Graafian Follicle
The Graafian follicle is characterized
by a large, fluid-filled antrum, and an eccentric oocyte. The
granulosa cells can be divided into two groups; the zona granulosa
is a thin layer along the periphery of the follicle and the corona
radiata surrounds the oocyte. The oocyte has undergone the first
meiotic division, giving rise to a secondary oocyte and the first
polar body. The secondary oocyte is now arrested in metaphase
of the second meiotic division and will so remain until fertilization.
The first meiotic division appears to be initiated by LH acting
on granulosa cells, however the exact mechanism of action is
unknown. A current model makes use of the observation that high
cAMP levels inhibit meiosis of the oocyte. In the absence of
LH, cAMP is transported from the granulosa cells into the oocyte.
It is theorized that LH blocks 
cAMP transport into the oocyte, thus removing inhibition of meiosis
and allowing meiosis to proceed. Oocytes also undergo cytoplasmic
maturation, a series of changes in protein synthesis and structure
which prepares the oocytes for embryogenesis in the event of
fertilization. The Graafian follicle represents the final stage
of follicular development before ovulation.