Reprint from SURGERY, GYNECOLOGY AND OBSTETRICS, May, 1050, Vol. 90, 368-579

 

ARTERIAL HOMOGRAFTS

I. The Fate of Preserved Aortic Grafts in the Dog

HENRY SWAN, M.D., F.A.C.S., HOWARD T. ROBERTSON, M.D., and
MARVIN E. JOHNSON, M.D., Denver, Colorado

HE fact that fresh and preserved ar-

terial homografts properly transplant-

ed may be expected to function in-

definitely was demonstrated experi-
mentally over 40 years ago by Carrel and
Guthrie. Largely because of technical pit-
falls, however, arterial homografts in the
human have never been extensively used.
The need for an effective method for bridging
a gap in major blood vessels was brought into
sharp focus during the recent war, and cur-
rent advances in the field of vascular surgery
have continued to stimulate research on this
problem. It has become increasingly appar-
ent that vein-lined tubes and plastic or metal
conduits are not a satisfactory solution, and
since the war renewed interest has been
aroused in the potentialities of the arterial
homograft.

It was our purpose in this study to investi-
gate by histological methods the fate of stored
aortic homografts in the dog, and to evaluate
the effect of the duration of storage upon the
biological and functional properties of the
transplanted vessel.

METHOD OF STORAGE OF GRAFTS

If a blood vessel ‘‘bank”’ is to be practical
for widespread use in either civilian or mili-
tary hospitals, a method of storage must be
devised which is simple and requires little or
no specialized or complicated equipment or
solutions. For this reason, we began by using
as a storage medium commercially obtained
. sterile Ringer’s solution to which was added
ro per cent by volume freshly prepared sterile
dog serum. This solution proved to be quite
successful, and grafts so stored would remain
in an excellent state of preservation for periods

From the Halsted Experimental Laboratory and the Depart-
ment of Surgery, University of Colorado Medical Center, Den-
ver, Colorado.

Presented in part in the Forum on Fundamental Surgical
Problems before the Clinical Congress of the American College
of Surgeons, Chicago, October 17 to 21, 1049.

Copyright, 1950, by The Franklin N, Martin Memorial Foundation

up to 6 months. At first penicillin and strep-

tomycin were added, but in the later phases of |

our work we found this to be unnecessary if
careful sterile precautions were observed in
the preparation of the solution. We had no
instance of known bacterial contamination oc-
cur in our laboratory.

The entire descending aorta from just be-
yond the arch to just above the bifurcation
was removed under sterile precautions within
1 to 4 hours after death of the donor animal.
Each efferent artery was carefully ligated by
a simple tie of No. ooo silk as close to the wall
of the aorta as possible. The entire artery was
then placed in a sterile Mason fruit jar con-
taining 450 cubic centimeters of the storage
solution, which was tightly stoppered, and
kept in a common electric refrigerator at a
temperature of 3'to 8 degrees C. By observing
care, this vessel could be used serially for
several transplants replacing the vessel into
the original bottle between implantations.

Pierce and his co-workers have recently re-
ported their careful studies on various meth-
ods for the storage of vascular segments. They
concluded that the use of a balanced salt solu-
tion containing 10 per cent plasma in stop-
pered bottles maintained at a temperature of
6 degrees to 11 degrees was the most satis-

_factory method studied. We believe, however,

that the carefully evolved but complex formu-
la for buffered salt solution recommended by
Pierce is not necessary, and that commercial
Ringer’s solution is quite adequate. ‘

Our method, which proved to be eminently
satisfactory, requires no agents or materials
not available in any civilian or military hospi-
tal, up to and including the field hospital.

