Trophoblast
The trophoblast represents the outer cell layer of a blastocyst and connects it to the wall of the uterus . It forms from blastomeres on the 5th to 12th day after fertilization , uses enzymes to soften the uterine lining and can thus adhere to it.
By dissolving the mucous membrane cells of the uterus, the germ is nourished for the first time. During this phase, the trophoblast invasively grows into the uterine lining with the help of its syncytiotrophoblasts , as the cells in question lose their boundaries and form irregularly networked trabeculae and lacunae (lacunar stage, 8th-9th day). The cell material required for the process is supplied by the cytotrophoblast , which is the portion of the trophoblast that surrounds the embryoblast .
From this stage on the first villi-like structures appear, one speaks of the primary villi or villi . From the 12th day cytotrophoblast cells penetrate into this, the syncytiotrophoblast portion is thinned out and only forms a thin covering layer. From this stage on one speaks of the secondary villi . From about the 20th day, the extraembryonic mesoderm penetrates the secondary villi, in which blood vessels form, which connect to the vessels of the sticky stalk, thus ensuring the definitive vascular supply of the embryo.
In the further course of the process, the so-called Hofbauer cells ( macrophages ) appear in these tertiary villi, which additionally stimulate villi growth and phagocytize foreign bodies. These villi trees, which are now growing, together with the remaining maternal parts, form the placenta , which takes on all vital functions for the embryo emerging from the germinal disc .
Since half of the trophoblast has male antigens, it is a potential target for defense reactions by the maternal immune system. In addition, its cells do not have any classic MHC class I molecules ( HLA-A or HLA-B ) on their surface. Although there are numerous natural killer cells in the placenta that specialize in the elimination of cells without HLA-A and HLA-B molecules ( missing-self hypothesis ), the trophoblast is spared. This is largely due to the non-classical MHC class I molecules that some of its cells express: HLA-C, HLA-E and HLA-G . Above all, HLA-E and HLA-G are recognized by inhibitory (ie reaction-inhibiting) receptors of the natural killer cells and thus prevent an attack. In addition, regulatory T cells in the placenta create a climate of immune tolerance that supports implantation.
In the course of placenta development, part of the trophoblasts, so-called extravillous trophoblasts , migrate into the mother's uterine lining in order to adapt the microenvironment to the needs of the growing embryo and to ensure that sufficient nutrients and oxygen are transported from mother to child. The spiral arteries of the uterine lining are actively remodeled and expanded by the extravillous trophoblasts in order to ensure the required higher blood flow. A reduced invasion of the extravillous trophoblasts into the uterine lining and a resulting inadequate remodeling of the blood vessels is associated with various pregnancy complications such as: B. associated with preeclampsia .
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Individual evidence
- ↑ J. Trowsdale, A. Betz: Mother's little helpers: mechanisms of maternal-fetal tolerance . In: Nature Immunology , Volume 7, Number 3, March 2006, pp. 241-246, doi : 10.1038 / ni1317 .
- ^ GF Clark, DJ Schust: Manifestations of immune tolerance in the human female reproductive tract. In: Frontiers in immunology. Volume 4, 2013, p. 26, doi : 10.3389 / fimmu.2013.00026 , PMID 23407606 , PMC 3570961 (free full text).
- ↑ R. Samstein, p Josefowicz, A. Arvey, P. Treuting, A. Rudensky : extrathymic Generation of Regulatory T Cells in Placental Mammals Mitigates Maternal-Fetal Conflict . In: Cell , Volume 150, July 2012, pp. 29-38, doi : 10.1016 / j.cell.2012.05.031 .
- ↑ P. Velicky, M. Knöfler, J. Pollheimer: Function and control of human invasive trophoblast subtypes: Intrinsic vs. maternal control . In: Cell Adhesion & Migration . tape 10 , no. 1-2 , September 29, 2015, ISSN 1933-6918 , p. 154–162 , doi : 10.1080 / 19336918.2015.1089376 , PMID 26418186 , PMC 4853032 (free full text) - ( tandfonline.com [accessed October 18, 2018]).
