Abstract

The development of the vitelline envelope during the oogénesis of the “needlefish" Pseudotylosurus microps was examined by transmission electron microscopy. In the previtellogenic oocytes, the vitelline envelope has a bílaminar structure. The inner electron-dense layer seems to be deposited by the oocyte, the outer, less electron-dense, and granule-fibrillar layer seéms to have an exogenous origin. In the vitellogenin oocytes, the vitelline envelope acquires a trilaminar appearance, The outer layer before mentioned become gradually compacted and electron-dense. In a subsequent step, it becomes central by the addition of an outer third stratus with loose aspect and low electron density. Al the same time, the inner electron-dense layer thickens ínwardly by farming domeshaped protuberances covered by less electron-dense cúpulas. The thickening of the median electron-dense and the outer layer seems to occur by the contribution of material arísing from large electron-dense bodies Incaled in (he intercellular spaces of the follicular epithelium. In the mature vitelline envelope the in­ner (thickest layer has become probably produced by the oocytes as a result of the apposition of endogenous mate­rial over the cúpulas, although the participation of exog­enous sources could not be discarded mainly at the fi­nal part of it deposition. The median layer seems to be co-produced by the oocyte and exogen sources and the outer layer is exogenous in origin. The large electron-dense bodies intermingled with the follicular cells give a very peculiar feature to the follicles, but its specific function is unknown.