The role of neurohumours in early embryogenesis

The 5-hydroxytryptamine concentration changes insignificantly in the course of each of the first cleavage divisions, except at prometaphase when a sharp short-term fall occurs. According to the data obtained by the simplified gaseous formaldehyde method of Falck and Hillarp, 5-HT is present in all embryonic and larval cells up to the stage of prism. In plutei 5-HT is detected by this method only in the digestive tube, ventral ciliary band and possibly in pigment cells. According to the data obtained by the aqueous formaldehyde method, at the gastrula stage 5-HT is localized in the archenteron and vegetative ectoderm and at the stages of prism and pluteus in cells of the digestive tube, ventral ciliary band and, possibly, in pigment cells. The aqueous method gives negative results at pregastrular developmental stages. On the basis of the data obtained a suggestion is put forward about the presence of two 5-HT pools in developing sea-urchin embryos and larvae. It is suggested that the first of them, not detected by the aqueous method and present in embryos beginning from the earliest developmental stages, performs intracellular regulatory functions whose character is discussed. The second pool detected by the aqueous method arises with the onset of gastrulation and appears to be related to morphogenetic processes.