Nutria or Coypu (Myocastor coypus)

Order: Rodentia

Family: Myocastoridae

1) General Zoological Data

This South American rodent, a member of the Suborder Hystricomorpha, is widely distributed through Southern South America and, because of importations, it is now resident in Southern USA (Louisiana etc.), England, Poland, and many more countries which it did not inhabit originally. Several (regional) subspecies have been nominated and there is also a considerable coat color variety in the many colonies. The huge colony in Louisiana (millions) is said to have derived from only 13 ancestors, indicating the success that this animal has in some habitats. It has replaced many local species where it was introduced.

It might here be mentioned, however, that the term Hystricomorphamust be correctly applied and understood. The nomenclature was preciselydefined by Simpson (1974) in his preliminary remarks to a symposium on thetopic of Hystricomorpha. It is not, as he cautions, an evolutionaryunit but an anatomical term. The animals reviewed here under that headingwould thus be more correctly grouped as Caviomorpha. There has alsobeen controversy regarding the placement under hystricomorphs. Thus, Lowery(1974) placed nutrias in the family Capromyidae. The most recentreview by Wilson & Reeder (1992) separates nutria as the only memberof the Myocastoridae. They discuss briefly the reasons for separatingthis species from other South American rodents. The name Myocastor derivesfrom the Greek mouse and beaver, while Coypu is anold South American native name (Gotch, 1979). It is said to mean "water-sweeper"(Lowery, 1974).

Coypus are extremely well adapted to water, are excellent swimmers, andprefer to feed in swampy areas off roots and other vegetable matter. Theirnostrils are "valvular" and thus adapted for under-water swimming(Kinler et al., 1987). They also have been well-adapted to form large breedingcolonies that are now widely exploited for fur production (Federspiel, 1941).Adults weigh between 3-14 kg (Hayssen et al., 1993), males being larger.Neonates weigh between 100 and 250 g; Newson (1966) gives a mean of 225g for neonates. There are commonly around five young born, but variationsoccur, with as many as 13 animals in a litter (Newson, 1966). Prenatal demiseis common in this species as well as in other hystricomorphs, and scarsmay be found of the uterus subsequently. On the other hand, on occasionfewer corpora lutea have been identified than there are implantations sites,thus leading to the speculation that occasional monozygotic offspring mayoccur. No direct evidence has been provided for this. A major reason formortality is cold weather and frost to which these South American rodentsare ill-adapted.

Nutria with young in Louisiana.

Adult nutria in their natural habitat, Louisiana.

Adult nutria in their natural habitat, Louisiana.

Skull of adult nutria with the characteristic large infraorbital foramen.

Dentition of adult nutria with the hypsodont molar and premolar teeth.

2) General Gestational Data

The chronology of pre-implantation stages in coypu was described by Felipeet al. (2002) from their daily colposcopic and cytological examinations.Flushing uteri yielded embryos to day 10 post coitus. Cleavage stages wereobserved from 3-6 days, morulae from 6-9 days, and blastocysts were obtainedafter day 8. Tubal specimens (morulae) were discovered that had up to 30cells. The length of gestation is around 130 days (128-135). Two littersare usually born annually. Newson (1966) has reviewed the general reproductivephysiology of the coypu in its British feral state and showed the growthrates of embryos, which is depicted in the next graph.

Fetal weights (Modified from Newson, 1966). Red dots represent gestational ages of animals used for this text.

Term, delivered nutria with uterus still attached.

3) Implantation

The anatomy of nutria genital systems, both male and female, is comprehensivelycovered by Hillemann et al. (1958). They also reviewed older publicationsand the anatomy of genitalia from some related taxa. The publication showsexcellent drawing of the genitalia. Males and females have bacula, althoughthe female's is cartilaginous. The uterus is duplex with two cervical openings;the ovaries are not enclosed in a bursa. Implantation of the placental diskis mesometrial but the initial nidation is anti-mesometrial.

Hystricomorph rodents have a complex, interstitial implantation and placentaldevelopment may be difficult to understand at first. Moreover, they arecharacterized by having a sizeable "subplacenta", the exact functionof which remains to be elucidated. Other rodents lack this structure. Inaddition, they possess, as do other rodents, an inverted yolk sac throughwhich proteinaceous substances are exchanged. All of these features makethis a difficult organ to comprehend, especially as it changes its appearancewith advancing gestational age.

