Endocrine Functions of the Placenta

Grow Taller 4 Idiots

How to Grow Taller

Get Instant Access

The placenta secretes both steroid hormones and protein hormones. The protein hormones include chorionic gonadotropin (hCG) and chorionic somatomammotropin (hCS), both of which have actions similar to those of some anterior pituitary hormones (table 20.7). Chorionic gonadotropin has LH-like effects, as previously described; it also has thyroid-stimulating ability, like pituitary TSH. Chorionic somatomammotropin likewise has actions that are similar to two pituitary hormones: growth hormone and prolactin. The placental hormones hCG and hCS thus duplicate the actions of four anterior pituitary hormones.

Pituitary-like Hormones from the Placenta

The importance of chorionic gonadotropin in maintaining the mother's corpus luteum for the first 5/2 weeks of pregnancy has been previously discussed. There is also some evidence that hCG may in some way help to prevent immunological rejection of the implanting embryo. Chorionic somatomammotropin acts together with growth hormone from the mother's pituitary to produce a diabetic-like effect in the pregnant woman. The effects of these two hormones promote (1) lipolysis and increased plasma fatty acid concentration; (2) glucose-sparing by maternal tissues and, therefore, increased blood glucose concentrations;

Intervillous pool of maternal blood

Chorionic Villi And Intervillous Space

■ Figure 20.50 The circulation of blood within the placenta. Maternal blood is delivered to and drained from the spaces between the chorionic villi. Fetal blood is brought to blood vessels within the villi by branches of the umbilical artery and is drained by branches of the umbilical vein.

Amniotic fluid

Amniotic fluid

Chorionic Villi

■ Figure 20.49 Amniocentesis. In this procedure, amniotic fluid containing suspended cells is withdrawn for examination. Various genetic diseases can be detected prenatally by this means.

Table 20.7 Hormones Secreted by the Placenta

Hormones

Effects

Pituitary-like Hormones Chorionic gonadotropin (hCG)

Chorionic somatomammotropin (hCS)

Similar to LH; maintains mother's corpus luteum for first 5M weeks of pregnancy; may be involved in suppressing immunological rejection of embryo; also exhibits TSH-like activity Similar to prolactin and growth hormone; in the mother, hCS acts to promote increased fat breakdown and fatty acid release from adipose tissue and to promote the sparing of glucose for use by the fetus ("diabetic-like" effects)

Sex Steroids

Progesterone

Helps maintain endometrium during pregnancy; helps suppress gonadotropin secretion; stimulates development of alveolar tissue in mammary glands

Estrogens

Help maintain endometrium during pregnancy; help suppress gonadotropin secretion; help stimulate mammary gland development; inhibit prolactin secretion; promote uterine sensitivity to oxytocin; stimulate duct development in mammary glands

and (3) polyuria (excretion of large volumes of urine), thereby producing a degree of dehydration and thirst. This diabetic-like effect in the mother helps to ensure a sufficient supply of glucose for the placenta and fetus, which (like the brain) use glucose as their primary energy source.

Steroid Hormones from the Placenta

After the first 5M weeks of pregnancy, when the corpus luteum regresses, the placenta becomes the major sex-steroid-producing gland. The blood concentration of estrogens, as a result of pla-cental secretion, rises to levels more than 100 times greater than those existing at the beginning of pregnancy. The placenta also secretes large amounts of progesterone, changing the estrogen/progesterone ratio in the blood from 100:1 at the beginning of pregnancy to close to 1:1 toward full-term.

The placenta, however, is an "incomplete endocrine gland" because it cannot produce estrogen and progesterone without the aid of precursors supplied to it by both the mother and the fetus. The placenta, for example, cannot produce cholesterol from acetate, and so it must be supplied with cholesterol from the mother's circulation. Cholesterol, which is a steroid containing twenty-seven carbons, can then be converted by enzymes in the placenta into steroids that contain twenty-one carbons—such as progesterone. The placenta, however, lacks the enzymes needed to convert progesterone into androgens (which have nineteen carbons). Therefore, androgens produced by the fetus are needed as substrates for the placenta to convert into estrogens (fig. 20.51), which have eighteen carbons.

In order for the placenta to produce estrogens, it needs to cooperate with the steroid-producing tissues (principally the adrenal cortex) in the fetus. Fetus and placenta thus form a single functioning system in terms of steroid hormone production. This system has been called the fetal-placental unit (fig. 20.51).

The ability of the placenta to convert androgens into estrogen helps to protect the female embryo from becoming masculinized by the androgens secreted from the mother's adrenal glands. In addition to producing estradiol, the placenta secretes large amounts of a weak estrogen called estriol. The production of estriol increases tenfold during pregnancy, so that by the third trimester estriol accounts for about 90% of the estrogens excreted in the mother's urine. Since almost all of this estriol comes from the placenta (rather than from maternal tissues), measurements of urinary estriol can be used clinically to assess the health of the placenta.

