Primary Unilaminar Follicle
By birth, the number of oogonia present
in the ovary has decreased to 1,000,000 and by puberty, that
number has decreased to 40,000. After the onset of puberty, between
15 and 20 primordial follicles develop into primary follicles
with each ovarian cycle. The primary follicle initially consists
of a primary oocyte surrounded by a layer of cuboidal or columnar
granulosa cells. The zona pellucida, a thick layer composed of
glycoproteins and acid proteoglycans, forms between the oocyte
and granulosa cells. At this stage in development, the stromal
cells surrounding the follicle also become more prominent. As
the follicle develops from a primordial follicle into a primary
unilaminar follicle, the primary oocyte completes it growth.
In order for an oocyte to grow properly, it must be surrounded
by a layer of granulosa cells. Gap junctions connect 
the oocyte and surrounding granulosa cells and transport amino
acids, nucleotides, and lipid precursors into the oocyte.Growth
factors such as kit-ligand and GDF-9 are also implicated in oocyte
growth. Kit-ligand presumably binds to tyrosine kinase receptors
on the oocyte membrane, which in turn activate transcription
of growth genes, while GDF-9 is produced by the oocyte itself.
All of follicular development up until the stage of the primary
unilaminar follicle is primarily driven towards enlargement and
differentiation of the oocyte.