The product of the spermatocyte in the testis when it has completed meiosis (by which the number of chromosomes is halved, so it's in the haploid state), and also enclosed in the testicular tubules by the Sertoli cells. Early spermatids are round-shaped, whereas late spermatids closely resemble mature spermatozoa.

(ROSI) Experimental form of testicular sperm extraction (TESE) followed by intracytoplasmic sperm insertion (ICSI) in which a round (very immature) spermatid is isolated for injection into the egg (or oocyte). Seems to be less successful than a similar procedure in which the nucleus is isolated from the spermatid and used instead. Not recommended clinically.

(ROSNI) Experimental form of testicular sperm extraction (TESE) followed by intracytoplasmic sperm insertion (ICSI) in which the nucleus of a round (very immature) spermatid is isolated for injection into the egg (or secondary oocyte). Intended to be used in treating non-obstructive azoospermia with severe maturation arrest, when more spermatozoa are not obtainable. Animal studies show higher pregnancy rates than with round spermatid injection (ROSI), but limited studies in humans still indicate very high rates of embryopathy. Not recommended clinically.

round spermatid injection.

The form of the sperm cell in the second stage of spermatogenesis (through which the sperm cells are formed in the testes), produced from primary spermatocytes in the first cell division of meiosis, and giving rise to spermatids, which have just half the normal cell's complement of chromosomes, through the second division of meiosis; enveloped by Sertoli cells in the testicular tubules.

The sperm cell equivalent to the oocyte stage of the egg. Further development in the testicular tubules results in a spermatid.

Development of a sperm cell, in the tubules of the testes (the testicular tubules), from spermatogonium and spermatocyte to spermatozoon (the last part of this process, from spermatid to spermatozoon, is called spermiogenesis). The process of spermatogenesis takes 56 days. At any one location in a tubule there are cells at four different stages of maturity, so mature sperm are released from a particular location into the lumen of the testicular tubules every 14 days. A systematic interruption of spermatogenesis results in azoospermia and is called maturation arrest.

The final part of the formation of a mature sperm cell, in which the round, early spermatid (already with a haploid number of chromosomes) loses its round shape to acquire the features of the mature, elongated spermatozoon. Contrast with spermatogenesis, which encompasses spermiogenesis as it's completed.