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The persisting right sixth aortic arch of mammals with a note on fetal coarctation.

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Department of Anatomy, University of Alberta, Edmonton, Alberta, Canada
Present day thoracic surgery is stimulating an interest in
the right sixth or pulmonary aortic arch. Edwards ( '48a) has
pointed out that this transient arch may persist as an impervious or slightly patent cord that may behave as a traction
band and deform the aorta. I n addition, the right sixth arch
may survive as a well developed functioning vessel and alter
the postnatal arterial pattern. At least 16 instances of this
sort have been recorded f o r man, not counting examples found
with a right-sided aorta and in situs inversus. Such a right
sixth arch recently turned up in the field of a heart operation
(Blalock, '48). A persisting functioning right sixth aortic
arch may be another of those rare anomalies that are not so
rare, and which have practical significance.
I n the course of an extensive study of abnormal embryonic
pig hearts (Shaner, '54) I found 17 embryos with the right
sixth aortic arch present after it should have disappeared.
All but one were embryos of from 20 to 50 mm C. R. length roughly equivalent to the third imonth in human development.
Many of the sixth arches in these embryos would doubtless
have disappeared later, but a few had established themselves
as vigorous vessels and had entered into unusual combinations with arteries derived from other arches ; combinations
which foreshadow those already recorded in man. A discussion of such abnormal right sixth arches in the pig, of the
arterial arrangements they enter into, and of the factors involved is the subject of this paper.
When is a right sixth arch abvzornzal.9 The kind of abnormal right sixth arch I am dealing with is not a vestigial cord,
but a well developed vessel functioning after it should have
disappeared. To distinguish such a vessel, one must review
the normal transfiguration of the aortic arches. A good starting point is the arch system of a 13-mm pig embryo (fig. 1).
At this stage there are four pairs of arches, pairs 111,IV, VT,
and traces of V. From the roots of the third pair spring the
external carotids; from the sixth sprout the tiny pulmonary
arteries. The original symmetry of the arch system has already been altered. The truncus is divided into an aorta
for the third and fourth pairs of arches, and a pulmonary
trunk for the sixth pair. The left fourth and sixth arches are
larger than the right ones. The left dorsal aorta is increasing
at the expense of the right. I n both aortae the segment between the third and fourth arches is degenerating. The tiny
pulmonary arteries are fused, preparatory to taking common
origin from the left sixth arch - a peculiarity of the pig not
found in man (Bremer, '01).
The symmetrical arrangement of the arches is further altered by the descent of the heart into the chest and the dextral twist of the conus. I n a 16-mm embryo (fig. 2) the left
sixth arch and the pu1,monary trunk are drawn out into a
straight vessel, about which is wrapped a spiral vessel made
up of the left fourth arch and the ventral aorta. The two
111, IV, V, VI, Aortic arches
A., Ventral aorta
A.d.d., s., Right and left dorsal aorta
B., Brachiocephalic artery
C., Locus of fetal coarctation
C.c.d., s., Right and left common carotid
C.e.d., s., Riglit and left external carotid
C.i.d., s., Right and left internal
D.a., Ductus arteriosus
In., Innominate artery
P., Pulmonary trunk
P.d., s., Right and left pulmonary
Sc.d., s., Right and left subclavian
T.,Truncus arteriosus
Pd. s.
I? d.+v I
Figures 1-5A
Fig. 1 Aortic arches of a normal 13-mm pig embryo. The right third, fourth,
and sixth arches are distinguished by crosses, circles, and striations, respectively.
From a wax reconstruction of the vessel cavities. X 131..
Fig. 2 Aortic arches of a normal 16-mm pig embryo. The right third, fourth,
and sixth arches are distinguished as in figure 1. From a wax reconstruction of
the vessel cavities. X 134.
Fig. 3 Aortic arches of a slightly abnormal 22-mm pig embryo, A.E.C. 927.
The right third, fourth, and sixth arches distinguished as in figure 1. From a wax
reconstruction of the vessel cavities. X 204.
Fig. 4, 4 A Persisting right sixth arch in a 10-month infant. 4 is from Kelsey
et al. ('53). 4 A is a diagram to explain the Kelsey ewe. Arches are marked
as in figure 1.
Fig. 5 , 5 A Distal segment of a right sixth arch forming the root of the
right pulmonary artery in a &day infant, after Ambrus ( '36). 5 A is a diagram
to explain the Ambrus case. Arches distinguished as in figure 1.
vessels are now the main outlets of the heart, are much enlarged, and pour blood into an enlarged left dorsal aorta. The
other parts of the arch system are reformed around these two
master vessels. The two third arches are now the first parts
of the internal carotids. Their roots proximal to the external
carotids are drawn out into common carotid arteries, as
Heuser ('23) has so well illustrated. The segments of the
dorsal aortae between the third and fourth arches are further
attenuated preparatory to atrophy. The remaining vessels
-the right fourth and sixth arches and the distal part of the
right dorsal aorta -are reduced to the status of competing
sources for the right subclavian artery.
