1) General Zoological Data
There are basically three species of zebra: The largest, and the most Northern African species, is Grevy's zebra (Equus grevyi). It is classified as being endangered. The most numerous zebra species is the Planes zebra with its several subspecies (Grant, Burchell, Damara - Equus quagga boehmi, etc.). The most Southern zebra species is the Mountain zebra (Equus zebra zebra), with Mrs. Hartmann's mountain zebra (Equus zebra hartmannae) a subspecies. Nowak (1999) provided an extensive consideration of the evolution of equidae, from their origin in North America, their ultimate increase in size and then their dispersion over Eurasia and Africa. So far as is known, their placental development and structure are essentially similar and also close to those of the better-studied horses.
Adult Grevy's zebras weigh around 400 kg, and newborns are around 40 kg. Gestation lasts from 358-438 days (average 409 days) according to Nowak. Their longevity is over 28 years.
Mountain zebras weigh around 300 kg, with neonates weighing 25 kg. Both subspecies are endangered, but Hartmann's mountain zebras may be seen in numerous zoological parks. Their longevity is over 29 years.
Planes' zebras have many more different phenotypes. This is most apparent in their differing striping patterns. They are probably conspecific with the now extinct quagga (Equus quagga) and are the most widely exhibited in zoos. Their weights are between 175 and 385 kg, neonates are 32 kg and are born after a gestational period of 360 to 396 days. Their longevity is as much as 40 years.
Most zebras produce a single young, but twins of opposite sex have been observed. We saw the placenta of such a pair of Hartmann's mountain zebra neonates, with one of the twins surviving. Twins and their placentas were significantly smaller than singletons and their placentas.
|Adult Equus grevyi at San Diego Zoo.|
|Grevy's zebras at San Diego Wild Animal Park.|
|Hartmann's mountain zebra in San Diego.|
|Note the typical "double chin" ("Wamme") of the mountain zebra, parent of the "zebronkey" in the background. (Manila zoo).|
|Burchell zebra in San Diego.|
General Gestational Data
|Implanted immature placenta of Grevy's zebra. Note endometrial glands (purple) and the thin endometrial septa that separate villous lobules.|
|Higher magnification of implantation site. The fetal villi are within the white spaces of endometrial glands.|
|Surface of zebra placenta with villous ramifications and interspersed maternal tissue. The chorionic membrane is on top. Note the space in the top center with cylindrical trophoblast and questionable red blood cell mass - the probable origin of the ensuing brown pigment collection.|
General Characterization of the Placenta
The placenta of all equidae is similar. It is a thin organ whose external surface is diffusely covered by villi. The thickness is actually difficult to measure precisely; it is maximally 0.5 cm, usually thinner. The outside has a velvety-red appearance. It is an epithelio-chorial placenta. There is a large allantoic sac which normally contains hippomanes and which is connected to the fetal bladder by the allantoic duct of the umbilical cord. The umbilical cord is relatively long and, in horses, entangling of excessively long cords has occasionally caused abortion. I have had the opportunity of studying several zebra placentas and pregnant uteri of all three species that are summarized here.
uterus of a nearly full term pregnant female Grevy's zebra was the most
recently available specimen. The mare had died from volvulus, a serious
condition of equidae. The female fetus weighed 15,750 g and had a crown-rump
length of 80 cm. The placenta weighed 2,980 g and was uniformly thin and
partially detached from the uterus. The fetus lay in the left horn, but
the allantoic sac extended into the right uterine horn.