The early stages of placental development are complex and shown here ina series of photographs from successive stages. Implantation in rodentsis preceded by significant decidualization of the endometrium, effectedthrough progesterone and estrogen. It is complex and has been extremelywell reviewed in great detail by Abrahamsohn & Zorn (1993). Althoughthese stages have not been described for the nutria, one may assume thatsome of the processes are similar to other rodents, but I remain cautiousat the same time.

The blastocyst implants (interstitially) in a greatly expanded mass of decidualizedendometrium. This initial attachment occurs anti-mesometrially. Once thetrophoblast makes contact with the endometrial epithelium and penetratesit, trophoblast cells infiltrate this large mass of endometrium shown inone of the next photographs. The trophoblast then also replaces some ofthe endometrium, spreads laterally and attaches (implanted in endometrium)on a very narrow pedicle that is also shown here. Older authors have spokenof a "double implantation", as there is initial attachment anti-mesometriallybut the disk develops mesometrially. Clefts develop in the endometrium sothat, essentially, a second endometrial cavity is created that thus explainsthe band-like extensions seen in the photographs.

Uterine horn with central implantation site.

This picture was taken under higher magnification from the previous photomicrograph. It shows, at left, the endo/myometrium and, on the right, the implantation decidual cylinder still covered on its left surface with recognizable endometrium, including its surface epithelium.

This photograph of the cylinder mass from the slides before shows the narrow stalk that connects to the endometrium which here still possesses some glands.

Thedecidua of this large ovoid cone shown above is quite pleomorphic. The questionwhy so few blood vessels are apparent was answered by Abrahamsohn &Zorn (1993) who indicated that the capillaries are probably collapsed (compressed).The eosinophilic granules shown to exist in some cells (next photograph)probably represent the so-called "granulated metrial gland" cells(GMG), also discussed by these authors and in further expansive detail byPeel (1989). These are widely studied in rodents and are currently consideredto derive from bone marrow, are "killer cells", and interact withinfiltrating trophoblast. I am not certain of this corollary, as nutriadecidua has not been studied in detail, but the morphology is highly suggestiveof their similarity.

Decidua of cone with granulated cells at arrows, perhaps representing the metrial gland cells.

Cross section through very slightly older implantation site.

Adjacent section from previous slide with embryo.

Anticipatingthat the development follows the other hystricomorph rodents (see Roberts& Perry, 1974) one must assume that also in nutria now the egg cylinderenlarges longitudinally and that the endoderm develops prematurely and elongatesmarkedly. The parietal portion of the trophoblast degenerates and that,thus, the visceral portion of the yolk sac becomes exposed to the endometrialsurface.

The juxtaposition of trophoblast to the endometrial surface and also its invasion into the decidua, in irregular columns, are shown next.

Embryo in the center of the implantation chamber with early blood formation, decidua laterally.

Embryo in the decidual implantation chamber with dark, syncytial trophoblast expanding from the bottom and at lower left into the decidua.

Composite of three cross sections of uterine horn with early nutria implantation. H&E left, cytokeratin right. The brown-staining-cells represent the early trophoblastic outgrowth toward the mesometrium where they ultimately form the disk.

Composite to show central embryo and outgrowth of cytokeratin-stained trophoblast towards forming the future disk.

H&E above, cytokeratin below. Embryo in its chamber and trophoblast growth towards mesometrium.

Later,the decidua capsularis (always quite thin) degenerates. This is the processof complete yolk sac inversion, characteristic of most rodents but takingplace unusually early and extensively in hystricomorphs. This membrane isvillous near the embryo but the epithelium becomes thinner the farther awayfrom the embryo; likewise, while very vascular, this decreases over thedome of the inverted yolk sac eventually. No early stages of nutria havebeen examined to support the existence of a vascular ring of yolk sac vesselsso beautifully described by Mossman (1987) for the guinea pig. It is, however,depicted by a drawing (Fig.1) of the nutria placenta by Hillemann &Gaynor (1961).

From the ectoplacental region of the blastocyst trophoblastic cells havepierced the single-layered endoderm and grow towards the decidual portionof the mesometrial region, thus establishing the precursor of the futuremesometrial placental disk. The embryo is always located towards the futureregion of the disk, away from the original anti-mesometrial implantationsite.

Uterus with earliest embryos available to me. The small areas of enlargement of the uterine horns are the sites from which the previous photographs were obtained.

The first stages of disk development with very small embryo at top. This also shows the delicate amnion and umbilical cord.

Placental disk at top, mass of invaded trophoblast in endometrium and degenerating edge of subplacenta and decidua.

