gametogenesis

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Overview :

Gametogenesis is gamete formation process in 2 sexes. It takes place in the genital glands, testes for males and ovaries for females. It concerns a particular cell line, germline (her seed), as opposed to all the other cells of the & rsquo; body, grouped under the term somatic cell (the Soma).

A- THE GERM CELL LINE :

They comprise successively :

1- the Gonies (spermatogonies I ovogonies) :

These are diploid stem cells. They multiply by mitosis equational.

2- Les cytes (oocytes or spermatocytes) :

These are the cells involved in the process of meiosis.

  • The cytes I or first order before the first meiotic division or reduction division.
  • The cytes II or second order are formed from the above during the first meiotic division .

3- the tides (spermatides I ovotides) :

They are formed during the second meiotic division. These are haploid cells that will divide more. Their chromosome number is the same as cytes II, either n chromosomes, but they contain half the d & rsquo; DNA, approximately half of the amount present in a somatic cell interphase.

B- MEIOSIS :

Meiosis is adapted to germline. It concerns the first and second order cytes and combines two successive cell divisions preceded by a single duplication of & rsquo; DNA.

Figure 1. General scheme of gametogenesis

It allows the formation of haploid gametes and & rsquo; exchange of chromosomal segments between the paternal and maternal genomes. This process that accompanies sexual reproduction allows genetic variability of & rsquo; species and allows it, over generations s & rsquo; adapt to changes in the & rsquo; environment.

Meiosis is a unique phenomenon of cell division, own to gametogenesis, in which she plays a key role in ensuring the reduction of chromosome number and the mixing of maternal and paternal genetic information The first division (reduction division) is preceded by duplication of & rsquo; DNA during the end of the & rsquo; interphase and includes successively : Prophase, metaphase, Anaphase and Telophase.

The second division (equational division) occurs very quickly and the & rsquo; we find ourselves directly in prophase.

It will be observed the usual stages of mitosis, for n pairs of chromosomes : étaphase-Anaphase-Télophase. The result will be the formation of 4 haploid gametes.

Meiosis allows :

  • halving the genetic content of & rsquo; a cell.
  • brewing the & rsquo; genetic information.
  • The transmission of the & rsquo; genetic information.
Figure 2. The stages of meiosis

spermatogenesis :

Spermatogenesis takes place in the male gonads or testes. It begins at puberty, peaked at 20-30 years, decelerates to the quarantine but may continue up & rsquo; in old age. Spermatogenesis is a slow and continuous process requiring 74 about days in the & rsquo; man ; C & rsquo; is the duration of the spermatogenic cycle.

A- STRUCTURE OF SPERM :

The sperm cell is a mobile, highly differentiated, whose & rsquo; organization is similar in most mammals.

Figure 3. Structure of the human spermatozoon

1- Morphology :

The sperm is about 60 long pm. In light microscopy, on distingue :

→ Head, elongated and flattened (4 at 5 Along pm and 2 pm d & rsquo; thickness). It contains the & rsquo; acrosome and nucleus .

  • The neck, narrowed and short (1 pm par 1 pm), typically corresponding to the & rsquo; space between the 2 centrioles.
  • The flagellum comprising :

→ The intermediate part (4 at 5 long pm), which contains the “mitochondrial spiral”.

→ The main room, long 45 pm.

→ The end piece, long 1 at 2 pm and fine.

2- Ultrastructure :

The fine structure of the sperm do s & rsquo; observes that & rsquo; electr microscopy

Figure 4. Ultrastructure of human sperm
  • The head : It is oval and flattened. At the boundary between 2/3 past and 1/3 posterior, a sofa, l & rsquo; nuclear ring, corresponds to the posterior edge of the & rsquo; acrosome.
  • The core : It is headed by the & rsquo; acrosome and occupies most of the head. The chromatin is highly condensed, essentially homogeneous, without nucleolus. The posterior pole of the core has a transversely oriented vacuum, the dimple d & rsquo; implementation.
  • L’acrosome : It is flattened and covers 2/3 previous core. C & rsquo; is a vesicle in homogeneous content, limited by a membrane. He understands 2 segments, d & rsquo; appearance and different physiological significance :

– The main segment, forward, the cap 1/3 prior the core and is 70 nm & rsquo; thickness. It contains hyaluronidase. The inner membrane is separated from the & rsquo; nuclear envelope by & rsquo; sub-acrosomal space, from 20 nm.
– The equatorial segment, backward, covers the 1/3 means the core. Its thickness is 25 nm. C & rsquo; is in this segment is primarily the & rsquo; acrosin. The inner membrane is about 40 nm of & rsquo; nuclear envelope.

