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Primary chorionic villi. Diagrammatic.
Secondary chorionic villi. Diagrammatic.
Latin syncitiotrophoblastus
Carnegie stage 5a
Days 8
Gray's p.47
MeSH A16.254.085.162
TE E6.
FMA 83040
Anatomical terminology

Syncytiotrophoblast (from the Greek 'syn'- "together"; 'cytio'- "of cells"; 'tropho'- "nutrition"; 'blast'- "bud") is the epithelial covering of the highly vascular embryonic placental villi, which invades the wall of the uterus to establish nutrient circulation between the embryo and the mother. It is a unique tissue in that it is a multi-nucleated, terminally differentiated syncytium, extending to 13cm .


It is the outer layer of the trophoblasts and actively invades the uterine wall, rupturing maternal capillaries and thus establishing an interface between maternal blood and embryonic extracellular fluid, facilitating passive exchange of material between the mother and the embryo.

The syncytial property is important since the mother's immune system includes white blood cells that are able to migrate into tissues by "squeezing" in between cells[citation needed]. If they were to reach the fetal side of the placenta many foreign proteins would be recognised, triggering an immune reaction. However the syncytium acts as a giant cell so there are no gaps for immune cells to migrate through[citation needed].

The syncytiotrophoblast secretes progesterone and leptin in addition to human chorionic gonadotropin (hCG) and human placental lactogen (HPL); hCG prevents degeneration of the corpus luteum. Progesterone serves to maintain the integrity of the uterine 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 the corpus luteum graviditatis.[1]


The syncytiotrophoblast lacks proliferative capacity and instead is maintained by fusion of underlying cytotrophoblast cells. This fusion is assisted by syncytin, a protein that was integrated into mammalian genomes from an endogenous retrovirus.[2]

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  1. ^ Langman's Medical Embryology, 10th Edition. T.W. Sadler. p. 34
  2. ^ Mi, S (Feb 17, 2000). "Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis.". Nature 403 (6771): 785–789. doi:10.1038/35001608. PMID 10693809. 

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