| Syncytiotrophoblast | |
|---|---|
 Primary chorionic villi. Diagrammatic. | |
 Secondary chorionic villi. Diagrammatic. | |
| Details | |
| Carnegie stage | 5a |
| Days | 8 |
| Identifiers | |
| Latin | syncitiotrophoblastus |
| TE | E6.0.1.1.4.0.2 |
| FMA | 83040 |
| Anatomical terminology | |
Thesyncytiotrophoblast (from theGreek 'syn'- "together"; 'cytio'- "of cells"; 'tropho'- "nutrition"; 'blast'- "bud") is theepithelial covering of the highly vascularembryonicplacentalvilli, whichinvades the wall of theuterus to establish nutrient circulation between the embryo and the mother. It is amultinucleate, terminally differentiatedsyncytium, extending to 13 cm.
It is the outer layer of thetrophoblasts and activelyinvades theuterine wall, duringimplantation, rupturing maternalcapillaries and thus establishing an interface between maternal blood and embryonicextracellular fluid, facilitating passive exchange of material between the mother and the embryo.
The syncytial property is important since the mother'simmune system includeswhite blood cells that are able to migrate into tissues by "squeezing" in between cells. If they were to reach the fetal side of the placenta, many foreign proteins would be recognized, triggering an immune reaction. However the syncytium acts as a giant cell so there are no gaps for immune cells to migrate through.[1]
One way in which it accomplishes this task is by suppressing the expression of immunity-related genesHLA-A andHLA-B, which are classically known to be expressed by all nucleated cells.[2] These genes normally express the MHC-I ligand that acts as a major binding mechanism for T-cells. By decreasing the translation of these gene products, the syncytiotrophoblast reduces the chances of an attack by the maternal immune system mediated by T-cells.[2]
The syncytiotrophoblast secretesprogesterone andleptin in addition tohuman chorionic gonadotropin (hCG) andhuman placental lactogen (HPL). hCG prevents degeneration of thecorpus luteum, and signals the corpus luteum to continue progesterone secretion. From this point on, the corpus luteum is called thecorpus luteum graviditatis.[3] Progesterone serves to maintain the integrity of theuterine lining and, until the syncytiotrophoblast is mature enough to secrete enough progesterone to support pregnancy (in the fourth month of embryonic development), it is aided by thecorpus luteum graviditatis.[4]
The syncytiotrophoblast lacks proliferative capacity and instead is maintained by fusion of underlyingcytotrophoblast cells. This fusion is assisted bysyncytin, a protein that was integrated into mammalian genomes from anendogenous retrovirus.[5]
Tony M. Plant, Anthony J. Zeleznik: "Knobil and Neill's Physiology of Reproduction: Two-Volume Set" p 1790
