Muscle differentiation in tentacles of Sepia officinalis (Mollusca) is regulated by muscle regulatory factors (MRF) related proteins

The tentacles of Sepia officinalis are muscular structures that can be quickly everted and 'super-elongated' to capture prey. The speed and super-elongation are achieved by the presence of both cross-striated and helical muscles. In the present study, the complex organization and differentiation of various fibers of the cuttlefish were examined from an early stage of development (stage 26), when the embryo is still inside the egg gel-coating, until the juvenile stage (two weeks after hatching).The muscles start to differentiate centrifugally from the area around the axial nervous system where two types of myoblasts can be recognized. Smooth fibers (referred to here as 'smooth-like' fibers because of their similarity to vertebrate smooth fibers) appear first, then bundles and layers of circomyarian helical and cross-striated fibers differentiate. In Sepia, two muscle-specific transcription factors (MRF), Myf5-like and MyoD-like, have been identified and they are differently expressed during development. Myf5 was detected at first in myoblasts, which give rise to helical smooth-like fibers, while MyoD was expressed later in the other population of myocytes from which circomyarian helical and cross-striated fibers derive. The effective role of these two MRF in tentacle muscle differentiation was confirmed by RNA interference experiments. Injection of double stranded (ds)RNA Myf5 inhibited differentiation of smooth-like fibers, whereas injection of dsRNA MyoD resulted in inhibition of cross-striated and circomyarian helical fibers.