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The Human Embryonic Heart in the Seventh Week'

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The Human Embryonic Heart in the Seventh Week' The Human Embryonic Heart in the Seventh Week' DONALD G. VERNALL The Department of Anatomy, The University of Michigan, Ann Arbor, Michigan Although cardiac development has in- Born's method of wax-plate reconstruc- terested investigators for more than 2,500 tion as modified by Kramer ('42) was years (see Kramer, '42 and Licata, '54 for employed. The model, reconstructed at a historical reviews) the discovery of many magnification of lOOX, is dissectable at of the details of cardiogenesis awaited the various levels in a plane coinciding with development of technics for the construc- the transverse plane in which the embryo tion of enlarged models of the embryonic was sectioned. Figure 3 illustrates the in- heart so that its anatomy could be seen ternal structure that is evident as several more easily. Such a technic was described succeeding segments of the model are re- by Born in 1883, but the method is so moved. The proximal portions of the ma- pains-taking and time-consuming that jor vessels entering or leaving the heart many gaps still exist in the available series have been included in the reconstruction of models illustrating the human heart at (figs. 1 and 2). various stages in its development. As Additional embryos of a comparable age these gaps become narrower, we are con- examined and compared with the model fronted with the problem of the individual are listed below: variability that occurs at each stage. The present paper, an attempt to fill u. of Crown-rump Streeter's Plane Mich. length horizon of one of the gaps in our knowledge of the Number mm number section embryology of the human heart, is based ~~ EH286 14.5 XVIII Transverse on an enlarged model of the heart in the EH314 14.8 XVIII Transverse seventh week of its development. This is EH407 14.5 XVII Sagittal the period during which the coronary ves- EH568 14.0 XVIII Transverse sels and the secondary interatrial septum EH583 14.6 XVII Transverse appear. It is also a period in which many of the structures and relationships of the OBSERVATIONS adult heart are recognizable. External configuration MATERIALS AND METHODS The definitive positions and interrela- The model upon which the present study tionships of the four chambers of the is based was made from the heart of an heart and the associated major vessels embryo' with a crown-rump length of are externally apparent at this stage. 14.5 mm. The embryo was in Streeter's The pulmonary trunk crosses anterior Horizon early XVIII. Several factors led to the ascending trunk (fig. l), arches to the selection of this particular heart dorsad and to the left of it, and is con- for reconstruction: (1) it fits well into nected by the temporarily robust ductus the existing series of reported reconstruc- arteriosus (fig. 2) with the ventral sur- tions, (2) the crucial changes that are face of the left dorsal aortic root. occuring during this period have often The left fourth aortic arch and dorsal been described on the basis of interpola- aortic root are larger than their counter- tions between more carefully observed parts on the right side. earlier and later stages, and (3) the heart was particularly well preserved to demon- 1 This study was supported in part by a grant from the Michigan Heart Association. strate internal features to best advantage. 2 The University of Michigan Collection No. EH-841. 17 18 DONALD G. VERNALL Fig. 1 Ventral view of a wax-plate reconstruction of the human embryonic heart in the seventh week of development. (Reconstruction X 100, illustration X 50.) The continuity between the primordial distal portions of these arteries can be pulmonary arteries (fig. 2) and the right seen projecting from this cut surface en- sixth aortic arch is obscured by the mass route to the developing lungs. of tissue deep to the cut surface of the The four pulmonary veins converge into dorsal mesocardium, although the more a common trunk, the pulmonary inlet, be- EMBRYONIC HEART IN THE SEVENTH WEEK 19 fore entering the left atrium (fig. 3). Los of the right, narrow (fig. 3) as they be- ('58) refers to this dilated pulmonary in- come continuous with the horns of the let as the spatium pulmonale. sinus venosus (venous sinus). The wide lumen of the left common The left horn of the venous sinus has cardinal vein, and to a lesser extent that become more elongated than the right and Fig. 2 Dorsal view of a wax-plate reconstruction of the human embryonic heart in the seventh week of development. (Reconstruction x 100, illustration X 50.) 