Primary Oocyte, Primordial And Early Primary Follicles Flashcards
Some oogonia enlarge and undergo nuclear changes preparing them for development into mature oocytes. These oogonia are now known as
primary oocytes and commence the first stage of meiotic division.
Meiosis: is not synchronous as evidenced by a variation in meiotic progression between
neighboring oocytes.
Oocytes in the inner gonad have been observed to commence
meiosis first (and initiate follicle formation and growth first) and those located at the periphery are last.
The DNA replicates and homologous sets of chromosomes pair up to form
tetrads.
The primary oocytes then lose their intracellular bridges and rapidly become encapsulated by
a single layer of flattened epithelial pre-granulosa cells, the cellular origin of which remains controversial (may be ovarian or mesonephric surface epithelial cells).
Finally, the somatic cell-surrounded oocytes arrest in meiotic prophase I at diplotene becoming
the primordial follicles.
This pool of primordial folicles forms the reservoir of female gametes in the fetal ovary from which
all mature oocytes will develop.
The size of the pool (ovarian reserve) may regulate the
reproductive lifespan.
Meiosis will not resume until after the female reaches
sexual maturity.
Some primary oocytes will remain in this suspended animation (meiosis I) until
menopause which could be as long as 50 years.
Indeed, the majority of the follicles in the ovaries of human adults are
primordial with a follicular diameter of 30 to 50 uM.
Not all primary oocytes survive to form follicles. Some of the ocytes may demise altruistically to ensure
that one or two from the cluster (nest) can survive.
The ‘well-being’ of an oocyte may be used as a measure
by which to select it for survival or cell death.
Estimates of the proportion of oogonia that enter meiosis in the human fetal ovary vary from
50%-90%.
Primordial follicles have been observed as early as
13 weeks of gestation in the human fetal ovary but they generaly become abundant around 16-20 weeks.
Regulation of primordial follicle formation is a complex developmental process that involves
the expression and coordination of many growth factors.
Neurotrophins, soluble polypeptide growth factors have various roles in non- neural tissues including
the ovary.
Locally produced factors including neurotrophins and their corresponding receptors, circulating hormones and steroid factors have been reported to regulate
nest breakdown and induce primordial follicle assembly in addition to factors involved in the apoptotic pathway.
The rate of follicular recruitment varies with age
increasing steadily during childhood and adolescence to a maximum during late teen years followed immediately by a continuous decline.
Primordial follicle activation occurs when
their flattened pre-granulosa cells become cuboidal; They are then known as primary follicles.
The follicle diameter doubles to about
0.1 mm.
Neurotrophins have also been shown to have a role in follicle activation , in particular,
nerve growth factor (NGF).
Molecular changes trigger the activation of a selected cohort of
dormant primordial follicles by inducing signaling changes in the intraovarian environment surrounding them.
From this point on oogenesis is intimately involved in
folliculogenesis.
