286

THE ELECTROCARDIOGRAM OF THEDEVELOPING CHICK

BY J. YULE BOGUE.

(From the Department of Pharmacology, University of Edinburgh.)

(Received 20th January, 1933.)

(With Two Plates and Two Text-figures.)

IN the investigation of the electrocardiogram of the chick embryo the same techniquehas been employed as that already described by the author (1932). The only point ofdifference is that in the present experiments after a record had been taken throughthe shell, the shell was opened, the embryo exposed and the electrodes placed inthe most favourable position for obtaining a record.

METHODS.

These experiments were carried out in an electrically screened room, with asingle stage amplifier incorporating a Mazda AC/SG valve. Text-fig. 1 shows thecircuit, the anode resistance and the applied voltage were varied according to theamount of amplification required. The tension of the galvanometer string wasalways 3 cm. for 3 millivolts, except in the case of the earliest records shown inPL I, fig. 1 a and b when the tension was 4-5 cm. for 3 millivolts. The standing anodecurrent was balanced out with a simple compensator consisting of two variableresistances and a battery before the galvanometer was switched into circuit.

The amplifier produced practically no distortion in the case of variations ofpotential occurring at a rate similar to those produced by the chick's heart. Thisfact was proved by two tests, a physical and a biological. In the physical test theamplifier was connected to a cathode-ray oscillograph and various frequencies from500 cycles per second down to 50 cycles per second were applied by means of avalve oscillator. Then a D.c. input was interrupted at various rates down to 1 persecond, the results were then compared with those done without an amplifier. Thisprocedure was then repeated with the string galvanometer in circuit but withfrequencies from 200 cycles per second downwards. When the D.c. input was inter-rupted every 2 sec. there was a slight tendency for the curve to die away a little.

In the biological test the effect of the amplifier was tested on the electrical responseof the frog's heart. Electrocardiograms were taken firstly without and secondly withthe amplifier in circuit. In the second case the current was reduced about 99 percent, by shunting. A comparison of the two records showed that the introduction ofthe amplifier did not alter the general shapes of the P, R and T waves. Measurements

The Electrocardiogram of the Developing Chick 287

of the rate of rise of the R wave showed, however, that the duration of the rise wasreduced by from 1 to 3 sigma when the amplifier was in circuit.

These experiments indicate that the amplifier does not distort the general shapeof the electrocardiogram of the chick embryo heart, but that it is unsafe to comparemeasurements of the rate of rise of the R wave taken with an ampUfier with thecorresponding measurements taken without an amplifier.

HT +SCREEN

UP to+6OOV HT~

ELECTRODES

-Ir "-

OUTPUT

Fig. 1. Circuit diagram of amplifier.

THE MORPHOLOGY OF THE CARDIAC DEVELOPMENT.

The first aim of this- investigation was to correlate the changes observed in theelectrocardiogram with the morphological development of the chick's heart. Thelatter has been described by Lillie (1908) and in drawings by Murray (1926), andmy observations agree with theirs as regards all essential points.

My findings are summarised in the series of diagrams shown in Text-fig. 2 aand b. One particularly obvious morphological change is the appearance of theauricular buds which occurs between the 25-somite and 30-somite stages {i.e.between 48 and 60 hours).

THE ELECTROCARDIOGRAM IN THE EARLIEST STAGES.

The chick's heart can be easily recognised at about 30 hours (10 somites), but Iwas unable to obtain any evidence of electrical variation at this age, moreover nocontractions could be detected by microscopic examination.

Occasional contractions were observed at about 33-35 hours and regular con-traction was established at 40 hours (15 somites). Many other workers have foundthat regular contractions first occur at the 15-17-somite stage (Wieman, 1907, andCongdon, 1918). The latter observer concluded from histological evidence that anirregular primitive form of contraction occurred at the 10-somite stage. The electrical

288 J. YULE BOGUE

variation even at the 15-somite stage consists of a rapid and a slow component as isshown in PI. I, fig. 1 a. The figure shows the slow component as an S-shapedvariation. Records at this stage were difficult to obtain, but at 45 hours (18-20somites) it was relatively easy to obtain a satisfactory record such as is shown inPI. I, fig. 1 b. This figure shows a diphasic rapid variation followed by a slowvariation. It is uncertain whether the latter is monophasic or diphasic.

