A 24-year-old woman comes to the clinic due to a missed menstrual period. She has had regular menstrual cycles every 28 days, and her last menstrual period began 7 weeks ago. Since then, she had sexual intercourse once 5 weeks ago. The patient has no chronic medical conditions. Urine pregnancy test is positive. Which of the following embryologic events is most likely occurring in the embryo at this time?
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By convention, gestational age is calculated from the first day of the last menstrual period, which was 7 weeks ago in this patient. However, ovulation typically occurs 2 weeks after a menstrual period (and coincides with her sexual history), marking the approximate time that fertilization (conception) occurred. Early embryonic development is described in weeks postfertilization, and, in this patient, the embryo is estimated to be 5 weeks.
Critical postfertilization stages include the following:
Implantation: The blastocyst implants in the uterine wall at the end of week 1 postfertilization (Choice A).
Gastrulation: After implantation, gastrulation occurs, which is the process that establishes 3 distinct cell layers (ectoderm, mesoderm, endoderm) from which all tissues and organs are derived. This process occurs during week 3 postfertilization (Choice B).
Neurulation: Formation of the neural tube (precursor to the brain and spinal cord) occurs as the lateral edges of the neural plate (ie, neural folds) migrate toward the midline and fuse by the end of week 4 postfertilization. During this process, neuroectodermal cells adjacent to the crest of the neural folds separate from the surface ectoderm to form neural crest cells.
Following neurulation and by the end of week 5 postfertilization, these neural crest cells migrate to distant sites and later develop into diverse tissues (eg, neural ganglia, melanocytes, adrenal medulla). Based on this patient's menstrual and sexual history consistent with an embryo of 5 weeks, neural crest migration is likely occurring.
(Choice C) Rapid expansion of the gastrointestinal tract with physiologic herniation of the intestinal loops occurs during the early embryonic period (typically weeks 4-8) and is likely happening in this embryo. However, fixation of the intestines to the abdominal wall occurs after their return to the abdominal cavity (around weeks 10-12).
(Choice E) Although the primary bronchial buds develop during the embryonic stage (weeks 4-5), primitive alveoli form during the saccular stage of lung development, beginning at approximately week 26; alveoli continue to mature throughout gestation and after birth.
Educational objective:
Early embryonic development is described in weeks postfertilization (ie, approximate time of ovulation), and major events during this time include implantation (week 1), gastrulation (week 3), and neurulation (week 4). As neurulation occurs, neuroectodermal cells differentiate into neural crest cells, which then migrate to distant sites during week 5 postfertilization.