Persistence of a functioning bulbus cordis homologue in the turtle heart

Abstract

The functional and structural characteristics of a discrete myocardial segment at the base of the turtle ventricle were studied. The study indicates that this segment is a homologue of the primitive bulbus cordis. Cinematographic techniques were utilized to observe the function of this segment. It contracts later than the main ventricular chamber and acts as a sphincter at the base of the pulmonary artery in late systole. It is suggested that this mechanism serves to regulate the normal left-to-right shunt into the lungs through the incomplete ventricular septum and to maintain adequate systemic blood flow. A resemblance between the arrangement in the turtle and that seen in certain cases of human congenital heart disease is briefly discussed.

Development of the Hearts of Lizards and Snakes and Perspectives to Cardiac Evolution

“…Also, the complete part and the free-standing part of the muscular ridge is said to resemble the trabecula septomarginalis (incomplete part) and moderator band (complete part) of the mammalian and avian right ventricle [15], [21], [70], [105], [107], [112]. Consistently, a small remnant of the myocardial outflow tract can be found ventrally in the reptile heart, at the pulmonary arterial base, like the right ventricular outflow tract (or conus, or infundibulum) of the mammalian right ventricle [71], [113]–[116]. It remains an intriguing question how the heart tube-like outflow tract myocardium acquires a ventricular phenotype upon being incorporated into the ventricle.…”

Sympathetic Control of Right Ventricular Dynamics

“…In this connec tion T obin et al [18] have shown that isoproterenol causes the infundi bular zone to narrow extensively in late systole and this geometric change is associated with the maximum pressure difference generated from the heart to the pulmonary arteries. Studies on the turtle heart have demon strated that the musculature of the conus arteriosus is anatomically distinct and can act as a functional stricture diverting blood through the intra ventricular foramen into the systemic circuit [8,21], In the mammalian heart, the conus arteriosus is incorporated into the right ventricle and contributes to the formation of the out-flow-tract musculature [7]. Recently, A rmour et al [1] have shown that during stellate ganglion stimulation or norepinephrine injection, muscle fibers in the infundibular region de veloped greater increases in the force of contraction than sinus fibers, while pressure recorded from the sinus greatly exceeded simultaneously measured conus pressure.…”

Cardiovascular Dynamics in Fishes, Amphibians, and Reptiles*

“…Bulbus tissue can, however, still be identified as a discrete muscular ring a t the base of the pulmonary artery. March (1961) has recently demonstrated that this muscular ring contracts later than the main ventricular chamber. H e argues that this phenomenon serves to regulate the normal left-to-right shunt pattern which has been demonstrated for the reptilian heart (White, 1956(White, & 1959Steggerda & Essex, 1957).…”

“…In reptiles it represents a discrete segment a t the base of the ventricle, with its distal portion absorbed into the truncus arteriosus (Greil, 1903). In birds and mammals with an anatomically complete double circulation, the proximal ridges of the bulbus participate in the completion of the intraventricular septum and the proximal bulbus is incorporated into the developing ventricle and forms the base of the pulmonary outflow tract in the right ventricular infundibulum (March, 1961).…”