Passage
Domestic pigs have been used as valuable preclinical models for scientific research due to their numerous anatomical, developmental, and physiological similarities to humans. For example, oogenesis and embryonic development in pigs are remarkably similar to the human processes and have facilitated the study of impaired fertilization due to decreased oocyte competence (quality) .Researchers have hypothesized that early termination of embryo development can arise from low mitochondrial DNA (mtDNA) copy number. Because mtDNA replication and reduction events have been extensively mapped in pigs, oocytes and sperm were isolated from their respective porcine gonads to test this hypothesis.Experiment 1To evaluate the developmental competence of porcine oocytes, researchers first measured the activity of glucose-6-phosphate dehydrogenase (G6PD) , an enzyme that is downregulated as the oocyte matures. The oocytes were stained with Brilliant Cresyl Blue (BCB) , a nontoxic blue dye that is converted to a colorless compound by G6PD. After 90 minutes of staining, oocytes were divided into two groups: one contained oocytes with colorless cytoplasm (BCB⁻) and the other contained oocytes with blue cytoplasm (BCB⁺) . Gene expression profiles were compared to evaluate the number of oocytes in the BCB⁺ and BCB⁻ groups that had developed to metaphase II.Experiment 2BCB⁺ and BCB⁻ oocytes underwent in vitro fertilization (IVF) , intracytoplasmic sperm injection (ICSI) , or supplementation with homogeneous mitochondria and ICSI (mICSI) . During IVF, an isolated oocyte was placed in a well plate and incubated with sperm at 39 C for 4 hours. In ICSI, a single sperm was selected and injected directly into the oocyte.Table 1 Developmental Rates for BCB⁺ and BCB⁻ Oocytes Fertilized Subjected to IVF, ICSI, or mICSI
Adapted from Cagnone GL, Tsai TS, Makanji Y, et al. Sci Rep. 2016;6:23229.
-Which of the following statements best describes a BCB⁺ oocyte that is arrested in metaphase II?

Question

Passage
Domestic pigs have been used as valuable preclinical models for scientific research due to their numerous anatomical, developmental, and physiological similarities to humans. For example, oogenesis and embryonic development in pigs are remarkably similar to the human processes and have facilitated the study of impaired fertilization due to decreased oocyte competence (quality) .Researchers have hypothesized that early termination of embryo development can arise from low mitochondrial DNA (mtDNA) copy number. Because mtDNA replication and reduction events have been extensively mapped in pigs, oocytes and sperm were isolated from their respective porcine gonads to test this hypothesis.Experiment 1To evaluate the developmental competence of porcine oocytes, researchers first measured the activity of glucose-6-phosphate dehydrogenase (G6PD) , an enzyme that is downregulated as the oocyte matures. The oocytes were stained with Brilliant Cresyl Blue (BCB) , a nontoxic blue dye that is converted to a colorless compound by G6PD. After 90 minutes of staining, oocytes were divided into two groups: one contained oocytes with colorless cytoplasm (BCB⁻) and the other contained oocytes with blue cytoplasm (BCB⁺) . Gene expression profiles were compared to evaluate the number of oocytes in the BCB⁺ and BCB⁻ groups that had developed to metaphase II.Experiment 2BCB⁺ and BCB⁻ oocytes underwent in vitro fertilization (IVF) , intracytoplasmic sperm injection (ICSI) , or supplementation with homogeneous mitochondria and ICSI (mICSI) . During IVF, an isolated oocyte was placed in a well plate and incubated with sperm at 39 C for 4 hours. In ICSI, a single sperm was selected and injected directly into the oocyte.Table 1 Developmental Rates for BCB⁺ and BCB⁻ Oocytes Fertilized Subjected to IVF, ICSI, or mICSI

Adapted from Cagnone GL, Tsai TS, Makanji Y, et al. Sci Rep. 2016;6:23229.
-Which of the following statements best describes a BCB⁺ oocyte that is arrested in metaphase II?

Quizplus · Accepted Answer

11ecba2d_0fcb_29be_a3bf_ab7c99e58643_MD0008_00 Female gametogenesis (oogenesis) is a hormone-driven process that occurs in the female gonads (the ovaries) and involves mitosis and two meiotic divisions. Oogenesis begins in utero at approximately 4 weeks gestation. In the female embryo, oogonia (germ cells) are diploid stem cells that first multiply quickly via mitosis and become primary oocytes. The primary oocytes then commence the first meiotic division but become arrested at prophase I. From infancy to puberty, the ovaries are functionally inactive and the primary oocytes remain stalled in prophase I of meiosis. The chromosome pairs are arranged in a tetrad during this phase, and their proximity allows for genetic recombination. At puberty, ovulatory cycles begin, and the female is capable of reproduction. During each menstrual cycle, stimulation by follicle-stimulating hormone (FSH) followed by a surge in luteinizing hormone (LH) causes some primary oocytes to resume meiosis I. The last stage of meiosis I (telophase I) in a primary oocyte yields two unevenly divided haploid cells. One cell is the secondary oocyte, which has almost all the cytoplasm of the primary oocyte; the other is the first polar body (Choice B), which is smaller and does not develop into a mature oocyte. The secondary oocyte begins meiosis II, but it halts in metaphase II and the polar body degenerates. The secondary oocyte remains frozen in metaphase II until fertilization occurs, at which point it completes its second meiotic division (telophase II) into an ootid and second polar body. The zygote that results from fertilization is diploid due to genetic contribution from both the ovum and sperm nuclei. In the given scenario, a BCB⁺ porcine oocyte is similar to that of humans. The metaphase II arrest of the BCB⁺ oocyte is the typical stage at which an oocyte can be fertilized by a spermatozoon (Choice C), and this completes the second meiotic division. (Choice D) Nondisjunction, the failure of one or more pairs of sister chromatids to separate, can occur during anaphase of either meiosis I or meiosis II. Although homologous chromosomes have already segregated during anaphase I, nondisjunction of sister chromatids may still occur during anaphase II after fertilization. Educational objective: Primary oocytes are present in female embryos and remain arrested in prophase I of meiosis from birth to puberty. During puberty, normal menstrual cycle hormones stimulate some primary oocytes to resume differentiating into a secondary oocyte. However, prior to fertilization, secondary oocytes are arrested in metaphase II of meiosis.