Young coypu gestation with seven embryos. The cervix is in the center below. Portions of kidneys are attached which are near the ovaries.

Near-term uterus containing five fetuses (+/- 150 g).

4) General Characterization of the Placenta

The nutria placenta is discoid, labyrinthine and hemomonochorial with aprominent subplacenta. It is mesometrially implanted in the duplex uterus.An excellent and comprehensive description of the advanced stages of thenutria placenta has been provided by Hillemann & Gaynor (1961). Theirpublication included specimens that were injected with various substancesin order to obtain a better understanding of the maternal and fetal vasculatures.It also includes measurements and weights of the disks. "In thefull-term gestation sac, the uterine epithelium completely lines the uterinelumen, except for the very restricted area occupied by the delicate andtenuous vessels of the basal plate", they stated. "The yolk sacplacenta is solely visceral but highly developed. It forms a tough reinforcinginvesting membrane on the outside of the amnion. It attaches to the outsideof the amnion."

I have had the great fortune to obtain four gravid uteri at different stagesof gestation. The largest uterus had five fetuses weighing between 140 and 155 g and measuring 12-13 cm CR length. Their placentas weighed (detached) 20-23 g and measured 4 x 1.2 cm. The cross sections of the disks show thecharacteristic pattern seen also in other South American rodents (see chapters on Porcupine and Pacarana).

In the other uteri of younger gestations obtained, the embryos were too small to be weighed, but measured 1.2 - 1.4 cm in CR length.

Diagrammatic representation of nutria placentation.

Near-term fetus (140 g) attached to placenta. The umbilical cord is always slung around the back of the fetus, leaving an impression seen here.

1.4 cm embryo still in situ within the amnion and vitelline membrane.

Amnion and vitelline membrane were opened and now expose the placental disk (center) and the embryo (below right).

Cross sections of placenta and embryo (in its vitelline membrane) of the previous photograph.

Cross section of fixed placenta; maternal aspect below. The white streaks through the darker labyrinth represent the trophospongium. The placenta consists of many lobules.

Maternal aspect of the detached placenta with its narrow stalk and peripheral maternal vessels.

From an earlier picture above: Embryo and umbilical cord above, early disk with maternally-perfused trophospongium and dark subplacental edge next to degenerating decidua.

Same as previous but with more of the subplacenta and degenerating basal decidua included.

Immature placental margin with vitelline membrane extending to the right.

Trophospongium of immature placenta. Maternal blood vessels.

Border of labyrinth (left) and trophospongium (right) in an immature placenta.

Central portion of implanted, near term, nutria placenta. Blue is the trophospongium, red is the labyrinth. Fetal chorioallantoic vessels above.

Subchorial region of giant cells of the trophospongium (blue) abutting the spongious labyrinth, near term.

5) Details of fetal/maternal barrier

The labyrinth comprises the feta/maternal barrier through which most nutrientsand gases are exchanged. Here, thin blood channels and trophoblastic tubes(containing maternal blood) extend from the center to the trophospongium.Capillary anastomoses were described to exist here by Hillemann & Gaynor(1961) through their injection studies. These vascular channels are essentiallycomposed of endothelial cells and most of the exchange would have to takeplace here. There is little connective tissue. The trophoblast is syncytialand extremely thin. Fine structural details of this complex labyrinth areonly available for the chinchilla placenta (King & Tibbitts, 1976).It is likely, however, that it is very similar if not identical in the nutria.That publication described the maternal blood-containing channels or tubesas being lined, as it were, by syncytiotrophoblast. This, in turn, connectswith fetal capillary endothelium or cytotrophoblast that can also be foundhere. The electron microscopic study of the chinchilla placenta by King&Tibbitts (1976) shows these cells very clearly.

The trophospongium consists mostly of syncytiotrophoblast and some sheathsof solid trophoblast, especially at the base. There, some giant cells arealso found which, otherwise, cover most of the outside of the disk and arepenetrated by a fine vascular meshwork.

Trophospongium trophoblast, maternal vessel at very edge, left.

Trophospongium left, bordering the labyrinth, right. Immature.

Margin of disk and subplacenta. This section also shows the trophoblastic infiltration around maternal blood vessels (left bottom).

Near-term disk (above) connected to decidua with narrow stalk, degenerating decidua and subplacenta below.

