Ovogenèse

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Gamete formation in the female s’called oogenesis.

It takes place in the ovaries and includes all changes that allow the formation of female gametes, oocytes from stem cells of the germ line or oogonia.

I- RUNNING OF’OVOGENÈSE :

L’oogenesis comprises propagating phases, d’growth and maturation.

The phase’growth and early maturation s’to perform’inside the ovarian follicle and are linked to’evolution of the follicle.

The end of maturation is delayed. it s’completed after fertilization.

There is no differentiation phase. The female gamete is a secondary oocyte before fertilization.

MULTIPLICATION PHASE :

She dérpule only during fetal life, of 3e T in month, in the cortical area of ​​the’fetal ovary.

the oogonia, diploid, multiply by successive mitoses. These are small cells (15 pm), round and relatively poor in organelles, without morphological difference next generations.

The number of successive divisions is not known, but forms around 7.106 primary oocytes. Each’surrounds of’a layer of follicle cells to form a primary ovarian follicle.

a- The primordial follicle :

C’is still the type of follicle most abundant on a cut of’ovary.

C’is a sphere of 50 microns in diameter which comprises :

– A primary oocyte or primary oocyte

C’is a round cell 20 diameter of 30pm..

During fetal life, it duplicates its DNA and begins meiosis. But’s evolution’stops at prophase of the 1st Division, the diplonema

  • The core is large, slightly off. It maintains its nuclear envelope, but chromosomes are apparent. There is a large nucleolus.
  • The cytoplasm is poor in organelles.

A layer of follicle cells flattened, endothéhformes.

In young women, you can sometimes find 2 oocytes in a follicle same, but their evolution is exceptional.

Since its formation, the primordial follicule Devient a quiescent structure here assures the survie the germ cells. At this stage, it is insensitive to external influences.

II- EVOLUTION OF OVARIAN FOLLICLE (FOLLICULOGENÈSE) :

For most follicles, s evolution’stop sooner or later by a mechanism called l’follicular atresia.

The follicular atresia s’accompanied by the death of’oocyte and follicle results in the absorption.

It concerns all the follicles that commit before puberty (the very small amount of’sex hormones does not complete revolution of the follicle).

Atresia For most follicles moving during active sexual life, from puberty to menopause.

It causes a rapid decrease in the number of follicles :
7 106 at 7 month development
1 106 at birth
350 000 at puberty
8000 at 45 years.

L’Complete development of follicles can be done during the active sexual life (from puberty to menopause). During this period :

  • About 450 follicles leading to the release’a mature gamete (13 per year for 35 at 40 years).
  • L’evolution of the follicles is synchronized with cyclical changes of sex hormones (estrogen and progesterone), responsible for the menstrual cycle 28 days.
  • The release’a mature gamete (l’ovulation) occurs milçîu the menstrual cycle, at 14e

The different stages of’follicle development are defined by their morphological : primary follicle, secondary follicle and follicle cavitary or antral.

The terminal stage of’evolution, after the resumption of the nuclear oocyte maturation, will be the follicle de Graaf.

a- The primary follicle :

His pass diameter 50 to 80pm.

– L’ovocyte I :

It is still blocked in prophase, begins the great increase phase. Its size increases to 50 pm.

The mitochondria and Golgi grow. The plasma membrane villi between which emits glycoproteins will form the outline of the zona pellucida. The latter is not visible ei microscopy.

– The follicle cells :

They become cut and are arranged in a single layer.

b- Secondary follicle (or pre-antral follicle or full) :

  • Its diameter gradually Password 80 at 200 pm.

– L’ovocyte I : continues to grow (and reached 80 g).

The syntheses are active. Mitochondria multiply. The Golgi apparatus and membrane microvilli grow.

– The pellucid membrane :

The visible light microscopy. This is a hyaline structure, composed of glycoproteins whose origin is essentially oocyte (but follicular cells in manufacture a portion).

– Follicular cells :

They multiply and are arranged in twenty layers around the oocyte. They are granulosa. The innermost layer, régulièrement disposée around the pellucid membrane, if nomme the Corona Radiata. The junctions between the membrane expansions of the oocyte and those of the cells of the corona radiata. The presence of these cell processes in the zona pellucida is why it appears finely striated in microscopy.

