Maternal Recognition of Pregnancy
During early pregnancy, the blastocyst must signal its presence to the maternal
system to stimulate CL maintenance for establishment of pregnancy.
The term “Maternal Recognition of Pregnancy” (first coined by Short in 1969)
is usually associated with prevention of CL regression when applied to domestic
species. However, the definition is inappropriate for marsupials and other
Eutherian species such as the dog and ferret.
Maternal Recognition of Pregnancy can be defined simply as a functional
relationship between the uterus, CL and embryo itself.
In most eutherian mammals, maternal recognition of pregnancy is established
when the length of the estrous cycle exceeds that of the normal cycle. The signal
which originates from the pre-attached blastocyst acts either directly at the
endometrial level (gilt, mare, cow, ewe) or indirectly at the ovarian level (human)
to block the action of prostaglandin F2a (PGF2a).
Function of the Corpus Luteum
Ludwig Frankel pioneered the role of the corpus luteum in pregnancy
maintenance when he demonstrated that removal of the CL (ovariectomy)
from a pregnant rabbit terminated pregnancy.
Gilt - Ovariectomy at any stage of gestation will terminate pregnancy.
However, Dziuk demonstrated that if CL were removed slowly, one CL
could support pregnancy.
Cow - Ovariectomy up to 210 days of gestation will terminate pregnancy. After
210 days no effect - adrenal and placenta are sources of progesterone.
Ewe - Ovariectomy up to 50 days of gestation will terminate pregnancy.
Mare - CL normally regress approximately Day 150 of gestation
Woman - No effect after Day 24.
Pituitary Support of CL Function
Gilt: There is necessity of secretion of a pituitary luteotrophin (LH) apparent
in the hypophysectomized-hysterectomized gilt. However, it is not
necessary until after Day 12 of the cycle. Prolactin can serve as a
luteotrophin after Day 70 of gestation.
Ewe: The presence of the pituitary gland is essential from maintenance of
pregnancy in the ewe during the first 50 days of gestation.
Hypophysectomy before Day 50 causes regression of the CL and
abortion. After Day 50 there is no effect.
WRONG
It is clear that the first signal from the blastocyst is the release
of hCG. The syncytiotrophoblast is the major source of hCG.
Hearn, J. Reprod Fert 76:806
hCG consists of two nonidentical glycosylated
subunits (a & b). Production of the two subunits
can vary throughout pregnancy.
a = similar to a chain of
FSH, LH etc
b = biologically active
chain
Peaks during
first trimester
Human Placental Lactogen 90 % homology with growth
hormone
Single nonglycosylated chain
Peaks near
term
Luteal function is needed only for the first 6-7 weeks of gestation.
After this period, the placenta produces enough progesterone to
maintain pregnancy.
Hormonal Control of Reproduction, Austin & Short, p 182
Human Pregnancy
Early Pregnancy Factor
Pregnancy Specific B
Glycoprotein - 29% CHO
Function: Immunosuppression
Placental Associated Protein - similar
of a2 macrogolubin - Only present in
plasma during pregnancy. Inhibitor of
leukocyte elastase - breaks down
basement membranes. Prevents maternal
proteolytic attack. Low levels usually
indicate pregnancy failure (2 wk prior)
Placental Protein-5 - Antiplasmin
and antitrypsin activity - may serve
as a coagulation inhibitor
Physiology of Reproduction, Ed. Knobil & Neill, p 1996
Approximately 2 days
before they can ovulate
18-22 days
30-32 days
4 day cycle
10-14 days
22 days
Delayed implantation
Marshalls Physiology of Reproduction, p460
Rat
In the rat, the corpus luteum is formed on the morning of estrus in response
to the LH surge released the previous day. However, unlike most other
mammals, the newly formed CL secretes progesterone for only 2 days after
formation. Rodent corpora lutea can be maintained (psuedopregncncy) by coital
stimulation for 10-12 days
Hormonal Control of Reproduction, Austin & Short, p 182
Mating in the rat induces two daily surges of prolactin, one at the end of the
light period (diurnal) and the other at the end of the dark period (nocturnal).
These surges are last seen on Day 9 (diurnal) and 10 (nocturnal) of pregnancy.
On Day 9, placental lactogen is secreted by the placenta which peaks on Day 12.
The prolactin surges do not require ovarian steroids. These surges will endure
for up to 14 days. A luteotrophic hormone of placental origin terminates the
surges of prolactin, while a shifting of the ratio of estradiol to progesterone at
the end to the pseudopregnancy terminates the prolactin surges in nonpregnant
animals.
