Placenta - Villi Development

Embryology - 18 Apr 2024 Expand to Translate
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Introduction

This page introduces an overview of aspects of the basic fetal subunit of the placenta, the placental villi development. In early placentation, each villi proceeds through a similar initial program of development. In later placentation, villi morphologically differentiate into a limited range of villi functional changes reflecting their specialization. The major initial contribution is from the trophoblast shell that surrounds the conceptus and later by the development of extraembryonic mesoderm and blood vessel differentiation.

There are three main types of trophoblast cells that differentiate:

villous cytotrophoblasts
extravillous cytotrophoblasts
syncytiotrophoblasts that form by fusion of villous cytotrophoblasts

Historic Embryology - Placenta
1883 Embryonic Membranes \| 1907 Development Atlas \| 1909 \| 1910 Textbook \| 1917 Textbook \| 1921 Textbook \| 1921 Foetal Membranes \|1921 human \| 1921 Pig implantation \| 1922 Single placental artery \| 1923 Placenta Review \| 1939 umbilical cord \| 1943 human and monkey \| 1944 chorionic villus and decidua parietalis \| 1946 placenta ageing \| 1960 first trimester placenta \| 1960 monkey \| 1972 Placental circulation \| Historic Disclaimer

Placental villi cartoon

Cytotrophoblast Layer

There is a new interpretation of the changes that are occuring in the cytotrophoblast (CTB) layer during early to full-term human placenta development. Traditionally the interpretation was that the cytotrophoblast layer thinned and became discontinuous towards term. The thinning is thought due to the epithelium surface expanding at a faster rate than its volume. Two recent studies suggest that while the cytotrophoblast layer does indeed thin, it does not become discontinuous.

Syncytiotrophoblast Layer

The syncytiotrophoblast (STB) layer forms the epithelial covering of the entire villous tree. These cells are multinucleated, terminally-differentiated syncytium formed by the fusion of the underlying progenitor cytotrophoblast (CTB) cells. The process is described as "syncytialization" and is mediated by syncytin-1, an envelope protein of a human endogenous retrovirus W (HERV-W). The differentiation is regulated by chorionic gonadotropin (hCG) and the fusion of cytotrophoblast cells is ongoing during placental development.

Cellular parts derived from the syncytiotrophoblasts (apoptotic nuclei and microparticulate debris) can be shed into the maternal blood in which they are bathed. The apototic process appears to be part of the fusion mechanism between cytotrophoblast and the overlying multinucleate syncytiotrophoblast layer.

Studies have suggested that these cells are transcriptionally inactive. A recent study using a number of different detection techniques now suggests that at least some of the cells nuclei may still be transcriptionally inactive.

Mesenchymal Villi

Mesenchymal villi generate all other villous types:

immature intermediate villi
stem villi
mature intermediate villi
terminal villi

Mesenchymal villi continuously form out of the trophoblastic sprouts throughout pregnancy and have been considered the basis for growth and differentiation of the villous trees.

Chorionic Villi

Primary villi

Week 2 - first stage of chorionic villi development, trophoblastic shell cells (syncitiotrophoblasts and cytotrophoblasts) form finger-like extensions into maternal decidua.

Secondary villi

Week 3 - second stage of chorionic villi development, extraembryonic mesoderm grows into villi, covers entire surface of chorionic sac.

Basal region will form chorionic plate.

Tertiary villi

Week 4 - third stage of chorionic villi development, mesenchyme differentiates into blood vessels and cells, forms arteriocapillary network, fuse with placental vessels, developing in connecting stalk.

stem villi - or anchoring villi, cytotrophoblast cells attached to maternal tissue.

branched villi - grow from sides of stem villi, region of main exchange, surrounded by maternal blood in intervillous spaces.

terminal villi - not active outgrowths caused by proliferation of the trophoblast. Passive protrusions induced by capillary coiling due to growth of the fetal capillaries within the mature intermediate villi (third trimester).

chorionic plate - region of membrane at the base of the villi through which placental arteries and vein passes.

Hofbauer Cells

Placental villi Hofbauer cells^[1]

human villous macrophages
highly vacuolated cells
located the core stroma of placental villi and cord
macrophages with micropinocytotic activity and phagocytosis ability
possible paracrine role for early stages of placental vasculogenesis

Villi Trimester Development

Trimester 1 and 2

In the first two trimesters they are the forerunners of the immature intermediate villi, whereas in the last trimester the mesenchymal villi are transformed into mature intermediate villi. Immature intermediate villi formed during the first two trimesters are developmental steps towards the stem villi.

