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Since Nature published the first report in 2002 on using immunodeficient mice as recipients and allogeneous or heterogeneous testes as donor tissues to study the ectopic development of spermatogenic cells, the technique has been widely applied in various species (including human). In comparison with other in vitro maturation methods for male germ cells, testicular allografting or xenografting technique has such advantages as similar environment for the development of germ cells in physiological conditions, and better reproducibility. Up to now, sperm has been successfully produced by this technique from the testicular tisues of the immature mouse, hamster, cat, rabbit, pig, goat, bovine and rhesus monkey, and their offspring have even been generated by ICSI technique using the mouse and rabbit sperm derived from testis grafts. This article comprehensively reviews the development of the technique by discussing the influencing factors on the germ cell development in grafts including the variety and age of donors, the sex, integrity and immunity of recipients, the graft location and grafting time. And the applications of the technique and the existing problems are discussed as well.

Objective: To establish the method to identify human testicular spermatogenic cells., Methods: Cells were dispersed by mechanic disintegration on testicular biopsy samples of obstructive azoospermic patients. Diff-Quik staining was applied to mixed cell smears. Live cells were observed under inverted microscope equipped with Hoffman modulation contrast optics, and classified according to their morphology. Chromogenic in situ hybridization (CISH) using chromosome 17 centromere probe and anti-c-kit immunocytochemistry staining were applied to classified cell smears., Results: Sertoli cell, primary pachytene spermatocyte (PPS), spermatogonia, round spermatid were the main four round cell groups that can be classified under Hoffman optics. In CISH, Sertoli cell, PPS and spermatogonia displayed 2 centromere signals, while round spermatid and elongated spermatid/sperm displayed 1 centromere signal. In immunocytochemistry staining, PPS and spermatogonia displayed positive staining, while Sertoli cell, round spermatid and sperm displayed negative staining., Conclusion: Dispersed human testicular cells displayed different characteristics in live/staining morphology, ploidy analysis, cell surface membrane antigen expression. All of these methods can be chosen to identify human testicular spermatogenic cells at different stages, while cell morphology classifying under Hoffman optics is a simple and effective method for live cell identification.

Aim: To investigate the stepwise development and germ cell gene expression in allografted neonatal mouse testes and the differentiation of immature human testicular cells in xenografted human testes., Methods: Immunodeficient nude mice were used as hosts for allografting of neonatal mouse testes and xenografting of human fetal testicular tissues. Stepwise development and stage-specific gene expression of germ cells in allografts were systematically evaluated and parallel compared with those in intact mice by periodically monitoring the graft status with measurement of graft weight, histological analysis and determination of five stage-specific genes. Human testicular tissues from 20 and 26 weeks fetuses were used for the xenografting study. Histological analysis of xenografts was performed 116 and 135 d after the grafting procedure., Results: In the allografting study, progressive increase in tissue volume and weight as well as in tubule diameter in grafts was observed; the appearance time of various germ cells in seminiferous tubules, including spermatogonia, spermatocytes, round and elongate spermatids and sperm, was comparable with that in intact donors; the initiation of gene transcription in grafts showed a similar trend as in normal mice. Graft weight ceased to increase after 7-8 weeks and degradation of grafts was observed after 5 weeks with progressive damage to seminiferous epithelium. In the xenografting study using immature human testicular tissues, graft survival and development was indicated by increasing graft weight, Sertoli cells differentiation into advanced stage, germ cells migration and location to the basal lamina and formation of a niche-like structure., Conclusion: The developmental course and gene expression pattern of germ cells in allografts were similar to those in intact mice. The best time point for retrieval of mouse sperm from grafts was 5-7 weeks after grafting procedure. An accelerated development of immature human testicular cells could be achieved by ectopic xenografting of human testes.

Objective: To investigate the differentiation of human testicular spermatogenic cells during in vitro culture., Methods: Testicular cells of obstructive azoospermic patients' testis biopsies were dispersed employing mechanic methods. Then, (1) mixed testicular cells were applied to in vitro culture, and changes of the ratio of elongating spermatids and all round cells were analyzed during mixed cell culture; (2) round spermatids were picked up from the mixed cells employing micromanipulator, followed by differentiation of the isolated round spermatids during microdrop culture., Results: The ratio of the elongating spermatids increased significantly (P < 0.05) after 24 hours of mixed cell culture in HTF medium supplemented with FSH and testosterone. During single round spermatid culture, transformation of the round spermatid to elongating spermatid with newly formed flagellum was observed, and the transformation ratio within 48 hours of microdrop culture was 3.54%. The differentiation of human testicular spermatogenic cells cultured in Vero cell conditioned medium was similar to that cultured in HTF medium., Conclusion: Human testicular round spermatids can differentiate to elongating spermatids during in vitro culture. Vero cell conditioned medium does not promote the differentiation of human testicular round spermatids to elongating spermatids.

Objective: To evaluate the predictive value of serum inhibin B (INH B) levels as an indicator of the presence of testicular spermatozoa in nonobstructive azoospermia., Methods: Forty patients with nonobstructive azoospermia (NOA), 20 patients with obstructive azoospermia (OA), and 10 fertile volunteers were involved in this study. A chemoluminescence method was used to measure the levels of FSH; Inhibin B was analysed by using sandwich enzyme-linked immuno-sorbent assay., Results: Patients with nonobstructive azoospermia has significantly higher levels of serum FSH [(21.34 +/- 12.15) IU/L] and significantly lower levels of inhibin B [(53.15 +/- 58.74) ng/L] than patients with obstructive azoospermia [FSH: (3.94 +/- 1.52) IU/L, INH B: (162.49 +/- 78.38) ng/L, P < 0.01] and fertile volunteers [FSH: (4.27 +/- 2.84) IU/L, INH B: (228.49 +/- 110.68) ng/L, P < 0.01]. Mean serum inhibin B were significantly higher in patients with nonobstructive azoospermia who had spermatozoa on TESE than in those in whom no spermatozoa was found on TESE [INHB: (90.31 +/- 72.18) ng/L vs (19.54 +/- 20.38) ng/L, r = 0.528, P < 0.01], but mean FSH levels did not have similar predictive power (P > 0.05)., Conclusion: Serum INH B level seems to be more accurate than serum FSH in the prediction of presence of testicular spermatozoa in patients with nonobstructive azoospermia. Serum inhibin B determination may be substitute of TESE as a diagnostic index.

Objective: To establish an in vitro model for the development of mouse spermatogenic cells into sperm by using the immunodefective mouse as the incubator., Methods: Tissue grafting was performed using testis from 1-2 days old Kun-ming mice as donor tissue and immunodefective mice as recipients; the expression of TESK1 mRNA in grafts was determined by RT-PCR and the spermatogenesis further observed with histological analysis of grafts., Results: Molecular biological and histological analyses showed that grafts post-grafting not only expressed TESK1 mRNA as in normal mouse testis, but also exhibited similarities in the structure of seminiferous tubules and component of spermatogenic cells, including sperms., Conclusion: Spermatogenic cells heterotopically grafted in vitro could continuously grow and complete spermatogenesis and finally develop into sperm.