Maturation of the oocyte in the dominant follicle shortly before ovulation
Review: The primary oocyte
In the first week of the cycle the maturation of the oocyte in its associated follicle depends on the progress of the maturation of the surrounding follicle cells. The fittest follicle with its oocyte becomes the dominant follicle in the second cycle week and, later, a graafian follicle (Fig. 9).
Up to just under two days before ovulation, the maturation of the oocyte consists in its ingestion of substances (growth of the yolk) that are supplied by the surrounding granulosa cells. This exchange of substances is mediated through cytoplasma processes of the granulosa cells that are anchored through the pellucid zone at the oocyte surface (Fig. 10). The oocyte nucleus [2n, 4C] is also matured in the last days before the LH peak. Up to that point it was arrested in the extremely elongated prophase (= dictyotene) of the first meiosis (the arrested condition that has existed since the fetal period). Through the «maturation» the nucleus changes in the diakinesis (of the prophase) and prepares itself for the completion of the first meiosis, which is triggered by the LH peak.
With the LH peak the following maturation steps are now triggered in and around the oocyte - up to ovulation:.
In the oocyte:
- Termination of the first meiosis with ejection of the first polar body
- Begin of the second meiosis with arrest in the metaphase
- Maturation of the oocyte cytoplasma by preparing molecules and structures that will be needed at the time of fertilization
In the follicle:
- The granulosa cells that sit just outside on the pellucid zone withdraw their processes from the oocyte surface back into the pellucid zone. Those processes were in charge of transferring substances to the oocyte.
- The perivitelline space forms between the oocyte and the pellucid zone. This space is necessary for allowing division of the oocyte and for harboring the first polar body formed in the division.
- Loosening of the granulosa cells in the vicinity of the cumulus oophorus and proliferation of the granulosa cells.
- Increasing the progesterone concentration in the follicle fluid via increased production in the granulosa cells.
The results of these processes are:
- The correct placement of the uterine tube infundibulum upon the ovarian surface.
- The rupture of the follicle wall and the flow of the follicle fluid with the oocyte into the infundibulum.
- The inhibition of the maturation of further follicles.
Termination of the first meiosis
The spindle apparatus for dividing the chromosomes has formed and oriented itself radially to the cellular surface. The first polar body will arise at the spot where the spindle apparatus is anchored on the cellular surface. Further, the processes of the granulosa cells have retracted from the oocyte surface into the pellucid zone. They have released themselves from the oocyte and this leads to the formation of the perivitelline space. In this space the ejection of the first polar body takes place as a sign that the first meiosis has ended.
With the end of the first meiosis the name of the oocyte changes from primary oocyte to secondary oocyte.
The secondary oocyte
Through the effects of LH on the granulosa cells, these have begun to loosen their cellular bonds and to multiply. They now also produce progesterone that is released into the follicle fluid.
Through the separation of the homologous chromosomes in the first meiosis a haploid (reduplicated) set of chromosomes (1n, 2C) is now to be found in the secondary oocyte. The first polar body also contains 1n, 2C. Via a fine cytoplasmic connection the polar body and oocyte remain bound together following the meiotic division, similar to what takes place when male gametes are formed.
The role of progesterone in the follicle fluid
According to present knowledge, progesterone has the following two main tasks in the follicle fluid:
- It stimulates the further maturation of the oocyte
- During ovulation it enters the fallopian tube and guides the formation of a concentration gradient for attracting the sperm cells
The follicle that is about to rupture
Besides the hormones, the granulosa cells also secrete an extra-cellular matrix, mainly hyaluronic acid, into the follicle fluid. Before ovulation the follicle fluid volume increases markedly. The cumulus cell bonds loosen further. In this way, together with the enclosed oocyte, they free themselves from where they were attached to the follicular wall and now swim in the follicle fluid. The wreath of granulosa cells that enclose the oocyte is called the corona radiata.
The oocyte has now ended all the steps of maturation that were set into motion by the LH peak. The molecular and structural preparations for the time following the penetration by the sperm cell have now been made in the cytoplasma. A spindle apparatus (2nd meiosis) has again been able to form with the chromosomes in the equatorial level (metaphase plate). The spindle is once more anchored radially to the cell membrane near the polar body. The same processes of spindle formation also take place in the polar body.
The second meiosis is arrested in this position. The final steps of the maturation, namely the freeing for the second meiosis, are first completed by the secondary oocyte when the spermatozoon has penetrated the oocyte.
The follicle and the oocyte are now ready for ovulation that takes place roughly 38 hours after the LH peak.