Maternal Recognition of Pregnancy

During early pregnancy, the blastocyst must signal its presence to the maternalsystem 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 domesticspecies. However, the definition is inappropriate for marsupials and otherEutherian species such as the dog and ferret.

Maternal Recognition of Pregnancy can be defined simply as a functionalrelationship 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 signalwhich originates from the pre-attached blastocyst acts either directly at theendometrial level (gilt, mare, cow, ewe) or indirectly at the ovarian level (human)to block the action of prostaglandin F2 (PGF2).

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 -

Cow -

Ewe -

Mare -

Woman -

Ovariectomy at any stage of gestation will terminate pregnancy.However, Dziuk demonstrated that if CL were removed slowly, one CLcould support pregnancy.

Ovariectomy up to 210 days of gestation will terminate pregnancy. After210 days no effect - adrenal and placenta are sources of progesterone.

Ovariectomy up to 50 days of gestation will terminate pregnancy.

CL normally regress approximately Day 150 of gestation

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

Comparative Placentation, Ed Steven,1975, p 192

It is clear that the first signal from the blastocyst is the releaseof hCG. The syncytiotrophoblast is the major source of hCG.

Hearn, J. Reprod Fert 76:806

Luteal function is needed only for the first 6-7 weeks of gestation.After this period, the placenta produces enough progesterone tomaintain pregnancy.

hCG consists of two nonidentical glycosylatedsubunits ( & ). Production of the two subunitscan vary throughout pregnancy.

= similar to a chain of FSH, LH etc = biologically active chain

Peaks duringfirst trimester

Human Placental Lactogen -90 % homology with growthhormoneSingle nonglycosylated chain

Peaks nearterm

Hormonal Control of Reproduction, Austin & Short, p 182

Human Pregnancy

Early Pregnancy Factor

Pregnancy Specific BGlycoprotein - 29% CHO Function: Immunosuppression

Placental Associated Protein - similarof 2 macrogolubin - Only present inplasma during pregnancy. Inhibitor of leukocyte elastase - breaks downbasement membranes. Prevents maternal proteolytic attack. Low levels usuallyindicate pregnancy failure (2 wk prior)

Placental Protein-5 - Antiplasminand antitrypsin activity - may serveas a coagulation inhibitor

Physiology of Reproduction, Ed. Knobil & Neill, p 1996

Approximately 2 daysbefore they can ovulate

18-22 days 30-32 days

4 day cycle

10-14 days 22 days Delayed implantation

Marshalls Physiology of Reproduction, p460

RatIn the rat, the corpus luteum is formed on the morning of estrus in responseto the LH surge released the previous day. However, unlike most other mammals, the newly formed CL secretes progesterone for only 2 days afterformation. Rodent corpora lutea can be maintained (psuedopregncncy) by coitalstimulation 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 thelight 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 endurefor 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 atthe end to the pseudopregnancy terminates the prolactin surges in nonpregnantanimals.

Soares et al. 1998, Biol Reprod 58: 273

Soares et al. 1998, Biol Reprod 58: 273

The decidual and placental tissue ofthe rat secretes a luteotropinPlacental Lactogen to maintainluteal secretion of progesterone.Placental lactogen or the decidualluteotrophin may maintain estradiolsynthesis by the corpus luteumwhich has an effect on progesteronesynthesis through the steroidogenicpathway

Placental Lactogen produced bygiant cells - is related to Prolactinand Growth Hormone.mPL-I - Glycoprotein, detected Day 6 peak Day 10.mPL-II - Gycoprotein, detected Day 10 increases till term.Regulated by fetal genotype, growthhormone, nutritional statusReceptors in ovary & liverBoth Lactogenic in mammary gland

Soares et al. 1998, Biol Reprod 58: 273

Systemic Versus Local Utero-ovarian Pathwayfor PGF2 Induced Luteolysis

In the absence of pregnancy, the uterus produces a luteolysin which passesto the ovaries and causes regression of the corpus luteum, thereby ending theestrous cycle. A number of studies have demonstrated the luteolytic propertiesof prostaglandin F2 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 farmspecies. For many substances, a local transfer from veins to arteries, thus short-circuiting the peripheral blood circulation, has been demonstrated. Thismaybe critical since much of the uterine PGF2 can be cleared from the circulation by metabolism to 15-keto-13, 14 dihydroprostaglandin F2metabolitein one passage through the lungs (cow, ewe).

Uterine PGF2 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 betweenthe 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 plasmaprogesterone concentrations declining to basal levels (1 ng/ml or less) byday 17 to 18.

Bilateral hysterectomy before day 16 results in prolonged luteal function forperiods equal to or longer than 114 days. However, unilateral pregnancy failsin pigs by 21 days in the presence of a non-gravid uterine horn. One fourthof the one uterine horn approached the minimal quantity of uterine tissuenecessary for bilateral luteal regression in the pig.

The results of studies indicating that concetuses must be present in both uterinehorns (2 per horn) for pregnancy to be established indicate a combined and local uterro-ovarian pathway. In swine, PGF2 is converted into an inactive metabolite,15 keto - 13, 14 dihydro-prostaglandin F2, by the lungs as it is in other species.However, only 18% of the PGF2 released is metabolized in one passage throughthe lungs in comparison to 99% in the lungs of the ewe. Therefore active PGF2 can reach the contralateral ovary by a systemic route in the pig thereby causingluteolysis on both ovaries.