FACTORS OF IMPORTANCE IN THE SUCCESSFUL
TRANSPLANTATION OF STORED HOMOGRAFTS

Gross and his co-workers have stressed three
factors which might influence the success or
2 SURGERY, GYNECOLOGY AND OBSTETRICS

 

Fig. 1.1.a, The graft stored 14 days has been in place for
3 days. b, At the suture line the tiny wedge-shaped throm-
bus has formed. c, The graft shows the absence of intima,

failure of an arterial graft: (1) the viability of
the vessel at the time of implantation; (2) the
degree of immunological reaction between the
host and the graft; and (3) the technique em-
ployed in making the anastomosis.

Insofar as the immediate (i.e. up to 1 year)
functional result of arterial homografts in the
dog is concerned, we believe the last named
factor is the one of prime importance and the
first two are of minor importance. The ‘‘via-
bility” of the vessel will be discussed at some
length in the following section of this report;

1In all the figures in this paper, a is a photograph of the gross
specimen, opened in the axis of the vessel, and viewed from the
intimal side; b is a photomicrograph of the anastomosis (hema-
toxylin and eosin stain) with the graft always on the right and
the host aorta on the left; c is a photomicrograph of the graft

(hematoxylin and eosin stain); and d is the same section as c but
with elastic tissue stain. All magnifications are & 45.

essentially normal appearing media and acute inflamma-
tion in the adventitia. d, The elastic tissue appears to be
normal.

the degree of immunological reaction which
may be great in heterogenous transplants, has
never appeared to have been of sufficient de-
gree in our experience with homogenous trans-
plants to jeopardize the survival of the graft.
The immunological factors relating to non-
autogenous grafts comprise a subject worthy
of extensive investigation. However, as far as
the functional success of the experimental
homogenous arterial transplant is concerned,
this has not appeared to be a decisive factor.
Technical aspects of the actual implanta-
tion of the vessel, on the. other hand, appear
to be of paramount importance. The well-
established techniques of arterial anastomosis
must, of course, be carefully observed. Rigid
asepsisy delicacy in the handling of tissue,

 

 

 
 

SWAN ET AL.:

Fig. 2. a, The graft stored 21 days has been in place 14
days. b, The wedge-shaped thrombus shows invasion by
fibroblasts and is beginning to extend down along the in-
ternal surface of the graft. c, Only a few of the smooth

avoidance of desiccation, the use of fine
atraumatic needles with fine silk, the careful
removal of the adventitia, and the avoidance
of too large or too small a cuff are all matters
of importance. We routinely used a series of
continuous everting mattress sutures resulting
in an intima-to-intima closure; other types of
suture for effecting anastomosis may be equal-
ly effective (5).

The use of grafts, however, brings up other
considerations than merely the technique of
anastomosis. Of supreme importance in our
experience is the size or diameter of the graft
in relation to the diameter of the host vessel.
The aorta when it is removed tends to con-
tract somewhat. There does not appear to be

ARTERIAL HOMOGRAFTS 3

 

muscle cells of the media remain in this specimen. On the
inner surface is a very thin plaque-like thrombus. The

adventitia is now formed of granulation tissue. d, The

elastic tissue appears normal.

further shrinkage during the time of storage.
The capacity for distention remains as a func-
tion of the vessel, and upon being subjected to
intraluminal pressure when implanted in the
host will stretch to its previous in vivo diam-
eter.

The creation of a vessel in continuity of uni-
form diameter under the hemodynamic condi-
tions of blood flood is a law of vascular trans-
plantation if uniformly successful results are
to be expected. If the graft is allowed to ex-
ceed in diameter the host vessel, an aneurysm-
like bulging occurs which results in turbulence
and eddy currents in the flow of blood through
the irregular channel. Intravascular throm-
bosis is favored, and clots may form usually
4 SURGERY, GYNECOLOGY AND

Fig. 3. a, The graft stored ro days has been in place for
30 days. b, The wedge-shaped thrombus at the suture line
has become further organized and has extended beyond
the field down the internal side of the graft. Endothelial-

just distal to the proximal anastomosis grad-
ually extending both distally and proximally
until the lumen is obstructed and its value as a
conduit ceases. Occasionally, degenerative
changes in the graft occur under these condi-
tions, with breakdown of tissue and secondary
hemorrhage.

