Abnormal sperm mid-piece morphology Thick spermatozoon midpiece insertion 2023-07-19T21:18:05Z Increased width of the midpiece of the spermatozoon. The midpiece is the structure that is located between the head and the flagella (tail) of the spermatozoon. This term applies if the width of the midpiece increases progressively from the flagella to the head and if the proportion of individual spermatozoa with this anomaly is above the upper limit of normal. See Figure 2.10i of the WHO laboratory manual for the examination and processing of human semen, Sixth edition (2021).