Abnormal sperm tail morphology Altered location of the longitudinal column in the fibrous sheath 2023-07-10T11:39:59Z The fibrous sheath is a structure in the midpiece of the sperm flagellum surrounding the outer dense fibers. It comprises two longitudinal columns and transverse ribs. In case of altered location of the longitudinal column in the fibrous sheath the symmetrical structure is lost, most commonly with the columns not being opposite to each other, which can be visualized by electron microscopy.