  • the cytoplasm : It is very small and is a blade surrounding the posterior third of the nucleus, behind the & rsquo; acrosome..
  • The neck : C & rsquo; is the junction area between the head and the flagellum. C & rsquo; is a complex region which contains the & rsquo; centriolaire apparatus and connective piece shaped & rsquo; inverted funnel.

Starting from the core can be described :

  • Le centriole proximal : Is arranged under the dimple of & rsquo; implanting. Its axis is almost parallel to the rear face of the core and forms an angle of 80 ° with that of the & rsquo; axonemal.
  • The 9 dense fibers : They are composed of cytoskeletal proteins. They doubled the inside of the segmented columns and extend to the rest of the flagellum.
  • The axial filamentary complex : It s & rsquo; is formed by lengthening the distal centriole and begins about halfway up segmented columns. Centrally located, inside dense fibers, it has the usual structure of & rsquo; ciliary axis : 9 doublets peripheral surrounding a central doublet.
  • mitochondria : to & rsquo; outside segmented columns, they constitute the beginning of the mitochondrial sleeve continues in the intermediate part of the flagellum.
  • The flagella comprises 3 rooms.

– The intermediate piece : C & rsquo; is the shortest section and the thicker the flagellum. It is limited, at its distal end, by a thickening of the membrane of the flagellum, l’annulus (corresponding to I Jensen ring light microscopy). Cross-sectional , on observe, from center :
→ L & rsquo; axoneme or filamentous axial complex has the typical structure of the & rsquo; ciliary axoneme : 9 microtubule doublets devices surround a central doublet.
→ The 9 dense fibers – Mitochondrial spiral sleeve. It surrounds the dense fibers and consists of & rsquo; a succession of mitochondria arranged in a single spiral line which is approximately 40 tours.
→ The plasma membrane.

– The main room : Its structure is the same throughout its length with, from center :
→ The axial filamentary complex which continues that of the intermediate piece.
→ The 9 dense fibers. They s & rsquo; taper towards the & rsquo; distal end
→ The fibrous sheath. It s & rsquo; is fibrillar proteins spirally wound. This sleeve has two diametrically opposite thickenings : longitudinal columns. In approaching the & rsquo; end of the main room, s & rsquo longitudinal columns erased and & rsquo; thickness of the fibrous sheath decreases.
→ The plasma membrane of the flagellum.

– The end piece : It has a simplified structure with :
→ The axial complex filamentous center.
→ The plasma membrane.

B- Spermatogenesis :

Spermatogenesis begins around 13 -14 years, at puberty, and continues until & rsquo; at a very advanced age. It takes place in & rsquo; within the seminiferous tubules. Its duration is fixed and characteristic d & rsquo; a species.

1- Phase multiplication :

spermatogonia, diploid strains (2n chromosomes et 2n ADN), multiply by mitosis. These are the most peripheral germ cells. They are in the basal compartment of the tube, in contact with the basement membrane and Sertoli cells.

  • Al spermatogonia or Ad (dark) are the true stem cells.
  • Spermatogonia A2 or Ap (pale) represent the first stage of spermatogenesis cell itself and have a life of 18 days before dividing into 2 spermatogonies B.
  • The B spermatogonia divide after 9 days to 2 spermatocytes I, the 2 spermatocytes do not separate completely, cytoplasmic bridges persist until & rsquo; at the end of spermatogenesis allowing exchanges between all cells deriving from & rsquo; one Gonie and enable synchronous evolution.

2- Phase d & rsquo; increase :

It concerns spermatocytes and schematically corresponds to the & rsquo; interphase and early prophase of the first meiotic division. The cell size increases by accumulation of material synthesized. When & rsquo; they reach their maximum size, from 25 pm, they are known by the name of & rsquo; auxocytes.

3- Maturation phase :

It concerns spermatocytes. It s & rsquo; d & rsquo basically, a nuclear maturation, this phase corresponds to the meiosis. Prophase is long (about 16 days). At the end of the prophase, les spermatocytes I renferment 2n chromosomes et 4n ADN.

