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Early Embryogenesis and Maternal Recognition of Pregnancy
Chapter 13 – Student Handout
Embryogenesis Terminology
Zygote
Embryo
Fetus
Conceptus
Early embryonic stage

Pre-implantation stage

Post-attachment stage

Fertilization
Four steps
1.
2.
3.
4.
Fertilized Oocyte (Figure 13-1)
Male and female pronuclei within the cytoplasm characterize a developmental stage
of the newly fertilized oocyte.
Ootid
Single-celled
embryo (zygote)
Undergoes
mitotic divisions called _______________.
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
First cleavage division
Blastomere
 Blastomeres from the 2-, 4-, and 8-celled
 Totipotency
Mitotic divisions of blastomeres
1)
2)
3)
Morula stage
Two
distinct populations form:
1. Inner cells
Gap junctions

2. Outer cells

After
Tight junctions
tight junctions are formed, fluid begins to accumulate inside the embryo.
Fluid filled cavity (Figure 13-2)

Hatching
Hatching of the blastocyst is governed by three forces.
1.
2
2.
3.
Blastocyst (Figure 13-1, 13-3)
Embryo
with a distinct cavity is called a _____________.
Due
to tight junctions and gap junctions the embryo is partitioned into two
cell populations.
1. Inner cell mass 2. Trophoblastic cells -
Continues
to undergo mitosis
Concurrent
The
with growth and fluid accumulation
blastocyst itself begins to contract and relax –
Pressure
pulses coupled with continued growth and enzymatic
degradation cause ______________________________.
A small crack or fissure in the zona pellucida develops
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 3
Pre-attachment Period
Mammalian
embryos are subdivided into two primary groups.
Most domestic animals

1. Preattachment period
Extensive extra embryonic membranes form


Primates
1. Preattachment period
Extraembryonic membranes form.

Extraembryonic membranes of the preattached embryo consist of:
1)
2)
3)
4)
Set
of 4 anatomically distinctive membranes which originate from: (Figure
13-14)
1)
2)
3)
4)
Specific
membranes give rise to the following:
Yolk sac -

Chorion -

Amnion -

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Primitive endoderm (Figure 13-3)
Yolk
sac
Chorion
Amnion
(Figure 13-4)
 Functions:
1)
2)
Allantois
 Function:
1)
Chorioallantoic membrane (Figure 13-4)
 Function
1)
Biochemical Recognition
The conceptus must provide a biochemical signal or pregnancy will terminate.
Progesterone
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The
embryo enters the uterus between days 2 and 5 after ovulation, depending
on the species.
Maternal
recognition of pregnancy
Inadequate
signal is not delivered (Figure 13-8)
Maternal Recognition of Pregnancy
Must
occur prior to luteolysis
CL
produces oxytocin which stimulates PGF2 synthesis.
Dependent
Pulsatile
on the number of oxytocin receptors.
secretions of PGF2 occur and luteolysis follows (Figure 13-
8).
A
threshold number of oxytocin receptors is required for secretion of PGF2
and luteolysis follows (Figure 13-4).
Clearly,
this mechanism must be prevented if a successful pregnancy is to
proceed.
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Ewe and Cow
In the ewe and cow the free-floating blastocyst produces specific proteins that
prevent luteolysis.
1)
Ovine trophoblastic protein (Figure 13-5)
2)
Bovine trophoblastic protein
 Produced by
o 1)
 Refered to as :
o 1)
o 2)
Present in uterus Functions include:
Bind to _________________and inhibits
Binds to apical portion of uterine glands
Sow (Figure 13-6)
Two major differences


1.

2.
Conceptus produces estradiol


Estradiol

Luminal PGF2

Two conceptuses are required
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Precise mechanism for rerouting of PGF2 is not completely
understood.
Estrogen
Prolactin
Mare
Presence
of conceptus prevents luteolysis
Endometrial
Conceptus
production of PGF2 is reduced.
must migrate within the uterus.
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