In contrast to the many investigations devoted to the morphological development of the atrioventricular specialized tissues (see James, 1970; Anderson and Taylor, 1972 for review) few researchers have studied the development of the sinoatrial node. Amongst the reports existing on this subject, descriptions of the initial histological differentiation of the node differ markedly. Some investigations have failed to distinguish nodal tissue until relatively late in development. Thus, Shaner (1929) first observed structurally distinct nodal cells in a 100 mm calf fetus while Robb, Kaylor and Turman (1948) first noted this tissue in a 160 mm human fetus. In contrast, Sanabria (1936), Walls (1947), Muir (1951), Van Mierop and Gessner (1970) and Yamauchi (1965) have all recognized specialized tissue at much earlier stages prior to completion of septation of the developing heart. Further disagreement concerns the origin of the node as either a paired (Patten, 1956) or as a unilateral structure (Van Mierop and Gessner, 1970). An entirely different area of controversy surrounds the presence or absence of specialized connexions extending from the sionatrial node through the atrial myocardium (James, 1970; Janse and Anderson, 1974).
KeywordsParaffin Cardiol Acetyl Crest Bedding
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