The goals of this project are to lay the foundation for further study on the developmental biology and neuronal plasticity of the VN neurons in a South American opossum (Monodelphis domestica) and to establish this animal as a new model for studying chemosensory communication. It investigates neuronal basis of the ontogeny of the vomeronasal (VN) chemosensory senses in prenatal and postnatal pups of marsupials obtained from a breeding colony in the investigator's laboratory. The opossum pups of this species are born at a primitive embryonic form in every system, especially in the nervous system and they attach to the teats of the pouchless dam during the early neonatal life. These unique features provide investigators an easy access to the pups for carrying out experimental manipulation on their postnatal "embryonic" development. The specific aims of this project are focused on the following aspects: (1) to reveal the sequence of embryonic differentiation of the primordial VN organ and the embryonic olfactory placode, (2) to elucidate when nasal chemosensory organs develop and mediate chemosensory reception during prenatal or neonatal life, and (3) to demonstrate when and how the developing VN neurons interact with the developing brain, in particular, the accessory olfactory bulb (AOB). Light, transmission, and scanning electron microscopy are utilized to study the morphological changes of cells in the VN organ and AOB during development. The fluorescent carbocyanine dye, DiI, will be used to trace the sprout and innervation of VN nerve to the AOB. It provides useful data to reveal some neuronal basis for the expression of vomeronasal and olfactory functions during the development of this macrosmatic marsupial.