Advanced Reproduction Physiology

(Part 4)

Isfahan University of Technology

College of Agriculture, Department of Animal Science

Prepared by: A. Riasihttp://riasi.iut.ac.ir

Placentation, the Endocrinology of

Pregnancy and Parturition

Reference: Pathways to Pregnancy and Parturition (Second revised edition)By: P. L. Senger, 2005

The phenomenon of intrauterine development

ensures that the developing conceptus will

receive adequate nutrient and protection during

its development.

Placenta formation

The placenta is a transient organ of metabolic

interchange between the conceptus and dam.

It is also a transient endocrine organ.

The placenta in composed from two parts:

Fetal component derived from the chorion

Maternal component derived from modifications of

the uterine endometrium

Placenta formation

The functional unit of the fetal placenta is the

chorionic villus.

Specific of mare placenta:

It has many specialized microzones of chorionic villi

known as microcotyledons

It contains unique transitory structure known as

endometrial cups.

Placenta formation

Placenta formation

Ruminant have cotyledonary placenta.

Placentome consists of:

Fetal cotyledon contributed by the chorion

Maternal cotyledon, originating from caruncular

regions of uterus

Placenta formation

The migrating binucleate giant cells (BGC) in the ruminat placenta

Placenta formation

The characteristics and functions of BGC:

They are quite large and two nuclei originated from

trophoblast cells.

They are formed continuously throughout gestation.

The BGC migrate from the chorionic epithelium and

invade the endometrial epithelium.

Placenta formation

The characteristics and functions of BGC (continue):

They are believed to transfer complex molecules

from the fetal to maternal placenta.

These cells secrete pregnancy specific protein B

(PSPB) and placental lactogen.

Placenta formation

The placenta regulate the exchange between

fetus and dam.

Placental exchange ways:

Simple diffusion

Facilitated diffusion

Active diffusion

Placenta functions

Placental transportation:

Gases

Water

Minerals and vitamins

Glucose and Amino Acids

Proteins

Lipids

Placenta functions

Placental transportation (continue):

Large peptide hormones (TSH, ACTH, GH, insulin,

glucagon)

Smaller molecular weight hormones (steroids, T3

and T4) and catcholamines

Toxic substances

Bacteria and viruses

Placenta functions

In addition to serving as a metabolic exchange

organ, the placenta serves as a transitory

endocrine organ.

Hormones from the placenta gain accesss to both

the fetal and the maternal circulation.

Placenta functions

The placenta produces hormones that can affect:

Stimulate ovarian function

Maintain pregnancy

Influence fetal growth

Stimulate mammary function

Assist in parturition

Placenta functions

Placenta functions

Placenta functions

Placenta functions

The placenta secretes progesterone and estrogen

Progesterone provides the stimulus for elevated

secretion by the endometrial glands

Progesterone inhibits myometrial contraction

(progesterone block)

Placenta functions

Placenta functions

In addition to progesterone, estrogens are also an

important product of the placenta.

The placental estrogens are particularly important

during the last part of gestation and in most species

signals the early preparturient period.

Placenta functions

The placenta produce placental lactogen or

somatomammotropin.

This hormone have two major effect:

Promoting the growth of fetus

Stimulating the mammary gland of the dam

Placenta functions

Placenta functions

Relaxin is also a product of placenta.

This hormone is produce in some species:

Humans

Mares

Rabbits

Pigs and monkeys

Cats and dogs

Placenta functions

Relaxin cause:

Softening of the connective tissue in the cervix

Promotes elasticity of pelvic ligaments

Placenta functions

The fetal hypothalamo-pituitary-adrenal axis is

obligatory for initiation of parturition.

During the conclusion of gestation, fetal mass

approaches the inherent space limitations of the

uterus.

Physiology of parturition

Fetus ACTH

Placental estrogen Uterus PGF2α

Progesterone

Limited space for fetus Fetus hypothalamus

Fetus cortisol

Induction of parturition

Physiology of parturition

Plasma cortisol concentration changes during sheep gestation (Yuen et al. 2004)

Physiology of parturition

Prepartum increase in sheep circulating cortisol

is required for the differentiation and maturation

of key fetal organs:

Lung

Liver

Kidney

Brain

Physiology of parturition

A positive relationship between the level of

activation of the fetal hypothalamus-pituitary-

adrenal axis and leptin synthesis and/or secretion

in late gestation.

Physiology of parturition

The endocrine changes cause two major events

to occur:

Removal of the myometrial “progesterone block”

enabling myometrial contractions to begin

Increased reproductive tract secretions particularly

by the cervix

Physiology of parturition

Physiology of parturition

Three stages of parturition are:

Initiation of myometrial contractions

Expulsion of the fetus

Expulsion of the fetal membranes

Effect of fetal corticoids:

Converting progesterone to estradiol

Synthesizing PGF2α by placenta

As both estradiol and prostaglandin become elevated, the

myometrium becomes increasingly more active

The CL of pregnancy regress

Physiology of parturition

Physiology of parturition

Physiology of parturition

Physiology of parturition

Relative hormone profiles in the cow during the periparturient period

Physiology of parturition

Physiology of parturition

Follistatin and activin

may be also involved in

the mechanism of

natural parturition (Xia et

al. 2009)

Physiology of parturition

In pre-parturition period birth canal is lubricated

by:

Mucus secretion in cervix and vagina

Fetal membrane rupture and loss of amniotic and

allantoic fluid

Physiology of parturition

As the fetus enters the birth canal, it becomes

hypoxic

Hypoxia promotes fetal movement that, in turn,

promotes further myometrial contraction

Physiology of parturition

In most species, expulsion of the fetal

membranes quickly follows expulsion of the

fetus

It is believed that vasoconstriction of arteries in

villi reduces pressure and thus allows the villi to

be release from the crypts

Physiology of parturition

Prolonged parturition can result in serious

complications to both the fetus and dam.

Different causes for dystocia:

Excessive size of fetus

Failure of proper fetal ritation

Multiple births in monotocous species

Physiology of parturition

Some research papers associated to this lecture

1. Leibovicha, H., et al. 2001. Effects of recombinant ovine placental lactogen and

recombinant ovine growth hormone on growth of lambs and milk production of ewes.

Livestock Production Science 68: 79–86.

2. Probo, M., et al. 2011. Peripartal Hormonal Changes in Alpine Goats: a Comparison

Between Physiological and Pathological Parturition. Reprod Dom Anim 46: 1004–

1010.

3. Leury, B. J., et al. 2003. Effect of insulin and growth hormone on plasma leptin in

periparturient dairy cows. Am. J. Physiol. Regul. Integr. Comp. Physiol. 285: 1107–

1115.

4. Murawski, M., et al. 2011. Cortisol, progesterone and estradiol secretion in vitro in

postpartum placental cotyledons of ewes that gave birth to single or twin lambs. Ann.

Anim. Sci., 11: 53–60.