A healthy, nulliparous 29-year-old woman with regular 28-day cycles comes to the office for a fertility evaluation. Over the past year, she and her 30-year-old husband have been trying to conceive. Her only medications are prenatal vitamins. She tracks her periods, cervical mucus, basal temperature, and intercourse with a smartphone app. The couple has intercourse every other day from cycle day 8 through the LH surge. The patient's urine pregnancy test is positive. The oocyte was most likely arrested in which of the following stages of meiosis immediately prior to fertilization?

Question

A healthy, nulliparous 29-year-old woman with regular 28-day cycles comes to the office for a fertility evaluation.  Over the past year, she and her 30-year-old husband have been trying to conceive.  Her only medications are prenatal vitamins.  She tracks her periods, cervical mucus, basal temperature, and intercourse with a smartphone app.  The couple has intercourse every other day from cycle day 8 through the LH surge.  The patient's urine pregnancy test is positive.  The oocyte was most likely arrested in which of the following stages of meiosis immediately prior to fertilization?

Quizplus · Accepted Answer

11ec160b_3787_54e9_9249_bbf5c63c8703_MD0001_00 Female gametogenesis begins in utero at approximately 4 weeks gestation when primordial germ cells migrate from the yolk sac region to the developing gonadal region. These germ cells then differentiate into oogonia and multiply by mitosis before beginning meiosis I. Now called primary oocytes, these cells arrest in prophase of meiosis I (Choice C) and remain there until ovulation. The chromosome pairs are arranged in a tetrad during this phase, and their proximity allows for genetic recombination. The full complement of oocytes is developed by 5 months gestation and then degenerates during the woman's lifetime until depleted at menopause. At puberty, ovulatory cycles begin and the female is capable of reproduction. During each menstrual cycle, FSH stimulation followed by an LH surge causes some oocytes to resume meiosis I. Telophase I (Choice E) is the last phase of meiosis I and yields secondary oocytes and polar bodies. The secondary oocyte begins meiosis II (the polar body degenerates) but halts in metaphase II. Approximately 36 hours after LH levels begin to rise, a secondary oocyte is released from the ovarian follicle. The secondary oocyte remains frozen in metaphase II until fertilization occurs, at which point it completes its division (telophase II, Choice F) into a mature oocyte (ovum) and second polar body. A normally fertilized embryo is then diploid due to genetic contribution from both the ovum and sperm pronuclei. (Choice A) Metaphase of meiosis I differs from metaphase of meiosis II and mitosis, as the spindles are not preparing to split the centromeres. Instead, the tetrads are lined up along the metaphase plate, and during anaphase the homologs separate from each other with the centromeres intact. (Choice D) Prophase of meiosis II is unique as it does not immediately follow a period of DNA synthesis; DNA was duplicated prior to prophase of meiosis I. Educational objective: Primary oocytes are completely developed in female embryos by the fifth month of gestation, at which point they are arrested in prophase of meiosis I. Normal menstrual cycle hormones stimulate the primary oocyte to resume differentiation. Prior to fertilization, secondary oocytes are arrested in metaphase of meiosis II. __________ References: Oocyte maturation: gamete-somatic cells interactions, meiotic resumption, cytoskeletal dynamics and cytoplasmic reorganization. (http://www.ncbi.nlm.nih.gov/pubmed/25744083)

A Healthy, Nulliparous 29-Year-Old Woman with Regular 28-Day Cycles Comes