Control of ovarian follicles The dynamics of follicular growth /

S:Growth of Follicles, Different developmental Phases of Follicular Development of Follicle.

See Book on IVF: - by Kay Elder 3^rd Ed pp.37.

Population of oogonia:-Oogonia uncovered by follicular cells à migrate from genital ridge to primary undiff gonadà Cells from the sex cords condense to form primordial follicles. It is the cells of the cortical sex cords will form the somatic component of follicular Granulosa cells, Theca cells, endothelial cells & supporting tissues. After reaching the gonadal ridge by 25-30 daysà oogonia divides rapidly to form by mitosis)à for couple of days only. Around 12 Th week of gestation the oogonia starts first meiosis. By contrast, in males the first meiosis commences at puberty. The population of oogonia at 8 weeks is 600 000 (6×10⁵  ) and later by mitosis becomes 6-8 million; 6×10⁶   at 16-20 weeks. After 20 Th weeks, the mitosis process stops. Moreover those oocytes which fail to acquire systemic cells envelope of gr. Cells do degenerate. Later this uncovered oogonia dies and total primary oocyte become 1-2 million at birth.

Population of Oogonia.

Again in females the naked oocytes are covered by systemic layer on layer of granulosa cells which are designated now as primary oocytes.àAfter initiation of meiosis 1, (this starts at 9-10 weeks), they become arrested at diplotone stage of prophase 1, (at about 16 weeks), and after chromatid exchange and crossing over (diakinesis) have taken place. This is last phase of prophase I. These arrested oocytes are termed as the Dictyate stage---Germinal Vesicle stage. GV means a large nucleus packed up by the visible chromosomal threads in a large nucleus.

At puberty: there is initiation of i.e. resumption of meiosis. This is process is a long drawn process and resumption of meiosis is not complete unless Graafian follicle (ANTRAL FOLLICLE) - is recruited at puberty. The process of oogenesis from primordial germ cells to pre-ovulatory oocyte take about minimum 11 years.

1)    Maturity of Primordial Cells: - oocyte (with its organelles, mitochondria, and other cellular factors) is covered by single layer of flattened pre-granulosa cells. These cells are linked to the oocyte by gap junctions and other cellular connections.  The surrounding stromal cells secrete Type IV collagen, laminin and fibronectin, these proteins form a basement mm. around each cluster of cellsà reproductive span of human female is about of 35 years.

2)    Growth of follicles at puberty: - Out of these follicles some will be called for growth and development-each phase span for about 6 month primordial follicles remain arrested for upto 50 years. Waiting for a signal; to resume development but from puberty each day few follicles are recruited on daily basis. These  enter into GROWTH  PHASEà There are three phases of such development once recruited :-

3)    A) Primaryàalso called preantral.

B) Secondary follicle (Antral Follicle)-also called Graafian follicle.-

C) Preovulatory. The size of the follicles increases from 7 mm to 18 mm by 10-20 days. And the no of gr cells increases from 2-5 million to 50 million by couple of days. The basement mm expand 400 times than original.

This also involves acquiring competency of theca interna cells into steroidogenic competency and theca externa connective tissue cells into outer layers.

The names of growth factors-products of genes that are expressed specifically in the oocytesà  are:-BMP-s, GDF-9, F1Galpha, FIGLA,BMP-15, AMD, cKit, Kit ligand etc.

Issue: A: What is the time span from initiation of growth of dormant follicles to become a mature dominat Follicle in humans? - The classification of different types of follicle is based primarily on morphological appearance of follicles as appear under the ordinary light microscopy, because any follicle < 2 mm cannot be imaged. As such  watching/ tracking the early developments/ growth  of follicles (say from resting phase àtransition to early growth initiation phase) can only be observed only under microscopy after the ovary is excised. To add to the problem, different physiologists, over the last century have issued separate nomenclatures for the same follicle and till date there is no uniformity of standards on microscopical staging of follicles.

 For instance, there is no set criterion as to which morphological appearance is ideal for “preantral follicle”. Therefore, so   far as morphological (microscopical appearance) classification of growing follicles are concerned there is no uniformity in definition till date. Some pay staging of follicles  on number of layers of granulosa cells, some on amount of Liquor folliculi, while still few physiologists depend heavily on number of theca cells as an index of morphological classification of Follicular development.

