79. Fetal membranes - amnion and chorion. Yolk sac. Allantois.
Covers the four extra-embryonic structures named in the topic: the amnion, chorion, yolk sac and allantois. For each, the note gives the structure, where it lies and what it does, from the amnion cushioning the fetus to the allantois becoming the urachus. Picture-based sections and the cord and placental roles are included.
Topic 79 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 5 sections, about 339 words (3 min of reading), with 13 flashcards.
In this note
Introduction
Amnion:
Chorion:
Yolk Sac:
Allantois:
Amnion:
Structure: The amnion is a thin, transparent membrane that surrounds the fetus.
Location: It forms from the epiblast layer of the bilaminar embryonic disc and grows to encase the fetus.
Function:
Contains amniotic fluid, which cushions and protects the fetus.
Maintains a stable temperature and pressure around the fetus.
Allows for fetal movement, which is important for musculoskeletal development.
Prevents adhesions between the fetus and the membranes.
Chorion:
Structure: The chorion is the outermost membrane surrounding the fetus and the amnion. It is formed by the trophoblast layer (specifically the cytotrophoblast and syncytiotrophoblast) and the extra-embryonic mesoderm.
Location: The chorion is located outside the amnion and forms the outermost layer of the fetal membranes.
Function:
Contributes to the formation of the placenta.
In the early stages of pregnancy, the chorionic villi (finger-like projections) invade the maternal endometrium to establish maternal-fetal blood exchange.
Produces human chorionic gonadotropin (hCG), which helps maintain the pregnancy by supporting the corpus luteum and stimulating the production of progesterone.
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