W/r - - 1

2

A3.10S.

32 HOURS15 S.

40HOURS18-20 3.45 HOURS

36 S. 72 HOURS

DORSAL VENTRAL

2--

RA--V-- --\--i.-i

--I--LV

t 0-25mnv <

25 S. 48 HOURS 30 S. 58 HOURS

Fig. 2 a.

40S. 96 HOURS

Fig. 2 b.

Fig. 2. Electrical records obtained at various stages of development of the chick embryo heart.10 S, etc., denotes the number of somites. Hours denote period of incubation. 1, ventral aorta.2, primitive cardiac tube. 3, vitelline vein. 4, developing atrium. 5, developing ventricle. 6 and 7,left and right auricular buds. LA and RA, left and right auricles. LV and RV, left and rightventricles. BA, bulbus arteriosus. Diagram at 72 hours partly after Lillie.

Wertheim-Salomonson (1913) and Cluzet and Sarvonat (1914) described theelectrical response of the chick heart commencing between 50 and 60 hours as aslow variation without a rapid component, but my results show that prior to thisstage both rapid and slow components are present in the electrical response. Indeedin the earliest records obtained both components were clearly shown to bepresent.

The Electrocardiogram of the Developing Chick 289

THE APPEARANCE OF THE P-WAVE.

The chick's heart develops rapidly between 48 and 60 hours (25-30 somites).At 48 hours it is a simple convoluted tube, whilst at 60 hours the auricles, ventricleand bulbus can be distinguished. The electrical records show corresponding changes.PI. II a shows a photograph of the heart shortly before differentiation (48 hours), andthe electrical response is the same as that shown in PI. I, fig. 1 b. The appearance ofthe heart at just over 60 hours is shown in PI. II b, which shows the auricular bulband the thickened ventricle wall. PI. I, fig. 1 c shows the electrical response obtainedfrom the heart photographed in PI. II b. The P and R waves are both clear and theP-R interval is about 0-12 sec.

THE ELECTROCARDIOGRAM AFTER 70 HOURS.

The electrical record of the chick embryo heart at 70 hours is essentially the sameas that of the adult hen. This is shown by the series of records in PI. I, fig. 2.

TIME RELATIONS IN THE EMBRYONIC AND ADULT HEART.

These relations are shown in Table I, and in general the outstanding fact is thatthe time relations of the earliest electrical response show relatively little differencefrom those observed in the adult hen. For example the earliest P-R interval iso-1 sec. and in the adult this is 0-06 sec. These figures agree with those of previousworkers. Spadolini and Giorgio (1921) noted a P wave at the 50th hour, the P-Rinterval being o-1 to 0-15 sec. with a heart rate of 70 per minute, which is muchlower than normal. Wertheim-Salomonson (1913) noted a P-R interval of o-i sec.on the eighth day of incubation.

Age

40-hour embryo2-5-day embryo5-Say embryo5-day chickAdult cock

Length ofheartmm.

o-81-220

1404O-O

Table

Heartrate

120163230295320

I.P-R

intervalsec.

O - I I

0090060-06

Duration ofrise of R

9—1208-100

1004—60

90

Duration ofR-S-T complex

sec.