  • The first meiotic division (editorial division). It occurs after 23 days. Prophase continues the metaphase, l & rsquo; anaphase and telophase. This first division results in the formation of two second-order spermatocytes or spermatocytes II. Each spermatocyte II has half the number of chromosomes present in I spermatocyte
  • The second division of meiosis (equational division). it occurs 24 hours after the first. Each secondary spermatocyte gives rise to 2 spermatides, haploid. They will divide more. spermatids (n chromosomes, n ADN) are round cells small.
Figure 5. General scheme of spermatogenesis

4- Differentiation phase (spermiogenèse) :

Spermiogenesis is specific for male gametogenesis. It corresponds to the transformation of each spermatid into a s permatozoïde. This phase lasts 23 days and ends with the release of sperm into the lumen of the seminiferous tubule, phenomenon called spermiation.

Spermiogenesis includes 5 basic phenomena that are virtually simultaneous :

a- Formation of & rsquo; acrosome from the Golgi :

The Golgi vesicles develop, become pro-acrosomal granules then merge to a single vesicle, the acrosome vesicle (or acrosome) that s & rsquo; spreads and covers the 2/3 core. It takes the name of & rsquo; acrosome and its contents became homogeneous. L & rsquo; acrosome is a large lysosome to be located at the anterior pole of the future sperm. Its enzyme content will be essential for the crossing envelopes of & rsquo; oocyte at fertilization (hyaluronidase, acrosine).

b- Training flagellum :

The 2 centrioles migrent BEN participants in the & rsquo; acrosome. Microtubules centriole Distal s & rsquo; s longer and & rsquo; organize into an axoneme which s & rsquo; extends and emerges from the cell, pushing the plasma membrane.

c- nuclear elongation :

Nuclear changes begin in mid spermiogenesis. The core s & rsquo; lengthens and becomes denser gradually. The 2/3 Kernel earlier are covered by & rsquo; acrosome. The posterior third is related to the cell cytoplasm.

d- mitochondrial sleeve training :

Mitochondria are arranged end to end in a spiral around the portion of the initial & rsquo; axonemal, constituting the mitochondrial sleeve workpiece Intermediate. At its distal end, the sleeve is limited by a dense ring described as the d & rsquo; annulus.

e- Elimination of the cytoplasm :

The majority of the cytoplasm is phagocytosed by Sertoli cells. Some stands, constituting the residual body (de Regaud). The rest will be eliminated in the final phase of maturation during migration in & rsquo; epididymis.

Figure 6. Stages spermiogénèse

C- NEURO-ENDOCRINE CONTROL :

With the pulsatile GnRH production (Gonadotropin Releasing Hormone) by neurons of & rsquo; s hypothalamus that & rsquo; installs testicular function. GnRH causes the pituitary secretion of two hormones, FSH I LH. At testicular level, these hormones have the following actions :

→ FSH allows the development of Sertoli cells and spermatogenesis (exocrine function of the testis : excretion of sperm). FSH binds to membrane receptors of Sertoli cells and plays a triple role :

  • it activates spermatogenesis by & rsquo; cytoplasm via sertolien;
  • it stimulates the formation of & rsquo; ABP (Androgen Binding Proteine);
  • it causes the FA-etión d & rsquo; inhibin, hormone exerting a negative rétrooontrôle on FSH secretion, either hypothalamic neurons by decreasing the secretion of GnRH, either directly on the pituitary cells gonadotropin.

→ LH provides multi folds * The one cell / dg and testosterone secretion (endocrine function of the testis) :

  • most of testosterone enters the sertolien cytoplasm where it binds to the & rsquo; ABP to condition the development of & rsquo; séminai epithelium and proper functioning of genital tract are (seminal fluid);
  • free testosterone passes dan s blood and ecerce December "Share: a positive effect on the reproductive tract and lesglandes ann © these and a negative-feedback on LH secretion, or indirectly on hypothalamic neurons ^ either directly on the pituitary cells gon adotropes
Figure 7. Synthesis scheme of horntonaui h f human reproductive function controls

D- CHARACTERISTICS OF SPERM :

1- Name :

  • The normozoospermie means a normal sperm count between 20 and 200.106/ ml.
  • L’oligozoospermie (or oligospermia) means a sperm count of less than 20.106/ ml.
  • L & rsquo; azoospermia is & rsquo; no sperm.
  • La polyzoospermie (ou polyspermie) refers too many, better than 200.106 / ml.

2- Mobility :

It allows the growth in the female genital tract and affects the fertilizing sperm. It includes the percentage of motile sperm, the speed of displacement and the direction of travel.

  • L & rsquo; asthénospermie means a lower percentage of sperm with normal mobility 40% and / or a decrease in the velocity.
  • L & rsquo; akinesia shall mean the that’ no sperm from moving.

3- Vitality :

It is scanned using a dye that enters the sperm

  • The necrospermia denotes a number of dead sperm than 30%.

