The marsupial shell membrane: an ultrastructural and immunogold localization study.

In the dasyurid marsupial, Sminthopsis crassicaudata, as the oocytes/embryos travel down the female reproductive tract two extracellular coats, the mucoid and shell membrane, come to surround them. Embryos recovered from the oviduct have a mucoid coat but no shell membrane which is only found surrounding uterine embryos. Initially, the shell membrane has a compact granular consistency but it later thins and becomes fibrous in texture with fibres oriented mainly in the plane of the membrane. Immunogold labelling with polyclonal antibodies raised against the extracellular coats was employed to determine the location and ultrastructural appearance of the secretory granules which contain mucoid and shell membrane precursors. Secretory granules in the luminal epithelium of the ampulla of the oviduct are of irregular electron density, while those in the isthmus are electron-dense and homogeneous. Both types give rise to the mucoid coat. Secretory granules in the epithelia of the utero-tubal junction and some endometrial glands are electron-lucent and contain some flocculent material which, after exocytosis, gives rise to the shell membrane.