0-28-0-320'2O—O-2I0-21-0-240-24-0-330-15-0-18

It is remarkable that the P-R interval changes so little during development, inspite of the very great changes in size that occur. A similar relative constancy in theP-R interval in spite of enormous variations in size has been noted in mammals.Clark (1927) noted that there was remarkably little difference in the P-R intervalof mammals of varying size, for instance the P-R intervals of the elephant(2,000,000 gm.) and of a small bat (5 gm.) were respectively 0-3 and 0-03 sec. Inthis case, however, there were two variables, namely, pulse rate and size, for theelephant was 400,000 times the weight of the bat, but the heart rate of the elephantwas 40 per minute and that of the bat 660 per minute. The adult hen and the chickembryo provide an example of two hearts which differ greatly in size but in which

290 J. YULE BOGUE

the smaller heart has the lower frequency. The relative constancy of the P-R intervalin this latter case would appear to be due chiefly to the effect of two opposingvariables. The hen's heart is about 40 times as long as the embryo heart, but on theother hand conduction in the embryo is much slower than that in the hen. Fano andBadano (1890) measured the mechanical response of hearts of chick embryos at thethird day and found rates of conduction of from 36 to 11-5 mm. per second. Therate of conduction in the adult hen's heart is about 2000 mm. per second (Mangoldand Kato, 1914; Lewis, 1916). It would therefore appear that in the embryo hearta very slow wave of excitation takes about as long to travel a short distance as doesthe rapid wave in the adult hen to travel a long distance.

From Table I it will also be seen that both the duration of the R-S-T complexand the rate of rise of the R wave show a remarkable constancy during development.The measurement of the rate of rise of the R wave is subject to a certain amount ofdistortion by the amplifier, but it is very remarkable that similar figures were obtainedof this measurement in the earliest records and in the adult hen. The similarity canbe partly explained on the same lines as that advanced to explain the constancy ofthe P-R interval, but in general it appears very difficult to interpret them by any ofthe usually accepted theories regarding the nature of the electrocardiogram. TheQ wave was usually absent, this was noted by Kiilbs (1920) who measured theelectrical response of the chick embryo. He also stated that the R and T waves wereat first monophasic and later diphasic, but my records show diphasic waves from thecommencement. He also concluded that from the third day onward the electro-cardiogram was similar to that of the hatched chick.

The suggestion by Wertheim- Salomonson(i9i3)andCluzetand Sarvonat (1914)that the form of the electrocardiogram commenced as a simple wave which becamemore complex as development proceeded does not agree with my results. But usingthe same tension for the string as Wertheim-Salomonson, namely, 25 cm. for 1 milli-volt, it was possible to obtain similar curves. This suggests that the form of the curveobtained by these workers was due to the employment of a very loose string.

DISCUSSION.

Two problems deserve consideration, firstly the relation between the generalform of the electrocardiogram of the embryonic heart and the microscopic structureof the heart cells, and secondly the time relations observed in the embryonic andadult hearts. The earliest records that I have obtained (15-somite stage) containa rapid and a slow component, whilst records as early as the 30-somite stage (60hours) show a P-R-T complex that is essentially similar to the electrical record ofthe adult. Wieman (1907) made a careful study of the histology of the chick embryoheart, and his conclusions have been confirmed by Congdon (1918). He found thatno fibrils were present before the fifth day (120-130 hours), after which fibrilsdeveloped rapidly and were fully developed by 140 hours. He noted that therewas little difference in the appearance of the cells from 30 hours (15 somites) to72 hours (36 somites).

JOURNAL OF EXPERIMENTAL BIOLOGY, X, 3. PLATE I.

(b)

(0

Fin. 1.

(6)

Fig. 2.

BOGUE—THE ELECTROCARDIOGRAM OF THE DEVELOPINGCHICK (pp. 286—292).

JOURNAL OF EXPERIMENTAL BIOLOGY, X, 3. PLATE II,

BOGUK—TIIK ELECTROCARDIOGRAM OF THE DEVELOPINGCHICK (pp. 286—292).

The Electrocardiogram of the Developing Chick 291

The heart of the chick embryo therefore shows a P-R-T complex essentiallysimilar to that of the adult fowl at a period (60 hours) long before any fibrils haveappeared, and when the cells show little difference from the histological appearanceseen when contraction is just commencing.

