Ontogeny of histidine‐decarboxylase‐immunoreactive neurons in the tuberomammillary nucleus of the rat hypothalamus: Time of origin and development of transmitter phenotype

The ontogeny of the histidine decarboxylase (HDC)‐immunoreactive neurons of the tuberomammillary (TM) nucleus was studied in the rat brain. The time of origin of TM neurons was studied by counting the percentage of HDC‐immunopositive neurons double labelled by autoradiography in adult progeny of dams injected with [³H]‐thymidine at various times during gestation. Neurogenesis began on embryonic day (E) 13, peaked on E16, and was complete by E18. HDC immunoreactivity was first detected in the fetal rat brain on E16. Experiments utilizing short‐survival [³H]‐thymidine autoradiography combined with HDC immunohistochemistry demonstrated that TM neurons undergo their final mitotic division prior to expression of their transmitter phenotype.