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Placental development & hormones

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Last staff update: 21 December 2021

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Cite this page: Ziadie MS. Placental development & hormones. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/placentaplacentaldevel.html. Accessed April 20th, 2024.

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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