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 in the’organization, 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 as much d’ADN, 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 the’ADN.

Figure 1. General scheme of gametogenesis

It allows the formation of haploid gametes and l’exchange of chromosomal segments between the paternal and maternal genomes. This process which accompanies sexual reproduction allows the genetic variability of the’species and allows it, over generations of s’adapt to variations in’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 the duplication of the’DNA during the end of l’interphase and successively understands : Prophase, metaphase, Anaphase and Telophase.

The second division (equational division) occurs very quickly and the’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’a cell.
  • brewing the’genetic information.
  • The transmission of’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, gradually slows down to quarantine but may continue until’at an advanced age. Spermatogenesis is a slow and continuous process requiring 74 about days at home’man ; C’is the duration of the spermatogenetic cycle.

A- STRUCTURE OF SPERM :

The sperm cell is a mobile, highly differentiated, dont l’organization is comparable 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 d pm’thickness). It contains’acrosome and nucleus .

  • The neck, narrowed and short (1 pm par 1 pm), conventionally corresponding to’space between 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 is not’observe qu’in electron 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’nuclear ring, corresponds to the posterior edge of the’acrosome.
  • The core : He is wearing his hair’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 of’implantation.
  • L’acrosome : It is flattened and covers 2/3 previous core. C’is a vesicle with homogeneous content, limited by a membrane. He understands 2 segments, d’different physiological appearance and significance :

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

  • the cytoplasm : It is very small and is a blade surrounding the posterior third of the nucleus, behind the’acrosome..
  • The neck : C’is the junction area between the head and the flagellum. C’is a very complex region which contains the’centriolar device and the d-shaped connecting piece’overturned funnel.

Starting from the core can be described :

  • Le centriole proximal : It is arranged under the dimple d’implantation. Its axis is almost parallel to the posterior surface of the nucleus and makes an angle of 80 ° with that of the’axoneme.
  • 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’is formed by elongation of the distal centriole and begins approximately halfway up the segmented columns. Centrally located, inside dense fibers, it has the usual structure of’a ciliary axis : 9 doublets peripheral surrounding a central doublet.
  • mitochondria : to’exterior of 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’is the shortest and thickest part of 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’axoneme or axial filamentous complex which has the typical structure of l’ciliary axoneme : 9 microtubule doublets devices surround a central doublet.
→ The 9 dense fibers – Mitochondrial spiral sleeve. It surrounds dense fibers and is made up of’a succession of mitochondria arranged in a single spiral file which is roughly 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’refine towards’distal end
→ The fibrous sheath. it s’acts as a fibrillary protein wound in a spiral. This sleeve has two diametrically opposite thickenings : longitudinal columns. Approaching the’end of main room, the longitudinal columns s’erase and l’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’at a very advanced age. It takes place at the’interior of seminiferous tubes. Its duration is fixed and characteristic of’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’at the end of spermatogenesis allowing exchanges between all the cells deriving from’the same region and allow synchronous evolution.

2- Phase d’increase :

It concerns spermatocytes I and corresponds schematically to the’interphase and early prophase of the meiotic first division. The cell size increases by accumulation of material synthesized. when’they reach their maximum size, from 25 pm, they are designated by the name of’auxocytes.

3- Maturation phase :

It concerns spermatocytes. it s’basically acts’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’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’acrosome from the Golgi :

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

b- Training flagellum :

The 2 centrioles migrate to the opposite pole’acrosome. Les microtubules du centriole distal s’lengthen and s’organize into an axoneme which s’lengthens and emerges from the cell by pushing back the plasma membrane.

c- nuclear elongation :

Nuclear changes begin in mid spermiogenesis. The core s’gradually lengthens and densifies. The 2/3 anterior of the nucleus are covered by l’acrosome. The posterior third is related to the cell cytoplasm.

d- mitochondrial sleeve training :

The mitochondria are arranged end to end in a spiral around the Initial portion of the’axoneme, constituting the mitochondrial sleeve workpiece Intermediate. At its distal end, the sleeve is limited by a dense ring described under the name of’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 stage of maturation during migration into the’epididymis.

Figure 6. Stages spermiogénèse

C- NEURO-ENDOCRINE CONTROL :

With the pulsatile GnRH production (Gonadotropin Releasing Hormone) by neurons of the’that's the hypothalamus’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 l’intermediary of the sertolian cytoplasm;
  • it stimulates the formation of’ABP (Androgen Binding Proteine);
  • it causes the separation of’inhibine, 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 the testosterone enters the sertolian cytoplasm where it binds to the’ABP to condition the development of’seminal epithelium and the proper functioning of the genital tract (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’azoospermia is there’absence of 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’asthenospermia means a percentage of sperm with normal mobility less than 40% and / or a decrease in the velocity.
  • L’Akinesis refers to the fact 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, of the’enzyme equipment’acrosome, or condensation of the core.

5- Morphology :

It is assessed by the spermocytogram which falls morphological atypical sperm after staining. all ejaculate, in L’human race, contains atypical sperm. Too large proportion is abnormal.

