Reproductive Yin and Yang (image) American Institute of Physics Share Print E-Mail Caption Progesterone and other fatty signaling molecules are critical for sperm fertility. Credit C. Cain Usage Restrictions This image may be used only with appropriate caption and credit. Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.