6) Umbilical cord

The umbilical cord of the largest fetus shown here measured 3 cm in lengthand had only a minimal spiral. It was always looped around the fetus' body.I assume that it grows as the fetus lengthens from stretch imposed uponit. Hillemann & Gaynor (1961) measured the umbilical cord at term gestationto be 6.5 x 0.45 cm in size. It branches near its insertion on the placentalsurface; the chorioallantoic portion then was 1 cm long, the vitelline portionwas less than 0.5 cm and this branched again almost immediately into thenumerous smaller vessels of the vitelline membrane. The amnionic surfaceof the cord was smooth and composed of a single-layered squamous epithelium.The complete cord had five blood vessels; the chorioallantoic part had threevessels, and the vitelline part contained only two blood vessels. Thereare no ducts and no smaller vessels are present as seen for instance inungulates. The surface is covered with amnion which has no squamous plaques.

Fetus in vitelline and amnionic membranes. The umbilical cord is typically tightly wound about the back of the fetus (at about 2 o'clock).

The short umbilical cord divides into its two portions immediately before its insertion on the placenta.

Cross sections through "complete" umbilical cord.

Cross section through the chorioallantoic split of the umbilical cord.

Cross section of the vitelline portion of the umbilical cord with attached vascularized vitelline membrane.

7) Uteroplacental circulation

Hillemann & Gaynor (1961) provided a most detailed description of thecomplex vasculature with the help of their injection studies. They foundno endovascular "plasmodia" (I interpret this to mean syncytiotrophoblast).My preparations, however, show that not only is trophoblast growing withinthe maternal blood vessels but it surrounds in great numbers the maternalvasculature of the myometrium and the mesometrium as well. In many vesselsit replaces the muscular wall completely. Moreover, both, arteries and (lessso the) veins are similarly infiltrated by trophoblast, as will be seenin the photographs below; nevertheless, the veins have much less trophoblasticinvestment than the arteries. The trophoblast is strongly cytokeratin positiveand resembles closely the picture shown in the pacarana (see Chapter on Pacarana).

A very similar picture is found in the guinea pig, and it is perhaps characteristicof Caviomorpha in general (see Nanaev et al., 1995). Indeed, Kaufmann(personal communication) informed me that one can observe this peritonealtrophoblastic infiltration when opening the abdomen of a pregnant caviomorph.There is a perceptible thickening in the size of mesometrial vessels inthose vessels that go to the disks, not in the remainder. Their Figure 1illustrates this aspect superbly. The complex perfusion of the labyrinthand trophospongium was illustrated for the guinea pig in diagrams by Kaufmann(1981). It is the same in nutria.

Near-term placenta with maternal blood vessels just below the subplacenta. Myometrial blood vessels are surrounded by dense infiltration of trophoblast.

Mesometrial ligament with maternal blood vessels.

Mesometrial blood vessels within adipose tissue. All blood vessels have a thick coat of dark blue trophoblast ensheathing them.

Cytokeratin staining of the trophoblast (brown) in one of the mesometrial arteries.

8) Extraplacental membranes

Luckett (1980) suggested that most caviomorph rodents studied form the amnioniccavity by folding, a point also supported by Roberts & Perry (1974).He also further described the vitelline placental formation of Caviomorpha.The membranous dome of the placental sac is extremely thin. It consistsfirst of the peripheral uterine muscle, then endometrium, finely vascularizedyolk sac membrane with thin epithelium, and finally the avascular amnion.At the interface to the next placenta, a significant ridge is formed thatis shown in the next photograph. The amnionic cavity contained practicallyno amnionic, especially in the largest gestations available to me. The vitellinemembrane remains very well preserved throughout gestation and, in analogywith other Rodentia, one would assume that some exchange betweenmother and fetus (immunoglobulins, other proteins) may be take place here(see King, 1977).

Placental (uterine) base after detachment of the placenta. The ridge indicates the site of attachment of the (left) placental membranes of the neighboring gestation.

Membranes directly next to the vitelline cord insertion. Note the debris (bottom left) in the endometrial cavity.

9) Trophoblast external to barrier

Trophoblast invades the uterus deeply and follows maternal myometrial arterieswhich it envelops in large numbers. This is not unlike the condition foundin some other South American rodents and especially the hystricomorpha (see Chapters on Pacarana, American Porcupine, Beaver; and Nanaev et al., 1995).These cells have a very varied appearance; most are unusually large, manyappear to be polyploid, have huge nuclei and large nucleoli. In addition,syncytial cells are present in this mantle of trophoblast around the maternalblood vessels.

Near term mesometrial blood vessels are surrounded and replaced by a thick layer of trophoblast.

Higher magnification of previous photo to show the complete replacement by trophoblast of the muscular wall and endothelium in a maternal mesometrial blood vessel.

