At what stage of meiosis does "first meiotic arrest of oogenesis" occur?

Meiosis consists of two divisions. Both are somehow similar to "ordinary" type of cell division - mitosis, but there is no DNA replication between them. As mitosis, each of two meiosis divisions might be divided into 5 stages:

1. Profaze (condensation of chromosomes, formation of microtubular spindle apparatus between two centrosomes)


2. Prometaphase (the nuclear membrane desintegrate and chromosomes and microtubules attach to kinetochores of chromosomes)


3. Metaphase (The chromosomes align at the metaphase plate)


4. Anaphase (segregation of chromosomes)


5. Telophase (restoration of nuclear membrane, cytokinesis)

The first division is crucial for reducing number of chromosomes. The prophase is subdivided to following stages:

1. Leptotene (condensation of chromosomes)


2. Zygotene (homologous chromosomes form tetrads )


3. Pachytene (crossing over)


4. Diplotene (chromosomes separate a little, sister chromosomes remain bounded in chiasmata)


5. Diakinesis (futher condensation of chromosomes, the nuclear membrane desintegrates)

In animals the function of meiosis is producing gametes. In case of oogenesis (in human) this process starts during prenatal development. Between 12-th and 25-th week of female fetus development cells called oogonia become primary oocytes and enter meiosis, but become arrested in stage of dictyotene, which is prolonged diplotene of prophase of first division. Then they wait 12 up to 50 years (too long waiting increase risk of chromosomal disjunction disorders such as Down syndrome in children of old mothers), covered by a layer of cells - primordial follicle. At the beginning of each menstruation cycle one or a few of the follicles become growing (the follicle cells divide, but oocyte is still waiting). At the stage of Graafian follicle the follicle is about 75 times bigger and there is a cavity full of liquid between oocyte and surrounding cells. The hormone signal - LH from pituitary gland - brakes the meiosis block, the primary oocyte divide into secondary oocyte and small first polar body (which quickly degenerates). At this time the follicle breaks - its ovulation. The oocyte starts second division, but stops at the metaphase. This second arrest ends when sperm cell penetrates into oocyte. The second, small polar cell is released and haploid nuclei of ovum and spermatozoon fuse together.

more:
http://www.ncbi.nlm.nih.gov/books/NBK10008/