General Gestational Data
A single young is born after a nearly year-long gestation (7-10 months according
to the review by Hayssen et al., 1993). Horses, Equus caballus, have
generally a length of gestation of between 329-345 days, depending on the "breed". This placenta came from the breeding group at San Diego
Zoo's Wild Animal Park and from a dam that had eight previous young. The
neonate was healthy and survived. We have had two neonates die to determine
weights: a male that weighed 36.5 kg and a female with a weight of 29.0
kg. In October 2007 another female delivered a healthy calf; its placenta weighed 2,200 g and the umbilical cord had three normal vessels.
Early stages of placentation have not been described, thus nothing is
known of the notorious placental (chorionic) "girdle" (of horses)
in this equine species. Implantation of the placenta in the horse - and
it is probably reasonable to draw this analogy - is relatively late, with
the blastocyst floating and sending signals to the endometrial surface
for days, whilst being shuttled back and forth in the bicornuate uterus.
Equidae have a "copious amount of uterine milk" (Amoroso, 1962;
Ramsey, 1982) in which the blastocyst can be shuttled for some days before
actual implantation. The trophoblast absorbs the nutrients from these
uterine secretions. They are composed of actual secretions and of endometrial
debris from degenerating epithelium that is later restored. Equine implantation,
and most relevant other aspects of placentation of the horse can be found
by Allen & Stewart (2001).
General Characterization of the Placenta
The placenta shown here weighed 3,450 g, and it was 100 cm long and maximally
60 cm in width. It was uniformly thin, about 0.2 cm. The cord was attached
near the center, the membranes had everted. Another placenta received
a few days later weighed 3,700 g, had similar measurements but the umbilical
cord was more completely present. Its allantoic portion was 37 cm long;
the amnionic portion was 30 cm long (67 cm altogether). A placenta that
had been collected earlier came from another dam and weighed only 1,500
g. Its cord, however, measured 44 cm in length and the whole specimen
measured 110 x 103 cm. Yet another placenta from a weak male newborn foal
weighed 2,025 g, measured 120x70 cm and had a 41 cm cord with four vessels,
many small vessels and allantoic duct. Much meconium (400 g) was attached
to the maternal aspect of that placenta, apparently from some neonatal
distress. In September 2003 another placenta became available. It weighed
3,000 g, had a three vessel cord of 84 cm length and marked spiraling.
At the amnionic portion was a "nipple"-like protrusion from
which a hair emanated. It is shown below.
Much work has been done in exploring the T-cell-defined alloantigens in
horse, donkey and their hybrids (see for instance Baker et al., 2001), but
no similar work has been reported on hemiones, so far as I can determine.
More likely than not, similar systems exist in this species.
Nothing has been written about kiang pathology. In our experience, trauma
was the cause of most death and a neonate died from omphalitis. The last
placenta obtained contained a typical teratoma, a "dermoid",
in its membranes that were adjacent to the mid-portion of the umbilical
cord. What is interesting is that this occurred with a male foal, as the
usual "dermoids" known are those derived from ovaries. One can
only speculate that this lesion did not derive from a germ cell but perhaps
from a stem cell. Its location precludes, I believe, the presence of an
aborted twin. Barr bodies could not be found in the lesion's cells. Aside
from these observations, the foal was weak after birth, had to be hand-raised
and had a thick, "meaty" umbilical stump.
Neonatal death with entanglement of the cord is described above.
16) Physiologic data
There has been an enormous amount of work on the physiology of the horse,
especially the reproductive biology. I have been unable, however, to ascertain
similar references for any hemione. The horse data can be accessed by
perusing the biographical references listed in Hayssen et al. (1993).
Numerous cell strains of this and related equids are available from CRES
at San Diego Zoo by contacting Dr. Oliver Ryder at firstname.lastname@example.org.
18) Other remarks - What additional Information is needed?
Early stages of the placenta need to be studied, especially in order to
elucidate whether a chorionic girdle exists. Likewise, endocrine studies
The animal photographs in this chapter come from the Zoological Society
of San Diego. I appreciate also very much the help of the pathologists
at the San Diego Zoo.
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