How are BDNF and Fruchtbarkeit related?

The provided context does not contain any information on the relationship between BDNF (Brain-Derived Neurotrophic Factor) and fertility. BDNF is a protein primarily known for its role in the nervous system, but it is also expressed in reproductive tissues. Some studies suggest a potential involvement in oocyte maturation and sperm function, but the evidence is limited and inconsistent. Without specific data from the given sources, no evidence-based conclusion can be drawn. Therefore, the evidence level is marked as unclear.