Transosomes Contribute to the Inner Perivitelline Layer of the Quail (Coturnix japonica) Oocyte: An Ultrastructural and Immunohistochemical Study

Abstract

The oocytes of vertebrates possess an extracellular layer known as the zona pellucida in mammals and the inner perivitelline layer (IPL) in birds. The avian IPL, a thin matrix, is essential for protecting the oocyte and facilitating fertilisation, although it does not prevent polyspermy as in mammals. It forms during folliculogenesis and is composed primarily of glycoproteins (zona proteins ZP1–ZP4, ZPD and ZPAX), along with numerous other proteins. Through electron microscopy and immunohistochemistry, we hypothesise that in quails (Coturnix japonica), transosomes, cellular organelles that occur exclusively in the granulosa cells of birds and have hitherto unknown functions, are involved in the transport of the tight junction proteins occludin and claudin. These proteins, synthesised in the granulosa cells of quails, are transported through the follicle cells and incorporated into the growing IPL during follicle maturation.