General Gestational Data
Twinning is not infrequent in cattle (0-5%) and the distribution over a variety of races has been summarized by Rutledge (1975). It results in smaller calves that are about 5 days premature and that have a much higher mortality (Cady & v. Vleck, 1978). Twin-bearing also increases with age and may thus be related to the known effects of maternal age and race, namely of elevated FSH/LH levels that have been defined in humans. It is thus also not surprising that twins and triplets can be obtained by the use of PMSG and hCG (Mauléon et al., 1970). Komisarek & Dorynek (2002) made an initial search for genes that may influence various twinning rates. They indicated that beef cattle have twins in 1% of their offspring while dairy breeds have 4-5%. It is an undesirable feature primarily because of the perinatal mortality but also because of the possibility of freemartinism (see section on endocrinology). Acardiac twins have also been reported in cattle twins (see section on pathology). Almost all twins are dizygotic (fraternal); monozygotic twinning appears to be rare in cows.
The cow has a 21 day estrus cycle with a 280 day length of gestation (270-285, depending on breed), much like humans. Singletons are commonest, twins occur in 1-5% and depending on breed; triplets are even rarer, and quadruplets are exceptional. The weight of cows and newborns varies very widely and depends largely on breed and nutritional state. Placentas weigh around 4-5 kg. A complete review of all aspects of cattle reproduction may be found in the chapter by Fogwell (1998).
Wild (1964) described the sequence of implantation thus: After ovulation and fertilization the 2-cell stage is attained within 46-48 hours, the 4-cell stage in 56 hours, 8-cell stage in 72 hours and, after 96 hours, the morula enters the endometrial cavity, containing 16-32 cells. First cavitation occurs after 8-9 days with loss of zona pellucida. For the first 14 days the morula lengthens to become tubular. Implantation is preceded by the development of tiny papillary trophoblastic projections and occurs on day 19-20 post conception. A beautiful fine-structural study on the early interactions of trophoblast and endometrium was published by King et al. (1982). It shows the microvillous surfaces and early trophoblast penetration into the uterine epithelium. They found no evidence for transuterine migration in cows and indicated that, because of one's ability to flush and transfer blastocysts until day 16, that irreversible attachment does not occur prior to that day. Close attachment was found on day 19, beginning near the embryonic tissues but then spreading over the entire chorion, including the intercotyledonary regions. The maternal epithelium often becomes much thinned and may be absent in some regions, as degenerative products are also observed in the trophoblast. In text figure 2 these authors presented a schematic time table of the implantation processes, including the time of appearance of hormones (from days 10 to 32) that should be consulted for details.
4) General Characterization of the Placenta
The cow has a typical ungulate polycotyledonary placenta. The often-used term placentome defines the combination of cotyledon and maternal caruncle - the placental unit if you wish. It is very variable in size, shape, thickness, histologic composition and other features. Nevertheless, it is the principal unit and comparative analysis of that unit among bovines, let alone other species, is considered to be the important aspect of adjudicating relationships and adequacy for fetal growth. Mossman (1987) considered the bovine cotyledon to be "concave". The placenta usually intrudes into the other horn but has fewer cotyledons at that site.
There is a very large number of publications on the bovine placenta that have been reviewed by Mossman (1987). The controversial aspects to Mossman were the question of epithelio-chorial (alone) or (combined with) syndesmochorial placentation, and the origin of the binucleate cells. Similarly, Amoroso (1961), who cited all the evidence for or against the maintenance of the endometrial surface epithelium. He suggested, for instance, that most of the intercotyledonary endometrial epithelium disappears (except for the near-glandular regions (areolae). Also, that the epithelium in between the maternal fibrous (endometrial/caruncular) septa is replaced by trophoblast. But, there is also much controversy as he cites abundantly. Wooding & Flint (1994) described the relationship as a "synepitheliochorial" one because of their distaste for the term syndesmochorial and their belief that the binucleate cell becomes fused with endometrial epithelium (more about that below). They were shown to be migrating towards the maternal tissue. Their chapter has a very detailed consideration of this aspect with much fine-structural support.
