;
that is, an organism in the course of its development goes through the
same successive stages as did the species in developing from the lower
to the higher forms of animal lifeTable of contents :
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that is, an organism in the course of its development goes through the
same successive stages as did the species in developing from the lower
to the higher forms of animal life,
the receptor for the morphogen Hedgehog
(Hh),
is active in the absence of ligand and blocks the expression of target
genes by inhibiting Smoothened (Smo), an essential transducer of the Hh
signal. Hh binding to Ptc abrogates the ability of Ptc to inhibit Smo,
thereby unleashing Smo activity and inducing target gene expression. A
cell's measure of ambient Hh concentration is not determined solely by
the number of active (unliganded) Ptc molecules. Instead, we find that
Hh-bound Ptc can titrate the inhibitory action of unbound Ptc. This effect
is sufficient to allow normal reading of the Hh gradient in the presence
of a form of Ptc that cannot bind the ligand but retains its ability to
inhibit Smo. These results support a model in which the ratio of bound
to unbound Ptc molecules determines the cellular response to Hhref.
differentiation : the first step is the alteration of the adhesive
properties of an individual cell. This is achieved by systematically changing
the complement of surface adhesion molecules used to attach to its neighbors.
Epithelial
buds are small bulges in undifferentiated embryonic epithelial layers
that develop into a variety of complex structures, including teeth, limbs,
and hair follicles. The formation of the epithelial bud is caused by a
decrease in E-cadherin
and an increase in P-cadherin
levels : only through combined noggin
and Wnt3a
signaling can the Lef1
transcription complex repress E-cadherin
expression, an intriguing outcome since Lef1 complexes typically activate
transcription in response to Wnt signaling.)
become capable of fertilizing an oocyte
(ovum)
after it reaches the ampullary portion of the uterine tube; a series of
biochemical transformations the spermatozoa undergo in the female reproductive
tract. Superoxide anion production output by spermatozoa, hydroxyl radical
and hydrogen peroxide initiate sperm capacitation. Cholesterol efflux
=> elevation of intracellular Ca2+ and HCO3-
=> activation of adenylyl cyclase (AC) => cAMP => activates PKA to indirectly
phosphorylate certain proteins on tyrosine. Uncertainties exist as to the
number of forms of AC that are present in the spermatozoon, whether they
are cytosolic or bound to subcellular structures, and which physiological
effectors they respond to (e.g. bicarbonate, Ca2+, or receptor-coupled
G-proteins). While net intracellular levels of cAMP in spermatozoa depend
upon the relative activities of AC and PDE, there are wide between-sample
variations within species, both in basal levels and in levels attained
after activation of the AC (e.g. after sperm treatment with HCO3-).
Moreover, minor changes in bulk cAMP levels can result in large changes
in cAMP-dependent functions. Finally, while cAMP levels respond very rapidly
to sperm treatment by effectors such as HCO3- and
Ca2+ (key components of capacitating media), there are big discrepancies
between the rates of functional response. For example, enhancement of motility
and collapse of phospholipid asymmetry take place within a few minutes,
whereas more than 1 h of exposure to capacitating conditions is needed
for cAMP-dependent protein tyrosine phosphorylation to become detectable
or for the sperm population to attain a capacitated state. During capacitation,
there is also an increase in protein tyrosine phosphorylation dependent
actin polymerization and in the membrane-bound PLC. The main tyrosine phosphorylated
proteins during the course of capacitation and fertilization are localized
to the flagellum, although tyrosine phosphorylation of less abundant proteins
may also be regulated in the sperm head. Spermatozoa bound to the zona
pellucida and fusing with the oocyte plasma membrane are characterized
by a tyrosine phosphorylated flagellum.
hyperactivation => hyperactivated motility : the movement of hyperactivated
sperm appears different under different physical conditions and in different
species, but basically it involves an increase in flagellar bend amplitude
and, usually, beat asymmetry. Hyperactivation may be critical to the success
of fertilization, because it enhances the ability of sperm to detach from
the wall of the oviduct, to move around in the labyrinthine lumen of the
oviduct, to penetrate mucous substances and, finally, to penetrate the
zona pellucida of the oocyte. Presumably, a signal or signals exist in
the oviduct to initiate hyperactivation at the appropriate time; however,
none has yet been identified with certainty. There is evidence that the
source of the signal is follicular fluid, yet spermatozoa are known to
hyperactivate before ovulation would release the fluid into the oviduct.
It requires sperm-specific voltage-gated calcium channels CatSper1
and CatSper2
in the flagellum => calcium ions interact with the axoneme of the flagellum
=> protein phosphorylation in the flagellum => switch on hyperactivation.
