Daily Variation in Mitotic Rate in Tail-Fin Epidermis of Larval <i>Xenopus laevis </i>and its Modification by Pineal Organ-Subcommissural Organ System and Photoperiods

Abstract

The mitotic rate in the ventral tail-fin epidermis of larval Xenopus laevis maintained in a natural day-night condition was higher during the nighttime than during the daytime. The rhythm persisted in blinded animals kept in the same conditions of light, but was lost when the animals were placed in continuous darkness. No rhythm was demonstrable in blinded larvae whose pineal organ (PO)-subcommissural organ (SCO) system had been cauterized, regardless of the light conditions obtaining. The rhythmic pattern as delineated by the mitotic rate was first detected in the larvae at stage 51 (stage of Nieuwkoop and Faber [1956]) and persisted in older larvae as late as stage 53. It was concluded that photic stimulus, perceived by the PO and transmitted presumably via tractus pinealis to the SCO, causes periodic release (and synthesis?) of the aldehyde-fuchsin (AF) positive material from the SCO, and directly or indirectly affects mitosis in the epidermis.