The macroscopic aspects of cow placentas have been superbly studied by Wild (1964). That study summarized results of 308 cow placentas, reviewed the literature and concerned itself especially with the possibility that abnormal placentation can be a cause of neonatal infectious problems. Wild concludes that the average weight of cow placentas' chorions is 4,501 g; amnions weighed 590 g, the allantois 699 g. He then measured the surface of all cotyledons with a median of 4,474 cm2 - from 4,424 to 4794 cm2. There was a significant correlation of small surfaces with neonatal pneumonia and diarrhea. There is a remarkable variation in the number cotyledons, in cow placentas from 80 to 120. Indeed, Wild quotes authors who found only very few (3), large cotyledons in healthy offspring, others have stated that as many as 150 may occur. The number of cotyledons is generally determined by the number of maternal caruncles in the bicornuate uterus. These occur primarily in four rows through the uterine body, perhaps lower at the very peripheries. Similarly, the size of the cotyledons varies greatly, also being smaller at the periphery. Their greatest diameter was given as 16 cm. But also, on rare occasion, a bovine placenta is described as being "diffuse" rather than cotyledonary.
Testard & du Buisson (1970) also studied a large number of bovine placentomes of singletons and twin gestations. In singletons, there were generally fewer cotyledons in the non-occupied uterine horn than in the pregnant horn (34 to 73 +/-) but there was great variation in numbers. Laven & Peters (2001) made a more recent morphometric study of bovine placentas that generally corroborates former descriptions. Another recent study is that by Schlafer et al. (2001) that concentrates on the development of the placental macrophages.
Most studies on bovine placentas have been directed to the details of the fine structure of the cotyledons. Most authors agree that the bovine placenta is primarily an epithelio-chorial organ with possibly small regions of a syndesmochorial character. Baur (1972) compared the overall villous surface of the cow placenta with human placentas and found that it gradually rises to term (falling in late gestation slightly) but that it is roughly 10 times larger than that of the human placenta (commensurate with fetal weight - notwithstanding possible deductions from the Grosser classification). Hradecky et al. (1988a) measured the length of villi in different artiodactyl cotyledons and found those of cows to be 17 mm with extensive branching. The villi became more compact with advancing age and were covered with single-layered epithelium. Their figures also show clearly that the crypts are lined with flat to cuboidal epithelium.
Schmidt et al. (2006) have recently studied the placenta of Syncerus in detail and compared it with that of cattle. The difference in the construction of the cotyledons is very apparent from their study.
Details of fetal/maternal barrier
Villi are covered with a single layer of cuboidal trophoblast that are more cylindrical under the chorionic surface. The striking and oft-discussed feature is the large number (15-20%) of binucleate cells, definitely trophoblast in origin but with changing fates. Trophoblastic binucleate cells formed from trophoblast explants on type I collagen substrate (not film!) and produced placental lactogen (Nakano et al., 2002). While prior in vitro studies had shown binucleate cell production, this is the first study showing lactogen production in vitro. That these cells fuse with endometrial epithelium ("synepitheliochorial") has been demonstrated by Wooding & Flint (1994) in decisive electron microscopic studies. There is no doubt now that these cells "migrate" (move) toward the maternal tissues and release placental lactogen and probably some progesterone. They also degenerate and are (focally) replaced by endometrial epithelium. It seems to me useless to have a disagreement over whether this placental type should be called an epitheliochorial, synepitheliochorial, or syndesmochorial organ. Various relationships may apply at one time or another and may then have a functional impact or not. What seems to me more important is the finding that such multinucleate cells (they often fuse with endometrial cells and become trinucleate syncytia) appear to have the propensity of infiltrative movement (Wooding & Flint, 1994). They do so in a variety of quite different placentations, as was pointed out by the authors. I should point out though that this demonstration required excellent electron microscopy for detection. Wooding et al. (1997) emphasized that the possession of binucleate cells is one of the most characteristic shared features of artiodactyl placentation, 15-20% of the epithelium being binucleate cells. Here and elsewhere did they demonstrate the fusion with caruncular epithelium to make "hybrid" trinucleate cells and syncytial plaques, mostly near the apex of the crypts, as they called them. The fact that the granular content of the binucleate cell cytoplasm "streams" into the maternally-derived cytoplasm of the trinucleate cell, suggested to the authors that this brings the placental lactogen (of the binucleate cell) closer to the maternal circulation where it needs to be effective. Aspects on placental lactogen are more fully discussed in the chapter on Sheep placentation.