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

OVOGENÈSE :

C’is the formation of gametes in the female sex. 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’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. it n’there is no differentiation phase.

1- Phase multiplication :

It concerns the oogonia, diploid stem cells and it is characterized by a succession of mitoses which will lead to the formation of’ovocytes I (first class), also diploid. This phase takes place, in women, during embryonic and fetal life. The ovogonies are observed in the cortical area of ​​the’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), who immediately after their training s’surround follicular cells and d’a peripheral membrane that separates them from the rest of the ovarian stroma, l’set designating the primordial follicle, Then they begin the first meiotic division, which hangs at the stage of prophase. L’the oocyte then enters a quiescent state in which it can remain for many years (oocyte I blocked in prophase of the first division of meiosis until’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’increase :

It is characterized by a very significant increase in the size of the’ovocyte I, passing of 20 at 120 pm diameter. Very long, she is not’complete that’at the time of maturation of the follicle and consists of syntheses of’RNA and proteins that will play a crucial role during fertilization and during the early stages of 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’to’ovulation during a woman's genital life.

3- Maturation phase :

Each month, at the time of’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 l’absence of fertilization, l’oocyte remains at this stage of meiosis and degenerates after 24 hours.

→ s’there fertilization, l’oocyte II will complete its maturation and will transform into a mature ovum(ovotide) with the emission of 2th polar body.

Figure 8. Phases oogenesis

B- EVOLUTION OF OVARIAN FOLLICLES (FOLLICULOGENÈSE) :

1- 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 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’oocyte I is still prophase blocked, begins the great increase phase.
  • The follicular cells become cut and are arranged in a single layer.
Figure 10. Primary follicle Microscopic observation of’rabbit ovary. magnification x 400

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

  • Its diameter gradually Password 80 at 200 pm.
  • L’oocyte I continues to grow (and reached 80 pim).
  • The zona pellucida becomes visible by light microscopy (c’is a hyaline structure, composed of glycoproteins of which l’origin is essentially oocyte but follicle cells make part of it).
  • The follicle cells multiply and are arranged in about twenty layers around the’oocyte constituting the 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 follicle Microscopic observation of’rabbit ovary. magnification x400.

4- tertiary follicle (or cavitary or antral) :

  • The follicle cells surrounding small cavities liquidiennes whose confluence will be Tantrum.
  • It is defined by’appearance of small cavities within the granulosa which contain a liquid called 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’oocyte is still blocked in prophase I and reached 100 pm diameter.
  • The pellucid membrane d 15 d pm’thickness. The continuous cell proliferation. L’oocyte is pushed back to the side of the follicle. He remains surrounded by’a cluster of faith cells constituting the Cumulus oophorus (Cumulus ou proliger) who maintains the’oocyte attached to the rest of the follicle.
  • The external library is formed around the previous. C’is fibrous connective tissue that condenses around the internal theca.
Figure 12. Tertiary follicle Microscopic observation of’rabbit ovary. magnification x 40

5- mature follicle (Graafian follicle) :

  • Its diameter is 18 or even 20 mm.
  • L’oocyte has a diameter of 120 at 150 pm.
  • The core migrates to the periphery of the cytoplasm and resumes the process of meiosis.
  • The division s reductional’complete and s’accompanies’expulsion of the first polar body. L’oocyte becomes an oocyte II.
  • After’ovulation, it will lock into metaphase 2th meiotic division.
  • The pellucid area increases slightly d’thickness.
Figure 13. De Graaf follicle Microscopic observation of’rabbit ovary. magnification x 40

C- OVULATION :

The liberation of the female gamete is l’ovulation. It occurs in the middle of the female cycle, at 14th day of the menstrual cycle. L’whole cumulus s’is then detached from the rest of the granulosa (under’action d’proteolytic enzymes) and floats in the follicular cavity, the follicle wall stays in the’ovary and forms the dehiscent follicle.

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

L’oocyte completes its cytoplasmic and nuclear maturation which results in the formation of’oocyte II which will lock in the metaphase of meiosis II.

At the time of’ovulation, the flag of the trunk, mobile, covers the’ovary. It recovers’oocyte II. Proper flag tubal mobility is essential to the reproductive function. Quickly, l’ovum migrates up’to’bulb tubal where fertilization is done. Migration is passive. It is facilitated by the liquid current going from the pavilion to the’uterus and the vibrating eyelash movements of the’tubal epithelium.

L’oocyte is a spherical cell of 150 pm diameter, relatively inert, surrounded by envelopes

D- YELLOW BODY TRAINING :

Immediately after the’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 luteales developing progesterone and little d’estrogen.

S’it n’there is no implantation, the corpus luteum regresses (lutéolyse). C’is the cyclic corpus luteum or progestogen body.

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

– S’there is gestation, the corpus luteum persists until’at 3th month. C’is the corpus luteum or gestative body. Its function is to produce progesterone to maintain pregnancy.

Figure 14. Schematic representation of the’human ovary : ovulation and corpus luteum formation

Course Dr A HECINI – Faculty of Constantine