PGF2 is luteolytic in the pig when given after Day 12 of the cycle or pregnancy.The CL of the pig may remain refractory to PGF2 until LH begins to dissociatefrom the luteal cell receptors. It is thought that conformational changes withinthe luteal cells facilitate PGF2 binding. The PGF2 alters adenylate cyclase system to inhibit progesterone secretion and activate lysosomal enzymes to causemorphological regression of the luteal cells.

Maternal Recognition of pregnancy occurs on about Day 12 in the pig. Flushingof embryos from the uterine horns after Day 12 results in an extension of thediestrous period.

Conceptus estrogen synthesis (estrone, estradiol-17 and estriol), as well as possible effects of catechol estrogens, on Day 12 coincident with rapid elongationof the blastocyst is the maternal recognition signal. This production of estrogenis reinforced by a second period of synthesis and release between Day 15 to 18which sustains pregnancy till term. Injection of exogenous estrogen on Day 11and 14 through 16 will stimulate pseudopregnancy (prolonged CL function inthe absence of any embryo) for an equivalent or slightly longer period thanpregnancy.

Control of luteolysis or timing is regulated by progesteronestimulation and the down-regulation of progesteronereceptor in the uterine surface epithelium and glands.

Mare

0

1

2

3

4

0 8 16 24

Day of Estrous Cycle

Cow

0

2

4

6

0 5 10 15 20

Day of Estrous Cycle

Sow

0

5

10

15

20

0 5 10 15 20

Day of Estrous Cycle

Ewe

0

1

2

3

0 5 10 15

Day of Estrous Cycle

PGF2(1000 pg/ml)

P4(ng/ml)

8

2

P4(ng/ml)

4

6

12

P4(ng/ml)

1

2

12

3

18

.5

1

1.5

P4(ng/ml)

PGF2(1000 pg/ml)

COX-2 Gene Expression During Estrous Cycle and Early Pregnancy

0

20

40

60

80

100

120

140

160

0 5 10 12 15 17

Cycle

Pregnant

aa

a

b

b

b

Day

Day Effect (P < .0001)

Days with differ superscripts differ significantly (P < 0.05)

Fo

ld D

iffe

ren

ce

in G

en

e E

xp

res

sio

n

NF-B Pathway

Progesterone receptor on Day 0 of the estrous cycle of the pig

Note the loss of progesterone receptor fromthe surface and glandular epithelium on Day 10

IL-1

IL-1R

IL-1RAP

IKK

IKK IKK

IB

IB

P50

P65 P50

P65

InducibleKinase Phosphorylation

of IB

Transcription

NF-B responsive genes

Nucleus

NF-BTranslocation

to Nucleus

Ubiquitinationand

Degradation

PR

IL-1

IL-1R IL-1RAP

IKK

IKK IKK

IB

IB

P50

P65 P50

P65

InducibleKinase Phosphorylation

of IB

Transcription

NF-B responsive genes

Nucleus

NF-BTranslocation

to Nucleus

Ubiquitinationand

Degradation

IL-8TNF-LIFKGFCOX-2Integrins

Estrogen

ER

Conceptus estrogen production appears to reduce the levels of PGF2 in theuter-oovarian vein in pregnant animals in comparison to nonpregnant gilts.However, the levels of PGF2 found in the uterine lumen during Days 12through 18 are extremely elevated in pregnant compared to nonpregnant gilts

Nonpregnant

Pregnant

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 increasefrom Day 15 to 30.

Loss of progesterone receptor in the uterine epithelium allow an increasein the presence of estrogen receptor.

How is the release of a large quantity of estrogen prevented from having a systemic effectlike stimulating uterine myometrial contractions?

Geisert et al. 1990, J. Reprod Fert Suppl. 40:293

The Theory of Maternal Recognition of Pregnancy in the Pigwas First Proposed by Bazer and Thatcher (1977)

In nonpregnant pigs, progesterone enhances and/or induces PGF2synthesisby the uterine endometrium and secretion is associated with the elevated plasma estradiol concentrations (ovarian) between Day 12 and 18 of the cycle.

Secretion of PGF2 in mid-luteal phase is primarily in an endocrine direction(uterine capillaries). The increased levels of PGF2 lead to luteolysis.

In pregnant pigs, progesterone stimulates PGF2 in the endometrium as innonpregnant pigs. However, estrogen production by pig conceptuses (localuterine effect) alters direction of PGF2 movement so that it remains in the uterine lumen (exocrine secretion). Maintenance of PGF2 in exocrine directionwould prevent PGF2 from entering the uterine venous drainage. Therefore,PGF2 synthesis is not inhibited but compartmentalized. It is also significantthat histotroph (protein in lumen) is also maintained in an exocrine direction.

Immunocytochemical staining foruteroferrin in the porcine endometrium

Day 15 of the estrous cycle

Day 15 of pregnancy

Note presence of stain in the stromaand not in the uterine lumen.This is an example of endocrinesecretion!

Note presence of staining in thelumen of glands.This is an example of exocrinesecretion!

Chen et al. 1975, Biol Reprod 13:304

Pig Conceptuses Protein Secretion

Day 10.5 Day 11.5

Retinoic Binding Protein

Interferon-

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

Keys et al, 1986, Biol Reprod 34:405

There are distinct changes in bloodflow during early pregnancy.

IGF-BP

IGF-I

Geisert and Yelich, 1997, J. Reprod Fert Suppl. 52:133