Domestic Pig (Sus scrofa domestica)

Order: Artiodactyla

Family: Suidae

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).

Male and female pigs (from pigsong@korea.com).

2) 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).

3) Implantation

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.

4) General Characterization of the Placenta

This is a diffuse epitheliochorial placenta with atrophy at the peripheral tips, a feature that was already known when Turner (1876) gave a detailed description of the pig placenta and uterus. He remarked on the ridge-like pattern of the chorion and on the presence of circular regions that he situated as corresponding to the "areolae" of the uterine mucosa into which glands open. They are shown in good detail by Patten (1947), Amoroso (1961), and by many other authors. Mossman (1987) remarked that the placenta of swine is often much more gelatinous in its appearance than that of other ungulates. This has certainly been my observation in the placentas of wild pig species (see chapters on warthog and red river hog). There is a large allantoic sac that is essential for nutrient transfer from the maternal uterine secretion (Geisert, 1999). Since no invasion occurs, much of the placenta/embryonic development depends on the "uterine milk or embryotroph", endometrial secretions that are dominated by progesterone and estrogen (Roberts & Bazer, 1998). The purple protein "uteroferrin", often confused with non-specific acid and alkaline phosphatases, is one of these important proteins. The acid phosphatase associates with uteroferrin, while the alkaline phosphatase has been localized to the microvasculature of the endometrium and is distinct from uteroferrin (Firth et al., 1986). The trophoblast also has a high content of acid phosphatase; other enzymes interacting between endometrium and trophoblast are considered in the study by Skolek-Winnisch et al. (1985). The importance of the hist(i)otroph is further demonstrated by the protein in the dome-shaped formations over the openings of uterine glands, the areolae, which were studied ultrastructurally by Friess et al. (1981).

An excellent and very detailed account of the pig placenta comes from Amoroso (1961). This very readable presentation also gives adequate reference to all former publications on pig placentation. Among many topics, he addressed the question as to whether the trophoblast and endometrial epithelium are separated by "milk" and find this to be highly unlikely. His photographs also show direct apposition. Binucleated trophoblast is not found, and there is normally no pigmentation.

Implanted pig placenta early in gestation. The interdigitation of villi and endometrium is apparent.

Higher magnification of young implanted pig placenta with endometrial gland and myometrium below.

5) 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.

Endometrial gland at left, villus at right.

The trophoblast in this young placenta is single-layered and cylindrical.

6) Umbilical cord

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.

Immature pig umbilical cord with three vessels, vein on top, allantoic duct at left.

7) Uteroplacental circulation

I am not aware of any detailed description but surely studies must have been done.

8) Extraplacental membranes

There is a large and highly vascularized allantoic sac that is lined by cuboidal to cylindrical epithelium. Much allantoic fluid collects early in gestation, and much work has been done to understand its role for protein transfer to the fetus (Geisert, 1999), especially of the uteroferrin. The allantoic sac is very large in early gestation and, as pregnancy advances, the amnionic sac enlarges more prominently and ultimately contains as much as 200 ml of fluid. Amoroso (1961) also stated that the allanto-amnion becomes vascularized during the course of pregnancy.

Free allanto-amnion with amnion above and vascularized allantoic membrane below.

9) Trophoblast external to barrier

No endometrial invasion by trophoblast occurs.

10) Endometrium

The endometrium has no caruncles and consists mostly of glands whose description during gestation has been detailed by Heuser (1927). Amoroso (1961) further described them in some detail and stated that they secrete iron; "the glandular orifices are covered by domes of trophoblastic cells - the areolae - which absorb the secretion (uterine milk) and transmit it as nutriment for the developing embryo by the allantoic vessels". The endometrium of pigs plays a major role in secreting various nutrients, and transfer is at least partially taking place through the allantoic fluid (Geisert, 1999). Among these nutrients, uteroferrin, an iron-transporting acid phosphatase, is an important aspect of fetal iron supply. It has been studied extensively (see also above) and arises from the areolar region of the endometrium (Dantzer & Nielsen, 1984). The role of endometrial cells was studied in vitro by Davis & Blair (1993) who investigated individual components of the endometrial epithelial cells in their secretory roles of uteroferrin and prostaglandin production. The localization to the non-ciliated endometrial glandular epithelial cells "overlaying the areolae" and the adjacent trophoblast was accomplished by Raub et al. (1985), using immunocolloidal gold-labels. The events of post partum discharge and uterine involution (macroscopic and histologic) were discussed extensively by McEntee (1990).

11) Various features

There is no subplacenta. The pig placenta is especially "gelatinous", but we do not know the reason for this feature, nor do we understand the nature of the content of this gelatinous material.

