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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