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Early Embryogenesis and Maternal Recognition of Pregnancy

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zygote
becomes an embryo
embryo
organism in early stage of development
fetus
potential offspring within uterus
conceptus
product of conception and includes
embryo during the early embryonic stage
embryo and extra-embryonic membranes during the pre-implantation stage
fetus and placenta during the post-attachment stage
fertilization four steps
1. development within the zona pellucida
2. hatching of the blastocyst from the zona pellucida
3. maternal recognition of pregnancy
4. extraembryonic membranes and formation
Fertilized Oocyte
Male and female pronuclei within the cytoplasm characterize a developmental stage of the newly fertilized oocyte
ootid
when male and female pronuclei are observed
Largest single cell in the body
undergoes mitotic division called cleavage
first cleave division generates a two-celled embryo, the cells are called blastomere
Blastomere
each has the potential to develop into separate healthy offspring
Blastomeres from the 2-, 4-, and 8- celled totipotent
Totipotency
is a term used to describe a single cell (blastomere) gives rise to a complete, fully formed individual
Mitotic division of Blastomeres
1. occurs simultaneously
2. no net increase in size
3. take place inside zona pellucida
Morula Stage
is characterized by solid balls of cells and indistinguishable blastomeres
Morula stage cells separate into:
two distinct population form
1. inner cells - gap junction allows for intracellular communication. May enable the inner cells to remain in a cluster
2. Outer Cells - tight junctions allow cell to cell adhesion
After tight junctions are formed
fluid begins to accumulate inside the embryo
Fluid filled cavity is called blastocoele
Hatching of the blastocyst
is governed by three forces
1. growth and fluid accumulation
2. production of enzymes by trophoblastic cells
3. contraction of the blastocyst
Blastocyst
embryo with a distinct cavity
Due to tight junctions and gap junctions
the embryo is partitioned into two cell populations.
1. inner cell mass - develops into embryo proper
2. Trophoblastic cells - give rise to the chorion
chorion
the fetal component of the placenta
Blastocyst continues to undergo mitosis,
fluid continues to fill the blastocoele and pressure begins to increase
Concurrent with growth and fluid accumulation
is the production of proteolytic enzymes by the trophoblastic cells
They weaken the zona pellucida so that it ruptures
The blastocyst itself begins to contract and relax
causes intermittent pressure pulses
The zona pellucida ruptures due to
pressure pulses coupled with continued growth and enzymatic degradation
A small crack or fissure in the zona pellucida develops
the cell of the blastocyst squeeze out
The blastocyst now becomes a free-floating embryo within the lumen of the uterus
Extra-embryonic Membranes
after hatching, the conceptus undergoes massive growth
due largely to development of a set of membranes - extra embryonic membranes
Pre-attachment period
Mammalian embryos are subdivided into two primary groups
Most domestic animals and Primates
Most Domestic Animals
preattachment period within the uterus is long (several weeks)
extensive extra embryonic membranes form by a folding process to generate the amnion, chorion, and allanto-chorion
Primates
Preattachement period is short
Blastocyst implant very soon it enters the uterus
Extraembryonic membranes form after implantation
Extraembryonic membranes of the preattached embryo consists of:
yolk sac
chorion
amnion
allantois
Set of 4 anatomically distinctive membranes which originate from
trophoblast
mesoderm
primitive endoderm
embryo
Specific membranes give rise to the following:
yolk sac
chorion
amnion
as the hatched blastocyst grow it develops an additional layer beneath but in contact with the inner cell mass
yolk sac
develops from the primitive endoderm
chorion
eventually attaches to the uterus
amnion
provide a fluid filled protective sac
Primitive Endoderm
Inside lining of the trophoblast
It also forms an evagination at the ventral
portion of the inner cell mass - yolk sac
yolk sac
regress as conceptus develops
chorion
as the blastocyst continues to expand it forms a double membrane
pushes upward in the dorsalateral region of the conceptus and begins to surround it
send wing like projectiong above embryo the aminon begins to form
amnion
complete sac around the embryo
when the chorion fuses over the dorsal portion
of the embryo, it then forms a complete sac around the embryo which constitutes the amnion
Allantois
during the same time the amnion is developing a small evagination from the posterior region of the primitive gut begins to form and is reffered to as allantois
Functions of the Allantois
1. fluid-filled sac that collects liquid waste
2. expands with embryo growth
3. fuses with chorion
Chorioallantoic membrane
when the allantois fuses with the chorion
Chorioallantoic Membrane Function
the membrane is the fetal contribution to the placenta and will provide the surface for attachment to the endometrium
Conceptus must provide a
biochemical signal or pregnancy will terminate
progesterone must be maintained at
sufficiently high levels so that embryogenesis and attachment of the developing conceptus to the endometrium can take place
the embryo enters the uterus
between 2 and 5 days after fertilization depending on species
Maternal Recognition of Pregnancy
the critical series of events by which the conceptus initially signals its presence to the dam enables pregnancy to continue. If an adequate signal is not delivered in a timely manner, the dam will experience luteolysis, progesterone concentrations will decline and the pregnancy will be terminated
Maternal recognition of pregnancy must occur prior to luteolysis
CL produces oxytocin which stimulates PGF2a synthesis.
PGF2a levels are dependent on the number of oxytocin receptors.
Pulsatile secretion of PGF2a occurs in response to luteal oxytocin secretion and luteolysis follows
The Production of PGF2a is dependent upon
a threshold number of oxytocin receptors that are synthesized by endometrial cells at a critical time during the estrous cycle.
When these receptors are available in sufficient numbers,
pulsatile secretion of PGF2a occurs in response to luteal oxytocin secretion and luteolysis follows.
Clearly, this mechanism must be prevented if a successful pregnancy is to proceed
In the ewe and cow
the free-floating blastocyst produces specific proteins that prevent luteolysis
Ovine trophoblastic protein
Bovine trophoblastic proteiin
Interferons produced by trophoblastic cells
Referred to as ovine interferon τ (oIFN-τ) and bovine interferon τ (biFN-τ)
oIFN-τ and bIFN-τ
present in uterus 13-21 days after ovulation
bind to endometrium and inhibits oxytocin receptor synthesis by endometrial cells
Binds to apical portion of uterine glands to promote protein synthesis critical to preimplantation embryonic survival
Sow
control of pregnancy recognition: two major differences
conceptus produces estradiol as signal for maternal recognition of pregnancy
PGF2a is rerouted into uterine lumen
sow conceptus produces
estradiol 11-12 days after fertilization
estrogen doesnt inhibit PGF2a but causes it to be secreted in different directions (away from submucosal capillaries and towards the uterine lumen)
Luminal PGF2a has little access to the circulation so cannot cause luteolysis
There must be at least two conceptuses present in each horn for pregnancy to be maintained
(sow) it is believed that estrogen causes
increased receptor production for prolactin in the endometrium
Prolactin promotes exocrine secretion of PGF2a (into the lumen of the uterus)
Production of estrogen also stimulates uterine contractions which causes proper spacing of the conceptuses along the uterine horn
Human
at the time of implantation (day 7-9 after ovulation) the conceptus secretes a hormone called Human Chorionic Gonadotropin (hCG)
acts like LH-inhibits intraovarian luteolysis
precise mechanism is not known
Queen
is an induced ovulator
if mating does not occur, corpora lutea are not formed and post-estrous period of several days exist before another estrus.
if she breeds, a CL forms and the duration is the same as gestation (60 days)
a signal from the conceptus is not needed because corpora lutea are not lysed before pregnancy is established