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:
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Mesenchymal villi generate all other villous types:
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.
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.
(Text modified from PMID: 2327595)
References:
Castellucci M, Scheper M, Scheffen I, Celona A, Kaufmann P. The development of the human placental villous tree. Anat Embryol (Berl). 1990;181(2):117-28
Castellucci M, Kosanke G, Verdenelli F, Huppertz B, Kaufmann P. Villous sprouting: fundamental mechanisms of human placental development. Hum Reprod Update. 2000 Sep-Oct;6(5):485-94. Review.
(PMID: 11045879)
During the first trimester of human pregnancy, extravillous trophoblasts (EVT) from placental villi invade the decidua temporarily occluding the spiral arteries, preventing maternal blood flow and creating a low-oxygen environment, which is believed to play an important role in the regulation of extravillous trophoblast outgrowth. The response to oxygen concentration occurred in placentae under 11 weeks of gestation, whereas in villi from placentae of 11 or 12 weeks, no differential response was observed. (text modified from: James JL, Stone PR, Chamley LW. The effects of oxygen concentration and gestational age on extravillous trophoblast outgrowth in a human first trimester villous explant model. Hum Reprod. 2006 Oct;21(10):2699-705. Epub 2006 Jun 28.)
Images below show the vascular organisation of the mouse placenta. Note the branching pattern of blood vessels and the relative distribution of arterial and venous vessels within the placenta. Mouse placenta, while being a good model of placentation, will differ in some features from the human placenta.
Fetal View |
Lateral View Venous side (red resin) and arterial side (blue resin). |
These resin casts are generated by filling the existing vascular beds (day 16.5 p.c.) with a resin that sets and the surrounding materials are then removed, leaving just the vascular beds. Venous side (red resin) and arterial side (blue resin). (More? Placenta Vascular Beds)
Villous Cytotrophoblast Marker - Hepatocyte growth factor activator inhibitor 1 (HAI-1) a membrane antigen.
Extravillous Trophoblast Marker - HLA class I, some CD105 positive, EBV-induced gene 3 (EBI3)
Syncytiotrophoblast Marker - CD105, EBV-induced gene 3 (EBI3), p28 a heterodimeric cytokine (PMID: 17659773) (PMID: 12657516)
Mucin 15 - (MUC15) protein was expressed by both cytotrophoblasts and syncytiotrophoblasts (PMID: 17720698)
"cytotrophoblast (CTB) layer that is subjacent to the syncytiotrophoblast (STB) and supported by a basal lamina is nearly complete, but later, it becomes discontinuous. In the present study, we investigated the structural integrity of the CTB layer in the normal villous tree by advanced microscopy techniques using an antibody to hepatocyte growth factor (HGF) activator inhibitor type 1 (SPINT1), a potent inhibitor of HGF activators expressed exclusively on villous CTB. In full-term placenta, the cell surface of the CTB layer was spread over the basal lamina but was not interrupted. Morphometric analysis showed that throughout the villous tree, 80% of the continuity of the CTB layer of full-term placenta and 90% of that of first-trimester placenta were preserved."
"initially cuboidal-shaped CTB cells were transformed to flat cells with many cellular processes that, together with those of the adjacent STB, eventually covered the trophoblast basal lamina in a complex network of interdigitations."
Mori M, Ishikawa G, Luo SS, Mishima T, Goto T, Robinson JM, Matsubara S, Takeshita T, Kataoka H, Takizawa T. [See Related Articles] The cytotrophoblast layer of human chorionic villi becomes thinner but maintains its structural integrity during gestation. Biol Reprod. 2007 Jan;76(1):164-72.
Demir R, Demir N, Ustunel I, Erbengi T, Trak I, Kaufmann P. [See Related Articles] The fine structure of normal and ectopic (tubal) human placental villi as revealed by scanning and transmission electron microscopy. Zentralbl Pathol. 1995 Apr;140(6):427-42.
