Female Reproductive Physiology- the oocyte/follicle

Development

All embryos have 2 primitive reproductive duct systems (Wolffian (male) and Mullerian (female)).  In females, absence of testosterone and mullerian inhibiting factor (present with Y chromosome) causes the Wolffian ducts to regress and Mullerian duct to continue development to become the female reproductive tract.

Functions of the female reproductive tract

Oogenesis

Oogenesis takes many years to complete, beginning in utero:

  • The first stage of meiosis occurs in in utero/early life with the doubling of genetic material.  However, the cells do not divide at this stage and are known as primary oocytes.  They are surrounded by a single layer of granulosa cells, and together make up a primary follicle.  There are around 2 million of these at birth (initially 6-7million, the rest undergo apoptosis).
  • Between birth and puberty, the majority will undergo atresia (degeneration and scarring), leaving just 300,000 at puberty.  Most of these will also undergo atresia too.
  • At puberty, primary follicles begin to develop in a cyclical manner.  The mechanism for selection is unknown.  In this process, the cytoplasm volume increases dramatically (for nutrition of a potential embryo) and the next stage of meiosis occurs (the first division).  Both daughter cells receive a full set of chromosomes but one will receive most of the cytoplasm.  It is this cell that will go onto become the ovum.  The other daughter cell (now known as the polar body), will degenerate soon after division.
  • The secondary oocyte is what is released as an egg during a period.
  • The menopause is when the supply of primary follicles has depleted.
  • Development into the mature ovum (haploid cell after second division) occurs at fertilisation.  It is only at this point that the secondary division takes place and another polar body (secondary PB) is released and degenerates.

The ovarian cycle 

Consists of two phases:

  • Follicular phase- maturation of the follicles

The follicles that are maturing at the beginning of the follicular phase will have had to undergo some major changes at the correct time (FSH surge) (those that are not will undergo atresia):

    • Proliferation of the granulosa cells and formation of the zona pellucida
      1. The single granulosa cell layer becomes multi-layered.  They also secrete a gel like substance surrounding the oocyte known as the zona pellucida.  Gap junctions through the zona pellucida connect the granulosa cells to the oocyte and provide a channel through which nutrients (glucose, amino acids etc) and hormones can flow.
      2. At the same time, specialised ovarian tissue proximal to the follicle differentiate to produce another ‘coating’ round the oocyte.  These are thecal cells and they secrete oestrogen along with the granulosa cells.
    • During the follicular phase, the follicle develops an antrum (to become a secondary follicle- capable of oestrogen secretion) which rapidly grows in size prior to ovulation.  One of the follicles (the mature follicle) does this more enthusiastically than the others, to develop first into the mature (Graafian) follicle with many FSH receptors.
    • Just before ovulation, the oocyte completes its first meiotic division.  At ovulation, there is a large surge of LH and FSH that causes the ovum to rupture from the follicle.
  • Luteal phase- presence of the corpus luteum
    • After ovulation, the thecal and granulosa cells collapse into the antrum space and undergo leuteinisation to become the corpus luteum.  This becomes highly vascularised and secretes lots of progesteron (and smaller amounts of oestrogen).
    • If the released ovum is not fertilised and doesn’t implant, the corpus luteum degenerates.

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