Maternal blood

Cholesterol

Progesterone

Estrogens

Placenta

Cholesterol

- Progesterone ■ Androgens " Estrogens

Fetus

Progesterone

■ Figure 20.51 Interactions between the embryo and placenta produce the steroid hormones. The secretion of progesterone and estrogen from the placenta requires a supply of cholesterol from the mother's blood and the cooperation of fetal enzymes that convert progesterone to androgens.

Was this article helpful?

+2 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • BELLISIMA
    What cooperates to form the placenta?
    8 years ago
  • carrie blanchette
    What are the hormones of the placenta in endocrine and functions?
    7 years ago
  • benedetta
    What is the endocrine function of the placenta?
    7 years ago
  • alyx douglas
    Is placenta an endocrine gland ?
    7 years ago
  • Michael
    What hormone of a pregnant woman produces a maternal glucose sparing effect?
    6 years ago
  • keke
    What gland secretes amniotic fluid?
    5 years ago
  • melanie vogel
    What is cooperate to form the placenta?
    5 years ago
  • Janina Freitag
    Why is placenta called an incomplete endocrine gland?
    5 years ago
  • kaari
    What are the endocrine gland functions of the placenta?
    5 years ago
  • yemane
    What are the primary endocrine functions of the placenta?
    4 years ago
  • jenni
    Why placenta is an incomplete endocrine organ?
    4 years ago
  • monica
    Why placenta is called endocrine tissue?
    4 years ago
  • abbondanzio
    Why is the placenta considered to be a gland?
    3 years ago
  • Fosco Ferri
    Why is the placenta considered incomplete endocrine organ?
    3 years ago
  • kris
    Why placenta is considered as an endocrine tissue?
    3 years ago
  • gloriana
    Why placenter is considered as an endocrine gland?
    3 years ago
  • Adalfredo
    Why placenta is considered as endocrine gland?
    3 years ago
  • rita teodros
    Why is the placenta regarded as an endocrine organ?
    3 years ago
  • simone
    What are the endorime fumction of the placenta?
    3 years ago
  • Mirrin
    Which gland secretets amniotic fluid?
    3 years ago
  • Aziz Girmay
    How placenta act as an endocrine gland after 3 month pregnancy?
    3 years ago
  • Gorbulas
    Why is the placenta called a endocrine organ?
    3 years ago
  • lukas
    What is the meaning of the question comment on the role of placenta as an endocrine gland?
    3 years ago
  • leonide
    What is the function of chorionic villi?
    3 years ago
  • Elfstan
    Why placenta is endocrine glands?
    3 years ago
  • leila meriluoto
    Why is the placenta said to b metabolic n incomplete endocrine gland?
    3 years ago
  • gene
    How placenta acts as an endocrine tissue?
    3 years ago
  • allen
    How endocrine function of placenta works?
    3 years ago
  • mirabella longhole
    Why placenta is considered as seasonal endocrine gland?
    2 years ago
  • Giovanna
    Why placeta incomplete endocrine organ?
    2 years ago
  • hiewan negassi
    What is the function of the placenter?
    2 years ago
  • Ermes
    Why placenta is also called an endocrine glands?
    2 years ago
  • Billy
    Why placenta is Called endicrine gland?
    2 years ago
  • marcella
    Why placenta act as endocrine system?
    2 years ago
  • avah
    Why placenta acts as endocrine system?
    2 years ago
  • Gorbaduc
    Why placenta. is. endocrine. in. nature, short point?
    2 years ago
  • pedro
    What is the importance of placentadoes it work as endocrine gland.?
    2 years ago
  • abbie
    Which grand secretes amniotic fluid in pregnant women?
    2 years ago
  • Daniel
    What are the roles of the placenta as an endocrine organ?
    2 years ago
  • Charles
    Which of the following is the major effect of placental hormones?
    2 years ago
  • Stella Miller
    Why Placenta Is Reffered As Endocrine Gland?
    2 years ago
  • damien paterson
    What is physiology of placental hormone?
    2 years ago
  • Aamos
    Why is placenta termed as incomplete endocrine gland?
    2 years ago
  • carmine
    Why is placenta onsidered as an incomplete endocrine organ?
    2 years ago
  • jenni kemppi
    How placenta acts as endocrine glansd?
    2 years ago
  • brodie mcintyre
    Why is placenta called an incomplete endocrine organ?
    2 years ago
  • TORSTEN
    How human physiologic protein acts in blood?
    1 year ago
  • Mary
    What the function of placenta hormones?
    1 year ago
  • Joe
    Why placenta is not a complete endocine organ?
    1 year ago
  • dieter
    Why placenta chang steriod to estrogen?
    1 year ago
  • Ryan
    What is the endocrine gland that is secreted from the hormone placental?
    12 months ago
  • Amaranth Bunce
    What hormones do that are released form placenta?
    7 months ago
  • andreas
    Why is the placenta regarded as an incomplete endocrine organ?
    5 months ago
  • anne
    How does the placenta act as an endocrine and exchange organ?
    5 months ago
  • everett crawford
    Which placental hormone acts as lh?
    4 months ago
  • topias filatov
    What is the function of the placenta in the endocrine system?
    1 month ago
  • Vernice Lentz
    What about the placenta makes it an endocrine organ?
    6 days ago

Post a comment