"Structures of placental villi between 28 and 34 days old (pc), early, 6-8 week normal and ectopic, and full term human placenta samples
Three-dimensional configurations of the developing chorionic villous trees were observed as large main villus groups, covered with abundant microvilli of different size and diameters.
Chorionic villous trees which emerged from the chorionic plate divided gradually into branches of which ramifications originated as buds.
Buds gradually grew and were transformed into shoots. The number of developing new villi appeared to increase gradually from 28 days to 9 weeks (pm) of gestation.
4th week onwards the massive trophoblastic sprouts were observed on the surface of main chorionic villi which transformed into primary, secondary and tertiary villous trees.
At term, some specialized structural modifications were observed on the free surface of the mature placental villi. The presence of some dome-like balloonings and many crateriform hollows were the most striking features of the mature intermediate and terminal villi.
According to the increasing physiological needs of the growing fetus, these special structures that are related to lung-like and kidney-like functions and named "nephropneumonic-like units", formed in the mature placental barrier. We have observed that these special units were showing a smooth surface similar to an inflated balloon."
In vitro studies of villi branching show
Fibroblast Growth Factor (FGF10) acting through receptor tyrosine kinase (RTK) receptors promotes invasion and outgrowth of trophoblasts.
Sprouty (Spry2) expression attenuates trophoblast sprouting.
Spry2 expression is also increased by FGF10
(PMID: 17496316)
Demir R, Demir N, Ustunel I, Erbengi T, Trak I, Kaufmann P. [See Related Articles] The fine structure of normal and ectopic (tubal) human placental villi as revealed by scanning and transmission electron microscopy. Zentralbl Pathol. 1995 Apr;140(6):427-42. Spry proteins are localized in the stroma of the chorionic villi, adjacent to cytotrophoblasts in areas of villous sprouting. placental macrophages (Hofbauer cells) express Spry. Hofbauer cells also expressed c-Cbl, a protein that interacts with Spry. (PMID: 15950061)
"The aim of this study was to examine the development of chorionic villous trees during early periods of normal intrauterinal and ectopic (tubal) pregnancies, and to study the structural specializations on the free surface of mature placental villi by scanning and transmission electron microscope (SEM and TEM). In order to study the structures of placental villi between 28 and 34 days old (pc), early, 6-8 week normal and ectopic, and full term human placenta samples were obtained from legal curettage and hysterectomized cases, and spontaneous deliveries, and tissues samples were prepared for SEM and TEM. Three-dimensional configurations of the developing chorionic villous trees were observed as large main villus groups, covered with abundant microvilli of different size and diameters. It appeared that the chorionic villous trees which emerged from the chorionic plate divided gradually into branches of which ramifications originated as buds. These buds gradually grew and were transformed into shoots. The number of developing new villi appeared to increase gradually from 28 days to 9 weeks (pm) of gestation. From the 4th week onwards the massive trophoblastic sprouts were observed on the surface of main chorionic villi which transformed into primary, secondary and tertiary villous trees. When the placental villi formation in ectopic pregnancy was compared with the intra-uterinal pregnancy, an arrested development was remarkable. The configurations of ectopic placental villi seemed to be disparate, such as curved lines or compressed and wrinkled positions so that the three dimensional aspect had been wizened. The ramification and new villi formation seen as in the normal placenta were not only decreased but also infrequent. Some placental villi samples displayed a gradually thinning terminal region. Trophoblastic degenerations were frequently found on the surface of ectopic villi ultrastructurally. According to these results, we comment that in ectopic pregnancy the placental villi formation and development could have been delayed. At term, some specialized structural modifications were observed on the free surface of the mature placental villi. The presence of some dome-like balloonings and many crateriform hollows were the most striking features of the mature intermediate and terminal villi. According to the increasing physiological needs of the growing fetus, these special structures that are related to lung-like and kidney-like functions and named "nephropneumonic-like units", formed in the mature placental barrier. We have observed that these special units were showing a smooth surface similar to an inflated balloon."
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This current page is an introduction placental villi development.
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