Amniogenesis in Schreiber’s long-fingered batMiniopterus Schreibersii Natalensis

Abstract

an amnion by folding as described for the tree-tailed bat Tadarida hrasiliensis cynocephala (Stephens 1962). Schreiber's long-fingered bat, Miniopterus schreibersii natalensis, is seasonally monoestrous producing a single off­ spring in November. It has a bicornuate uterus with asymmet­ rically sized horns. The right horn is twice the length of the left one. All implantations occur in the right uterine horn, with 90% of the conceptuses originating from the left ovary (Van der Merwe 1986). Implantation is superficial and the blastocyst expands within the uterine horn. Orientation of the point of initial trophoblastic attachment, like the orientation of the embryonic disc, is anti-mesometrial (Van der Merwe 1982). Soon after the endodermic cells have started to delam­ inate from the inner cell mass during the pre-implantation period, a single cavity, the primitive amniotic cavity, devel­ ops between the remainder of the inner cell mass by means of cavitation (Van der Merwe 1980). However, this cavity as well as its roof (composed of cells of the embryonic mass and overlying trophoblast) do not participate in the formation of the definitive amniotic cavity or definitive amnion respec­ tively. At the stage when the trophoblast covering the embry­ onic pole of the blastocyst starts to proliferate to form the invading syncytiotrophohlast, the floor of the primitive amni­ otic cavity flattens off (Van der Merwe 1982). Owing to this action, the roof of the primitive amniotic cavity, as well as the overlying cytotrophoblast, becomes stretched and progres­ sively thinner until it eventually ruptures and disappears. The embryonic disc at this stage is roofed only by the syncytiotro­ phoblast (Van der Merwe 1982). The purpose of the present study was to determine how the pleuramnion in the superficially implanting Schreibers' long-fingered bat is formed. The main aim was to determine whether the cephaliclhead, lateral and caudal folds develop simultaneously and contribute equally to its formation. There has been some controversy as far as this aspect is concerned because, although authors refer to these folds, some do not elaborate on the contribution made by each of these folds dur­ ing amniogenesis. They normally only mention that the free edges of the folds meet dorsally of the embryo. In the pig these circumferential amniotic folds are in reality all directly continuous with each other (Patten 1948). This, indeed, is expected to be the case in the majority of mammals where a pleuramnion exists. In Schreibers' long-fingered bat these folds eventually become continuous, but this occurs only at a later stage of amniogenesis (see below). Twenty Schreiber's long-fingered bats carrying embryos were collected during July and August at Sandspruit Cave No.1 (24'37'S, 27°40'E) in the northern Transvaal bushveld. Following paraffin-wax embedding, all the genital tracts were serially sectioned at 5 11m and the mounted sections stained:with Ehrlich's haematoxylin and counter-stained with eosin. The sections revealed that upgrowing folds from the peri­ meter of the embryonic disc in Schreiber's long-fingered bat, brought with them ectoderm and extra-embryonic mesoderm (Figures la, b). Because this is a double fold of soma top leur a, the outer part comes into close contact with the syncytiotro­ phoblast. As growth of the folds progresses, the outer layer of the folds eventually covers more of the syncytiotrophoblast overlying the embryonic pole. Finally the folds unite dorsally,