Yolk transport in the ovarian follicle of the hen (Gallus domesticus): lipoprotein-like particles at the periphery of the oocyte in the rapid growth phase.

Thin sections of the oocyte periphery and surrounding granulosa layer from 1--5 day preovulatory follicles were examined by transmission electron microscopy. With the use of certain procedures in tissue preparation, notably the tannic acid method, numerous particles in the range of 15--40 nm with a mean diameter of 27 nm were observed in both extra- and intracellularly. The particles were abundant in the granulosa basal lamina, in the spaces between the granulosa cells and in the perivitelline space. They appeared to adhere to the oolemma as a continuous double layer which was also observed to line the coated vesicles, 200--350 nm in diameter, invaginating from the oolemma. The layer of particles was not found on the plasma membranes of the granulosa cells, nor were particles present within the cells. In the peripheral cytoplasm of the oocyte the yolk spheres, ranging upwards from 250 nm diameter, were membrane-bound and contained tightly packed particles similar to those on the oolemma. Bodies displaying features intermediate between coated vesicles and yolk spheres suggested that, on entry into the cell, loss of the cytoplasmic coat and obliteration of the vesicular lumen gave rise to nascent yolk spheres which then fused together to form the larger spheres. The extracellular layer, coated vesicles and smaller yolk spheres were absent in oocytes fixed after a 10-min delay. The evidence indicated that 27-nm particles were transferred from the basal lamina to the oocyte surface via the intergranulosa cell channels, incorporated into the cell by adsorptive endocytosis and then transferred to the yolk spheres with little morphological alteration. The identity of the particles with very low density lipoproteins, the major components of the yolk solids, was discussed.