Human spermatids. Transmission electron micrograph (TEM) of a section through a human testis (testicle), showing two elongating spermatids. Spermatids mature into spermatozoa (sperm cells). The wedge-shaped nuclei (black) contain the DNA/chromatin condensed and clustered as it is compacted into the emerging shape of the sperm head. A centriole is seen behind each head, from which the tail (flagellum) will grow. Magnification: x17,000, when printed 10 centimetres wide.