4- fertilizing :

It is reduced if abnormalities in the plasma membrane, s & rsquo; s enzymatic equipment & rsquo; acrosome, or condensation of the core.

5- Morphology :

It is assessed by the spermocytogram which falls morphological atypical sperm after staining. all ejaculate, in & rsquo; human species, contains atypical sperm. Too large proportion is abnormal.

  • The teratospermia designates a percentage of typical sperm below 40%.

OVOGENÈSE :

C & rsquo; is the formation of gametes in the female. It takes place in the ovaries and allows the formation of female gametes, oocytes from stem cells of the germ line or oogonia.

A- RUNNING OF & rsquo; Oogenesis :

L & rsquo; oogenesis include the multiplication phase, d & rsquo; growth and maturation. The phase & rsquo; growth and early maturation s & rsquo; & rsquo to perform, within the ovarian follicle and are related to the & rsquo; evolution of the follicle. The end of maturation is delayed. It s & rsquo; completed after fertilization. There is no differentiation phase.

1- Phase multiplication :

It concerns the oogonia, diploid strains and is characterized by a succession of mitosis which will result in the formation of & rsquo; oocytes I (first class), also diploid. This phase takes place, in women, during embryonic and fetal life. The oogonia are observed in the cortical area of ​​the & rsquo; embryonic ovary, have a spherical shape and are small (15pm), degenerate, for the majority, to the 7th month of intrauterine life (atresia), I give oocytes (2n chromosomes, 2q ADN), larger cells (20 at 40 pm), immediately after training s & rsquo; surrounding follicular cells and & rsquo; a peripheral membrane which separates them from the rest of the ovarian stroma, l & rsquo; together designating the primordial follicle, Then they begin the first meiotic division, which hangs at the stage of prophase. L & rsquo; oocyte then enters a quiescent state in which it can remain for many years (I blocked oocyte in prophase of first meiotic division until & rsquo; at puberty).

So, at the end of this phase of multiplication (birth), one-time stock egg I (about a million) consists, each contained in a primordial follicle.

2- Phase d & rsquo; increase :

It is characterized by a very significant increase in the size of the & rsquo; I oocyte, passing of 20 at 120 pm diameter. Very long, it s & rsquo; & rsquo concludes that, at the time of follicle maturation and consists of summaries of & rsquo; RNA and proteins that play a key role during fertilization and during early embryonic development.

The primordial follicles regress in large numbers between birth and puberty :

  • it will only 400 000 at puberty ;
  • less of 500 will grow up & rsquo; to & rsquo; ovulation during the sexual life of women.

3- Maturation phase :

Each month, at the time of & rsquo; ovulation, l’ovocyte I (2n chromosomes, 4q ADN) completes the first meiotic division and gives an oocyte II (n chromosomes, 2q ADN) with the emission of 1is globule polar Immediately after, starts 2th meiotic division. But the process still hangs once (in metaphase 2th meiotic division) A is conditioned by the occurrence or non-fertilization :

→ in the & rsquo; lack of fertilization, l & rsquo; oocyte remains at this stage of meiosis and degenerates after 24 hours.

→ s & rsquo; there fertilization, l & rsquo; oocyte II will complete its maturation and egg will turn into wall(ovotide) with the emission of 2th polar body.

Figure 8. Phases oogenesis

B- EVOLUTION OF OVARIAN FOLLICLES (FOLLICULOGENÈSE) :

1- primordial follicle :

  • C & rsquo; is still the type most abundant follicle on a cutting d & rsquo; ovary.
  • C & rsquo; is a sphere of 50 microns in diameter which comprises an oocyte I and a layer of flattened cells follicular.
Figure 9. Ovary rabbit primordial follicle. Stade 1: primordial follicle. x1000 magnification immersion objective.

2- primary follicle :

  • His pass diameter 50 to 80pm. L & rsquo; I oocyte is still blocked in prophase, begins the great increase phase.
  • The follicular cells become cut and are arranged in a single layer.
Figure 10. primary follicle Microscopic observation d & rsquo; ovarian rabbit. magnification x 400

3- secondary follicle (or pre-antral follicle or full) :

  • Its diameter gradually Password 80 at 200 pm.
  • L & rsquo; I egg continues to grow (and reached 80 pim).
  • The zona pellucida becomes visible by light microscopy (c & rsquo; is a hyaline structure, composed of glycoproteins whose & rsquo; origin is essentially but oocyte follicle cells in manufacture a portion).
  • The follicular cells multiply and are arranged in twenty layers around & rsquo; oocyte constituting granulosa. The innermost layer, régulièrement disposée around the pellucid membrane, if nomme the Corona Radiata.
  • The membrane separates Slavjanski granulosa theca interna which forms around the basement membrane by differentiation of cortical stroma.
  • Cells, initially fusiform, become cubic.
Figure 11. secondary follicles Microscopic observation of & rsquo; ovarian rabbit. magnification x400.

