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Fig. 4. Control of spermatogonial stem cell differentiation and self-renewal by
GDNF dosage. GDNF is expressed by the Sertoli cells in the seminiferous
tubules. These cells control sperm differentiation in a paracrine manner. When
the GDNF level is low, the spermatogenic stem cells enter the differentiation
pathway. When its level is high, the stem cells only self-renew and are unable
to differentiate. In transgenic mice with loss- and gain-of-function of GDNF,
both conditions lead to disturbed spermatogenesis.