– La membrane de Slavjanski

– The theca interna

It forms around the basement membrane by differentiation of cortical stroma. Cells, initially fusiform, become cubic. They acquire receptors for LH. They will gradually take the characters cells developing steroid hormones.

Theca interna includes several cell layers and is richly vascularized.

At this stage, synthesis’steroid hormones, both granulosa by the theca interna is insignificant

At the end of the stage of full follicle (antral follicle pre-), it appears, au sein granulosa, small rosette formations, the « body and Call Exner ». A level, the follicular cells surrounding small cavities whose liquidiennes confluence will constitute Tantrum.

c- tertiary follicle (or cavitary or antral) :


It is defined by’appearance of small cavities in the granulosa containing a liquid called « Liquor folliculi »

The follicular diameter continues to increase to 10 at 15 mm at the end of this stage. L’evolution of tertiary follicles can be divided into 2 steps :

– Follicle not hormonogen. His pass diameter 0,2 at 2 mm. At this stage of growth remains little hormone and hormone production is negligible.

– Follicle tertiary hormonogen. From a diameter 2 mm, growth hormone and follicle becomes s’accelerates under’influence of hormonal stimulation. Hormonal production appears and quickly becomes important.

L’endocrine activity of’ovary will be the sum of the activities of follicles present in hormonogènes’ovary at one time.

All structures of the follicle involved in the development.

– L’oocyte :

It is still blocked in prophase I and reached 100 pm diameter.

In the cytoplasm, Golgi fragments and accumulation of ribosomes observed, small vesicles and multi vesicular body.

L’oocyte accumulates a large number of mRNA that used for protein synthesis after fertilization (during segmentation). L’oocyte also developing a decondensation factor that will allow the sperm nucleus to unpack after entering the’oocyte.

– The pellucid membrane achieved 15 d pm’thickness.

– granulosa

The continuous cell proliferation. Despite the follicle size, granulosa n’is not vascularized.

From the diameter of 2 mm. the sensitivity of cells to FSH increases. The number of FSH receptors increases under’action of’activine (also produced by granulosa, c’is a dimer of the chain of inhibin) Cell proliferation and s’speeds up to form several million cells, then the number of mitosis and decreases as the size increases follicular.

– Cells, strongly stimulated by FSH, become hormonogènes. The production’estradiol is by transformation of androgens produced by the theca interna under the effect of’aromatase

– L’antrum :

It grows by accumulating the liquor folliculi produced by granulosa cells. It is a liquid rich in hyaluronic acid

During the development of’antrum, the oocyte is discharged on the side of the follicle. It remains surrounded by a cluster of follicular cells constituting the Cumulus oophorus (Cumulus ou proliger) which keeps the egg attached to the rest of the follicle.

– La membrane de Slavjanski

– The theca interna :

well vascularized, it produces androgens converted into estrogen. The synthesis of the theca interna activity is stimulated by LH and by’inhibine (produced by granulosa).

– The external library :

It is around the previous. It is fibrous connective tissue which condenses around the theca interna. Its boundaries with theca interna are poorly defined. It has no activity hormonogen.

MATURATION PHASE :

Unlike what happens in the male sex, the maturation of female gametes ends in case of fertilization. At the moment of fertilization, the oocyte is at the stage of oocyte II.

The maturation phase begins in response to hormonal stimulation by the l.h. (luteinizing hormone or luteinizing hormone). The increased production of estrogen triggers a significant increase in LH secretion (LH peak), which begins 36 h before ovulation.

  • The follicle becomes a Graafian follicle.
  • The oocyte completes its first meiotic division.
  • Moreover, LH surge control ovulation.

The mature follicle ( Graafian follicle) :

Its diameter is 18 or even 20 mm. 11 bomb on the surface of the ovary and the connective between the follicle and the ovarian epithelium becomes very small.

Its size is linked to the development of Tantrum and the various external structures decreased thickness (granulosa and theca internal and external). Its evolution is conditioned by the LH surge, who (in synergy with F.S.H.) control the’ovulation.
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– L & rsquo; oocyte :

It has a diameter of 120 at 150 \xm.

