Perinatal organization of a sexually dimorphic aromatase enzyme‐containing immunoreactive nucleusVeney, Sean L.1; Rissman, Emilie F.1,3; Freeman, Louise M.2Neuroreport: 20 October 2000 - Volume 11 - Issue 15 - p 3409–3412 Neuroendocrinology Abstract Author Information Sexual dimorphisms have been identified in the brains of many vertebrates. The adult musk shrew brain contains a sexually dimorphic nucleus of aromatase (AROM) immunoreactive (ir) neurons within the medial preoptic area (mPOA). This cell group is larger and contains more neurons in males than in females. We examined the effects of perinatal hormone manipulations on the organization of this nucleus. Administration of testosterone (T) to female pups (days 1–5 postnatal) increased the size of the AROM-ir nucleus. Males castrated on postnatal day I had a smaller AROM-ir nucleus as adults than did males that remained gonadally intact during development. These data show that T and/or its estrogenic metabolite act during an early critical period to organize the development of AROM-ir neurons in the mPOA. 1University of Virginia, Department of Biology, Charlottesville, VA 22903, USA 2Present Address: Mary Baldwin College, Psychology Department, Staunton, VA 24401, USA 3Corresponding Author: Emilie F. Rissman Acknowledgements: The authors would like to thank Xia Li for expert technical assistance with the immunocytochemcial staining and Aileen Wills for excellent animal care. This work was supported by NIH grants R01 MH57759 and K02 MH01349 (E.F.R.) and NIH training grants HD072323 (S.L.V.) and DK07464 (L.M.F.). Received 2 August 2000; accepted 17 August 2000 © 2000 Lippincott Williams & Wilkins, Inc.