In choosing a suitable section of the stored
aorta for implantation therefore, a segment
should be selected which is slightly smaller in
diameter than that of the normally pulsating
host aorta. Under no circumstances is it wise
to use a segment which is slightly or definitely
larger than the host vessel. Care must then be
taken to turn up as small a cuff as possible in
order to avoid constriction at the suture line.

OBSTETRICS

 

like cells seem to line its surface. c, In this graft, smooth
muscle survival in the media is very abundant. The ad-
ventitia is now composed of dense scar tissue. d, The elas-
tic fiber pattern is relatively unchanged.

In the early phase of our work the importance
of this factor was not recognized, and no par-
ticular care was exercised in the choice of the
graft for size, with the result that in the first
few experiments thrombosis or hemorrhage
supervened in over half of the animals. After
realization of the phenomena involved, 36
aortic homografts have been performed with
only 4 failures irrespective of the duration of
storage of the graft.

EFFECT OF DURATION OF STORAGE UPON THE
FUNCTIONAL SUCCESS OF THE HOMOGRAFT

Gross and his co-workers (2, 4) studied the
effect of the duration of storage upon the
“viability” of the blood vessel by tissue cul-
SWAN et at.: ARTERIAL HOMOGRAFTS

TABLE I.—EXPERIMENTAL AORTIC HOMOGRAFTS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Duration Duration in
Dog No. storage of host before : Picturedia
graft sacrifice Results report
days days
CR ee Se a
49-64 7 5 : Successful graft; microscopic study
48-270 8 oy 240 eS Successful graft; microscopic study ae Fig. 6
49-3 8 7 ~| Successful graft; microscopic study i
49-109 = 8 45 oe Successful graft; microscopic study 2
8 Cae 8 ie ao Successful graft; microscopic study Fig. 5
a 48-351 ee 190 4 30 Successful graft; microscopic study Fig. 3
ees Pe ee 0 oe 3 Died; rupture proximal suture line (graft too big) oo
45-4 ay te ES 73 Successful graft; microscopic study oro en Fig. 1
48-344 eo 15 2 Successful graft; died of intussusception ee
40-77 ae wee I Successful graft; microscopic study ae
49-136 : 21 14 Successful graft; microscopic study Fig. 2
40-89 i a 2 go Successful graft; (spec. lost down sink intact 2 a 2 eed
iG aL ee oe | Loe “Successful graft; microscopic study ee
2 48-332 2 an len ee : Bucceatl graft; microscopic study ee
: Dato. aoe no Gee Successful graft; microscopic stidy Oe ee oo Hig
Taiea46 = a a6 = oe Successtal wentes microscopic study Lee 0 ae Et See
oe ee | iS 4 Series B 3
a ao a 3 = a a 5 : : a Bacchi graft; microscopic study ee
Oe 2 Lene cae 66 : | Successful graft; no photo obtained Z ee i
o gieacé oe ea oe Beek ao) Successful graft; microscopic study i eee Dig ee
#e 48-3 fe ae nb : ] i Tee ; Successful graft; microscopic study Pe ee ee
a igo ae ag oF ey Successful graft, dog stil] living (transplanted January 109, iea0) oe 2
E 40~21 ae Sa ae 7 : Successful graft; died of intussusception; microscopic study
: 497-1 <a ae : ae oy 60 ‘Successful graft; microscopic study ae
a eS are 65° 18 i Successful graft; microscopic study
49-4 64 180 Successful graft; microscopic study Fig..7
48-233 65 Successful graft; dog still living (transplanted January 26, 1949)
4Q-15 65 220 Successful graft; microscopic study
7 eS ey aes oS | Successful graft; dog still living (transplanted January 18, 1949)
S 48-110 70 | go Successful graft; microscopic study
49-16 1% 7 “Died; rupture of suture line
- 49-244 76 2 Died; rupture and thrombosis
49-26 81 Successful graft; dog still living (transplanted February 11, 1949)
49-23 84 4 Died; thrombosis, microscopic study
40-57 : 12i 42 Successful graft; animal killed in fight, microscopic study
49-115 180 | 75 Successful graft; microscopic study Fig. 8

