11.3:
Meiosis II
Cells formed during meiosis one divide during meiosis two, creating new ones that will develop into mature gametes, like sperm.
In humans, prophase two initiates meiosis two, where dense, X-shaped chromosomes begin to appear in a haploid cell's nucleus. Every X consists of a centromere and joined sister chromatids, each containing a unique combination of maternal and paternal material caused by crossing over during meiosis one.
Meanwhile, in the cell's cytoplasm, the centrosomes travel to opposite poles, and the microtubule-based meiotic spindle apparatus again develops between them.
Prometaphase two is characterized by nuclear envelope disintegration and free-floating chromosomes. Kinetochores also materialize on the centromeres and serve as anchors for extending microtubules. This results in the meiotic spindles securing sister chromatids in a pair to opposing centrosomes.
Continuing to metaphase two, the microtubules orient the chromosomes along the equator and during anaphase two, these structures truncate, tugging sister chromatids apart while the cell also stretches.
With telophase two, the chromatids are delivered to opposite edges and each such aggregation constitutes one haploid unreplicated set of chromosomes, which untangle. New nuclear envelopes appear and the cell divides via cytokinesis.
Thus, meiosis two results in haploid cells, each with a distinct mixture of parental information that yield gametes capable of fertilization, and giving rise to genetically unique siblings.
11.3:
Meiosis II
Meiosis II is the second and final stage of meiosis. It relies on the haploid cells produced during meiosis I, each of which contain only 23 chromosomes—one from each homologous initial pair. Importantly, each chromosome in these cells is composed of two joined copies, and when these cells enter meiosis II, the goal is to separate such sister chromatids using the same microtubule-based network employed in other division processes. The result of meiosis II is two haploid cells, each containing only one copy of all 23 chromosomes. Depending on whether the process occurs in males or females, these cells may form eggs or sperm, which—when joined through the process of fertilization—may yield a new diploid individual.
Meiosis II, Human Egg Cells and the Meiotic Spindle Apparatus
Although the goal of meiosis II is the same in both males and females—to produce haploid egg or sperm cells—there are some critical differences in this process between the sexes. For example, in a woman’s egg precursor cells, the meiotic spindle apparatus responsible for separating sister chromatids forms off to one side, near the periphery. This asymmetry allows for two cells of unequal sizes to be produced following meiosis II: a large egg, and a smaller polar body that dissolves. This division of cytoplasm ensures that the egg contains enough nutrients to support an embryo.
The position of the meiotic spindle apparatus is of concern for scientists involved in assisted reproductive technologies, like intracytoplasmic sperm injection (ICSI). ICSI—used to aid couples experiencing infertility—involves a needle to insert a single sperm directly into an egg’s cytoplasm. Embryologists must take care to avoid injection into the area of the meiotic spindle apparatus, as this could damage the microtubule framework and lead to an abnormal number of chromosomes in the resulting embryo. Therefore, embryologists performing ICSI typically predict the location of the spindle based on the position of the polar body or directly visualize the structure using techniques like polarized light microscopy.
Another unique feature of female meiosis is that the egg precursor cells undergo cell cycle arrest, first in prophase I, and then in metaphase II. At puberty, female sex hormones release the egg cells from prophase I arrest, and meiosis II begins. Subsequently, egg cells arrested in metaphase II are released from the ovary into the fallopian tube, where meiosis only resumes if fertilization occurs. This means that the meiotic spindle apparatus is formed and associated with chromosomes, but does not complete the process of separating sister chromatids until after a sperm and egg precursor cell join.
The arrest of meiosis II poses a unique challenge to women who choose to have their eggs frozen, as many in vitro fertilization protocols require that these cells be isolated during metaphase II and then frozen. Given that problems with the meiotic spindle can cause chromosomal abnormalities like trisomies, considerable research has been dedicated to determining which egg-freezing procedures have only minimal effects on this structure. To diminish damage to eggs, techniques have been developed where sugar or other cryopreservation agents are added to the freezing medium, which limits the formation of ice crystals that can harm cells upon thawing.
Suggested Reading
Casper, Robert, Jigal Haas, Tzu-Bou Hsieh, Rawad Bassil, and Chaula Mehta. “Recent Advances in in Vitro Fertilization.” F1000Research 6 (August 31, 2017). https://doi.org/10.12688/f1000research.11701.1.