Development of serotoninergic neurons from ventricular cells of the mouse neural plate in vitro

Cephalic neural plates and neural tubes of mice (pros- and rhombencephalic anlagen), developmental stages Theiler 11–18 [Th 11–18; embryonic day $\text{7}\text{1}\text{2}$–11 (E$\text{7}\text{1}\text{2}$–11)], were prepared and cultured in a plasma clot with horse serum-containing MEM medium. Differentiation of the ventricular cells was studied in order to investigate the expression of serotoninergic properties. Serotoninergic neurons were not detected in preparations derived from neural plates of stage Th 11 (E$\text{7}\text{1}\text{2}$), but were demonstrated in increasing numbers from the early stage Th 12 (E8) onwards. They exclusively originated from the rhombencephalic floor caudal to the mesencephalic flexure. The serotoninergic neurons developed from these areas, irrespective of whether being cultured in their natural position within the neural plate, or separated as microcultures, or transplanted into the prosencephalic anlage. Every other region of the neural plate remained free of serotoninergic neurons. The in vitro findings are highly reproducible due to the following properties: the morphological and immunocytochemical peculiarities of the serotoninergic neurons, their tendency to appear in increasing numbers with age, their localization within the cultured neural plates and their appearance in all cultures from stage Th 12 (E8) on. Due to these findings it is considered possible that the progenitor cells of serotoninergic neurons might already have been determined within distinct areas in the mouse neural plates as early as stage Th 12 (E8).