The Foetal Membranes, Placenta And The Gonadogenital Organs Of The Female Root-rat Tachyoryctes Splendens (ruppell): A Morphological And Morphometric Study
Abstract
The foetal membranes, placenta and some of the
gonadogenital organs of the root-rat ( Tachyqryctes splendens ) were
fixed and examined by routine histological and ultrastructural
methods. Specimens of the late term placenta were fixed and used
for a morphometric analysis. The root-rat ovulates more than one
egg and this usually occurs from both ovaries. There is apparently a
high incidence of foetal resorption in these rodents leading mostly
to a single embryo growing to full term. Twinning does occur
occasionally in these rodents. Implantation of the blastocyst is
antimesometrial and the embryonic disc is mesometrially oriented.
The mode of implantation is eccentric and secondarily interstitial.
Like in most other rodents there is a primary decidual reaction
involving the uterine stroma at the site of implantation.
Amniogenesis is by-cavitation with a temporary open epamniotic
cavity. Inversion of the yolk sac is complete but the disappearance of
the parietal segment occurs relatively late. With the complete
breakdown of both the capsular and parietal deciduas, the yolk sac
wall at the margins of the placenta becomes bathed in a pool of
maternal blood and tissue debris resembling the haemophagous
organ. The yolk sac villi bordering the placental disc are well
vascularized. The allantoic vesicle is persistent up to the limb bud
stage.
The definitive chorioallantoic placenta is haemochorial with
the interhaemal membrane showing a distinct arrangement of three
layers of trophoblast between the maternal blood spaces and the
foetal capillary endothelium (haemotrichorial). The outer layer of
trophoblast enclosing the maternal blood space is cellular while the
next two layers are syncytial. The outer layer of trophoblast is rich in
granular endoplasmic reticulum whereas this organelle is less
abundant in subsequent layers. The placenta shows conspicuous
endodermal sinuses or placental pits (of Duval).
Using a morphometric model of oxygen diffusion it was
demonstrated that the overall weight specific diffusing capacity of
the root-rat late-term placenta (4.73 cm.3 min.-1 mmHg-l. kg.-l) is
two and half times that of man (1.86 cm.3 mirr ! mmHg.-l kg.-l) The
root-rat also shows a marked irregularity and compensatory
thinning of the interhaemal membrane leading to a low harmonic
mean thickness. It is suggested that the low harmonic mean
thickness and the high oxygen diffusing capacity may probably be a
structural and functional adaptation of the root-rat placenta to the
hypoxic-hypercarbic conditions occurring in the burrows where
these rodents live.
Even though the root-rat shares a few foetal membrane
development features with the Geomys I most of the morphological
and developmental characteristics of the foetal membranes and
placenta of the root-rat -suggest that this rodent is closely related to
the myomorphs
The observations made in this study suggest that the root-rat
should be placed in its own minor family (Tachyoryctoidae).
Citation
Master of Science (Veterinary Anatomy)Publisher
University of Nairobi