 

 

ture methods. They found that “viability” as
measured by fibroblastic proliferation in tissue
culture media was enhanced by the storage of
a large segment of vessel and by the use of an
airtight stopper on the storage bottle. Under

these conditions, vessels stored in balanced
salt solution with 1o per cent serum showed a
high percentage of fibroblast viability up to 37
days. Thereafter, the percentage of such via-
bility appeared to decrease. Their series of ex-
6 SURGERY, GYNECOLOGY AND OBSTETRICS

Fig. 4. a, The graft stored 40 days has been in place for
60 days. This graft shows more extensive surface throm-
bosis than is usual. b, At the suture line, the continued
organization and differentiation of the wedge thrombus
has resulted in a fibrocellular layer which smooths the con-
tour of the lumen. c, The fibrocellular layer which grows

periments with storage of 50 days or more was
quite small (3 observations) and it is possible
that fibroblast viability may occasionally per-
sist for even longer periods of time. However,
in their experimental transplantations, 2 grafts
stored longer than 50 days failed to function
and became thrombosed. They conclude that
a graft is more apt to be successful if it is
“viable” when transplanted.

Our experience has led us to believe that the
immediate success of the properly transplanted
graft is not materially affected by the duration
of storage, at least for periods up to 6 months.
A series of aortic grafts stored from 4o to 180
days were implanted and allowed to remain in

 

in from the ends now covers the interna! surface of the
graft. In this specimen, no smooth muscle cells are seen
in the media. The adventitia is becoming hyalinized. d,
The elastic fibers appear very dense, possibly because with
the loss of smooth muscle cells the total thickness of the
media is diminished and the elastic fibers are more compact.

the host for various lengths of time before
sacrificing the animal for histologic examina-
tion. Nineteen such transplantations were
performed, with only 3 failures from throm-
bosis or hemorrhage. At the present time, a
graft stored 70 days has been functioning for a
period of ro months in an animal apparently
in perfect health. Table I is a summary of the
pertinent data of our series of aortic homo-
grafts.

To say that the duration of storage has
little apparent effect on the immediate success
of the homogenous arterial transplant is not to
say that it may not have a significant effect
upon the ultimate outcome. Histologic evi-
 

 

SWAN ET AL.:

Vig. 5. a, The graft stored 8 days has been in place for
120 days. b, At the suture line the fibrocellular layer has
smoothed the contour of the lmmen. The surface layer of
endothelium appears quite definite. Differentiation of the
fibrocellular layer is apparent. c, The media is thin and

dence suggests that there are differences in the
cellular structure of the resultant vessel be-
tween grafts stored for short periods of time
and those stored for long periods. The “‘old”
grafts demonstrate more pronounced degen-
erative changes earlier than those seen in
“young” grafts. The nature of these changes
is described in more detail in the following sec-
tion of this paper, but we wish to emphasize
that, although the immediate functional re-
sults using “‘old”’ grafts is equivalent to those
obtained using ‘‘young”’ ones, our period of
observation at this time (10 months) is too
short to allow opinion as to the ultimate fate
of these grafts, and there is reason to believe

ARTERIAL HOMOGRAFTS 7

 

d

compact. No cells are visible. The adventitia is a thick,
dense, hyalinized scar. d, The elastic fibers appear less
dense and the layer is thinner. It is apparent that new
elastic fibers have been formed in the layer of fibrocellular
tissue.

that “young” grafts (i.e. less than 40 days
storage) may ultimately prove to be more
durable.

