Birefringence and fine structure of spindles in spermatocytes of Nephrotoma suturalis at metaphase of first meiotic division

Spermatocytes of Nephrotoma suturalis fixed at metaphase of the first meiotic division were observed with both the polarizing microscope and the electron microscope. The distribution of birefringence in fixed cells is similar to that in living cells: chromosomal fibers are detected as birefringent bands extending from kinetochores to poles, but retardation of fixed fibers is less than retardation of fibers in living cells. Thin sections show spindles to be composed of microtubules and granular matrix. Transverse sections of spindles show that kinetochore microtubules are more densely concentrated (i.e., more microtubules per unit area) than continuous microtubules. A comparison of the distribution of microtubules in transverse sections with the distribution of birefringence in the spindle suggests that microtubules are the basis for birefringence in fixed spindles at metaphase. The results are discussed with regard to the possibility that other factors may influence birefringence of the spindle in vivo.