A low-molecular weight factor from colonial hydroids affects body proportioning and cell differentiation

Treatment of metamorphosing planulae of Hydractinia echinata with proportion altering factor (PAF) causes oversizing in the hypostome of developing primary polyps. With increasing concentrations more specimens fail to acquire the appearance of a primary polyp but remain as cones lacking tentacles. Yet, such specimens are true primary polyps as they contain neurons of RF-amide-like immunoreactivity that are typical of the polyp stage. Numbers of RF-amide-positive cells are increased up to 2.5 times compared with the normal value. In intact Hydra vulgaris, PAF increases the number of neurons that differentiate from cycling precursor cells, or even from stem cells. The effect is dose dependent.