HISTOLOGIC EVIDENCE ON THE FATE OF
THE ARTERIAL HOMOGRAFT

In our experiments, we used the abdominal
aorta just above the inferior mesenteric artery
as the recipient area. The aorta was mobilized
for a distance of about 4 centimeters, divided
between special broad-bladed noncrushing
clamps, and the ends allowed to contract. Into
the gap a graft 2 centimeters in length was
placed. This graft was carefully matched for
size as described, and therefore might be taken
8 SURGERY, GYNECOLOGY AND OBSTETRICS

 

Fig. 6. a, The graft stored 8 days has been in place for
240 days. b, The anastomosis is firmly healed. c, The fi-
brocellular layer shows further differentiation and some
degenerative changes. There is noticeable a spotty depo-
sition of calcium throughout the layer and there are

from the thoracic end of the donor aorta or the
abdominal end, as the host vessel demanded.
There are some minor histologic differences
between the thoracic and the abdominal aorta
in dogs relating chiefly to the thickness of the
media and the abundance of elastic tissue,
both of which are greater at the thoracic end.
This factor introduced some minor quantita-
tive variations in the resultant vessels but ap-
peared to have no qualitative effect.

We have not thoroughly investigated what
influence the length of the graft has on the
ultimate survial. A few pilot experiments led
us to believe that longer grafts survive and
function equally as well as short ones and this

some smooth muscle cells visible in the media. d, The
basal portion of the fibrocellular layer contains many
elastic fibers. The elastic tissue of the media shows
relatively normal architecture although the fibers are
somewhat thin.

was corroborated by a recent clinical exper-
ience in which one of us (H.S.) successfully
resected a mycotic aneurysm of the thoracic
aorta in a 17 year old boy and restored con-
tinuity of the vessel by the use of a stored
graft 8 centimeters in length. The histologic
data presented in this report, however, are all
derived from transplants approximately 2
centimeters long.

Since the fate of the 3 layers of the
aortic graft is apparently different, it seems
convenient to describe them separately. The
course of events is reconstructed from serial
observations made on grafts which had been
allowed to remain in the host from 1 to 240

 
SWAN ET AL.:

Fig. 7. a, The graft stored 64 days was in place for 180°

days.

\ large calcium plaque is visible in the center of the
graft.

b, The suture line is similar to those seen with

days. The fate of “young” grafts stored from
8 to 40 days is described first.

Grafts stored less than go days. 1. The
adventitia. Within 24 hours there is evidence
of an acute inflammatory reaction on the part
of the host evidenced by edema, fibrin deposi-
tion, extravasation of red blood cells, and in-
filtration of polymorphonuclear neutrophiles.
This reaction reaches its height on about the
third to fifth day. Degeneration of the donor
fibroblasts in the adventitial layer is appar-
ently extensive. Thereafter, healing begins by
the infiltration of lymphocytes and capillary
buds and fibroblasts. This granulation tissue
gradually becomes more fibrous and even-
tually, by the end of the second month, quite
collagenous. This whole process does not ap-

ARTERIAL HOMOGRAFTS

‘oO

 

younger grafts. c, The fibrocellular layer is well differenti-
ated and is sprinkled with calcium. No cells are visible in
the media. d, The elastic tissue is dense and compressed.

pear to differ in any significant way from the
usual reponse to inflammation and degenera-
tion, namely, healing by formation of scar tis-
sue. Eosinophiles and plasma cells are not in
evidence. This scar, for the first 3 months,
may be much thicker than the original layer,
but gradually thins out until by the sixth
month it approximates the thickness of the
original layer.

Thus, the adventitia of the donor vessel ap-
parently degenerates and is entirely replaced
by scar formation from the host. By the end
of the second month, or perhaps earlier, it ap-
pears to be capable of contributing strength to
the grafted segment.

