Apolipoprotein A-I production by chicken granulosa cells

In avian species such as the chicken, development of the oocyte is associated with massive deposition of yolk in this cell. Oocytes grow within the follicle, a compartment consisting of a very specialized set of cells and acellular structures. The oocyte is surrounded by the perivitelline layer and granulosa cells, which are separated from the thecae by a pronounced basement membrane. In addition to the production of yolk precursors in the liver, we have long implied that cells within the follicle make a direct contribution to the growth of the oocyte. Here we show that chicken granulosa cells express and actively secrete apolipoprotein A-I (apoA-I) as a part of particles with very high density. The granulosa cell-derived, apoA-I-containing material is different from the small portion of yolk high density lipoprotein that arises via transfer from the peripheral circulation. We propose that the ApoA-I-containing particles secreted by granulosa cells 1) support the growth of the rapidly growing germ cell, possibly by direct lipid transfer to the plasma membrane of the oocyte, and/or 2) deliver cholesteryl esters to the steroid-producing cells of the theca layer. These findings are discussed with respect to the proposed functions of apoE (an apolipoprotein not found in chicken) within the mammalian follicle.