The process may also involve increases in intracellular cAMP, which at
least is required to support motility in general. The edding of antioxidant
enzymes inhibits the spontaneous and induced sperm hyperactivation : anyway
high doses of reactive oxygen species block the sperm motility through
the inhibition of ATP synthesis by the mitochondrial enzymes and cell membrane
compounds injury. Although hyperactivation often occurs during the process
of capacitation, divergent pathways regulate the 2 events. Sperm hyperactivation
might be a moiety of capacitation or the result of capacitation.
.
In human clinical practice, pregnancy is timed from onset of the last normal
menstruation and lasts 40 weeks (280 days). Elsewhere the onset may be
timed from estrus, coitus, artificial insemination, vaginal plug formation,
fertilization, or implantation.
and uterus. The hardy survivors face a grueling swim up the effectively
endless Fallopian tube thanks to thermotaxis (the upper reaches
cradling the egg are around 2°C warmer than those at the entrance).
Once in the right vicinity, sperm are thought to switch on chemotaxis,
sniffing out chemicals secreted by the egg. Without this navigation mechanism
in the female genital tract the chances of chaotic collision is negligible.
Fertilization occurs in the outer third of the Fallopian
tube.
The egg remains able to be fertilized for 24 hours after ovulation
;
apoptosis percentage was measured by the TUNEL technique. The contamination
by somatic and germ cells was assessed by looking for specific molecular
markers of these cells, respectively CD45 and E-cadherin for somatic cells
and c-kit for germ cells. The viability of sperm was unchanged in high
and low motile fractions, as well as DNA fragmentation percentage. The
amount of Prm-1 mRNA was significantly higher in low density motile than
in the high motile fraction. In most of high motile sperm samples eNOS
and nNOS transcripts were undetectable whereas they were present in the
low motile sperm. In contrast, no significant variation was found in the
c-myc/Prm-2 mRNA ratio between the two populations. Moreover, a partial
or complete disappearance of c-myc transcripts was observed after capacitation.
Thus analysing mRNA profiles could be helpful as a diagnostic tool and
prognosis value for fertilizationref.
.
While mitochondria from the sperm are introduced into the oocyte at fertilization,
almost all of the mitochondria are excluded during the first cleavage,
and so these mRNAs may not be useful. When one made cloned animals, one
wouldn't be delivering those mRNAs. This fingerprint technology could provide
a screen for different toxicological effects to which a male may have been
exposed.
and, after 3-5 days, reaches the endometrium.
for extraembryonic maternal tissues involved in development) : the production
of an embryo
in the endometrium,
wider at the chorionic surface of the placenta and tapering to a point
within the intertwin membrane.
,
derived not from the fetus but from the trophoblast that envelops the fetus;
from within outward, it consists of.
It secretes :.
It secretes
(from maternal cholesterol, then secreted into both maternal and fetal
bloodstreams)
and estriol
(due to placental lack of 17a-hydroxylase, they
are synthetised from androgenetic precursors in maternal circulation up
to pregnancy wk 20, then from fetal adrenal precursors (DHEA-S) processed
by placental aromatase to estrone or estradiol or by hepatic 16a-dehydroxylase
and placental sulfatase into dehydroxyepiandrosterone, then converted into
estriol by aromatase),
and persists until birth.)
: uterine blood flow increases fromm 50 mL/min to 500 mL/min. After pregnancy
week 20 cytotrophoblast disappears and only syncytiotrophoblast remains.
of the embryo.
maintains the pluripotent state of inner cell mass cells—at the blastocyst
stage of preimplantation development—that develop into the fetus after
implantation. The precise level of Oct-3/4 governs 3 distinct fates of
ES cells,
respiratory
tract
(except the nose),
digestive
tract,
bladder,
and urethra
and cartilage,
muscle,
blood
and blood vessels,
lymphatics
and lymphoid organs,
notochord,
serous membranes (pleura,
pericardium,
peritoneum,
kidney,
and gonads
and segmental musculature
and the epidermal tissues, such as the nails,
hair,
enamel
of teeth,
and glands
of the skin,
the nervous system,
the external sense organs such as the ear
and eye,
and the mucous membrane of the mouth
and anus)
and are incorporated in the thyroid
gland.
In submammalian vertebrates they remain as discrete masses in the neck
or mediastinum throughout adult life. The parafollicular
cells
of these bodies produce calcitonin..
or the rete testis
develops,
urethra
and lower vagina
in the female and prostate
and posterior urethra
in the male
or the clitoris.,
urethra
and lower vagina
secrete estrogens
that promote maturations of,
which induces degeneration of homolateral Muller ducts, apart from vestigial
appendix
testis
or hCG
to produce testosterone,
which induces,
is taken over as an excretory duct by the mesonephros
and metanephros);
later it differentiates into :
and its ampulla)
and posterior urethra
and anterior urethra