Soares et al. 1998, Biol Reprod 58: 273
Soares et al. 1998, Biol Reprod 58: 273
The decidual and placental tissue of
the rat secretes a luteotropin
Placental Lactogen to maintain
luteal secretion of progesterone.
Placental lactogen or the decidual
luteotrophin may maintain estradiol
synthesis by the corpus luteum
which has an effect on progesterone
synthesis through the steroidogenic
pathway
Placental Lactogen produced by
giant cells - is related to Prolactin
and Growth Hormone.
mPL-I - Glycoprotein, detected Day 6
peak Day 10.
mPL-II - Gycoprotein, detected Day 10
increases till term.
Regulated by fetal genotype, growth
hormone, nutritional status
Receptors in ovary & liver
Both Lactogenic in mammary gland
Soares et al. 1998, Biol Reprod 58: 273
Systemic Versus Local Utero-ovarian Pathway
for PGF2a Induced Luteolysis
In the absence of pregnancy, the uterus produces a luteolysin which passes
to the ovaries and causes regression of the corpus luteum, thereby ending the
estrous cycle. A number of studies have demonstrated the luteolytic properties
of prostaglandin F2a and proposed that it is the uterine luteolysin.
Blood vessesls of the female reproductive tract form a unique network with a
large common contact area between veins and arteries in most domestic farm
species. For many substances, a local transfer from veins to arteries, thus
short-circuiting the peripheral blood circulation, has been demonstrated. This
maybe critical since much of the uterine PGF2a can be cleared from the
circulation by metabolism to 15-keto-13, 14 dihydroprostaglandin F2a metabolite
in one passage through the lungs (cow, ewe).
Uterine PGF2a can the reach the corpus luteum by
three possible pathways
1. Systemic utero-ovarian (passage through lungs)
2. Local utero-ovarian (ovarian artery and uterine vein)
3. Local uterine lymphatic vessels
Why must the
equine embryo
migrate between
the uterine horns?
Veterinary Scope 1, 1976
Veterinary Scope 1, 1976
Veterinary Scope 1, 1976
Bovine Uteroovarian Vasculature
Veterinary Scope 1, 1976
Veterinary Scope 1, 1976
Maternal Recognition of Pregnancy in the Gilt
In the cyclic gilt, luteal regression begins on about day 15 with plasma
progesterone concentrations declining to basal levels (1 ng/ml or less) by
day 17 to 18.
Bilateral hysterectomy before day 16 results in prolonged luteal function for
periods equal to or longer than 114 days. However, unilateral pregnancy fails
in pigs by 21 days in the presence of a non-gravid uterine horn. One fourth
of the one uterine horn approached the minimal quantity of uterine tissue
necessary for bilateral luteal regression in the pig.
The results of studies indicating that concetuses must be present in both uterine
horns (2 per horn) for pregnancy to be established indicate a combined and local
uterro-ovarian pathway. In swine, PGF2a is converted into an inactive metabolite,
15 keto - 13, 14 dihydro-prostaglandin F2a, by the lungs as it is in other species.
However, only 18% of the PGF2a released is metabolized in one passage through
the lungs in comparison to 99% in the lungs of the ewe. Therefore active PGF2a
can reach the contralateral ovary by a systemic route in the pig thereby causing
luteolysis on both ovaries.
PGF2a is luteolytic in the pig when given after Day 12 of the cycle or pregnancy.
The CL of the pig may remain refractory to PGF2a until LH begins to dissociate
from the luteal cell receptors. It is thought that conformational changes within
the luteal cells facilitate PGF2a binding. The PGF2a alters adenylate cyclase
system to inhibit progesterone secretion and activate lysosomal enzymes to cause
morphological regression of the luteal cells.
Maternal Recognition of pregnancy occurs on about Day 12 in the pig. Flushing
of embryos from the uterine horns after Day 12 results in an extension of the
diestrous period.
Conceptus estrogen synthesis (estrone, estradiol-17b and estriol), as well as
possible effects of catechol estrogens, on Day 12 coincident with rapid elongation
of the blastocyst is the maternal recognition signal. This production of estrogen
is reinforced by a second period of synthesis and release between Day 15 to 18
which sustains pregnancy till term. Injection of exogenous estrogen on Day 11
and 14 through 16 will stimulate pseudopregnancy (prolonged CL function in
the absence of any embryo) for an equivalent or slightly longer period than
pregnancy.
Control of luteolysis or timing is regulated by progesterone
stimulation and the down-regulation of progesterone
receptor in the uterine surface epithelium and glands.
