9th Postgraduate Course for Training in Reproductive Medicine and Reproductive Biology

 Gametogenesis

F. Urner
Department of Obstetrics and Gynecology, Geneva University Hospital

• PGC are undifferentiated germ cells
• PGC are identifiable in 3-weeks old embryo, in the epithelium of the yolk sac
• PGC migrate from the epithelium of the yolk sac to the genital ridge by amoeboid movements
• PGC proliferate following colonization of the genital ridge

Differentiation of primordial germ cells in males

Y chromosome

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Differentiation of the male gonad

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Inhibition of mitosis /meiosis of the PGC

Differentiation of primordial germ cells in females

The absence of theY chromosome

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Differentiation of the female gonad

XX chromosomes

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Differentiation of the PGC into oogonia and oocyte

• Takes place in seminiferous tubules of the testis
• Begins at puberty
• Time for completion in man : 64 days

Mitotic phase

A proportion of male germ cells (spermatogonia) enter mitosis and, after 5 mitosis, become type B spermatogonia, which divide mitotically into primary spermatocytes

Spermatogonia A1

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Spermatogonia A2

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Spermatogonia A3

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Spermatogonia A4

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Intermediate spermatogonia

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Spermatogonia B

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Primary spermatocytes

Meiotic phase

Primary spermatocytes enter meioisis. The first meiotic division leads to secondary spermatocytes and the second division to the haploid round spermatids

Primary spermatocytes (4n)

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Secondary spermatocytes (2n)

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Round spermatids (1n)

Spermiogenesis

Spermatids differentiate into spermatozoa :

• DNA is associated with nuclear basic proteins called protamines and become highly condensed
• The acrosome, tail and the midpiece (containing mitochondria) are formed
• Most of the cytoplasm is eliminated

Round spermatids (1n)

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Elongated spermatids (1n )

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Spermatozoa (1n)

3. Oogenesis

• Is initiated in follicles in the ovary but is completed in the Fallopian tube following fertilization
• Begins during fetal life
• Time for completion : 15 to 50 years

Mitotic phase

During fetal life, primordial germ cells become oogonia which divide mitotically. At the end of the first trimester, oogonia undergo one last round of DNA and become oocytes

Meiotic phase

Phase I

During fetal life, the oocytes enter meiosis and stop at the diplotene stage of meiosis I. They display a large nucleus - the germinal vesicle- which encloses the decondensed oocyte chromatin.

Phase II

From puberty and during the adult reproductive life, the preovulatory LH surge induces reinitiation of oocyte meiosis which will arrest again at the metaphase II stage. Metaphase II oocytes are characterized by the presence of the first polar body which contains half of the oocyte chromosomes. The oocytes are ovulated at the metaphase II stage.

Phase III

Fertilization of the metaphase II oocyte induces the resumption of meiosis which lead to an haploid oocyte, with the extrusion of the second polar body.

Primordial follicles

One layer of granulosa cells surrounds the diplotenic oocyte. Primordial follicles apparition is coincident with the initiation of meiosis and are present in the ovary during the whole reproductive life

Follicle recruitment

Recruitment of primordial follicles is characterized by follicular and oocyte growth. The numbers of layers of granulosa cells surrounding the oocyte are increased, the thecal layer is formed. During its growth, the oocyte accumulates molecules important for embryogenesis (mRNA )and synthesizes and secretes the components of the zona pellucida. During its growth phase, the oocyte is still arrested at the diplotene stage of meiosis I.

Antral follicles

Folllicles become antral when a cavity filled with follicular fluid is forming. When the size of the follicle and the antrum is maximum, the follicle is called Graafian follicle. It contains a fully grown oocyte arrested at the diplotene stage and is able to respond to LH. The preovulatory LH surge will induce the reinitiation of meiosis and ovulation of the oocyte.

References

• Johnson M and Everitt B. Essential Reproduction. Blackwell Scientific Publications, 1984

• Picton H, Briggs D and Gosden R. The molecular basis of oocyte growth and development. Mol Cell Endocrinol 145 : 27-37, 1998

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Edited by Aldo Campana,