List of placental weights of different species of zebra:
|Grevy's zebra||Term||1,580 g|
|Grevy's zebra||Term||1,560 g|
|Burchell's zebra||Term||1,200 g|
|Damara zebra||Term||2,980 g|
|Damara zebra||Term||1,550 g +750 g membranes|
|Hartmann's mountain zebra||Term||1,400 g|
|Hartmann's mountain zebra||Term||1,700 g|
|Hartmann's mountain zebra||Term||2.350 g|
|Grevy's zebra uterus with foal. The right uterine horn is lying on top.|
|Near-term Grevy's zebra uterus with foal.|
|Placenta of near-term Grevy's zebra.|
|Placenta of Damara zebra. Hippomanes at arrows.|
|Chorionic plate of delivered zebra placenta with villi below. Note the cylindrical epithelium beneath the vascularized chorion.|
|This picture exhibits a chorionic surface of placenta with a much more cylindrical trophoblast between the villous stems and accumulation of deep brown pigment.|
|This picture also exhibits a chorionic surface of placenta with a much more cylindrical trophoblast between the villous stems and accumulation of deep brown pigment.|
|Villous ramification of zebra placenta. A cuboidal epithelium covers the villi which are dominated by fetal capillaries. The endometrial surface is single-layered and cuboidal.|
|Higher magnification of the villi of a delivered zebra placenta. The fetal capillaries dominate the villous structure and are located directly beneath the single-layered trophoblast.|
of fetal/maternal barrier
List of umbilical cord lengths of different species of zebra
|Grevy's zebra||Term||68 cm|
|Grevy's zebra||Term||68 cm|
|Burchell's zebra||Term||62 cm|
|Damara zebra||Term||58 cm|
|Damara zebra||Term||65 cm allant.+17 cm amn.portion|
|Hartmann's mountain zebra||Term||68 cm|
|Hartmann's mountain zebra||Term||55 cm|
|Hartmann's mountain zebra||Term||20 cm (?complete)|
|Hartmann's mountain zebra||Term Twins||60 cm each|
|Hartmann's mountain zebra||Term stillborn||84 cmx2.5 cm|
The length of the umbilical cord has been studied in a large sample of thoroughbred horses (Whitwell, 1975). It varies from 36 to 83 cm (55 cm mean), with complications often ensuing from excessively long and, less often, short cords. Horses also have two arteries, one vein and an allantoic duct that may become constricted by fetal motions. The long cord allows the fetus to reside in the uterine horn in which the placenta is not implanted because of the large quantity of amnionic fluid. As a result of tactic stimulation, the fetus moves actively and, when it has a long cord, it may strangulate.
|Photograph of the undersurface of the chorionic plate.|
|At left is the amnionic membrane, at left is the vascularized allantoic membrane with its cylindrical epithelium.|
Trophoblast external to barrier
|Uterus and large ovaries of the Grevy's zebra fetus|
|Cross-sectioned ovary of Grevy's zebra fetal ovary.|
|Ovarian cortex of the ovary of the Grevy's zebra foal. At arrows are a few oocytes within the fibrous cortex; the majority of the ovary is replaced by pigmented interstitial cells.|
|Higher magnification of pigmented interstitial cells in the neonatal ovary.|
|The fetal testis has a similar degree of interstitial cell stimulation. A germ cell is indicated by the arrow.|
|In very young fetuses, the interstitial cell stimulation is extensive, but the pigmentation is less apparent.|
A detailed study of fecal estrogen and progesterone secretion in Grevy's zebra was undertaken by Asa et al. (2001) as well as Kirkpatrick et al. (1990) Asa et al. also studied the urinary gonadotropin (eCG) excretion. Progesterone levels were found to be relatively low when compared with those of numerous other species. Urinary estrone levels became elevated and were then diagnostic of pregnancy at about 8 months before the end of pregnancy (Czekala et al., 1990). In horses, it showed a rise in serum levels as early as the 40th day of gestation. Higher levels were found in horses than in Hartmann's zebras. Asa et al. (2001) found eCG (equine chorionic gonadotropin) to be present after 35-40 days, as is the case in the domestic horse. It completely disappeared at 195 days of their 425 day gestation. The steroid profiles were similar to those of the horse.
Horses and, presumably all equidae, have special sites of eCG secretion in their fetally-derived "endometrial cups". These unique structures have been studied by numerous investigators, both structurally and functionally, but they have not been studied in zebras, and their remains are not recognizable in term placentas. In horses, eCG secretion from these cups begins on day 32; some time after day 100 the cups are destroyed by an intense maternal lymphocyte reaction (reviewed in detail by Wooding et al., 2001). While it is easy to speculate that this hormone may also be the cause of the fetal gonadal stimulation, its rapid decline after day 100 in maternal serum argues against this possibility, as the gonads remain large and apparently stimulated until term. Nevertheless, the unique fetal gonadal development and the uniqueness of endometrial cups suggest a relationship.
As is usual for all equidae, the fetal gonads of my Grevy's zebra gestation were much enlarged and dark brown. They were autolyzed and somewhat diffluent. They weighed 42 and 39 g.
wide variety of zebra hybrids have been recorded (Gray, 1972). This includes
hybrids among zebra species, as well as with other equidae. Depending
on their chromosome number, most are sterile offspring (Benirschke et
al., 1964; 1967).
remarks - What additional Information is needed?