The form of the electrocardiogram is therefore not dependent on the develop-ment of fibrillary structure.

The relation between the electrical response and the presence of a syncytialstructure is equally obscure. Wieman describes the heart cells commencing asdiscrete cells, which gradually elongate and begin to interlace and form a syncytium-like structure at about 130 hours. He mentions that in the adult heart it is difficultto define the cell boundaries, but that no structure analogous to the protoplasmicbridges of the mammalian heart tissue are seen. My records show therefore that anadult type of electrical response is developed by the heart of the chick embryo notonly before any fibrils have appeared but also before there is any evidence for theformation of a syncytium.

These conclusions can indeed be deduced from the work of previous authors,Kiilbs (1920) and Spadolini and Giorgio (1921), who did not use an amplifier, forthey found the adult type of electrocardiogram on the third day, which is two daysearlier than the appearance of fibrils or syncytial structure.

SUMMARY.

Electrical records of the heart of the chick embryo can be obtained at 40 hours(15 somites). These earliest records show a fast and a slow component making upan R-T complex. The electrocardiogram of the third-day embryo is practicallyidentical with that of the adult. The development of fibrils and of syncytial-likestructure have been shown, however (Wieman, 1907), to appear first at the fifth dayand therefore the characteristics of the electrical record of the chick's heart cannotdepend on the presence of these structures.

The time relations of the earliest electrical records show a remarkable resemblanceto those found in the adult hen although the adult heart weighs 14 gm. and the earlyembryonic heart not more than 1 mg.

I wish to take this opportunity of thanking Prof. Clark for his invaluable helpand criticism throughout the course of this work. Also to thank Dr Gray of theZoology Department for his help in preparing the diagrams and in mounting andphotographing the embryos. Part of the expenses of this research were defrayed bygrants from the Moray Fund and from the Ministry of Agriculture and Fisheries.

292 J. YULE BOGUE

REFERENCES.

BOGUE, J. Y. (1932). J. Exp. Biol. 9, 351.CLARK, A. J. (1927). Comparative Pkytiology of the Heart, p. 50. Cambridge University Press.CLUZET and SARVONAT (1914). J. Phytiol. Path. gin. 16, 802.CONGDON, E. D. (1918). Anat. Rec. 15, 135.FANO, G. and BADANO, F. (1890). Arch. ital. Biol. 13, 387.KOLBS, F. (1920). Beitr. chem. Phytiol. 1, 439. Quoted from Ber. ges. Phyriol. (1920), 2, 554.LEWIS, T. (1916). Pkilot. Trans. B, 207, 221.LILLIE, F. R. (1908). The Development of the Chick, pp. 119 and 348. New York: Henry Holt and Co.MANGOLD, E. and KATO, T. (1914). Pfltigers Arch. 160, 91.MURRAY, H. A. (1926). Journ. Gen. Phytiol. 9, 603.SPADOLINI, D. I. and GIORGIO, A. (1921). Arch. Fitiol. 19, 479.WERTHEIM-SALOMONSON, J. K. A. (1913). Pfliigert Arch. 163, 558.WIEMAN, H. L. (1007). Amer.J. Anat. 6, 191.

EXPLANATION OF PLATES.

PLATE I.Fig.1. Electrocardiograms of the chick embryo, (a) 40 hours of incubation. (6)48 hours. (e)63 hours,shell opened. Time in 02 sec. (a), (A) and (c) read left to right.Fig. 2. Electrocardiograms of the developing chick from 72 hours onwards, (a) 72 hours of incuba-tion, embryo exposed. (6) 100 hours, (c) 8 days after hatching, (d) Adult cock. Time in 02 sec.(a), (6) and (c) read left to right; (d) right to left.

PLATE II.

(a) 48-hour embryo heart, total length of tube 143 mm. (6) 63-hour embryo heart, length of heart131 mm.