[Topography of the serotonin neurons in the brain stem of human fetus: an immunohistochemical study].

The distribution of serotonin neurons in the central nervous system (CNS) has been intensively examined in mammals, such as rats, cats and monkeys. However, the details of serotonin neuron system have been remained uncertain in human CNS, although two fluorescence histochemical studies were reported in human fetus. In this study, we performed immunohistochemical examination on the distribution of serotonin neurons in the brain stem of human fetuses. The brain stems from five human fetuses (CRL: 120-275 mm, GA: 15-27 wks) were fixed with 4% paraformaldehyde, dehydrated with graded ethanol, and embedded in paraffin. Serial sections, 6 micron in thickness, were cut from seven different levels of the brain stem of each fetus. The initial several sections were used for usual histological observations. The following serial ones were stained by peroxidase-antiperoxidase (PAP) technique using anti-serotonin sera. The anti-serotonin sera used were raised in rabbits by the methods of Ranadive and Sehon (1967), Grota and Brown (1974), and Steinbusch et al (1978). Before the use, the specificity of the antisera was confirmed by the immunohistochemical examination of the CNS of rat embryos and adults. Positively stained serotonin neurons were clearly demonstrated in the brain stems of all cases examined (Figs. 1A-1H). They were small to medium in size, 10 X 20 micron to 20 X 40 micron, and varied in shape, showing round to oval cell somata with unipolar, bipolar and multipolar processes. The distribution of serotonin neurons in the brain stems was almost the same among five human fetuses (Figs. 2A-2K). A large number of serotonin neurons were located in the midline raphe nuclei. In addition, numerous serotonin neurons were observed widely in the other tegmental area. The nuclei containing serotonin neurons were listed in Table 2 according to the terminology by Olszewski and Baxter (1982). The distribution of the serotonin neurons in the raphe nuclei of human fetuses was fundamentally similar to those of many mammals reported previously. However, the lateral extension of serotonin neurons to the other tegmental area beyond the midline raphe nuclei in human fetuses was much greater than in any other mammals. This distribution pattern of serotonin neurons was considered to be peculiar to human fetus. Since the histological architecture of the brain stems of five fetuses examined was very similar to that of human adults, the distribution of serotonin neurons demonstrated here may also represent that of human adults.