10) Endometrium

The remarkable finding in this and some other hystricomorphs is the relativelynarrow pedicle that connects the subplacenta to the uterine musculature.Significant decidualization of the endometrium occurs early in gestationand becomes less prominent later. The extensive review of rodent decidualizationby Abrahamsohn & Zorn (1993) needs to be consulted to fully comprehendthe complex development of rodent decidua. Some trophoblast can be seento infiltrate the endo- myometrium lateral to the implanted disk. Near theend of gestation the entire uterine cavity is lined by a thin endometriumwithout glands over the dome of gestational sacs.

Endo-myometrium of pregnant coypu with some (dark) trophoblast infiltration.

Immature gestational endo-myometrium. It shows focal (dark blue) trophoblast infiltration.

The distal portion of the endocervix (below) is duplex in this rodent.

Portion of endocervix that is filled with thick mucus during pregnancy.

Narrow uterine (endometrial) site of implantation after detachment of the placenta.

11) Various features

Coypus have a pronounced subplacenta beneath the single disk and below ita "junctional zone" of debris. They are penetrated by small maternalblood vessels, the larger ones passing at the periphery of the subplacenta.The subplacenta consists of large numbers of trophoblastic cells, includinggiant cells, below which is much fibrin and degenerated material, includingsome foci of calcification. Scattered trophoblast and giant cells are presentwithin the subplacental debris. Its function has been debated for a longtime and is not yet completely resolved. Mossman (1987) considered it extensively.He maintained that it had to represent a zone of growth, a "germinativeregion", for placental expansion. A secretory function was assignedto this region (in the guinea pig placenta) by the fine structural studiesof Wolfer & Kaufmann (1980). They considered the region to be metabolicallyhighly active but not an area of fetal/maternal exchange. Others have speculatedthat it might be the source of "PBG" (progesterone binding protein- see subsequent section on Endocrinology).

12) Endocrinology

The maternal ovary is small and ovoid, with scattered corpora lutea anda plethora of luteinized follicles. They are not folded as in Pacaranaor Chinchilla. Appreciably more luteinization of Graafian follicles isfound as gestation advances. An extensive description of the ovaries canbe found by Rowlands & Heap (1966). Felipe et al. (1999) have alsodescribed them in great detail. The ovaries of both, fetuses and adults,are located remarkably close to the kidneys. The fetal adrenal glandsare diminutive. Weir & Rowlands (1974) depicted an essentially similarovary as that shown below of late pregnancy with numerous luteinized follicles.The seemingly excessive luteinization of the follicles of hystricomorphrodents during pregnancy is perhaps one means of prolonging the gestationin these unusual animals, as Amoroso (1974) suggested in his highly readablesummarizing remarks to that symposium already mentioned. There is no ovarianbursa. The fetal testes of the specimens I was able to study had onlya moderately active population of interstitial cells, both in young andthe older embryos. The fetal ovaries showed no endocrine stimulation whatever.

There is a dramatic rise in progesterone in mid-pregnancy and a fall someweeks before parturition, perhaps coincident with follicular luteinization.Rowlands & Heap (1966) speculated that the luteotropic activity mayderive from placental secretions but no direct evidence was then presented.Since the coypu, as the guinea pig, is a member of the larger group ofHystricomorpha, these authors suggested that similar mechanismsof the maintenance of gestation may prevail. In guinea pigs, pregnancycontinues when, in later gestation, the ovaries are removed, suggestingthat placental hormone (?progesterone) secretion is sufficient for theendocrine support of gestation. This was not the case with coypu. Whentheir ovaries were removed during gestation, the pregnancies aborted.

Schallyet al. (1969) injected LRF into normal and oophorectomized coypu and founda significant rise in circulating LH. They cautioned that the coypu is nota good study animal for this modality of experimentation, however, becauseit's LH-rise following oophorectomy is so irregular. They speculated thatthis may, in part, be the result of "stress" from captivity. Theprogesterone disappearance rate in coypu during pregnancy (as well as thatin some other hystricomorph rodents) is slow. This has been attributed tothe presence of a progesterone-binding globulin (PBG) by Heap & Illingworth(1974).

Wang et al. (1990) experimented with passive immunization of mice with amonoclonal progesterone antibody (B3) and found it to block pregnancy ina number of species. It was found in uterine lumens 36 hours after injectionand it binds also to progesterone in the DB3-protein of coypu.

