MEDICAL EMBRYOLOGY 502
Lecture 4 Summary

Abnormalities associated with the third week

1. Primitive streak

   The primitive streak usually disappears without a trace. Occasionally remnants may persist in the sacrococcygeal region of the embryo to give rise to a tumor called a TERATOMA. These tumors are more common in females than in males and may become malignant. The tumor appears as an external mass at the base of the spine. Because the primitive streak contains pluripotent cells, these tumors often contain tissue derivatives of all three germ layers. (Note: Teratomas may also result from abnormal migration of primitive germ cells.)

2. Persistence of the neurenteric canal

   In the reorganization of the notochordal process to form the definitive notochord, a transient connection forms between the amniotic cavity and the yolk sac. Rarely, this connection persists after birth as a connection between the central canal of the spinal cord and the lumen of the intestine.

Highlights of development early in the third week - embryo only

1. The primitive streak appears.
2. The notochordal process appears and reorganizes into the notochord.
3. The embryonic axis is established.
4. The embryo is transformed from a bilaminar to a trilaminar germ disc composed of the three germ layers, ectoderm, mesoderm, and endoderm.
5. The neural plate appears and gives rise to neural folds.
6. The allantoic diverticulum appears.
7. Intraembryonic mesoderm appears.
8. This is the week of the first missed menstrual period.
9. Later in week 3 organ formation begins.

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Copyright 1999 The University of Michigan Unauthorized use prohibited. Please send comments to Dr. Tom Gest (gest@umich.edu)

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