17. Review of Week 2 and Abnormal Development

  1. Review of week 2
    1. BLASTOCYST IMPLANTATION begins at the end of week 1 and ends during week 2
      1. The zona pellucida disappears (days 4 to 5) probably due to enzymatic lysis released by the acrosomes of sperm that penetrate it
      2. The blastocyst attaches to the endometrial wall (days 5 to 6)
      3. The trophoblast erodes the uterine epithelium as it differentiates (days 7 to 8) into an inner cytotrophoblast and an outer syncytiotrophoblast
      4. The lacunae are seen in the syncytiotrophoblast (day 9)
      5. The lacunae fuse to form lacunar networks (days 10 to 11)
      6. The trophoblast invades the endometrial sinusoids permitting the maternal blood to flow into the lacunar networks, thus, a uterop1acenta1 circulation is formed (days 11 to 12)
      7. The endometrial surface defect disappears (days 12 to 13)
      8. One sees a marked decidual reaction in the endometrium around the implanting embryo (days 13 to 14)
  2. Abnormal development
    1. ABOUT 15% OF OOCYTES fail to become fertilized
    2. ABOUT 10-15% OF OOCYTES begin cleavage, but fail to implant
    3. OF THE 70-75% OF OOCYTES that do implant, only about 58% survive until week 2
      1. Of the above, about 16% are abnormal
    4. WHEN THE FIRST EXPECTED MENSTRUATION IS MISSED, only about 42% of the eggs exposed to sperm usually survive and, of these, a number will abort in subsequent weeks, and some will be abnormal at birth
    5. ABOUT ONE-THIRD TO ONE-HALF OF ALL ZYGOTES never survive to implant
      1. May be the result of a poorly developed endometrium
      2. Most likely caused by chromosomal abnormalities in the zygote itself
    6. EXPOSURE OF THE EMBRYOS TO TERATOGENS during the first 2 weeks usually does not result in congenital malformations, but some do kill the blastocysts or cause early abortions
    7. ABOUT 25% OF EARLY ABORTED EMBRYOS (days 7 to 17) have abnormal chromosomes and are lost during the menstrual flow, which occurs later than usual and the woman may not even be aware of the fact that she had been pregnant
    8. IF THE TROPHOBLAST BECOMES HIGHLY PROLIFERATIVE (or even after embryonic death), we may find a noninvasive hydatidiform mole or a highly malignant tumor, a chorioepithelioma (from chorionic epithelium) which may produce high levels of human chorionic gonadotropin

review of week 2 and  abnormal development: image #1