Placenta
General
Placental development & hormones


Topic Completed: 1 September 2011

Minor changes: 28 October 2020

Copyright: 2002-2021, PathologyOutlines.com, Inc.

PubMed Search: Placental development[title] "loattrfree full text"[sb]

Mandolin S. Ziadie, M.D.
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Cite this page: Ziadie MS. Placental development & hormones. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/placentaplacentaldevel.html. Accessed January 24th, 2021.
Implantation
  • Blastocyst implants on postovulation day 6 - 7; by day 10, ovum is implanted in stroma
  • Trophoblasts proliferate and erode the maternal capillaries and venules to form the intervillous space
  • Extraembryonic mesoderm grows into the primary villi and capillary formation occurs
  • By 5 - 6 weeks, the villous vessels are formed
  • At 8 weeks, they contain nucleated red blood cells (nRBC) which diminish to 10% by 10 weeks and are absent at 12 weeks
First trimester
Implantation and arterial plugs:
  • Extravillous intermediate trophoblasts invade the endometrium while endovascular trophoblasts grow into arteries and form cellular plugs

Villous morphology:
  • Early mesenchymal villi are large (170 microns) with scant connective tissue, few Hofbauer cells, no thick walled vessels
  • They have a complete outer layer of syncytiotrophoblast and an inner cytotrophoblast layer
  • During mid trimester, they mature to immature intermediate villi which have loose stroma with many Hofbauer cells and a complete trophoblastic coat
  • Then transform to stem villi, which have denser stroma and thick walled vessels
  • This process continues through the second trimester

Microscopic (histologic) images:

Images hosted on other servers:

Chorionic villi covered
by cytotrophoblast and
syncytiotrophoblast

Second trimester
Implantation and vascular remodeling:
  • Invading trophoblasts extend into the myometrium while endovascular trophoblasts invade the arterial walls and destroy their endothelium and media, replacing them with fibrinoid material, creating a low pressure circulation

Villous morphology:
  • Mesenchymal villi give rise to mature intermediate villi, from which terminal villi sprout
  • Mature intermediate villi are large with loose stroma, capillaries, arterioles and venules
  • Terminal villi appear near the end of the trimester and are much smaller (70 microns) with denser stroma surrounded primarily by syncytiotrophoblasts and a thin cytotrophoblast layer that may have syncytial knots
  • Vessels are numerous (3 - 5 capillaries per villous) and in contact with the trophoblastic coat

Microscopic (histologic) images:

Images hosted on other servers:

"Syncytial knots" and pink fibrin

Third trimester
Villous morphology:
  • Mature intermediate and terminal villi are now more prevalent and smaller than second trimester with thin stroma
  • More have syncytial knots (approximately 30%) and vasculosyncytial membranes (fused fetal capillaries with syncytiotrophoblasts)
  • Trophoblastic inclusions are common

Microscopic (histologic) images:

Images hosted on other servers:

Vascularized chorionic villi and hCG immunostain

Mean placental weight by gestational age
  • Prior to 28 weeks: 253 grams
  • 28 - 32 weeks: 314 grams
  • 33 - 36 weeks: 391 grams
  • 37 - 40 weeks: 456 grams
  • > 40 weeks: 496 weeks
Placental hormones
Steroid hormones: estrogens and progesterone
  • Trophoblasts synthesize estrogens and syncytiotrophoblasts synthesize progesterone, which maintains a noncontractile uterus and fosters development of an endometrium conducive to pregnancy
  • By the end of the first trimester, placental production of these hormones replaces the corpus luteum

Peptide hormones
Activin and inhibin:
  • Produced by trophoblast
  • Regulate hCG production

Cytokine growth factors (TGF-alpha, TGF-beta, EGF):
  • Produced by trophoblast
  • Stimulates proliferation of trophoblast and production of fibronectin

Human chorionic adrenocorticotropin (hACTH):
  • Small amounts produced
  • Believed to function similar to ACTH

Human chorionic gonadotropin (hCG) :
  • Glycoprotein similar in structure to pituitary LH
  • Synthesized primarily by the villous syncytiotrophoblast
  • Synthesis begins before implantation and is detectable 7 - 10 days after implantation, forming the basis for early pregnancy tests
  • Peak levels reached at 8 - 10 weeks
  • Maintains maternal corpus luteum that secretes progesterone and estrogens

Human chorionic thyrotropin (hCT):
  • Small amounts produced, probably by the syncytiotrophoblast
  • Believed to function similar to TSH

Human placental growth hormone:
  • Differs from pituitary growth hormone by 13 amino acids
  • Regulates maternal blood glucose levels so that the fetus has adequate nutrient supply

Human placental lactogen (hPL) :
  • Polypeptide similar to growth hormone
  • Synthesized by the villous syncytiotrophoblast
  • First detectable by 4 weeks
  • Peak levels at end of third trimester
  • Acts as an insulin antagonist to influence growth, maternal mammary duct proliferation and lipid and carbohydrate metabolism

Insulin-like growth factors:
  • Stimulate proliferation and differentiation of cytotrophoblast

Placental alkaline phosphatase (PLAP):
  • Alkaline phosphatase normally produced by syncytiotrophoblast and primordial germ cells
  • Also produced in seminoma, intratubular germ cell neoplasia, rarely in other non germ cell tumors
  • May be involved in migration of primordial germ cells in developing fetus

Relaxin:
  • Produced by villous cytotrophoblast
  • Softens the cervix and pelvic ligaments in preparation for childbirth

SP1:
  • Pregnancy specific beta-1 glycoprotein
  • Present in syncytiotrophoblast and extravillous trophoblast
  • Not in cytotrophoblast
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