Under’influence of LH, syntheses s’stop and meiosis resumes.

– The core migrates to the periphery of the cytoplasm and resumes the process of meiosis. An achromatic unit (without centrioles) appears.

– The division s reductional’ends et s’accompanied by the expulsion of the first polar body (substantially simultaneously with the laying ovular). L’oocyte becomes an oocyte II. After ovulation, it will lock into metaphase 2e meiotic division.

– Cytoplasmic maturation completes the establishment of mechanisms that s’oppose the Polyspermy. L’oocyte remains bound to the cells of the corona radiata by membrane extensions which pass through the zona pellucida.

– The areas pellucid increases slightly from’thickness.

– Le Cumulus oophorus :

Cumulus cells secrete mucopolysaccharides, in particular the’hyaluronic acid (responsible for filance Cumulus vitro), dissociate and eventually detach from the rest of the granulosa. At the time of ovulation, l’oocyte, surrounded the pellucid membrane and the Corona Radiata, is already free in Tantrum within’a cumulus more or less dissociated.

The Corona Radiata et quelques cumulus cellules du sont encore presentes au moment de la fécondation, the outer third of the fallopian.

– L’antrum :

Its volume increases rapidly. He reaches 3 at 5 ml while the diameter of the follicle can reach 20 mm.

– granulosa :

– The LH peak directs hormonal synthesis to the production of progesterone. C’is the beginning of the luteinizing specifying if during the formation of the corpus luteum. – The proliferation s’stopped, likely due’a paracrine effect of progesterone (the follicular cells possess progesterone receptor). A the moment, there near 50.106 follicle cells. They are arranged in several layers to the inner face of the basement membrane.

– La membrane de Slavjanski :

It is still present. It opposes the penetration of vessels. However, at this stage, macrophages across the basement membrane. They will be involved in the development of vascularization of the corpus luteum after’ovulation.

– The theca interna :

As with granulosa, hormone synthesis changes under’effect of LH and is moving towards the production of estrogen. After’ovulation, these elements participate in the formation of the corpus luteum.

– The external library :

It becomes denser compression of tissue structures due to the’rapid increase in the follicular volume.

Oocyte maturation preovulatory

The first meiotic division gives birth to 2 Daughter cells of very unequal value :

  • A secondary oocyte which has retained substantially all of the cytoplasm
  • The first polar body.

Jones II cells un de petite taille (4 at 5 pm) whose genome is equivalent to that of’oocyte II, but the cytoplasm is extremely small.

Usually, l’nuclear envelope does not reform and the cell does not divide. The division (or « expulsion polar body ») occurs at about the time of’ovulation. The polar body rest against the’oocyte in the peri-oocyte space, to’Near the pellucid membrane.

The 2e meiotic division immediately began, without inter kinetic stage, s more’stop metaphase, at the end of 6 at 7 h. C’is at the stage of’oocyte in metaphase II as gamete can be fertilized.

III- INVOLUTION OF OVARIAN FOLLICLES :

L’follicular atresia For most follicles s’commit. Ce s phenomena’observed from fetal life and continues until’to’stocks follicles to menopause.

IV- OVULATION :

The release of the female gamete is ovulation.

It occurs in the middle of the female cycle, 36 h after the onset of the LH surge, at 14e day of the menstrual cycle (for a normal cycle 28 days).

L’whole cumulus s’is then detached from the rest of the granulosa (under’action d’proteolytic enzymes) and floats in the follicular cavity. C’is a viscous mass of several mm3.

L’Ovulation occurs only s’there was an LH surge before. C’is a complex mechanism, teaming up :

– Vascular phenomena :

Under pressure from the mature follicle, the surface portion of the’ovary undergoes ischemia. L’stop vascularization was observed macroscopically on the surface of’ovary by’appearance’a small roundish area, stigma, color and transparency different from the rest of the ovary wall.

– Enzymatic mechanisms :

The F2-D ovarian prostaglandins induce the release, by cells of’ovarian epithelium, d’lysosomal enzymes which lyse connective ovarian wall.

