The discovery of the formaldehyde-induced fluorescence of biogenic amines in different cell lines, made available a most valuable marker for detecting cells endowed with the property of storing one or other of these amines6. Among amine-storing cells the mast cells, which in normal histological preparations are often overlooked in view of their small number and uneven distribution in peripheral tissues and organs, come sharply in relief in tissue sections prepared according to the Falck-Hillarp technique, thanks to the bright yellow fluorescence developed by 5-hydroxytryptamine stored together with other amines in the cell granules?. The present studies were prompted by the observation that tissues and organs in neonatal rats injected with nerve growth factor (NGF) and prepared for histofluorescence studies, revealed at the ultraviolet microscope the presence of a much larger number of fluorescent cells that the same tissues of control littermates. These findings were the starting point of more extensive and detailed investigations directed at the identification of these cells and at answering the question whether this NGF effect was specific of this protein molecule or was also elicited by other like or unlike effectors. Litters of newborn rats (Sprague-Dawley strain), reduced to 8 pups each, were injected with: NGF, physiological solutions or other agents as indicated in Table I. The NGF was purified according to the method of Bocchini and Angeletti from the mouse salivary glands4 and injected in the amount of 10 µg/g body weight (bw) as described in a previous articles. Bovine serum albumen (BSA), Cytochrome-c, lysozyme and insulin, were purchased from Sigma Co. St. Louis, USA; 6-hydroxydopamine (6-OHDA) was obtained from Fluka AG, Chemische Fabrik, Switzerland; epidermal growth factor (EGF) was prepared according to the method of Cohen5. Ninety neonatal rats were divided in coded groups, injected daily with one or another agent listed in Table I and sacrificed by decapitation at the end of the second or third week.

Twenty young rats, 150 g bw, were injected intraperitoneally for 14 consecutive days with 7 µg/g bw of NGF or physiological solution. Experimental and control rats were sacrificed one day after the last injection and tissues and organs were prepared for histofluorescence studies and examined with the UV microscope. Tissues