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The disposition and innervation of atrioventricular ring specialized tissue in rats and rabbits.

Although Davies & Francis (1946) expressed the belief that the specialized tissue was a neomorphic development in homoiothermic animals, others have described such tissue in poikilothermic hearts (Gaskell, 1883, 1900; Keith & Mackenzie, 1910; Robb & Petri, 1961; Robb, 1964). Keith & Mackenzie (1910) thought that the mammalian AV bundle was a remnant of the more widespread specialized atrioventricular junctional musculature of such lower animals, whilst Lewis (1920) suggested that the additional specialized tissue described by Kent (1893, 1913, 1914) might also be a remnant of this musculature. A similar explanation would account for the extranodal specialized tissue adjacent to the AV junction described by Shaner (1929), Truex, Bishof & Hoffman (1958), Lev, Fielding & Zaeske (1963), Gossrau (1971), and others. However, Robb & Petri (1961) indicated briefly that in human and monkey fetal hearts, and in adult guinea-pig hearts, specialized tissue was related to the entirety of the primitive atrioventricular junctions. Subsequent investigation has confirmed the presence of rings of specialized tissue related to the AV orifices in guinea-pig and human fetal hearts (Anderson, 1972a; Anderson & Taylor, 1972). The present investigation was performed in order to establish the presence of such tissue in other mammalian hearts. The species selected were the rabbit, in which specialized AV rings have been reported in an electrophysiological investigation (Paes de Carvalho, 1961) and the rat, in which junctional tissue was mentioned by Kent (1893).