Placental lactogen

• Some species– Type of placentation

• Ruminants• Humans• Rodents

• Produced by fused cells– Syncytiotropoblast– Binucleated/

trinucleated cells

• Proteins related to pituitary GH and prolactin– Close to GH in

humans– Close to prolactin in

rodents– 50-50 in cows

• Function– Regulation of metabolism

• GH-like activity– Much weaker than pituitary GH

– Development of mammary gland• Prolactin-like activity

– Maintenance of CL function• Rodents during early stage of pregnancy

• No known receptor(s) for placental lactogen– Interacts with GH receptor– Interacts with prolactin receptor

• Pattern of secretion– Different between

cattle and sheep• Degree of fusion

between tropoblasts and endometrium

• GH-Variant– Human only– Acts like GH

• Tissue growth• Nutrient metabolism

– Affects function of insulin• Pregnancy-induced diabetes

Placental steroidogenesis

• Cholesterol– Lipoproteins from circulation

• No De Novo synthesis

• Progesterone– Replace CL in some species

• Maintenance of pregnancy• Precursor for fetal adrenal steroids

• Estrogens– Limited production

• Limited 17-hydroxylase activity– Abundant in fetal adrenal gland

– Androgens from fetal adrenal gland• Converted to estrogens in the placenta

– Production of estriol rather than estradiol

– Secretion of estrone• Majority of placental estrogen in some species

Parturition

• Three stages– First stage

• Initiation of myometrium contraction• Initiated by the fetus

– Second stage• Expulsion of the fetus

– Third stage• Expulsion of fetal membrane

Parturition

• First stage– Initiated by fetus– Distress

• Maximum size that can be maintained– Lack of nutrients– Hypoxia

• Activation of fetal hypothalamus-pituitary-adrenal axis

– Release of ACTH by the fetal pituitary gland

• Fetal ACTH– Release of cortisol from

fetal adrenal gland• Fetal cortisol

– Removal of “progesterone block”

• Contraction of myometrium

• Conversion of progesterone to estradiol

– 17a-hydroxylase– 17-20 lyase– Aromatase

• Fetal cortisol– Increase production of PGF2a by the

endometrium• Further removal of progesterone block

– Cause luteolysis

• Stimulates contraction of myometrium– Synergism with estradiol

• Contraction of myometrium– Pushes the fetus toward cervix

• Activation of the CNS– Secretion of oxytocin from the posterior pituitary gland

• Oxytocin– Increased contraction of the uterine smooth muscle

– Positive feedback• More contraction (pushing of fetus), increased oxytocin

secretion• Entering of the fetus in the cervical canal

– End of the first stage

Expulsion of fetus

• Relaxation of the cervix and pelvic ligaments– Relaxin

• Secretion stimulated by PGF2

• Role of estradiol on parturition– Increased overall secretory activity

• Mucus production by cervix and vagina– Removal of cervical plug– Lubrication

• Rupture of fetal membrane– Loss of amniotic and allantonic fluid

• Further lubrication

• Fetus becomes hypoxic– Movement of the fetus

• Stimulates further contraction of the myometrium

– Increased strength of contraction

Expulsion of the fetal membrane

• Shortly after expulsion of the fetus– Dislodging of fetal membrane from the

uterus– Vasoconstriction

Mammary gland development

• Cyclic changes in ovarian steroid hormones– Essential for mammary

development• Estradiol

– Development of mammary alveoli (secretory unit)

• Progesterone– Development of

mammary ducts

• Final stage of mammary gland development– Pregnancy

• Ovarian/placental steroid hormones

– Periparturient period• GH and prolactin

– Placental lactogen?

• Glucocorticoids

Lactation

• Parturition– Metabolic shift

• Fetal growth to synthesis and secretion of milk

– Mobilization of nutrients

• Storage within the body

– Fats

• Triggered by changes in hormones

Lactation and re-initiation of reproductive cycle

• Lactating women– High prolactin– Low LH and estradiol

• No ovulation– Lactation-induced anovulation/amenorrhea

(infertility)

– Women remain anovulatory as long as they breast-feed their babies

• Increased survivability of the infants

• Lactational anestrus in the domestic species– Divert nutrients for lactation

• Ensure the survival of offspring

– Lactation stimuli/physical contact between mother and offsprings

– Animals remain in anestrus (absence of estrus) until nutrient intake exceeds nutrient demand