Ewe
P4
(ng/ml)
Cow
P4
P4
(ng/ml)
(ng/ml)
PGF2a
(1000 pg/ml)
18
3
2
PGF2a
(1000 pg/ml)
6
2
4
1.5
2
1
12
1
.5
6
0
0
0
5
10
0
15
5
10
15
20
Day of Estrous Cycle
Day of Estrous Cycle
P4
(ng/ml)
Sow
3
20
15
Mare
12
4
3
8
2
2
10
4
5
1
1
0
0
5
10
Day of Estrous Cycle
15
20
0
0
8
16
Day of Estrous Cycle
24
Fold Difference in Gene Expression
COX-2 Gene Expression During Estrous
Cycle and Early Pregnancy
b
160
140
Cycle
120
Pregnant
100
b
80
Day Effect (P < .0001)
60
b
40
20
a
a
a
0
0
5
10
12
15
17
Day
Days with differ superscripts differ significantly (P < 0.05)
NF-kB Pathway
Progesterone receptor on Day 0 of the
estrous cycle of the pig
Note the loss of progesterone receptor from
the surface and glandular epithelium on Day 10
IL-1b
IL-1R
IL-1RAP
Inducible
Kinase
Phosphorylation
of IkB
IkBa
Ubiquitination
and
Degradation
IKKg
IKKa
IKKb
P50
Nucleus
IkBa
P65
P50
Translocation
to Nucleus
NF-kB
P65
PR
Transcription
NF-kB responsive genes
IL-1b
IL-1RAP
IL-1R
Estrogen
Inducible
Kinase
Phosphorylation
of IkB
IkBa
Ubiquitination
and
Degradation
IKKg
IKKa
IKKb
P50
Nucleus
IkBa
P65
P50
Translocation
to Nucleus
P65
NF-kB
ER
Transcription
NF-kB responsive genes
IL-8
TNF-a
LIF
KGF
COX-2
Integrins
Conceptus estrogen production appears to reduce the levels of PGF2a in the
uter-oovarian vein in pregnant animals in comparison to nonpregnant gilts.
However, the levels of PGF2a found in the uterine lumen during Days 12
through 18 are extremely elevated in pregnant compared to nonpregnant gilts
Pregnant
Nonpregnant
Bazer et al. 1982, Control of Reproduction in the Pig, p227
There are two periods of estrogen release by the conceptuses in the pig.
A sharp but short release on Day 12, followed by a more sustained increase
from Day 15 to 30.
Loss of progesterone receptor in the uterine epithelium allow an increase
in the presence of estrogen receptor.
How is the release of a large quantity of estrogen prevented from having a systemic effect
like stimulating uterine myometrial contractions?
Geisert et al. 1990, J. Reprod Fert Suppl. 40:293
The Theory of Maternal Recognition of Pregnancy in the Pig
was First Proposed by Bazer and Thatcher (1977)
In nonpregnant pigs, progesterone enhances and/or induces PGF2a synthesis
by the uterine endometrium and secretion is associated with the elevated
plasma estradiol concentrations (ovarian) between Day 12 and 18 of the cycle.
Secretion of PGF2a in mid-luteal phase is primarily in an endocrine direction
(uterine capillaries). The increased levels of PGF2a lead to luteolysis.
In pregnant pigs, progesterone stimulates PGF2a in the endometrium as in
nonpregnant pigs. However, estrogen production by pig conceptuses (local
uterine effect) alters direction of PGF2a movement so that it remains in the
uterine lumen (exocrine secretion). Maintenance of PGF2a in exocrine direction
would prevent PGF2a from entering the uterine venous drainage. Therefore,
PGF2a synthesis is not inhibited but compartmentalized. It is also significant
that histotroph (protein in lumen) is also maintained in an exocrine direction.
Immunocytochemical staining for
uteroferrin in the porcine endometrium
Day 15 of the estrous cycle
Note presence of stain in the stroma
and not in the uterine lumen.
This is an example of endocrine
secretion!
Note presence of staining in the
lumen of glands.
This is an example of exocrine
secretion!
Day 15 of pregnancy
Pig Conceptuses Protein Secretion
Day 10.5
Day 11.5
Retinoic Binding Protein
Interferon-g
Godkin et al. 1982, Biol Reprod 27:977
Endometrial fluorescence following systemic treatment of gilts with Evans Blue
Day 12 of Pregnancy
Day 15 of Cycle
Day 15 of Pregnancy
There are distinct changes in bloodflow during early pregnancy.
Keys et al, 1986, Biol Reprod 34:405
IGF-I
IGF-BP
Geisert and Yelich, 1997, J. Reprod Fert Suppl. 52:133