2. The intima. At the end of 24 hours no
visible trace of the intima of the donor vessel
10 SURGERY, GYNECOLOGY AND OBSTETRICS

 

Fig. 8. a, The graft stored 180 days was in place for 75
days. b, The anastomosis is well healed. The fibrocellular
layer is thin in this section, yet in other areas, as seen in
d, is much thicker. c, The media shows no survival of
smooth muscle cells. There is diffuse and patchy deposition

remains. The luminal surface is apparently
formed by the internal elastic membrane of
the media. There is no evidence of inflamma-
tory reaction; occasionally patchy areas of
very thin thrombi occur on the internal elas-
tic membrane. A tiny wedge-shaped ring
thrombus appears in the angle where the graft
was everted for anastomosis. About the sev-
enth day a growth of fibroblasts appears in
this tiny thrombus, cells apparently derived
from the media of the host, although perhaps
from the intima. From these cells a pannus-
like growth progresses from each end of the
graft centrally forming a fibrocellular layer
which eventually, usually by the third month,

of calcium throughout the media of the stained section
which does not show well in this black and white photo-
micrograph. d, The elastic tissue appears very dense and
compressed. It apparently has remained almost totally
intact.

covers the entire luminal surface. This layer
is not perfectly uniform but varies in thickness
from a single cell in some areas to many cells
in others. It tends to obliterate the angle at
the anastomosis and to smooth the luminal
contour of the vessel at this point. It rarely
grows onto the recipient aorta’s intima for any
significant distance but usually becomes fused
to it at the arc of eversion. Thus, by the end of
the second month, the vessel has acquired
from the host a new intima, albeit somewhat
thicker and irregular.

Subsequently this layer shows remarkable
powers of differentiation. The surface cells
tend to become flattened and resemble endo-
eect
fees

SWAN ET AL.:

thelium. Immediately beneath this layer, the
cells have a foamy appearance. Hyalin matrix
is laid down and even new elastic fibrils are
formed. By the fifth month spotty areas of
calcification and some deposition of lipoids,
presumably cholesterol, may be observed.

It appears, therefore, that the donor intima
disappears entirely only to be replaced by a
fibrocellular layer derived from the host by
centripedal growth from the ends. This layer
by the fifth month shows both differentiation
and degeneration, and by this time may be
contributing to the total strength of the
grafted segment.

3. The media. There appears to be more
variation in the fate of the media than in that
of the 2 previously described layers, but
these differences seem to be merely quantita-
tive variations of a basic pattern. For a few
days, the media looks entirely normal. About
the fifth day the nuclei of the smooth muscle
cells begin to disappear as if the cells were de-
generating. Very little inflammatory reaction
accompanies this process, which continues un-
til about the end of the fourth week. There-

after, no further loss of smooth muscle appears -

to take place. The quantitative extent of this
loss varies from roo per cent in some grafts to
only an estimated so per cent in others. Cer-
tainly in some specimens, smooth muscle cells
“take” in abundance and survive for at least
8 months. By the end of the third week a
pink-staining hyalin material has appeared
diffusely throughout the media. The greater
the smooth muscle degeneration the greater
the hyalinization. This is probably hyalin de-
generation rather than the elaboration of a
fibrous tissue matrix as no fibroblastic pro-
liferation is apparent during this episode.
Meanwhile, the elastic tissue fibers survive
intact and continue to show little tendency to
degenerate or disappear. Because of the loss
of smooth muscle cells, the total thickness of
the media decreases and the elastic fibers are
compressed closer together. In extreme ex-
amples the media, after 4 months, consists
exclusively of densely packed elastic tissue em-
bedded in a hyalin matrix. At the opposite
extreme, where there has been good survival of
smooth muscle, the elastic tissue is only mod-
erately compressed, and the general archi-

ARTERIAL HOMOGRAFTS ir

tecture of the narrowed media appears almost
normal. Plaque-like calcium deposits appear
in the media by the fourth to sixth month in
vessels in which smooth muscle degeneration
has been extensive. In the presence of such
calcium depositions, elastic fibers may become
locally fragmented.