The morphologic aspects of uterus, cervix and Fallopian tube were describedin detail by Felipe et al. (1998), and an excellent description of the grossanatomy of both sexes can also be found by Hillemann et al. (1958).

One ovary in near term pregnancy with single corpus luteum and numerous markedly luteinized follicles. Note the lack of folding that is so striking in Pacarana.

Ovary of immature gestation with corpus luteum at right, atretic follicle below left, and less significantly luteinized follicle above left.

Fetal testis with interstitial cells at red arrows.

13) Genetics

Nutrias have 42 chromosomes, all metacentrics with the exception of theY chromosome (Fredga, 1966; Tsigalidou et al., 1968). I am not aware thatcoypu have hybridized with other rodents, and no hybrids are mentioned byGray (1972). Garcia-Meunier et al. (2001) prepared a technique of PCR amplificationof the Sry gene in order to sex coypu embryos efficiently. There is considerablecolor variation of nutrias within breeding colonies, an aspect that affectsmarketability (Kinler et al., 1987). These authors also give reference tosome protein polymorphisms that have been identified.

Karyotypes of male and female nutrias.

14) Immunology

I am not aware of any immunological studies that have been published oncoypu. It would be an interest, however, to learn more about the MHC situationin all hystricomorphs, as there is such extensive infiltration of trophoblastinto the maternal vasculature and exposure of fetal antigens to the maternalimmune system should prevail. Enders & Welsh (1993) have speculatedon the mechanisms that might prevent "rejection" of placentas.A consideration of this aspect, however, is deemed to be inappropriate here.

15) Pathological features

Epizootic pneumonia has become a major disease of nutria and was studiedby Martino & Stanchi (1994) in Argentina. They considered that Streptococcuszooepidemicus was a major cause, although other organisms (but noviruses) were also isolated in pneumonia cases. Howerth et al. (1994)studied some exposure to diseases in coypu that had been trapped in Louisiana.They found sporadic antibodies against toxoplasma, chlamydia, and leptospira,but none against tularemia and encephalomyocarditis virus. Giardia wereisolated in 22 of 30 nutria captured in East Texas by Dunlap & Thies(2002). Griner (1983) observed endometrial polyps, metritis, nephritis,and hemopericardium from an aortic rupture. Kinler et al. (1987) makereference to the frequent dermatitis (due to a tick) and other parasitesaffecting nutria.

As in some other rodent gestations, there are occasional fetal/embryonicdemises that are readily manifest macroscopically by a smaller than expectedbulge of the uterine horn. One such fetal death is shown in the next fewpictures.

Young nutria gestation with empty space, left after fetal demise, at arrow. The next picture shows it opened and its normal neighbor.

The smaller implantation site shown previously contains only degenerating placenta/endometrial tissue. The embryo had been resorbed.

Itis difficult to know what the cause of embryonic death has been, as no remnantswere found; even most of the placenta had been resorbed. Only trophoblastremained at the site of placentation and also peripheral to the maternalblood vessels. It is possible that the embryo was chromosomally abnormal,the commonest cause of such events in human gestation. Several step sectionsof the failed pregnancy are shown next.

The hemorrhagic mass is located at the former site of the fetus; adjacent are remnants of placenta and trophoblast. The whole mass in enclosed by endometrium and the intense vascular congestion is evident.

An even more lateral section of the failed gestation, also showing the edges of the uterine wall.

More lateral still, the cavity contains only decidual tissue. At bottom left the maternal uterine arteries with their trophoblastic coat are shown.

The former implantation site within the endometrium, no embryo.

16) Physiologic data

Bo et al. (1994) have described the techniques for immobilization of copywith ketamine and xylazine. The structure of coypu insulin was determinedby Smith (1972). Jelinik & Glasrova (1982) described the red cell findingsin postnatal coypu and other biochemical studies are referred to in Kinleret al. (1987).

17) Other resources

Cell lines of several nutria at CRES of the San Diego Zoo can be made available through contacting Dr. O. Ryder at oryder@ucsd.edu.

18) Other remarks - What additional Information is needed?

Immunological studies are lacking and especially the MHC gene expression during gestation would be of interest because of the massive trophoblast infiltration into the uterus. More endocrine studies of early pregnancy and, especially, insight into whether the placenta produces hormones would be of interest.

Acknowledgement

The animal pictures and tissues were kindly made available to me through the generosity of Dr. Val Lance (CRES), and Ruth M. Elsey, Louisiana Department of Wildlife & Fisheries, Rockefeller Wildlife Refuge, Grand Chenier, Louisiana.