Mammalian oocyte organelles with special reference to pleomorphic mitochondria and vacuole formation in marsupials

Abstract

Marsupial oocytes are larger and have a thinner zona than eutherian oocytes; and the ooplasm becomes almost completely filled with empty-looking vacuoles the contents of which have, so far, defied histochemical analysis. In the opossum, Monodelphis domestica, apart from orthodox mitochondria, a 'hooded' form is found occasionally in young primary oocytes, and a novel 'spiked' form-which is very elongate and has longitudinally-running filaments attached to the outer membrane--is found in mature oocytes. On the genesis of the ooplasmic vacuoles in mammals, information is available only for two marsupials. In Monodelphis, the vacuoles originate from endoplasmic, endocytotic and Golgi vesicles which generate multivesicular bodies; these give rise to the vacuoles. For the bandicoot, Isoodon macrourus, evidence is presented for the formation of the vacuoles from enlarged, transformed mitochondria which undergo a complex evolution during development. Primordial oocytes of Isoodon contain three ooplasmic localizations--a paranuclear complex, a vesicle microtubule complex and an aggregate of tubular cistenae-which have not been described for other mammalian oocytes. The origin, fate and function of these organelle localizations is unknown. In this paper, problems with respect to the definition of 'yolk' are described and the extent of our ignorance concerning oocyte organelles is discussed.