The formation, septation and fate of the truncus arteriosus in man.

The results of our studies enable us to draw the following conclusions. The truncus appears in the human embryo, between Stages XII and XIII, as a portion of the aortic sac which invaginates into the interior of the pericardial cavity. Therefore it is an arterial portion which is added to the heart. It lengthens progressively. The sigmoid valves form in the angle between the bulbus cordis and the truncus. Septation of the truncus begins when the sixth arterial arches appear in embryos of 6 to 8 mm. The process is very rapid; commencing in embryos of 6 to 7 mm, it is complete in embryos of 10 to 11 mm, that is to say, during only five days. The septation mechanism is extrinsic. The peribranchial mesenchyma which accompanies the aortic sac in its invagination advances principally on the right inferior part and insinuates itself between the fourth and sixth arterial arches, separating the truncus pulmonalis from a portion of the ascending aorta. An aorticopulmonary communication exists for a certain period prior to fusion of the two blocks of mesenchyma; there is a mesenchymal island. On the contrary, in the bulbus cordis septation is effected by the bulbar ridges. Septation of the truncus, which does not exist in the primitive cardiac tube, occurs prior to that of the bulbus cordis. While septation of the truncus has been already completed in embryos of 10 mm, septation of the bulbus cordis is completed only in embryos of 14 to 16 mm. Therefore the bulbus and the truncus are two portions, different in respect of both structure and septation. There is no continuity between the bulbar ridges and the septation of the truncus. They are separated by the sigmoid valves. This makes it possible to observe independent malformations in the bulbus and in the truncus. In the truncus the mesenchyma passes between the two vessels. They do not have a common septum, and it is for this reason that the surgeon can separate them in the mature heart.

[Somitogenesis of the human embryo: appearance of myotomes]. / La somitogenèse chez l'embryon humain: apparition des myotomes.

In the human embryo, somitic segmentation takes place between stage 9 and 13, that is from 19-21 days to 28-32 days of age. Somites are initially formed of clusters of elongated epithelial cells. In embryos with 10-12 somites, a cavity (the myocele) appears in the central portion of the somites while the neural crest (with its migratory cells) begins to develop along the closed portion of the neural groove. Later on, the medial face of the somites gives rise to the sclerotomes in their ventral portion and to the myotomes in their dorsal portion. In embryos with 30 somites, the cells of the neural crest have developed and they push the myotomes in a ventral direction. The spinal ganglia that increase in volume form a relief on the surface of the embryo. The segmentation that is now visible is ganglionic and no more somitic.

The morphogenesis of the ventricular flow pathways in man.

This work which is based on the study of eight human embryos of 4,5 to 13,6 mm from the Madrid collection, and also on six plastic reconstructions made by the BORN method, and on experiments on chick embryos (ligature of the left vitelline vein in embryos, 3 days of incubation) enables us to reach the following conclusions: no true rotatiorial movements take place during normogensis. Only ventromedial rotation of the right ventricle is observed in stages 12 to 14. This rotation will place the right ventricle, and therefore the vestibulum pulmonalis in front of the aortic infundibulum. We consider that the initiation of cardiac septation is to a great extent due to hemodynamic. causes. During normogenesis in all cases, from the earliest stages of development the presumptive pulmonary outflow tract is always anterior and to the right of the presumptive aortic tract. We do not agree with the statement of GOOR and EDWARDS and VAN PRAAGH R. and VAN PRAAGH S. (1965), that in some stages the presumptive coni of the aorta and pulmonalis are arranged inversely to the way in which they will arranged later.

Testoviron is a potent inducer of chick blastoderm.

Testoviron is a potent neural inducer, acting on the epiblast of the chick embryo during the period of competence (Hamburger-Hamilton stages 3 and 4). Under the influence of Testoviron, the epiblast differentiates in the neural direction. The structure that develops is typically neural in appearance with a 'dorsoventral' and a mediolateral pattern. The notochord is not formed; consequently, there is no lengthening and therefore no craniocaudal pattern. Hence, Testoviron is an 'inducer' and not an 'evocator' even though it differs from the 'primary organizer' in that it does not organize the neural groove in the axial direction as does the dorsal lip of the blastopore of the amphibians or as with Hensen's node in birds. Therefore, it would be very interesting to ascertain whether there is a biochemical difference in the constitution between Hensen's node and the notochord of the bird. The inducing action is localized in a small region aroung the testoviron implant. The steroids might be of major importance in the early morphogenetic processes. We feel steroids deserve more attention than they are receiving at the present time.

Innervation of the sinu-atrial node and neighbouring regions in two human embryos.

In human embryos of 20 to 23 mm (36 to 40 days) it is possible to identify on the right side a nerve that we may call the sinusal, which originates by several roots from the nervus vagus dexter (Figs. 1A, B, D), descending through the right ventrolateral face of the primary trachea and right bronchus (Fig. 2, arrows). Beaded in appearance, it gives a fine anastomotic branch which, passing in front of the arteria pulmonalis dextra, passes to the left side (Figs. 2B, C, D; AN). At this level it gives the large branch for the nodus sinoatrialis which, penetrating through the wall of the superior vena cava, provides a rich innervation for the nodus sinoatrialis which is already in an advanced stage of differentiation (Fig. 3, 2; Cy, D, AN). Afterwards it gives fine branches which, following the atrial fold, are distributed throughout the posterior face of the atrium dextrum (Fig. 3). It increases in diameter and, passing through the angle formed by the right pulmonary veins with the atrium dextrum, reaches the intrapericardial portion of the inferior vena cava in the vicinity of its outlet from the atrium (Fig. 3, arrows). The whole innervation is parasympathetic at the stages studied.

Origin and development of the septum primum.

Based on the study of ten hearts of human embryos from the Orts Llorca collection in Madrid, comprising stages 12-17 of O'Rahilly, the origin of the septum primum was analyzed. The septum referred to has its origin in the dorsocaudal part of the common auricle starting from the cells of the lower cushion which makes contact with the wall of the auricle referred to. The closure of the atrioventricular canal is due to the introduction of the spina vestibuli of the septum primum between the two endocardial cushions. The foramen primum is formed in the human species in the same way as in the rest of the most highly developed vertebrates.

Arterial vascularization of the sinuatrial node in the embryonic rat heart.

We studied the embryological development of the extracoronary cardiac vessels, determining their origins, courses and terminations in embryonic rat hearts. The sinuatrial node is supplied by an extracoronary artery, which can originate from (a) the right internal mammary artery, as a nodal artery, based on its origin; (b) the internal mammary artery, or (c) the right subclavian artery as a collateral artery of the cardiac branch of the cardiomediastinal trunk. The intimate relations of the nodal artery with the sinus node make it possible to study, at the same time, the development of the sinuatrial node in the rat embryo. The sinuatrial node always develops before the nodal artery, which does not appear until 16 or 16.5 days of development.

[Evolution of the homotypic graft of the presumed cardiac area of the quail in the chick]. / Evolution de la greffe homotypique de l'aire cardiaque présomptive de caille chez le poulet

Employing quail cells as biological labelers, we could easily follow the evolution of the presumptive cardiac area of the quail in the chick blastoderm. If the embryos are operated on in stage 5, the cardiac area fuses completely to that of the host embryo forming a large part of the corresponding half in the definitive heart. On the other hand, in earlier stages the grafted cardiac area contributes only to the formation of island of cells in the resultant heart.