Analysis of Events Governing the Meiotic Division in Mouse Spermatocytes

The meiotic division is essential for successful gametogenesis. However, many events occurring during male and female meiotic development remain poorly understood. While it is known that chromosomes must pair, recombine, and segregate to form gametes, critical questions remain. How and when do these events occur with respect to each other? What mechanisms monitor their developmental success? Insight into these questions is provided in this dissertation, using the mouse spermatocyte as a model. The purpose of this work is to aid in the overall understanding of mammalian meiosis.

After an introduction into mammalian meiosis in Part I, a temporal order of events occurring during meiosis in the mouse spermatocyte is provided in Part II. The development of events such as chromosome pairing, spindle formation, and localization of cell cycle proteins was monitored using immunofluorescence. This work established a framework for which developmental progress can be monitored in normal and abnormal environments.

In Part III, the MLH1-deficient mouse was used to study an abnormal G2/M transition. Spermatocytes lacking the DNA mismatch repair protein MLH1 are characterized by univalent chromosomes at metaphase I, and do not progress into the first anaphase. Apoptosis, or programmed cell death, was seen in Mlh1-/- metaphase spermatocytes.

In Part IV, spermatocytes heterozygous for Robertsonian-chromosome translocations were also used to study abnormalities in the G2/M transition. Many of these spermatocytes failed to properly pair homologous chromosomes during MI.

Many metaphase spermatocytes also contained unaligned/lagging chromosomes. Apoptosis was seen in a large portion of the spermatocytes containing unaligned chromosomes, possibly in response to the activation of a spindle checkpoint mechanism. Although functional sperm are produced in these mice, many were found to be aneuploid for chromosomes involved in Robertsonian translocations.

The findings of this dissertation aid in the overall understanding of meiotic development and regulation, which is discussed in the closing Part V. By the establishment of a meiotic timeline, genetic abnormalities can be and were studied in the context of normal meiotic progression. Situations in which meiotic abnormalities arise are provided in Parts IV and V. These findings provide insight into the consequences of chromosomal abnormalities and failure in the DNA repair mechanism during meiosis, possibly reflecting the creation of errors in our own species.