Localization and transport of lipids in the avian ovarian follicular layers and the structural relationship of theca and granulosa to the basement membrane

We describe the localization of lipids in the wall and superficial ooplasm of the largest avian ovarian follicles by the use of different fixatives and light and electron microscopy. We demonstrate that each yolk globule is always accompanied by one or more highly osmiophilic and sudanophilic alcohol insoluble yolk masses, which we have called satellite yolk. Together with the protein containing yolk globule it forms an integral morphological part of a compartmentalized, bipartite yolk system. Cytochemical, histoautoradiographic, biochemical, and light and electron microscopical aspects of satellite yolk were studied. At the start of satellite yolk formation in the 3–4 mm diameter follicle (when the oocyte begins to yellow) the distribution of the microcirculation of the follicle wall becomes printed on the underlying superficial ooplasm of the oocyte. The oocyte then presents so‐called yolk mountains (containing satellite yolk), only localized below the thecal capillary sinus and not below the efferent and radially perforating thecal veins (black hole regions). We also describe the structural continuity between the thecal intercellular spaces and the microvilli‐associated extracellular spaces of the granulosa cells via the basement membrane. The thecal cells present centripetal extensions into the basement membrane and the basement membrane material extends centripetally into the granulosa microvillar channels. Therefore, at least two cellular barriers are crossed when fat or fat precursors are transported from the thecal capillary sinus to the ooplasm.