Azoospermia may be obstructive (blockage of the genital ducts) or non-obstructive (a lack of testicular production). The distinction is based on an ensemble of clinical, spermiological, hormonal, ultrasound, genetic and histological data. Azoospermia is the main indication for testicular biopsy for therapeutic and diagnostic purposes. Testicular spermatozoids are processed in the reproductive biology laboratory (simultaneously with oocyte retrieval or not) for in vitro fertilization with intra-cytoplasmic sperm injection. The histological study of spermatogenesis is usually performed on a testicular biopsy sample taken at the same time and provides additional diagnostic information on infertility. Histological alterations in the testicular tissues are frequently observed in azoospermic men. In non-obstructive azoospermia, three histological situations prevail: hypospermatogenesis, Sertoli-cell-only syndrome and germ cell arrest. One can distinguish between pure forms (in which all the seminiferous tubules have the same appearance) and mixed forms (in which the tubules' aspects are heterogeneous). Hypospermatogenesis is highly prevalent in azoospermia and is characterized by a low, basal level of spermatozoid production. The prevalence of Sertoli-cell-only syndrome varies from 27 to 68% and the mean spermatozoid recovery rate is between 16 and 33%. Germ cell arrests are rare phenotypes and have a poor prognosis for spermatozoid recovery. Overall, histological examination (still the only way to fully describe spermatogenesis) must be qualitative and quantitative, with the adoption of a standardized, universally understood terminology. It is essential to compare the histological data with (i) recovery of testicular spermatozoids, (ii) clinical, ultrasound, hormonal and genetic data and (iii) the outcome of IVF/ICSI procedures.
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