– Muscle twitching :

The smooth muscle cells of’ovary contract

The break is brutal. It is about all the follicle wall (granulosa, membrane Slavjanski and thecae), the thin layer of connective stroma overlying the follicle outer side and’ovarian epithelium.

Upon’opening, due to the tension of the follicle and the contraction of smooth muscle cells of’ovary, content Tantrum flows abruptly to’outside, in the peritoneal cavity. The assembly formed by the oocyte II and its envelope is driven. It is recovered by the flag of the trunk, covering the ovary at this time. L’ovulation is accompanied by a small amount of bleeding of the ovary wall.

Fertilization takes place in the fallopian. If it does not occur, the egg degenerates after 24 ish.

V- EVOLUTION FOLLICLE dehiscent :

Immediately after the’ovulation, the follicle becomes a temporary endocrine gland. I iaunet body luteinizing). He wrinkles due to sagging. The cell mass is penetrated by vessels. The granulosa cells and theca interna alter their hormonal syntheses and become luteal cells, and para developing luteal progesterone and some estrogen. This luteinization elements is controlled by the peak l.h. preceding’ovulation.

WE- MIGRATION IN THE OVUM GENITAL TRACT :

At the time of’ovulation, the flag of the trunk, mobile, covers the’ovary. It recovers’oocyte IL Proper mobility tubal pavilion is essential for reproductive function.

Quickly, egg migrates up’to’bulb tubal where fertilization is done.

Migration is passive. It is facilitated by :

  • The current fluid from the pavilion to the’uterus,
  • The movements of cilia’tubal epithelium.

During this trip, cumulus partially dissociates.

VII- THE FEMALE GAMETES :

/Users/adlane/Downloads/médecine/1ère année/Embryologie/Constantine/16/media/image7.jpegA- L’OVOCYTE :

It is a spherical cell 150 pm diameter, relatively inert, no synthetic activity. it n’has not completed its nuclear maturation. It is a secondary oocyte (n chromosomes et 2n ADN), blocked in metaphase 2e meiotic division. It is surrounded by envelopes

  • The plasma membrane bristles with microvilli. Junctions with killed cells- oocyte disappeared and the extensions crossing the zona pellucida is retracted. The membrane has receptors for sperm.
  • the cytoplasm contains few endoplasmic reticulum, but contains many free ribosomes, poor mitochondria ridges and large quantities’inactive RNA, set aside for early development. In peripheral, cortical granules (from 1 at 2 pm diameter) are arranged in 2 at 3 layers beneath the plasma membrane.
  • The nuclear material is blocked in metaphase 2e meiotic division. Chromosomes are haploid number (but with 2n DNA). All oocytes are similar (22 autosomes you a gonosome X). Mitotic figure is very eccentric. Its dimensions are small compared to the size of the’oocyte (10 pm). it n’there is no centrioles to spindle poles, but more or less disorganized microtubule.

Oocyte vitality is reduced. In the absence of fertilization, l’oocyte survives 24 at 48 h. It ages gradually : more fertilization occurs late, the greater the risk of’du concept anomalies.

Fecundability of’oocyte (ability to merge with the male gamete) is conditioned by the degree of nuclear and cytoplasmic maturation (blocking metaphase 11).

2- THE ENVELOPE OF OVOCYTE :

Of the’oocyte towards the periphery, on distingue :

a- The perivitelline space :

This is a clear very small space, except in the area of 1is polar body. This contains 23 separate chromosomes, without replenishment of its nucleus.

b- The areas pellucid :

It is a layer 15 at 20 d pm’thickness, consisting of glycoproteins. Electron microscopy, it has a very fine fibril structure. It involves 2 different density layers, the densest layer of the side of the oocyte. It is crossed by gaps, which are the locations of the cell processes which are retracted.

c- The cells died oocyte :

Against the outer face of the zona pellucida, a contiguous layer of cells forms the corona radiata. Around, are cells of Cumulus. Intercellular spaces are wide and occupied by a viscous substance composed of glycoproteins and’hyaluronic acid

Courses of Dr. Hannachi – Faculty of Constantine