MEDICAL EMBRYOLOGY 502
Lecture 3 Summary

Abnormalities associated with the second week of development
  1. Spontaneous abortion
    A "blighted" embryo will not develop. Early loss is difficult to detect.
Highlights of the second week of development
  1. The embryo is interstitial, lying within the endometrium.
    The epithelium reforms over the point of entrance and decidual cells appear in the endometrium.
  2. The embryo is composed of two layers, the epiblast and the hypoblast.
    The epiblast is confluent at its margins with the amnioblasts enclosing the amniotic cavity. The hypoblast is confluent at its margins with the endodermal cells lining the yolk sac.
  3. The extraembryonic mesoderm appears (from endoderm).
  4. The primitive trophoblast becomes two-layered.
    Inner mitotically active cytotrophoblast, outer secretory syncytiotrophoblast.
  5. The uteroplacental circulation begins when maternal blood flows through the trophoblastic lacunae.
  6. Placental villi begin to form.
    Hillocks of cytotrophoblast invade the syncytiotrophoblast creating the primary villi. The extraembryonic mesoderm invades the primary villi creating the secondary vill.
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