending on the race (Asdell, 1946). Wild European pigs have on average 4 offspring. A much more comprehensive review of the reproductive physiology is provided by Geisert (1999). Ovulation and fertilization occur on the first day of estrus; blastocyst hatching occurs 6 days later; two days thereafter the blastocysts reach the uterus and elongate with initial adhesion of the trophoblast occurring in the endometrium 12 days after fertilization. On day 14, the allantois develops, and placental development commences 17 days after fertilization. Placental fusion of adjacent placentas is "very rare". But, when it happens, freemartins (blood chimerae) may develop if the litters are of opposite sex according to Hughes (1929). A high percentage of embryos are lost at different stages of gestation, estimated to be at least 30%. A detailed review of reproduction and embryonic development, as well as placentation may be found in the classic book on the pig by Patten (1947).
Initial implantation is accomplished primarily by the yolk sac epithelium, which is then the dominant membrane, but it lasts only a short time. There is much uterine "milk" with absorption through this epithelium. By day 24 of the domestic pig pregnancy, the allantois attaches all around the periphery and the yolk sac shrinks (Ramsey, 1982, Mossman, 1987). Additional details of early phases of domestic pig reproduction are available from the review by Geisert (1999). The uterus is bicornuate, and multiple implantations occur in all pigs, first at a mesometrial site. The cord locates mesometrially.
General Characterization of the Placenta
Details of fetal/maternal barrier
The placenta of all suidae is a typical diffuse epitheliochorial organ without invasion of the endometrium (Amoroso, 1961; Mossman1987; Macdonald & Bosma, 1985). The latter authors have compared a number of suid placentas and pregnant uteri, including those of the warthog. They described the chorion as being rippled and having indentations of trophoblast by capillaries. The single-layered trophoblast interdigitates with endometrial epithelial folds. Mossman (1987), and Macdonald & Bosma (1985) both described the arcades and areolae and the importance of uteroferrin transfer to the fetus. This progesterone-induced glycoprotein ("pig purple acid phosphatase" - Twitchett et al., 2002; Klabunde et al., 1995; Nuttleman & Roberts, 1990) from the endometrium has been studied extensively (Renegar et al., 1982) and is apparently transferred to the fetus through the allantoic fluid (Buhi et al., 1983; Geisert, 1999; further discussion below). Administration of iron and tetrahydrofolate had no effect on fetal iron concentration (Vallet et al., 2001). Binucleated cells as known from ungulates do not occur. Ludwig (1968) compared numerous species' interaction of trophoblast-endometrial surface relations and grouped the Suina with the placentas that have an "enteroid" function.
The umbilical cord contains three blood vessels and a widely patent allantoic duct. In this immature specimen, few smaller allantoic vessels are found in the cord; later in gestation more are formed. Also, because of the immaturity of this specimen, there is no squamous metaplasia which, again, develops later. I cannot find any detailed reference in the literature on the length of pig umbilical cords.
I am not aware of any detailed description but surely studies must have been done.
Trophoblast external to barrier
No endometrial invasion by trophoblast occurs.
Immunological studies have been mostly related to infectious diseases occurring in swine or of diseases that are thought to be transmitted through them. Recognition has also been of importance because of the transplants of pig cardiac valves. Thus, foot-and-mouth disease has been identified from swine in abattoirs in England (Alexandersen et al., 2003). Immunohistochemistry is being employed for porcine reproductive and respiratory syndrome virus disease (Yaeger, 2002), and this disease has been experimentally studied, among others by Shibata et al. (2003). These papers will give access to numerous other immunological studies.
Other remarks - What additional Information is needed?
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