20 DONALD G. VERNALL passes diagonally across the dorsal wall of the inferior caval vein and from the left the heart in what approximates its ulti- and right horns of the venous sinus. mate position as the coronary sinus. In the floor of the sinal bay, a ridge is The anterior surface of the heart is visible between the orifices of the inferior devoid of attachment to the ventral body caval vein and the coronary sinus (fig. 3). wall. Ventral attachments are found on This ridge extends from the basal tip of the anterior surfaces of the ascending the left venous valve in a dorsolateral di- of the aorta and inferior caval vein. rection toward the medial surface right venous valve. The originally continuous dorsal meso- The unguarded orifice of the pulmonary cardium has separated into a venous and inlet (spatium pulmonale) is located in an arterial segment. The cut surfaces of the floor of the left atrium in a bay which these dorsal attachments are indicated by is similar to the sinal bay of the right the light areas in figure 2. atrium (fig. 3). The medial wall of this The vessels of the coronary circulation bay is partly formed by the primary inter- are not apparent externally as yet although atrial septum. The atrial wall lateral to the proximal portions of the more major this pulmonary bay exhibits a local pro- channels can be found in sections of the liferation which Los ('58) refers to as the epicardium in the atrioventricular sulcus. crista pulmonalis sinistra (left pulmonary crest). This crest deepens the pulmonary Internal structure bay and marks the area where the left The most conspicuous structures within horn of the sinus venosus was absorbed the atrial chambers are the primary inter- into the atrial wall externally. The corre- atrial septum and the right and left venous sponding, but less clear, right pulmonary valves. There is no evidence of the sec- crest, which Los found at this stage is not ondary interatrial septum which if present recognizable in this model. would lie between the left venous valve The ventricles at this stage are of simi- and the primary interatrial septum in a lar size with walls that are richly trabecu- recess referred to by Tandler ('12) as the lated and of equal thickness. The conus interseptovalvular space. ridges have not fused and the ventricles The primary interatrial septum is dis- are in open continuity in this area. In the tinguished by the large secondary inter- region of the atrioventricular canals, how- atrial ostium which occupies almost the ever, the interventricular septum has met entire cephalic half of this partition. The the fused endocardial cushions, and the primary interatrial ostium has been com- ventricles are separated from each other pletely obliterated by the fusion of the at this point. primary septum with the endocardial cush- DISCUSSION ion masses which have partitioned the atrioventricular canal. External configuration The left and right venous valves are The discrepant size relation between the both well developed, the left being almost left and right fourth aortic arches and as prominent as the right. The right ve- between the left and right dorsal aortic nous valve forms a baffle which directs roots heralds the development of the left the blood leaving the sinus venosus toward channel into the main systemic channel the interatrial orifice and away from the and the development of the right into the chamber of the right atrium. proximal portion of the right subclavian Both venous valves unite in the cephalo- artery. dorsal portion of the right atrium, above The convergence of the four pulmonary the level shown in figure 3, to form the veins into a common channel before en- prominent, but transitory, septum spur- tering the left atrium gives little indica- ium. From this ridge, the valves flare out tion that they will subsequently enter this (fig. 3) to enclose the deep sinal bay chamber separately after the common (Licata, '54) of the right atrium. Into trunk has been absorbed into the back of this chamber is discharged the blood from the heart at later stages of development. EMBRYONIC HEART IN THE SEVENTH WEEK 21 B Fig. 3 Internal structure revealed as succeeding segments of the model are removed. (Recon- struction X 100, illustration X 32.) The lumina of the common cardial veins The position and the elongation of the are surprisingly wide relative to the caliber left sinus horn, as well as its point of of the anterior and posterior cardinal entry into the right atrium, graphically veins. The boundary between common illustrate the path followed by the venous cardinal vein and sinus horn is indicated sinus in its shift to the right of an original by the conspicuous narrowing of the chan- midline position. nel that has been described by Los ('58) The cut attachments to the ventral body in even younger embryos (4.4 mm cr wall that are visible on the anterior sur- length). faces of the ascending aorta and inferior 22 DONALD G.
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