The presence of melanin-containing cells in the heart has been documented in tetrapods,
but not in fish. It has been even suggested that dark pigmented cells are exclusively
associated with hearts having two atria and two ventricles. The aim here is to report the
occurrence of pigment cells in the heart of the bichir, an extant representative of the
polypteriformes, an ancient ray-finned fish lineage that split from the stem of the
actinopterygians soon after their divergence from the sarcopterygians. The bichir heart
is composed of sinus venosus, atrium, ventricle, conus arteriosus and bulbus arteriosus
arranged sequentially within the pericardial cavity. Dendritic-shaped cells containing
melanosomes were found in the five cardiac components of the 12 bichirs included in
this study. Numerous melanophores were distributed regularly over the surface of all
segments having myocardium in their walls, thus resulting in a marked pigmentation of
the whole heart. The bulbus arteriosus, which in the bichir is reduced in size, showed an
even more intense pigmentation. In all instances, the melanophores were localized in
the subepicardial space. Pigment cells also occurred in the pericardium and ventral
aorta. The functional role of melanocytes in the tetrapod heart remains obscure. Antiinflamatory
activity, cytoprotection and effects on the viscoelastic properties of the
cardiac tissue have been adduced as possible actions of such cells. The role of pigment
cells in the bichir heart constitutes a new open question. Interestingly, however, the only
cells that have been shown to form melanin-containing cells in the heart derive from the
neural crest. If the melanophores of the bichir heart are indeed of neural crest origin, it
would suggest a much more extensive contribution and persistence of elements from the
neural crest in the primitive heart of jawed vertebrates as assumed so far in most papers
devoted to vertebrate heart embryology.
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