The healing between the media of the graft
with that of the host at the site of anastomosis
is accomplished by a thin layer of fibrous tis-
sue. The ends remain everted, and there is no .
tendency for the two to flatten out and become
a single layer in continuity. There is no fusion
of elastic tissue between the donor and the
recipient vessels.

Thus, in sharp contrast to the intima and
adventitia which are replaced by the host,
some or most of the media of the graft appears
to survive and remains relatively intact for at
least 8 months. The elastic tissue always sur-
vives and, in these ‘‘young”’ grafts, smooth
muscle may persist in varying degrees. Cal-
ciication occurs in inverse proportion to
smooth muscle survival. During the first 30 to
60 days, the strength of the graft must reside
primarily in the elastic tissue of the media.

Grafts stored longer than go days. The chain
of histological events in the adventitia and
intima of transplants which have been stored
longer than 4o days appears to be identical
with that previously described for “younger”
grafts. The media, however, differs in that
there is no survival of smooth muscle cells.
The smooth muscle degeneration seen in the
second, third, and fourth weeks progresses to
completion and the media is left as a thin
layer of compact elastic tissue in a hyalin
matrix. By the third or fourth month, calcium
deposition is commonly seen in patchy distri-
bution, and in these areas degeneration of elas-
tic fibers may be evident.

It is this evidence of early significant de-
generative changes in the media which should
lead to caution in the use of grafts stored
longer than 4o days in spite of their consistent
effectiveness as conduits of blood in the
months immediately following implantation.
We have several living animals both with
“young” and “old” grafts which we propose to
follow over a period of years in an attempt to
evaluate the outcome of these transplants.
+2 SURGERY, GYNECOLOGY AND OBSTETRICS

SUMMARY

1. Canine arterial (aortic) homografts can
be successfully stored in Ringer’s solution con-
taining 10 per cent homogenous serum in an
ordinary electric refrigerator at 3 to 8 de-
grees C.

2. For consistent successful transplanta-
tion of aortic homografts, the proper matching
in size of the donor and recipient vessels is a
factor of paramount importance.

3. The duration of storage (up to 6 months)
does not appear to be a significant factor gov-
erning the immediate functional success of the
transplant. However, microscopic evidence
suggests that degenerative changes appear
earlier and are more extensive in grafts stored
longer than 40 days than in those stored for a
lesser period.

4. Serial microscopic studies indicate that
the adventitia and the intima of the graft are

PRINTED BY R. R. DONNELLEY & SONS CO..

totally replaced by similar layers derived from
the host.

5. At least a part of the media of the graft
apparently survives. The elastic tissue in-
variably remains for periods up to 10 months.
Some smooth muscle cells in varying degree
survive in grafts stored less than 40 days; no
smooth muscle cells survive in grafts stored
longer than 4o days.

REFERENCES :

1. CARREL, A. The surgery of blood vessels. Bull. Johns
Hopkins Hosp., 1907, 18: 18-28.

2. Gross, R. E., Brit, A. H. Jr., and Pierce, E. C., 2nd.
Methods for preservation and transplantation of ar-
terial grafts. Surg. Gyn. Obst., 1949, 88: 689-701.

3. Gururiz, C. C. End-results of arterial restitution with
devitalized tissue. J. Am. M. Ass., 1919, 73: 186-187.

4. Pierce, E. C., 2nd, Gross, R. S., Brit, A. H., Jr., and
Merritt, K., Jr. Tissue-culture evaluation of via-
bility of blood vessels stored by refrigeration. Ann.
Surg., 1949, 129: 333-348.

5. Situ, S. Studies in experimental vascular surgery.
Surgery, 1945, 18: 627-043.

AT CHICAGO, ILLINOIS, U.S.A.