Subcommissural organ-associated neurons in fetal and neonatal rabbit.

Neurons which initially lie in the basal region of the subcommissural organ (SCO) were investigated in 20 rabbit fetuses from day 15 to 30 of gestation, and in eight neonatal, 4 and 8 day old rabbits. These SCO-associated neurons, first observed on day 17 of gestation, develop into (1) a rostral mesodiencephalic nerve cell group situated in an area dorsal to the rostral-most part of the SCO and (2) a more caudal layer of single neurons extending throughout the length of the SCO. The present findings are discussed in relation to recent histochemical studies that demonstrated AChE-positive neurons in the pineal complex and subcommissural area of frogs and to recent fluorescence microscopic studies in fetal and adult rats in which a 5-HT system is known to extend from the nucleus raphe dorsalis (B7) along the SCO to the pineal stalk and habenular region. The term "SCO-associated neurons" is a purely morphological way of describing the neurons in question as the neural interconnections of these neurons are still a matter of speculation.