From WikiLectures

Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.

Chorion is external fetal membrane (due to its appearance it is called "membrane villous"). From the inside, it is formed by extraembryonic mesoderm (subsequently it differentiates into mesenchyme, which then differentiates into fibrous layer), from the outside, it is formed by cytotrophoblast and syncitotrophoblast. It participates in the nutrition of the embryo, it is part of the fetal part of the placenta, villi and trophoblast lacunae. It creates the hormone hCG. From the second month of pregnancy, it is possible to take a sample from the chorion (through the vagina) for examination of the embryo (tissues are identical). During development, the chorion divides into two regions:

  • chorion frondosum − rich in villi, in the region of the embryonic pole, part of the future placenta;
  • chorion leave − it has degenerating villi, abembryonic pole, area outside the placenta.

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  • VOKURKA, Martin – HUGO, Jan. Velký lékařský slovník. 9. edition. Maxdorf, 2009. 1159 pp. ISBN 978-80-7345-202-5.

  • SADLER, Thomas, W – SINHA, M.D. Langmanova lékařská embryologie. 1. české edition. Grada, 2011. 414 pp. ISBN 978-80-247-2640-3.