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).