One Truncal Valve or Two Semilunar Valves: An Unusual Case of Truncus Arteriosus

In the accounts of the development of the heart of vertebrate animals given by various embryological writers, we find an apparently clear description of the mode in which the permanent aorta and pulmonary artery are formed by the longitudinal division of a single large vessel, the truncus arteriosus, into two vessels. The truncus arteriosus is, as is well known, the large arterial trunk that commences in the originally single ventricle of the heart, and terminates by splitting up into the branchial arteries. It conveys the whole of the blood from the ventricle to the system of the embryo, and is also known by the name of bulbus aortæ (see Plate XXXI. fig. 1). In studying the descriptions given by different authors of its division into two vessels, it appeared to me very strange that with such a clear description of the mode of division nothing at all, or only very little, should be said about the mode of development of the semilunar valves attached to the commencement of these vessels. Kölliker is the only author I have been able to find who makes any mention of their mode of development, and his account, which I shall presently quote, is very brief and unsatisfactory. Nowhere have I found any drawings of these parts in their rudimentary state. I was therefore obliged to conclude that very little was known about this point, probably in consequence of the difficulty of accurately examining such minute parts at an early period of development, and I was hence led to attempt the observations recorded in the present paper. They were made during the years 1865, 1866, and 1867, on the embryo of the common domestic fowl, artificially incubated. Though not nearly as complete as I could have wished them to be, they nevertheless demonstrate certain new and interesting facts connected with the development of the semilunar valves, and the formation of the aorta and pulmonary artery in the bird’s heart. These appear to me to be valuable, as possibly throwing light on some of the congenital malformations of this part of the heart. In working at the development of the semilunar valves, I was also obliged to examine very closely into the mode of division of the truncus arteriosus into two vessels, and found that the manner in which it becomes divided differs from that usually described to occur in some very important particulars. The development of the semilunar valves is so closely connected with the process of division of the truncus arteriosus that I have found it best to unite the description of each stage of the one with that of the corresponding stage of the other.

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Observations on the development of the semilunar valves of the aorta and pulmonary artery of the heart of the chick

Proceedings of the Royal Society of London ◽

10.1098/rspl.1867.0067 ◽

1868 ◽

Vol 16 ◽

pp. 329-335

Keyword(s):

Pulmonary Artery ◽

Congenital Malformations ◽

The Other ◽

Truncus Arteriosus ◽

Semilunar Valves ◽

Arterial Trunk ◽

Mammalian Embryos ◽

The Common ◽

Pass Through ◽

Branchial Arteries

Kölliker is the only embryological author in whom I have found any information about the development of the semilunar valves of the aorta and pulmonary artery, and I have not been able to discover any observations later than his. After speaking of the formation of the aorta and pulmonary artery by the division of the truncus arteriosus into two vessels this being, as is well known, the large single arterial trunk conveying the blood from the rudimentary ventricle into the branchial arteries, he says, “Simultaneously with the division the semilunar valves also become developed, and I saw them already present in both arteries in an embryo of the seventh week. They are, however, at first nothing but horizontally projecting crescentic growths of the middle and of the epithelial coats by which the lumen at this spot receives the form of a three-rayed star. At what time they first become visible as distinct pockets I have not yet investigated.” The division of the truncus arteriosus is described by Rathke as occurring in birds and mammalia by the formation on its interior of two oppositely situated longitudinal ridges, which then grow together throughout its whole extent and completely divide the vessel into two lateral halves, one representing the commencement of the aorta, and the other that of the pulmonary artery. Though the semilunar valves are said by Kölliker, and quite correctly, to develope simultaneously with the division, he gives no information about the manner in which they are connected with it, or the part of the vessel in which they originate, and nowhere are any drawings given of them in their rudimentary state. I was hence led to conclude that very little was known about this point, and to make the observations the results of which are here recorded. They seem to me valuable, as throwing light on some of the congenital malformations of this part of the heart. They were made during 1865, 1866, and 1867, on the embryos of the common fowl, and I have had no opportunity of investigating human or other mammalian embryos with reference to this point. But from the great likeness between the hearts of birds, mammalia, and man at different periods of their development, it seems pretty certain that the arterial semilunar valves in man and mammalia generally must pass through the same stages of development as those of the bird, which, in the fully developed state, quite resemble them.

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