16. Neural Development: Sexual Differentiation Mechanisms and Androgen Insensitivity

In this lecture I review how the Sry gene on the Y chromosome initiates testis development, leading to secretion of anti-Müllerian hormone and testosterone that sculpt Müllerian and Wolffian ducts into male internal genitalia. I discuss androgen insensitivity syndrome as a natural experiment in hormone action, then introduce the organizational hypothesis—how early androgen exposure permanently masculinizes brain structures like the sexually dimorphic nucleus of the preoptic area (SDN-POA) and the spinal nucleus of the bulbocavernosus (SNB) via apoptosis and neurotrophic mechanisms. Finally, I describe my pioneering digit-ratio (2D:4D) research with Andrew Huberman, showing more masculine ratios in lesbian women but no difference between gay and straight men, explore butch–femme variations, and highlight the extensive replication literature. This lecture is part of the Development of the Nervous System course by Dr. Mark Breedlove at MSU. Be sure to check out the full series for more in-depth neuroscience content! 📝 Check Your Knowledge: https://forms.gle/w5RRQsASMgnJdUju6 🎥 More Lectures:    / @dr.breedlove   🔔 Subscribe for more neuroscience content! #SexualDifferentiation #OrganizationalHypothesis #DigitRatios #PrenatalHormones #AndrogenInsensitivity #SexualOrientation #MSU #MarkBreedlove #Neuroscience #BrainDevelopment #NeuralDevelopment #epigenetics #Mosaic #Mendel #neuroscience #neurosciences #developmentalbiology #NatureVsNurture #mitosis #andrewhuberman #andrewhubermanpodcast #hubermanlab