Spermatids from the ejaculate or those obtained by testicular biopsy can be used to fertilize oocytes, by means of round spermatid injection (ROSI) or elongating spermatid injection (ELSI). If possible, the use of spermatids from the ejaculate is preferred, in order to avoid the potentially traumatic biopsy procedure. However, in many cases the spermatids isolated from the ejaculate have a poor viability (to be determined by a previous examination) and recourse to a biopsy is unavoidable.
In the absence of both spermatozoa and spermatids in the sample obtained by testicular biopsy, the development of spermatids may be achieved by in vitro culture, if at least some spermatocytes are present. In all cases, the fertilization attempt is carried out by means of a microinjection of spermatids, either fresh or derived from spermatocytes through in vitro culture.