Oogenesist

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OOGENESIS

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Polar bodies

Ovum

Primary

sper matocyte

Primary

oocyte

Secondaryoocyte

Secondary

sper matocytes

Sper matids

Sper matozoa

Meiosis IMeiosis I

Meiosis II

Meiosis II

Sper miogenesis

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Differences between sper matogenesis and oogenesis.

1. Four gametes from each primary

sper matocyte

1. One gamete from each primary

oocyte

Sper matogenesis Oogenesis

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

3. Most cytoplasm is shed from

sper matocyte

3. Cytoplasm conser ved 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 f unctions in fertilization before

meiosis is complete

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Specializations of sper m and egg

Sper matozoan

1. Transfer of genetic infor mation to next generation

2. Locomotion

3. Penetration of barriers surrounding the egg

4. Fusion with oolemma

5. Receptor mediated recognition of egg

Ovum

1. Transfer of genetic infor mation to next

generation

2. Chem

oattraction of sper m

atozoan3. Prevention of polysper my

4. Storage of nutrients (importance varies)

5. Storage of cytoplasmic infor mation (importance

varies)

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All oocytes remain in the*

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The Dictyate State

1. A sort of ³stasis´ 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 menstr ual cycle a number of

oocytes in primordial follicles are stimulated by pituitary

gonadotropins to continue their maturation.

a. Mainly due to leuteinizing hor mone (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 menstr ual cycles per year ~ 12

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

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

the menstr ual 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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*

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Storage of energy, raw materials and infor mation in the egg

1. Previtelligenic phase

Occurs through early diplotene (during the dictyate state, diplotene may be as

long as 50+ years)

Lampbr ush chromosomes for m - RN A transcription including cytoplasmic

infor mation for development

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

Occurs d

uring

middle to late diplotene

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Nucleus swells to for m ger minal vesicle

1. Pre-vitellogenic phase

The nucleus will continue as a ger minal

vesicle into the vitellogenic phase.

Pictures of frog follicles in digital lab manual.

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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 str ucture of vitellogenins modified in the egg. Deposition of

yolk in cytoplasm mediated by enzymes, endoplasmic reticulum, golgi

bodies, mitochondria. Yolk platelets for med.

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

phosphor us

http://www.luc.edu/depts/biology/dev/vitellog.htm

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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:

http://en.wikipedia.org/wiki/Image:Salmoneggskils.jpg

htt ://www.ex loratorium.edu/cookin /e s/e com osition.html

3. Microlecithal, oligolecithal - very little yolk (mostmammals)

2. Mesolecithal - medium amount of yolk (amphibians)

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Cytoplasmic Infor mation

Where does it come from?Some of the RN A transcribed from DN A during

diplotene of the first meiotic prophase and stored in

cytoplasm in inactive for m until needed during

development.

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

2. Can deter mine fate of specific groups of cells, e.g.

primordial ger m cells in amphibians and insects.

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Ovulation

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/femalerepro_3.gif

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http://www.talbotcentral.ucr.edu/mammalianfert.htm

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/reprod/fert/gxport.html

Bovine secondary oocyte surrounded by

cumulus follicle cells

Secondary oocyte of hampster from which the

cumulus cells have been removed by treatment

with hyaluronidase.

Ovulation

In mammals, the egg is ovulated as a

secondary oocyte that is atm

etaphaseof the second meiotic division and is

surrounded by layers of cumulus follicle

cells.

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http://www.talbotcentral.ucr.edu/mammalianfert.htm

http://www.obgyn.net/medical.asp?page=/english/pubs/features/mcgill-student-projects/ovulation-image

Ovulation [research perfor med 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 oophor us containing

the oocyte toward the weakened follicle wall,

which r uptures

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

http://www.em

c.m

aricopa.edu

/facu

lty/farabee/BIO

BK/fem

alerepro_3.gif

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.

Oogenesist

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