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Polar bodies


Primary spermatocyte

Primary oocyte

Secondary oocyte

Secondary spermatocytes



Meiosis IMeiosis I

Meiosis II

Meiosis II


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Differences between spermatogenesis and oogenesis.

1. Four gametes from each primary spermatocyte

1. One gamete from each primary oocyte

Spermatogenesis Oogenesis

2. Four small gametes of equal size 2. One large gamete + 2-3 polar bodies

3. Most cytoplasm is shed from spermatocyte

3. Cytoplasm conserved in one large gamete - may increase. Thus, in the final gamete there is a large amount of cytoplasm.

4. Diplotene relatively short 4. Dipotene very long - dictyate state

5. Functions in fertilization only after meiosis is complete

5. Often functions in fertilization before meiosis is complete

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Specializations of sperm and egg


1. Transfer of genetic information to next generation

2. Locomotion

3. Penetration of barriers surrounding the egg

4. Fusion with oolemma

5. Receptor mediated recognition of egg


1. Transfer of genetic information to next generation

2. Chemoattraction of spermatozoan

3. Prevention of polyspermy

4. Storage of nutrients (importance varies)

5. Storage of cytoplasmic information (importance varies)

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All oocytes remain in the*

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All oocytes remain in the*

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The Dictyate State

1. A sort of “stasis” where the developing oocyte arrests in diplotene of the first meiotic prophase following initial maturation while the mother is an embryo in her mother’s womb.

2. Mediated by a meiosis stabilizing factor that is secreted by the follicle cells of the primordial follicle.

3. At the beginning of a menstrual cycle a number of oocytes in primordial follicles are stimulated by pituitary gonadotropins to continue their maturation.

a. Mainly due to leuteinizing hormone (LH) concentration

b. Either blocks or deactivates the meiosis stabilizing factor

c. As a result, egg maturation continues and meiosis I occurs

4. In many species (including humans), the oocyte then arrests at metaphase of meiosis II until after fertilization.

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Number of eggs stimulated to begin maturation in a females lifetime.

Number of years between puberty and menopause ~ 40

Number of menstrual cycles per year ~ 12

Number of eggs stimulated to continue maturation at each menstrual cycle ~ 9

So, if a woman is never pregnant, the number of eggs she will loose due to the menstrual cycle is about:

40 x 12 x 9 = ~ 4320 eggs

Thus, the vast majority of eggs (~495,680) lost during her life degenerate without every being stimulated to continue their maturation.

All oocytes remain in the *

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Follicular development

Pictures of follicle stages in digital lab manual.

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Storage of energy, raw materials and information in the egg1. Previtelligenic phase

Occurs through early diplotene (during the dictyate state, diplotene may be as long as 50+ years)

Lampbrush chromosomes form - RNA transcription including cytoplasmic information for development

2. Vitellogenic phase - starts after the egg is stimulated to continue its maturation

Occurs during middle to late diplotene

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Nucleus swells to form germinal vesicle

1. Pre-vitellogenic phase

The nucleus will continue as a germinal vesicle into the vitellogenic phase.

Pictures of frog follicles in digital lab manual.

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Lampbrush chromosomes present - RNA synthesis





Pre-vitellogenic and vitellogenic phases

Pictures of lampbrush chromosomes and frog follicle.

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Vitelligenic phase - Yolk deposition

1. Vitellogenins - estrogen stimulates synthesis of vitellogenins in liver or equivalent organ. Transported to ovary by circulatory system. Follicle cells may mediate transfer into egg.

2. Molecular structure of vitellogenins modified in the egg. Deposition of yolk in cytoplasm mediated by enzymes, endoplasmic reticulum, golgi bodies, mitochondria. Yolk platelets formed.

3. Vitellins - definitive yolk. Composition - lipid, protein, carbohydrate, phosphorus


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a. In species with yolky eggs, size of oocyte increases dramatically

2. Vitellogenic phase - yolk deposition

Pictures of frog follicle stages in digital lab manual.

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1. Polylecithal, megalecithal - a huge amount of yolk (birds, reptiles, bony fish)

Egg classification by distribution of yolk:

1. Telolecithal - yolk distributed in gradient, concentrated toward one pole of egg, usually the vegetal pole (e.g. amphibians).

2. Isolecithal - yolk evenly distributed throughout egg cytoplasm (e.g. sea urchins, human)

Egg classification by amount of yolk:



3. Microlecithal, oligolecithal - very little yolk (most mammals)

2. Mesolecithal - medium amount of yolk (amphibians)

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Cytoplasmic Information

Where does it come from?

Some of the RNA transcribed from DNA during diplotene of the first meiotic prophase and stored in cytoplasm in inactive form until needed during development.

What’s it for?1. Fast start for development.

2. Can determine fate of specific groups of cells, e.g. primordial germ cells in amphibians and insects.

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Bovine secondary oocyte surrounded by cumulus follicle cells

Secondary oocyte of hampster from which the cumulus cells have been removed by treatment with hyaluronidase.


In mammals, the egg is ovulated as a secondary oocyte that is at metaphase of the second meiotic division and is surrounded by layers of cumulus follicle cells.

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Ovulation [research performed on hampster follicles (Martin et al., 1981; Schroeder and Talbot, 1982)]

1. Enzymes weaken the follicle wall

2. Smooth muscle cells at base of follicle contract

3. This forces the cumulus oophorus containing the oocyte toward the weakened follicle wall, which ruptures

4. The oocyte + surrounding cumulus cells are forced out of the follicle.

Arrows indicate weakened follicle wall

Arrowheads indicate base of follicle where cumulus + oocyte are or were located

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Corpus luteum


stratum granulosum + theca interna give rise to the corpus luteum.

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stratum granulosum + theca interna give rise to the corpus luteum.

Picture of cat corpora lutea in digital lab manual.