I n normal pig embryos the right sixth arch drops out after
the 16-mm stage and the distal segment of the right dorsal
aorta at the 20.7-mm stage (Heuser, 23) ; thereafter the reduced and-foreshortened right fourth arch alone supplies the
right subclavian. The same arrangement is reached in an
18-mm human embryo ( Congdon, '22).
A right sixth arch may therefore be considered abnormal
when it occurs in an embryo much over 20 mrn in length. Additional anomalies within the heart would confirm such a conclusion.
Simple rete.vltion of t h e right sixth arch. An example of an
early abnormal right sixth arch is shown in figure 3, from a
22-mm pig embryo. The sixth arch can be recognized by its
course and a peculiar covering of light staining tissue which
it shares with the ductus arteriosus. This right sixth arch is
admittedly a borderline case mhich may he either abnormal
or just delayed normal. There are, however, other slight
anomalies : incomplete rotation of the conus, rather late persistence of the distal part of the right dorsal aorta, and a left
subclavian that arises farther down the aorta than it should.
Such a functioning right sixth arch is one of the anomalies
found in a 10-month human infant by Kelsey et al. ('53),
whose figure is reproduced in figure 4. The heart of the Kelsey
case suffered from complete transposition of the aorta, open
interventricular foramen, and basal pulmonary stenosis. The
human right sixth arch gives rise to the right pulmonary
artery; it does not lose the artery as does the pig. The arch
became indispensible with the closure of the ductus arteriosus.
Nature performed a Blalock operation, using the right sixth
arch instead of the usual subclavian.
The vessel encountered by Rlalock ( '48)is probably another
functioning right sixth arch, although its origin could not be
observed. It was large enough to graft into the right pulmonary and to reproduce the Kelsey heart arrangement. Another instance of a functioning right sixth arch has been reported by McCullough and Wilbur ('44). Here the ductus
arteriosus was also patent. Several older examples with and
without a patent ductus arteriosus have been recorded by
Poynter ( '16).
ContiinuiNg right sixth arch with atrophy of its prox;imal
segmeNt. In man the retention of the entire right sixth arch
provides a double origin for the right pulmonary artery (fig.
4,4 A). Should the proximal segment of the sixth arch secondarily disappear, the right pulmonary artery would be supplied from the innominate artery through the iourth arch and
distal remnant of the sixth arch, and the condition found in
the Ambrus ( '36) case would result (figs. 5, 5 A). Similar
right pulmonary arteries arising from the distal segment of
the right sixth arch in man are recorded by Doering ( '14),
Miiller ( '27), and Jew and Gross ( '52). I n each instance there
are additional defects in other great vessels or in the heart
ContiNuing right sisth arch with loss of the innominate.
When the right sixth arch persists as a functioning vessel,
as in the pig embryo 'shown in figure 3, the arch forms a vascular loop with the aid of the innominate artery and right
fourth arch. The loop supplies the right subclavian and right
common carotid arteries. Such an arrangement of embryonic
vessels is unstable; some part of the loop tends to becotme a
stagnant vessel and drop out.
I n the abnormal 35-mm pig embryo whose vessels are shown
in figure 6, the innominate artery has disappeared. A vigorous
right sixth arch (identified by its course and relations) supplies the right subclavian through the usual segment of the
right dorsal aorta. It also supplies the internal and external
carotids through a long vessel that must be a right fourth arch
(with reversed flow) and a common carotid. A blind remnant
of the vanished innominate is preserved in the wall of the
ascending aorta.
The heart of this 35-mm pig embryo has extensive defects:
half-completed rotation of the conus, rudimentary semilunar
cusps, nonfusion of proximal and distal conus ridges, aorta
from the right ventricle, and patent interventricular foramen.
A rather similar arrangement of the great vessels occurs
in the 14-month infant described by Shapiro ( ’30). The heart
was very abnormal. The only vessel leaving it was a “solitary
truncus arteriosus” with three cusps and two reversed coronary arteries. A sketch of the great vessels, based upon
Shapiro’s photographs and text, is shown in figures 7, 7 A.
‘Two centimeters above the cusps,” writes Shapiro, “two
vessels were given off. One directly supplied the left lung.
The other promptly divided into an innominate artery and a
vessel led to the right lung. The left common carotid and
subclavian arteries arose at a slightly higher level.”
A comparison of figure 6 with figure 7 will bring out that a
true innominate artery is missing in the Shapiro case. What
Shapiro calls the “innominate” is really a persisting right
sixth aortic arch.