 Many theories still prevail in the text books of reproductive physiology. Regarding your query how many days it takes from one resting follicle to a mature follicle my overall impression after going through different text books is about 84-90 days. But, I know there are limitations in my statement -as there is no uniformity in nomenclature of follicles itself. Physiologists have documented and opined different time frames in this regard (maturity eggs and factors controlling each type of cells growth in a unit in different stages of development). Both of us are right. But my considered view is that it takes about 90 days sometimes 120 days from “initiation of growth phase”-to achieve to “Dominant Follicle”. It is like variation in commencement of age of menarche of different girls of same race in same geographical area.

Issue B: - Intraovarian Regulators of Follicular Growth I presume that both of us are aiming at factors controlling development of “single and quality egg “per cycle in the reproductive age group. This branch of science is nowadays n broadly designated as “Intraovarian Regulators of Follicular Growth”. When we will be in a position to explore this part of physiology, I hope, we will be able to conquer most unexplained subfertile couple & poor responders. The same proposition holds good for RECEPTIVITY ENDOMETRIUM. At least that is our objective rather than exploring how many days or weeks it takes when a “Resting Phase” or “dormant follicle “is called for “Growth Initiation Phase” which is < 2 mm in diameter-therefore cannot be imaged by USG but can be documented by microscopyà finally become Graafian Follicle. Therefore, we the clinicians who read the growth of follicles in USG only but basic scientists interpret and designate the follicular growth phases are as per microscopical feature. This is the field in which we all are interested i.e. from the growth initiation to mature egg formation, irrespective of time frame.

There are still different phases of active phase (growing phase).Growing phase of follicles are divided into a) slow growing phase which is equivalent to gonadotrophin independent phase. & rapid growing phase i.e. Gonadotrophin dependent phases. Some biologists have described the growth of follicles in growing phases in eight phases, (Class 1 to Class 8) -again as per microscopical appearances as per physiologists classification, in which we are less interested.

Issue C: What are the stimulating chemical regulators of Follicles? What are inhibitory bioactive substances of Follicular growth? This is area where we the clinicians are most interested.  We will happy to know details  of the promoters of follicular growth and inhibitors of follicular growth (as is seen in poor responders). We do not know what causes apoptosis (programmed cell death in the follicular cells) of some particular follicle! These are the grey areas where molecular biologist should come forward and directly guide us and indirectly to subfertile couples and women seeking contraception.

Issue D: The role of Molecular biologists & Growth & development of follicle:We are still unaware of the factors(growth factors, hormones, proteins & enzymes ) which give a call to a particular DORMANT FOLLICLE -> to go in to “phase of growth(Growth initiation”. Which particular genes control such proteins / enzymes/ Growth factors are also unknown. Similarly we are still unaware of the different biochemical agents which allows apoptosis of some follicles as the process of growth continues from Primordial Follicleà Preantral FollicleàAntral FollicleàGraafian follicle. We do not why a healthy woman of aged 30 yrs, mother of three children do not ovulate on 23% of cycles even if physically & endocrinologically normal. Similary we do not know why in the process of growth of follicles why in some follicles the total number of granulosa cells is less or why Thecal cells are much more in another growing follicle in the same cycle. Three different types of cells (Oocyte, Gr Cells & theca cells) are collectively called one UNIT by the physiologists.

Issue E: - Will we be able to synthesize Growth enhancers in infertility cases? Will it be possible to synthesize Follicular growth inhibitors without using oral contraceptives which involves Pituitary: These are the grey areas of reproductive physiology? On the intraovarian regulators of theca three different types of cells (OOCYTE, Granulosa Cell & Theca Cells) are largely unknown. Admittedly research is on the way. When we will be in a position to identify markers of biochemical agents responsible for lagging of growth and able to manufacture such agent in factory then much of use of clomiphene & gonadotrophins will decreases. Possibly by another two decade we will be using such wonderful molecules which will synchronize growth of three different types of cells as observed in a Growing follicle. Admittedly, researchers have pointed out that defective GDF-9 gene is an important cause of female subfertility but still many more defectives genes are involved. Not all ovum nutrients are known to us not all the molecules involved in autocrine-Intracrine –paracrine factors in growth of follicles are known to molecular biologists.