4- tertiary follicle (or cavitary or antral) :

  • The follicle cells surrounding small cavities liquidiennes whose confluence will be Tantrum.
  • Il se définit par l’apparition de petites cavités au sein de la granulosa qui renferment un liquide appelé Liquor folliculi””. These cavities are small formations rosettes, the “body and Call Exner”.
  • The follicular diameter continues to increase to 10 at 15 mm at the end of this stage.
  • L & rsquo; oocyte is still blocked in prophase I and reached 100 pm diameter.
  • The pellucid membrane d 15 pm d & rsquo; thickness. The continuous cell proliferation. L & rsquo; oocyte follicle is discharged on the side. It remains surrounded d & rsquo; a mass of cells constituting the Cumulus foiiicuieuses oophorus (Cumulus ou proliger) which keeps the egg attached to the rest of the follicle.
  • The external library is formed around the previous. It is fibrous connective tissue which condenses around the theca interna.
Figure 12. Microscopic observation of tertiary follicle & rsquo; ovarian rabbit. magnification x 40

5- mature follicle (Graafian follicle) :

  • Its diameter is 18 or even 20 mm.
  • L & rsquo; oocyte diameter 120 at 150 pm.
  • The core migrates to the periphery of the cytoplasm and resumes the process of meiosis.
  • The division reductional s & rsquo; s end and & rsquo; s accompanies & rsquo; expulsion of the first polar body. L & rsquo; oocyte becomes an oocyte II.
  • After ovulation, it will lock into metaphase 2th meiotic division.
  • The areas pellucid augmente leggerezza d & rsquo; online.
Figure 13. Follicule de De Graaf Observation microscopique d’ovaire de lapine. magnification x 40

C- OVULATION :

The release of the female gamete is ovulation. It occurs in the middle of the female cycle, at 14th day of the menstrual cycle. L & rsquo; s entire cumulus & rsquo; is then detached from the rest of the granulosa (under the & rsquo; & rsquo action; proteolytic enzymes) and floats in the follicular cavity, the follicle wall remains in the & rsquo; ovarian follicle and is the dehiscent.

  • the mature follicle protrudes the surface of the & rsquo; ovary.
  • at the theca interna, vasodilation causes an increase in volume of the cavity follicular.
  • collapse of the membrane Slavjanski
  • l & rsquo; oocyte surrounded by follicular cells fleet in follicular fluid
  • in the region of & rsquo; apex, vasoconstriction and necrosis of the ovary wall

L & rsquo; oocyte completes its nuclear and cytoplasmic maturation leading to the formation of the & rsquo; oocyte II which will lock in metaphase of meiosis II.

At the time of ovulation, the flag of the trunk, mobile, covers the & rsquo; ovary. It recovers the & rsquo; oocyte II. Proper flag tubal mobility is essential to the reproductive function. Quickly, egg migrates up & rsquo; to & rsquo; bulb tubal where fertilization is done. Migration is passive. It is facilitated talks fluid flow from the pavilion to the & rsquo; uterus and the movement of the cilia of the & rsquo; tubal epithelium.

L & rsquo; oocyte is a spherical cell 150 pm diameter, relatively inert, surrounded by envelopes

D- YELLOW BODY TRAINING :

Immediately after the & rsquo; ovulation, the follicle becomes a temporary endocrine gland, lutea (luteinizing). The granulosa cells and theca interna alter their hormonal syntheses and become luteal cells, and para developing luteal progesterone and some estrogen.

O & rsquo; n & rsquo it; there is no implantation, the corpus luteum regresses (lutéolyse). C & rsquo; is the cyclic corpus luteum or progestin body.

After the & rsquo; involution of the corpus luteum, persists in & rsquo; ovary a small mass of fibrous tissue, white body or corpus albicans.

– O & rsquo; there gestation, the corpus luteum persists up & rsquo; to 3th month. C & rsquo; is the corpus luteum of pregnancy or gestational body. Its function is to produce progesterone to maintain pregnancy.

Figure 14. Schematic representation of the & rsquo; human ovary : ovulation and corpus luteum formation

Course Dr A HECINI – Faculty of Constantine