Continuing right sixth arch with loss of the right fourth
arch. Should the right sixth arch in figure 3 be a strong
vessel-stronger than it really is in this particular pig embryo-then the short fourth arch may become a stagnant
link between two vessels: one made up of the right sixth
arch and the right subclavian and the other of the innominate
and the right common carotid. Under such conditions the
right fourth arch might atrophy.
I have found just this condition in a 100-mm pig embryo
(fig. 8). The heart of this pig embryo is much deformed. The
aorta arises from the right ventricle, which is nearly divided
Cc.d. C.C.S.s c-.s.
’ In.= VI
7 T .
Figures 6-10 A
Fig. 6 Persisting right sixth arch in an abnormal 35-mm pig embryo, A.E.C.
1004. The right third, fourth, and sixth arches distinguished as in figure 1.
From a wax reconstruction of the vessel cavities. x 9 K,.
Fig. 7, 7 A “Innominate” artery from right pulmonary in a 14-month infant, from Shapiro (’30). Sketch based upon photographs and text. 7 A is a
diagram to explain the Shapiro case. Arches distinguished as in figure 1.
Fig. 8 Persisting right sixth arch in an abnormal pig embryo, A.E.C. 832.
The sixth arch is distinguished as in figure 1. Wax reconstruction of the vessel
cavities. X 5 %.
Fig. 9, 9 A Right subclavian artery from the pulmonary i n a fetal pig. After
%egg (’46). 9 A is a diagram t o explain the Gregg case. The arches are distinguished as in figure 1.
Fig. 10, 10 A Persisting right sixth arch and retroesophageal right subclavian
i n a 17-month infant. After Edwards ( ’48b). 10 A is a diagram to explain the
Edwards case. The arches are distinguished as in figure 1.
in two by an hypertrophied moderator band ; in addition there
is a patent interventricular foramen. The right subclavian
arises from the pulmonary trunk through what is clearly a
persisting right sixth arch.
Such an anomalous right-sixth-arch-subclavian seems to be
a favorite with pigs. I have found a second case in a 254mm
embryo, and possibly two more of 35 and 40 mm, but the vessels were trimmed too closely for certain identification. Gregg
( '46) has described the same arrangement in a 21-ern pig. His
figure is reproduced in figure 9. The heart in Gregg's case
is normal. Another example from a new-born pig is given by
Kitchell and Stevens ( ' 5 3 ) ; the condition of the heart is not
mentioned. No human case is known to me. The closest
parallel in man are three cases of a left subelavian arising
from the left pulmonary artery, cited by Ghon ('08).
As is well known, the most distal part of the right dorsal
aorta - the part distal to the right subclavian in figure 3 may survive and act as the first part of a retroesophageal
right subclavian. Should this be added to the Gregg scheme,
then the vascular arrangement found in a 17-month infant by
Edwards ('48b) results. Edwards' figure is reproduced in
figure 10. The vessel linking the right pulmonary with the
subclavian is a persisting sixth arch and a length of right
dorsal aorta. The retroesophageal root of the right subclavian
is very short. The heart is normal; the only other recorded
anomaly is an imperforate anus.
The cause of the unusual vessel arrangements I have described seems to be the abnormal retention of the transient
right sixth arch followed by secondary changes in other vessels. The retention of the arch was probably the consequence
of a general slowing down in embryonic growth when the arch
was flourishing. That some general disturbance in growth
did take place, the anomalies in the heart of nearly every embryo testify. Under these conditions, the right sixth arch
would tend to persist. When growth is resumed after a stoppage, the normal sequence of events is not necessarily followed ; disappearing structures 'may acquire unnatural dominance and survive, as Stockard ( ,Zl) pointed out long ago.
Once established, the right sixth arch together with the anomalies within the heart must upset the hemodynamic forces
within the arch system. Normally important vessels, such as
the right fourth arch and the innominate artery, might become
stagnant vessels and disappear. Such degeneration would be
the passive result of disuse ; no active obstruction is needed.
Fetal coarctatio9z. Nevertheless, a reading of the stimulating paper by Bremer ('48) led me to some observations on
norlmal pig embryos which suggest that an active obstructive
growth into the riglit dorsal aorta between the fourth and
sixth arches - at the point C in figures 9 9and 10 A -might
be accessory to the obliteration of the fourth arch in the Gregg
and Edwards cases.
Bremer, following Bonnet ( '03), distinguished between
adult coarctation of the aorta opposite the mouth of the ductus arteriosus, and fetal coarctation situated a bit upstream.
Both writers thought that fetal coarctation required some
definite growth into the aorta. Bonnet suggested that the degeneration of the transient fifth arch extended into the aorta
-at the point x in figure 11. Bremer found in rat embryos
some indication of a growth into the aorta from the degenerating segment of the dorsal aorta between the third and
fourth arches-at
the point y in figure 11. I n addition,
Bremer found the ingrowth on both sides of the embryo, and
affecting both dorsal aortae.