Asa, C.S., Baumann, J.E., Houston, E.W., Fischer, M.T., Read, B., Brownfield, C.M. and Roser, J.F.: Patterns of excretion of fecal estradiol and progesterone and urinary chorionic Gonadotropin in Grevy's zebras (Equus grevyi): Ovulatory cycles and pregnancy. Zoo Biol. 20:185-195, 2001.
Benirschke, K. and Kaufmann, P.: The Pathology of the Human Placenta. Springer-Verlag, NY, 2000.
Benirschke, K., Low, R.J., Brownhill, L.E., Caday, L.B. and de Venecia?Fernandez, J.: Chromosome studies of a donkey/Grevy zebra hybrid. Chromosoma 15:1?13, 1964. (Benirschke, K.: Corrigendum. Chromosoma 15:300, 1964.)
Benirschke, K. and Malouf, N.: Chromosome studies of Equidae. In: Equus, Vol. 1 & 2. H. Dathe, ed, pp. 253?284, 1967.
Czekala, N.M., Kasman, L.H., Allen, J., Oosterhuis, J. and Lasley, B.L.: Urinary steroid evaluations to monitor ovarian function in exotic ungulates: VI. Pregnancy detection in exotic equidae. Zoo Biol. 9:43-48, 1990.
Enders, A.C. and Liu, I.K.M.: Lodgement of the equine blastocyst in the uterus from fixation through endometrial cup formation. J. Reprod. Fertil. Suppl. 44:427-438, 1991.
Enders, A.C. and Liu, I.K.M.: Trophoblast-uterine interactions during equine chorionic girdle cell maturation, migration, and transformation. Amer. J. Anat. 192:366-381, 1991.
Enders, A.C., Jones, C.J., Lantz, K.C., Schlafke, S. and Liu, I.K.M.: Simultaneous exocrine and endocrine secretions: trophoblast and glands of the endometrial cups. J. Reprod. Fertil. Suppl. 56:615-625, 2000.
Enders, A.C., Meadows, S., Stewart, F. and Allen, W.R.: Failure of endometrial cup development in the donkey-in-horse model of equine abortion. J. Anat. 188:575-589, 1996.
Gadi, I.K. and Ryder, O.A.: Distribution of silver-stained nucleolus-organizing regions in the chromosomes of the Equidae. Genetica 62:109-116, 1983.
Gallagher, P.C., Lear, T.L., Coogle, L.D. and Bailey, E.: Two SINE families associated with equine microsatellite loci. Mammalian Genome 10:140-144, 1999.
George, M. and Ryder, O.A.: Mitochondrial DNA evolution in the genus Equus. Mol. Biol. Evol. 3:535-546, 1986.
Ginther, O.J.: Mobility of the early equine conceptus. Theriogenology 19:603-511, 1983.
Gray, A.P.: Mammalian Hybrids. A Check-list with Bibliography. 2nd edition. Commonwealth Agricultural Bureaux Farnham Royal, Slough, England, 1972.
Griner, L.A.: Pathology of Zoo Animals. Zoological Society of San Diego, 1983.
Keenan, I.R., Forde, D., McGeady, T., Wande, J. and Roche, J.F.: Endometrial histology of early pregnant and nonpregnant mares. J. Reprod. Fertil. Suppl. 35:499-504, 1987.
Kirkpatrick, J.F., Lasley, B.L. and Shideler, S.E.: Urinary steroid evaluations to monitor ovarian function in exotic ungulates: VII. Urinary progesterone metabolites in the equidae assessed by immunoassay. Zoo Biol. 9:341-348, 1990.
Mossman, H.W. and Duke, K.L.: Comparative Morphology of the Mammalian Ovary. University of Wisconsin Press, Madison, 1973.
Nowak, R.M.: Walker's Mammals of the World. 6th ed. The Johns Hopkins Press, Baltimore, 1999.
Ryder, O.A. and Hansen, S.K.: Molecular cytogenetics of the equidae. I. Purification and cytological localization of a (G+C)-rich satellite DNA from Equus przewalskii. Chromosoma 72:115-129, 1979.
Whitwell, K.E.: Morphology and pathology of the equine umbilical cord. J. Reprod. Fertil. Suppl. 23:599-603, 1975.
Wooding, F.B.P., Morgan, G., Fowden, A.L. and Allen, W.R.: A structural and immunological study of chorionic gonadotrophin production by equine trophoblast girdle and cup cells. Placenta 22:749-767, 2001.
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