Mammalian sexual phenotype is regulated by gonadal hormones
All mammals begin development as gender neutral, the presence of the
Y chromosome induces testis development that produce hormones that switch
the development of somatic tissues into the male pathway.
This means that the sex of only the gonads is genetically determined
but the rest of the cells are neutral (whatever their chromosome complement
is).
Their fate depends upon hormones.
The mesonephros (embryonic kidney) contribute
to both male and female reproductive organs.
Wolffian & Mullerian ducts
On the sides of the mesonephros are the Wolffian ducts and Mullerian
ducts that open into the cloaca.
In females (in the absence of the testes), the Mullerian ducts develop
into the oviducts (Fallopian tubes) and the Wolffian ducts degenerate.
In males, the Wolffian duct becomes the vas deferens.
The genital region differentiates after gonad development with the
action of the gonadal hormones.
Drosophila sex determination
The primary sex-determining signal is the number of X chromosomes.
Gynandromorphs are genetic mosaics in which
one X is lost in half of the organism(ie. left XX is female and right XO
is male).
The Sex-lethal (Sxl) protein acts as a stable binary
genetic switch.
The presence of two X chromosomes results
in the production of Sxl .
The presence of Sxl results in the proper splicing of the tra mRNA
production of the transformer protein.
Downstream, male and female versions of the double sex gene product
are made by sex-specific splicing of dsx mRNA.
Tra protein (plus Tra2) leads to the splicing of the female dsx mRNA.
The male dsx mRNA is the default product and is made in the absence
of female signaling.
Most flowering plants are hermaphrodites but some produce unisexual
flowers.
In ~10% of flowering plants, flowers are one sex (some appear on the
same plant, others on differ plants).
In single sex plants the other sexual organs are selectively reabsorbed.
In maize, male (tassle)and female (ears) flowers develop in different
sites along the shoot.
The plant hormone, gibberellic acid may be involved in plant sex determination
as the stamen and pistels show differences in its concentration.
Germ cell sex determination may depend on cell signals and genetic
constitution.
In the mouse embryo, pre-germ cells
enter the genital ridge (where the gonads form) and continue
dividing.
In the female, cells enter meiotic prophase and arrest until the mouse
matures.
In the male, the cells continue to divide but eventually arrest in
the G1 phase.
After the mice are born, the cells start dividing again and enter meiosis
with maturity.
Mouse germ cells that enter meiosis before birth become eggs and those
that enter later become sperm.
Mouse germ cells that fail to enter the genital ridge, start to develop
as oocytes (in both males and females) but later development is abnormal.
In Drosophila,
1) XY germ cells transplanted to a female enter the embryo and
2) XX germ cells transplanted into a testis both develop as non-functional
sperm to demonstrate both cell autonomy and environmental signals.
Various strategies are used for dosage compensation of X-linked genes.
The imbalance of X linked genes between males and females must be corrected
(dosage compensation).
Mammals achieve this by inactivating one of the two female X chromosomes
after the blastocyst has been implanted in the uterine wall.
The inactive X can be seen in the nucleus as a Barr body.
Xist, is a genetic switch, which produces an RNA that interacts with
the "X inactivation region" of the X chromosome.
In Drosophila, the opposite approach is used when Sxl is off, transcription
from the single X chromosome is doubled (translation increases as well).
Development of germ cells
Germ fate is specified by germ plasm.
In Drosophila, the egg contains large polar granules contain RNA's
& proteins.
Exposure to UV will destroy the pole plasm and germ cells will not
develop.
If pole plasm is transplanted to the anterior
of another embryo, pole cells will develop there & will develop normally
when transplanted to the posterior of a developing embryo.
In C. elegans, P granules of the fertilized
egg are sequestered to the pre-germline.
In Xenopus, distinct yolk-free patches aggregate in the vegetal pole
and are distributed into the pre-germ cells.
Pole plasm is localized at the posterior end
of the Drosophila egg through the action of maternal genes (such as oskar).
Differentiation of the germ cells depends upon a reduction
in the number of chromosomes but oogenesis and spermatogenesis
have different approaches.
Fertilization involves cell-surface interactions between egg and
sperm.
The sperm has to pass several barriers to
enter the egg.
In mammalian eggs, the sperm first passes
through a layer of cumulus cell embedded in hyaluronic acid aided by the
hyaluronidase actvity on its surface.
The 2nd layer is the zona pellucida, a layer of glycoproteins.
The acrosomal reaction (release of enzymes in the sperm head) is mediated
by interaction of the ZP3 species-specific receptor and adhesion molecules
in the sperm head.
The acrosome releases acrosin (a protease) and an acetylglucosaminidase
(which degrades glycoprotein side-chains).
The sperm surface which contains proteins (i.e. fertilin) that can
bind the egg's surface are exposed during the acrosomal reaction.
Fertilin binds an intergrin-like receptor of the egg plama membrane
to initiate sperm-egg fusion.
In some invertebrates (i.e. sea urchins), an actin filament-driven
acrosomal projection allows the sperm and egg to meet through a coat of
jelly.
Changes in the egg membrane at fertilization to block polyspermy include,
in the sea urchin, depolarization and the release
of cortical granules.
Only one sperm may enter an egg.
To prevent polyspermy, enzymes that prevent other sperm from binding
to the zona pellucida are released.
At fertilization a number of events occur to activate development
(such as increase in protein synthesis, structural changes [cortical rotation]).
Main event is the completion of meiosis, fusion of the nuclei to form
a diploid zygotic genome and entry into mitosis.
A calcium wave initiated at fertilization
results in egg activation.
The sharp increase in calcium initiates the cell cycle by acting upon
proteins that control the cell cycle.
The Xenopus egg is kept in metaphase II by maturation-promoting
factor (MPF) a complex including cyclin.
The Calcium wave activates a kinase which results in the degradation
of cyclin which allows the meiosis to finish & the nuclei to
fuse.
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