So far as pig embryos are concerned, I find some evidence
that the distal end of the fifth arch is the site of such a growth
into the dorsal aorta - as Bonnet thought -and further that
the growth occurs into both dorsal aortae, as Bremer found
in rat embryos. To begin with, pig embryos show considerable traces of a transient fifth arch. A complete arch is rare,
but an ephemeral vessel arising from the fourth arch and ending in the dorsal aorta between the fourth and sixth arches
is common (fig. 11). Similar traces of a fifth arch appear in
human embryos. A section through the distal ends of the last
three arches on the left side of a 13-mm pig embryo (fig. 11L)
shows a peculiar creasing of the aortic wall around the mouth
of the degenerating fifth arch. The lateral crease, marked by
an arrow, becomes an indentation in a 16-mm eimbryo (fig.
12 L), and then a well marked fold in older embryos (fig. 13).
The fold becomes a pseudovalve over the outlet of the ductus
arteriosus as the ductus enters the aorta obliquely on its
lateral aspect. A similar structure has been found in the fetal
guinea pig by Kennedy and Clark ( '41), and in the rabbit
and dog by Hamilton et al. ( '37).
Fetal coarctation appears early. I have found it in a 25-mm
abnormal pig embryo. An example from an abnormal 35-mm
pig is shown in figure 14. The narrowing develops just proximal to the fold just described. It seems reasonable that the
fold has solmething to do with the fetal narrowing of the left
dorsal aorta.
Furthermore - and this is of immediate concern -the
same creasing and indentation appears on the right side of
the same embryos (figs. 11R, 12 R). I n normal embryos,
where the sixth arch is disappearing at the same time, a true
fold cannot develop. Rut in abnormal embryos with a persisting right sixth arch the fold could grow, block the aorta
and obliterate the very short right fourth arch. Such a right
sided coarctation combined with a persisting sixth arch would
explain the vessel pattern found in the Gregg case (fig. 9 A ) ,
and in the Edwards case (fig. 10 A).
The segment of the dorsal aorta between the fourth and
sixth arches must be rather unstable on both sides of the embryo. The fifth arch drops off its ventral aspect. The nearby
segment of the aorta between the third and fourth arches
withers away. I n addition the left aorta suffers from a general colmpression (Barry, '51) and a migration of the left
subclavian over it. Anomalies might be expected in a vessel
undergoing such a transformation.
A simple narrowing of the right dorsal aorta where the
fourth and sixth arches join it would produce the peculiar
isthmus in the right subclavian described by Love and Holms
( '39). The figure of Love and Holms is reproduced in figure
i II
Figures 11-16
Fig. 11 Sections through the distal ends of the fourth, fifth, and sixth arches
of a 13-mm normal pig embryo. Arrows point to lateral indentations. X 27.
Fig. 12 Sections through the distal ends of the fourth and sixth arches of
a normal 16-mm pig embryo. Arrows point t o lateral indentations. X 27.
Fig. 13 Section through the distal ends of the left fourth and sixth arches
of a normal 54-mm pig embryo. Arrow points to lateral fold. x 13%.
Fig. 14 Fetal coarctation on the usual left side of a 35-mm a.bnorma1 pig
embryo, A.E.C. 842. X 113.
Fig. 15 Fetal coarctation on the right side of the embryo affecting the first
part of the right subclavian artery. Adult form of coarctation also present on
left side. Adult case, after Love and Holms ('39).
Fig. 16 Diagram of aortic arches to show how fetal coarctation on the right
side ooould prodGce retroefiophageal right subclavian artery.
15. This constricted part of the right subclavian is derived
from the old fourth arch and a bit of the dorsal aorta. A
more severe coarctation of the right dorsal aorta might cut
off the right subclavian from the fourth arch entirely and
compel the artery to arise from a persisting distal segment of
the right aorta, thus producing the well-known retroesophageal right subclavian (fig. 16).
1. The right sixth aortic arch may persist as a functioning
vessel and enter into unusual combinations with the great
vessels above the heart. Embryonic examples froim abnormal
pig embryos are described and correlated with neonatal and
adult examples in man and the pig.
2. Such abnormal vessel arrangements could be produced
by an early developmental arrest leading to the retention of
the right sixth arch, and followed by disuse atrophy of those
parts of the aortic arch system that have become stagnant
segments under the unusual hemodynamic conditions.
3. Normal fetal coarctation of the left aortic arch map
arise from several causes ; in the pig it appears behind a fold
in the dorsal aorta at the mouth of the transient fifth aortic
4. A like fold begins on the right dorsal aorta. Either
alone, or in co.mbination with a persisting right sixth arch, the
fold may be a contributing cause for the unusual vessel arrangements of the region.
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note, mammal, persisting, arch, coarctation, aortic, right, sixth, fetal
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