The development of the ventricular myocardial trabeculae occurs in three steps: emergence,
trabeculation and remodeling. The whole process has been described in vertebrates with two different
myocardial structural types, spongy (zebrafish) and compact (chicken and mouse). In this context, two
alternative mechanisms of myocardial trabeculae emergence have been identified: (1) in chicken and
mouse, the endocardial cells invade the two-layered myocardium; (2) in zebrafish, cardiomyocytes
from the monolayered myocardium invaginate towards the endocardium. Currently, the process has
not been studied in detail in vertebrates having a mixed type of ventricular myocardium, with an
inner trabecular and an outer compact layer, which is presumptively the most primitive morphology
in gnathostomes. We studied the formation of the mixed ventricular myocardium in the lesser spotted
dogfish (Scyliorhinus canicula, Elasmobranchii), using light, scanning and transmission electron
microscopy. Our results show that early formation of the mixed ventricular myocardium, specifically
the emergence and the trabeculation steps, is driven by an endocardial invasion of the myocardium.
The mechanism of trabeculation of the mixed ventricular myocardium in chondrichthyans is the one
that best reproduces how this developmental process has been established from the beginning of
the gnathostome radiation. The process has been apparently preserved throughout the entire group
of sarcopterygians, including birds and mammals. In contrast, teleosts, at least those possessing a
mostly spongy ventricular myocardium, seem to have introduced notable changes in their myocardial
trabeculae development.