12) Endocrinology

Austin & Short (1972) wrote: "In this species, the nature of the luteotrophic complex changes during gestation. Early in pregnancy, the corpora lutea are relatively independent of pituitary control and their survival depends on the ability of the conceptus to neutralize the uterine lytic factor. Later, up to about day 40, LH is the predominant luteotrophic factor, but thereafter prolactin becomes increasingly important. Oestrogens also may play a part in the maintenance of the corpus luteum, possibly acting indirectly to stimulate pituitary prolactin secretion; the placenta of the pig produces appreciable amounts of oestrogen in late pregnancy." The changes in hormone production during pregnancy are graphically represented by Geisert (1999). Involution of the corpora lutea during pregnancy is prevented by the conceptus' secretion of PGF2a. Pregnancy is much dependent upon the secretion of on the timely production of estrogens and is readily disrupted by environmental; agents (see Tarleton et al., 2003). Bazer & Roberts (1983) related that pig blastocysts begin estrogen production on day 11 of pregnancy, thus inhibiting the otherwise prostaglandin-induced luteolysis. The glandular epithelium is also responsible for the secretion of a variety of endometrial proteins. Mossman & Duke (1973) described the ovaries of various pigs and found them to be enclosed in a partial bursa, with a wide opening. Fewer corpora lutea (4) were found in wild European pigs than in domestic pigs.

13) Genetics

The domestic pig has 38 chromosomes, but its wild ancestors may have 38 or 36 chromosomes, with fertile hybrids possessing 37 (McFee et al., 1966; Gropp et al., 1969). Robertsonian fusions have generally played an important role in suid speciation (Bosma et al., 1991). Several chromosomal errors have been described and are listed in the pathology section and are also added in the appendices to Hsu & Benirschke (1967). A variety of Sus species have hybridized (Gray, 1972). The gene for uteroferrin secretion has now been localized to chromosome # 2 (Yasue et al., 1995), and it has been further characterized by Vallet & Fahrenkrug (2000).

Karyotypes of male domestic pigs, 2n=38.

Karyotypes of female domestic pigs, 2n=38.

14) Immunology

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.

15) Pathological features

A variety of chromosomal errors have been identified in domestic pigs. In a boar with a statistically lower litter sizes, Locniskar et al. (1976) found a reciprocal translocation (1p-;6q+). Madan et al. (1978) identified a translocation t(6p;15q-) in a boar with "intersex" genitalia (bilateral ovotestes) whose ovarian portions had ovulated. Booth & Polge (1976) reported chimeric pigs. Hancock & Daker (1981) reported on a sterile 39,XXY boar. In pigs with anal atresia, Vogt (1967) found no chromosomal errors in lymphocyte cultures. Henricson & Bäckström (1964) did meiotic studies on normal and subfertile boars, without finding correlations. McFeely (1967) reported on studies of early blastocysts in which he found numerous chromosomal errors, such as triploidy (XXX & XXY), tetraploidy and chimeras. It is possible that some of these abnormal embryos begin implantation but degenerate and leave the spaces in the uterus noted in earlier studies (33% according to Hanley, 1961). Hydrometra, intersexes, and some other reproductive pathologic features were reviewed by McEntee (1990).

Numerous infectious diseases occur in pigs and are discussed in texts of veterinary pathology, e.g. Smith et al. (1972). Thus, brucellosis is more fulminant in this species and may lead to chronic placentitis. Foot-and-mouth disease became a problem in England after infection of a group of pigs in 2001 (Alexandersen et al., 2003). Swine suffer many other diseases such as leg weakness and, importantly, an infection with the reproductive and respiratory syndrome virus (Yaeger, 2002).

16) Physiologic data

Some physiologic data are discussed by Asdell (1946), including the migration of eggs in the uterus which is also discussed by Geisert (1999). As has already been alluded to above, much work has been undertaken to clarify the role of uteroferrin (UF) in iron transport to the fetus' hematopoietic organs. A quantitative relation exists of UF and hematopoiesis (Ducsay et al., 1986). Iron treatment during pregnancy had no significant effect on neonatal hematologic values (Ducsay et al., 1984). These investigations have been extended to encompass neonatal pigs by injection of uteroferrin and these identified its role in stimulating hematopoiesis (Laurenz et al., 1997; Bazer et al., 1991). In utero measurements by Vallet et al. (1996) indicated a relationship to protection against lipid peroxidation of various components of this transfer system. Hematopoietic binding sites were investigated by Michel et al. (1992) who found them to be invariably present and perhaps unrelated to neonatal anemia. Doi et al. (1986) concluded that direct iron transfer to apotransferrin is an unlikely physiological role of UF.

17) Other resources

There is a huge literature on domestic pigs. Cells are stored in a variety of personal cell banks and cells of domestic pigs and pot-bellied pigs are also available from the cell bank at the San Diego Zoo's "CRES", by contacting Dr. Oliver Ryder at oryder@ucsd.edu.

18) Other remarks - What additional Information is needed?

Domestic pigs have first been cloned in 2000. Since "absorption" of fetuses is relatively common in domestic pigs (up to 30% or perhaps even more), it would be of interest to know whether these "resorbed" fetuses have similar chromosomal errors as those that have been described for blastocysts by McFeely (1967). This is of special interest because a variety of chromosomal errors have also been identified in adult domestic pigs.- There is a deficiency in our knowledge of the length of pig umbilical cords.

Acknowledgement

The photograph of the sow and boar at the beginning of this chapter comes from the internet: pigsong@korea.com