Cytology · MU Pleven

Topic 56 - Formation and development of embryonic envelopes

Covers the formation and development of the four extraembryonic membranes: chorion, amnion, yolk sac and allantois. It begins with a brief overview of the embryonic period, then takes each membrane in turn, noting its germ-layer origin and structure. It also describes amniotic fluid and its protective functions, and gives the timing of the yolk sac and allantois, including the yolk sac's role in early haematopoiesis.

Inside: 8 sections · 7 labelled pictures · 14 flashcards · 2 min

Topic 56 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 8 sections, about 296 words (2 min of reading), with 14 flashcards.

In this note

  1. Topic 56 - Formation and development of embryonic envelopes
  2. Embryonic Period
  3. Embryonic Membranes
  4. Chorion
  5. Amnion
  6. Amniotic Fluid
  7. Yolk Sac (Umbilical Vesicle)
  8. Allantois

Embryonic Period

  • Major organ systems have begun development.
  • Period in embryo is most susceptible to teratogens.
  • Haematopoiesis begins around the yolk sac at week 3 and is later taken over by the liver, thymus, spleen, and finally, lifetime production occurs in bone marrow.
  • By week 7, the embryo begins sexual differentiation.
  • By week 7, the upper and lower limbs and digits are formed.

Embryonic Membranes

  • During embryonic development, the embryo forms 4 extraembryonic membranes that are ultimately discharged after birth. These membranes are:
  • Chorion
  • Allantois
  • Yolk sac (umbilical vesicles)
  • Amnion

Chorion

  • Formed from cells that come from the two layers of the trophoblast and mesodermal germ layer.
  • Consists of two layers:
  • Inner layer – formed by somatic mesoderm.
  • Outer layer – formed by primitive mesoderm.
  • The chorion has two parts:
  • Non-placental part
  • Bushy chorion part

Amnion

  • Epiblast derivative.
  • Thin, transparent memebrane surrounding the fetus.
  • Cells of the amnion secrete a watery fluid that fills the amniotic cavity.
  • Directly surrounds the developing embryo – amniotic sac.

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