**Abbreviations**

)

transplantation into testis of infertile mice [66]. Nayernia et al. reported 300–500-fold increase in spermatogonial population after transplantation of mouse bone marrow (BM) derived SSCs into germ cells depleted recipient mice, but the cells were arrested at pre-meiotic stage [64]. A similar study has shown that transplanted green fluorescence protein positive (GFP<sup>+</sup>

**Table 1.** A summary of selected studies designed to differentiate male germ cells from different sources of stem cells.

**Source of cells Method used Observations References**

Typically round germ-like cells were appeared with time. The cells were positive for early germ cell specific *STELLA* & *VASA* and male germ cells specific *DAZL*

Completed the normal seminiferous cord formation. Induction of steroidogenesis was observed with hCG treatment. Germ cells differentiation was confirmed by decreasing *OCT4* and increasing *Vasa* expressing

Small number of germ cells expressing *SCP3* and *VASA* were

Confirmed meiosis in 84% of colonies and erasure of genetic imprinting. ICSI procedure using spermatids like cells produced viable and fertile offspring

Xie et al. [62]

Mitchell et al. [93]

Hua et al. [94]

Zhou et al. [95]

markers

cells

formed

MSCs were co-cultured with mitotic inactivated newborn mouse Sertoli cells for 3 weeks

Testis tissues were inserted subcutaneously into mice. Mice with second-trimester xenografts were randomly given hCG injection

Cells were induced with 2 × 10−6 M RA and 10 ng/ml BMP4 in DMEM for 7–14 days

were cultured with testicular somatic ells in the presence of Knockout serum, RA, BMP 2, 4, 7, Activin A, FSH, Testosterone and bovine pituitary extract for 14 days. Resultant spermatids like cells were used for ICSI

Mouse ESCs ESCs derived PGCs like cells

Human Wharton's jellyderived MSCs

40 Spermatozoa - Facts and Perspectives

Human testicular tissues from first and second trimester fetuses

MSCs from human umbilical cord Wharton's jelly

mouse BM cells can differentiate into both somatic and germ cell lineages in a favorable testicular niche [79]. Transplanted adipose tissue-derived MSCs could induce spermatogenesis in busulfan-treated recipient rats in another study [80]. Toyooka et al. cultured *Mvh* knockin GFP or *Lac-Z* mouse embryoid bodies with *BMP 4,8* expressing embryonic trophoblast cells, and they could achieve morphologically normal sperm after transplantation of *MVH* overexpressing cells under the testis capsules of nude mice [81]. Use of combination of gene transfection and subsequent germ cells transplantation techniques could results live sperm (with reduced motility) and offspring after intracytoplasmic sperm injection (ICSI) in mice. Though the newborns died within few months due to imprinting defects, this finding paves the way to promising *in vitro*- and/or *ex vivo*-derived functional gametes in the future [82]. Additional treatment with hormones (hCG) after stem cell transplantation may be a powerful tool for increasing the efficiency of transplantation [83]. It is suggested that FSH and testosterone favor the survival of germ cells by regulating both intrinsic and the extrinsic apoptotic pathways. Furthermore, FSH is needed to initiation of meiosis and androgen is necessary for

the completion of meiosis and spermiogenesis [84].