Recent evidence from cloning the gene of bovine placental heparanase and hybridization studies, Kizaki et al. (2003) have shown convincingly that this cell produces heparanase during early and late bovine gestation. These authors suggest that heparanase "may take migratory roles in placentogenesis for degrading the extracellular matrix".
|Binucleate cells often possess a strikingly large nucleus. Its DNA content has now been studied using Feulgen staining and microspectrophotometry (Klisch et al., 1999). The nuclei were found to be tetraploid, and polyploidization was found to be generally common in touch preparations of bovine trophoblast. The authors speculated on the mechanism of this polyploidization and the possible benefits - increase in gene copies for hormone production.|
|The nature and origin of the trophoblastic pigment beneath the chorionic plate are not fully understood and this aspect has been discussed in several other chapters on artiodactyl placentation. This is the so-called "hematophagous" (or haemophagous) organ and it has been speculated that it derives from focal subchorionic hemorrhages that initiate at the tips of maternal septa by degeneration and that the iron is useful to the fetus. That does not explain the absence of iron in special stains and, most commonly, there is no hemorrhage to be found nearby. In the subsequent photographs, however, foci of hemorrhage are adjacent to the pigmented trophoblast in a cow placenta. Such hemorrhages have also been shown in sheep and some other species.|
The umbilical cord has 4 large blood vessels, two arteries and two veins, in addition to a moderate number of smaller allantoic blood vessels. It also contains an allantoic duct connecting bladder via urachus with the allantoic sac. There are many squamous plaques on the surface of cattle umbilical cords.
umbilical artery is a common abnormality of human placental development
(Benirschke & Bourne, 1960) and it is often accompanied by a wide variety
of fetal anomalies. Such a case was described in a neonatal Holstein bull
calf by Stein & Martin (1985). Aside from some vascular anomalies with
this single umbilical artery, there were rectal atresia, rectourethral fistula,
unilateral anorchia and renal abnormalities.
Trophoblast external to barrier
There is no infiltration of the maternal tissues unless one considers the fusion with focal migration of the binucleate cells in the epithelium of the clefts as invasion; I do not.
There is no subplacenta and no other special features exist.
Hill et al. (2002) showed that the trophoblast of cloned cow embryonic placentas express frequently a high degree of MHC-I expression (in contrast to normal implantations) and that this is accompanied by a strong lymphocytic (T-cell) infiltration of the endometrium. Schlafer et al. (2000) studied the development of placental macrophages in cow placentas and showed a steady and remarkable increase of them with advancing gestation. They are the equivalent of human "Hofbauer cells."
15) Pathological features
Holm et al. (1965) studied eight cotyledons of different cows with prolonged pregnancy. They showed fibrosis of maternal septa, atrophy of trophoblast, fibrosis of villi and, at times, sclerosis of maternal arteries. I have seen several acardiac fetuses in cattle twins and show one such anomaly here. They result from a reversal of the circulation in one of twins via placental anastomoses and are well-studied by Schatz and others (references can be found in Benirschke & Harper, 1977). They take many forms but are not to be confused with teratomas (see Stephens et al., 1989). Most acardiacs have a short umbilical cord, but it may be so short that it is missed. They cannot be confused with teratomas because their outside is composed of skin, a feature not found in teratomas. Dunn et al. (1967) studied the chromosomes of one such female twin that was blood chimeric to a normal male co-twin.
et al. (2002) presented a case of "hydatidiform mole" in a bovine
freemartin, but the case is quite confusing. It shows a stillborn male fetus
(60,XY) with large attached 47.8 kg "twin mole" with a 60,XX karyotype.
There were no fetal remnants and the "mole" was composed of fluid-filled
cisterns. To equate this to an androgenetic origin (as are human moles)
seems to be overinterpreting the findings. Indeed, whether the few cases
cited by the authors really are similar in their pathogenesis to human moles
Other remarks - What additional information is needed?
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