OOGENESIS
Primary
oocyte
Primary
spermatocyte
Meiosis I
Meiosis I
Secondary
spermatocytes
Secondary
oocyte
Meiosis II
Spermatids
Polar bodies
Meiosis II
Spermiogenesis
Spermatozoa
Ovum
Differences between spermatogenesis and oogenesis.
Spermatogenesis
1.
Four gametes from each primary
spermatocyte
Oogenesis
1.
One gamete from each primary
oocyte
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
Specializations of sperm and egg
Spermatozoan
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
Ovum
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)
*
All oocytes remain in
All
remain
theoocytes
dictyate
statein the
(quiescent) until
stimulated by hormones
during the menstrual
cycle to continue their
maturation
*
All oocytes remain in
All oocytes remain in the
the dictyate state
(quiescent) until
stimulated by hormones
during the menstrual
cycle to continue their
maturation
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.
All oocytes remain in
All oocytes remain in the
the dictyate state
(quiescent) until
stimulated by hormones
during the menstrual
cycle to continue their
maturation
*
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.
Follicular development
Pictures of follicle stages in digital lab manual.
All oocytes remain in
the dictyate state
(quiescent) until
stimulated by hormones
during the menstrual
cycle to continue their
maturation
*
Storage of energy, raw materials and information in the egg
1. 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
1. Pre-vitellogenic phase
Nucleus swells to form germinal vesicle
Pictures of frog follicles in digital lab manual.
The nucleus will continue as a germinal
vesicle into the vitellogenic phase.
Pre-vitellogenic and vitellogenic phases
Frog
Lampbrush chromosomes present RNA synthesis
Pictures of lampbrush chromosomes and frog
follicle.
Chicken
http://www.luc.edu/depts/biology/dev/lampbr.htm
http://rat.inst.bio.spbu.ru/posters/Paris2001/gsa_p1f1.jpg
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
http://www.luc.edu/depts/biology/dev/vitellog.htm
2. Vitellogenic phase - yolk deposition
a. In species with yolky eggs, size of oocyte increases dramatically
Pictures of frog follicle stages in digital lab manual.
Egg classification by amount of yolk:
1.
Polylecithal, megalecithal - a huge amount of yolk (birds, reptiles, bony
fish)
2.
Mesolecithal - medium amount of yolk (amphibians)
3.
Microlecithal, oligolecithal - very little yolk (most mammals)
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)
http://en.wikipedia.org/wiki/Image:Salmoneggskils.jpg
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.
Ovulation
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/femalerepro_3.gif
Ovulation
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.
Secondary oocyte of hampster from which the
cumulus cells have been removed by treatment
with hyaluronidase.
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
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
http://www.talbotcentral.ucr.edu/mammalianfert.htm
http://www.obgyn.net/medical.asp?page=/english/pubs/features/mcgill-student-projects/ovulation-image
Corpus luteum
stratum granulosum + theca interna give rise to
the corpus luteum.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/femalerepro_3.gif
Picture of cat corpora lutea in digital lab manual.
stratum granulosum + theca interna give rise to
the corpus luteum.