Trimester 3

Mature intermediate villi develop during the last trimester, produce numerous terminal villi. Terminal villi are not active outgrowths caused by proliferation of the trophoblast, but rather passive protrusions induced by capillary coiling due to excessive longitudinal growth of the fetal capillaries within the mature intermediate villi. The arrangement of the capillary bed in the terminal villi can vary from simple U-like loops to a richly branched network due to capillary elongation and sprouting.

Some text modified from ^[2], see also ^[3]

Placental Villi Blood Vessels

Some of the following data is from a histological study of human placental villi.^[4]

macrophage-like cells first cells to differentiate at day 21 (post-conception) from mesenchymal precursors.
haemangioblastic cell cords (angiogenic cell cords, ACC) at day 21 (post-conception) also from mesenchymal cells, are the precursors of the capillary endothelium and haematopoietic stem cells
pericytes form later and are a third cell population derived from the mesenchymal cells
first main vascular patterns grow towards the longitudinal axis of the developing villi
capillary basal lamina cannot be detected earlier than in the third trimester
third trimester - fetal villous angiogenesis occurs by proliferation of the existing endothelium and pericytes rather than through haemangioblastic cells.

Human Villi Timeline

The placental vill development data below is based upon a recent immunochemistry confocal laser scanning microscope (CLSM) study.^[5]

Note that the paper uses clinical gestational age (GA) from last menstrual period (LMP) and has been corrected for post-conception (fertilization) age, approximately 14 days later.

Fertilization Age (weeks)	Gestational Age (weeks)	Vessel Lumen Diameter (range in microns, µm)	Features
3 to 4	5 and 6	10 - 15	a complex network of cords and vessels with redundant connections network comprises mainly cords, already connected together vessels and cords are connected to each other without any interruptions chorionic villus dominated by this network of vascular elements vessels and cords are located centrally as well as peripherally and as a consequence contact the overlying trophoblastic layer
5 to 6	7 and 8	10 - 26	villi dominated by capillary network of vessels and cords capillary network contains more vessels than cords chorionic villus tip - regular small branched off (mesenchymal) chorionic villi are present containing CD31 positive cells
7 to 8	9 and 10	60 - 75 two central vessels 26 - 34 capillary network	villi have two large centrally located vessels surrounded by and connected to a peripheral capillary network capillary network contains vessels with a lumen in tight contact with overlying trophoblastic layer villous projections also contain blind ending capillary sprouts
9 to 10	11 and 12	70 - 90 two central vessels 26 - 34 capillary network	immature intermediate villi characterized by two large vessels surrounded by a capillary network capillary network has few cords blind ending capillary sprouts off the capillary network

CD31 - (PECAM-1, Platelet Endothelial Cell Adhesion Molecule) is a cluster of differentiation molecule found on endothelial and other blood cells.

Extravillous Trophoblast Outgrowth

During the first trimester of human pregnancy, extravillous trophoblasts (EVT) from placental villi invade the decidua temporarily occluding the spiral arteries. This occusion prevents maternal blood flow and creates a low-oxygen environment, that may play a role in the regulation of extravillous trophoblast outgrowth.

A study has shown that the early placenta ( under 11 weeks of gestation) responds to oxygen concentration, whereas villi from older placentae (11 or 12 weeks) show no differential response.^[6]

References

↑ <pubmed>22606231</pubmed>| PLoS One.
↑ <pubmed>2327595</pubmed>
↑ <pubmed>11045879</pubmed>
↑ <pubmed>2481376</pubmed>
↑ <pubmed>17545656</pubmed>
↑ <pubmed>16807282</pubmed>

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Cite this page: Hill, M.A. (2024, April 18) Embryology Placenta - Villi Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Placenta_-_Villi_Development

What Links Here?

[1] <pubmed>22606231</pubmed>| PLoS One.

[2] <pubmed>2327595</pubmed>

[3] <pubmed>11045879</pubmed>

[4] <pubmed>2481376</pubmed>

[5] <pubmed>17545656</pubmed>

[6] <pubmed>16807282</pubmed>

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