Human spermatozoa with various types of morphological defects. a, Normal spermatozoa. b, An immature sperm with a cytoplasmic droplet. c, A head-defective sperm with loss of nuclear content. d, A neck-defective sperm with a bent neck. e and f, Tail-defective sperm with bent (e) or coiled (f) tails. The defects are indicated by the white arrows. Blue, nuclei stained with DAPI; red, mitochondria stained with MitoTracker; green, SEPT7