U.S. patent application number 09/735522 was filed with the patent office on 2001-05-03 for tissue decellularization.
This patent application is currently assigned to CRYOLIFE INC.. Invention is credited to Black, Kirby S., Goldstein, Steven.
Application Number | 20010000804 09/735522 |
Document ID | / |
Family ID | 25278218 |
Filed Date | 2001-05-03 |
United States Patent
Application |
20010000804 |
Kind Code |
A1 |
Goldstein, Steven ; et
al. |
May 3, 2001 |
Tissue decellularization
Abstract
The present invention relates, in general, to tissue
decellularization and, in particular to a method of treating
tissues, for example, heart valves, tendons and ligaments, so as to
render them acellular and thereby limit mineralization and/or
immunoreactivity upon implementation in vivo.
Inventors: |
Goldstein, Steven; (Atlanta,
GA) ; Black, Kirby S.; (Acworth, GA) |
Correspondence
Address: |
Nixon & Vanderhye P.C.
1100 N. Glebe Rd., 8th Floor
Arlington
VA
22201
US
|
Assignee: |
CRYOLIFE INC.
|
Family ID: |
25278218 |
Appl. No.: |
09/735522 |
Filed: |
December 14, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09735522 |
Dec 14, 2000 |
|
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|
08838852 |
Apr 11, 1997 |
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Current U.S.
Class: |
623/23.72 ;
435/378; 600/36; 623/918; 8/94.11 |
Current CPC
Class: |
A61L 2430/40 20130101;
A61F 2/2415 20130101; A61L 27/3687 20130101; Y10S 623/918 20130101;
A61L 2400/02 20130101; Y10S 623/915 20130101; Y10S 623/916
20130101; A61L 27/3604 20130101; A61L 27/3695 20130101; A61L
27/3683 20130101 |
Class at
Publication: |
623/23.72 ;
435/378; 8/94.11; 623/918; 600/36 |
International
Class: |
A61F 002/02; C12N
005/00; A61L 017/00 |
Claims
What is claimed is:
1. A method of producing a decellularized tissue comprising: i)
contacting a biological tissue with a hypotonic solution under
conditions such that lysis of cells of said tissue is effected, and
ii) contacting the tissue resulting from step (i) with nuclease
under conditions such that nucleic acid is degraded, said
decellularized tissue thereby being produced.
2. The method according to claim 1 further comprising contacting
the decellularized tissue with a physiologically isotonic
solution.
3. The method according to claim 1 further comprising fixing said
decellularized tissue.
4. The method according to claim 1 wherein said decellularized
tissue is at least 70% decellularized.
5. The method according to claim 1 wherein the tissue is a
mammalian tissue.
6. The method according to claim 1 wherein the tissue is a heart
valve, tendon, ligament, artery, vein, diaphragm, pericardium,
umbilical cord, facia, dura mater, tympanic membrane, or portion
thereof.
7. The method according to claim 6 wherein said tissue is a heart
valve.
8. A decellularized tissue produced according to the method of
claim 1.
9. The tissue according to claim 8 wherein said tissue is a heart
valve, tendon, ligament, artery, vein, diaphragm, pericardium,
umbilical cord, facia, dura mater, tympanic membrane, or portion
thereof.
10. The tissue according to claim 9 wherein said tissue is a heart
valve.
11. The tissue according to claim 8 wherein the tissue is at least
70% decellularized.
12. The tissue according to claim 8 wherein the tissue is
fixed.
13. A method of mitigating mineralization of a biological tissue
transplant comprising: i) contacting a biological tissue with a
hypotonic solution under conditions such that lysis of cells of
said biological tissue is effected, and ii) contacting the tissue
resulting from step (i) with nuclease under conditions such that
nucleic acid is degraded, wherein the tissue resulting from step
(ii) is decellularized and said mitigation is thereby effected.
14. The method according to claim 13 wherein mineralization is
mitigated by at least 30 percent relative to non-decellularized
tissue.
15. A method of reducing the immunogenicity of a biological tissue
transplant comprising: i) contacting a biological tissue with a
hypotonic solution under conditions such that lysis of cells of
said biological tissue is effected, and ii) contacting the tissue
resulting from step (i) with nuclease under conditions such that
nucleic acid is degraded, wherein the tissue resulting from step
(ii) is decellularized and said reduction in immunogenicity is
thereby effected.
Description
TECHNICAL FIELD
1. The present invention relates, in general, to tissue
decellularization and, in particular to a method of treating
tissues, for example, heart valves, ligaments and tendons, so as to
render them acellular and thereby limit mineralization and/or
immunoreactivity upon implantation in vivo.
BACKGROUND
2. Cardiac valve disorders can be serious and in fact are often
fatal. Treatment may require replacement of the valve with a
prosthetic valve--mechanical or bioprosthetic. Bioprosthetic valves
typically include a leaflet portion and a vascular conduit portion,
both generally of a biological material, and possibly a stent.
3. While bioprosthetic valves have a number of advantages over
mechanical valves, including a lower risk of complications
resulting from thrombus formation, they are associated with a
higher risk of mineralization. This increased risk significantly
limits the durability of the replacement valve. The present
invention provides a method of rendering tissues, including heart
valves, resistant to mineralization while preserving biomechanical
properties of the tissue. The present invention also provides a
method of reducing immunoreactivity of transplanted tissues which
are not fixed by chemical or physical means, or combinations
thereof, prior to implantation.
OBJECT AND SUMMARY OF THE INVENTION
4. It is a general object of the invention to provide a method
treating tissue so as to limit mineralization and/or
immunoreactivity post implantation.
5. It is another object of the invention to provide a method of
decellularizing a tissue and thereby enhancing its durability
and/or reducing its immunoreactivity.
6. It is a further object of the invention to provide a tissue, for
example, a heart valve bioprosthesis, that retains mechanical
integrity, is resistant to calcification and is characterized by
reduced immunoreactivity upon implantation.
7. The foregoing objects are met by the present invention which
provides a method of effecting decellularization of tissues,
including heart valve tissues (eg leaflets and valve associated
vascular conduit). The method comprises contacting the tissue to be
decellularized with a hypotonic solution under conditions such that
cell lysis occurs, and subsequently subjecting the tissue to
nuclease treatment under conditions such that the tissue is
rendered histologically acellular.
8. Further objects and advantages of the present invention will be
clear from the description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
9. FIGS. 1A and B show the effect of decellularization on the
extensibility of and elastic modulus of aortic and pulmonary
leaflets.
10. FIGS. 2A and B show the effect of decellularization on rates of
stress-relaxation of aortic and pulmonary leaflets.
11. FIGS. 3A, B and C show the effect of decellularization on
failure load, maximum stress and elastic modulus of aortic and
pulmonary leaflets.
12. FIGS. 4A, B, C and D show the effect of decellularization on
calcification of porcine heart aortic and pulmonary heart valve
tissues.
DETAILED DESCRIPTION OF THE INVENTION
13. The present invention relates, in one embodiment, to a method
of rendering a biological tissue acellular. The method comprises
exposing the tissue to a hypotonic solution under conditions such
that cell lysis results, and subjecting the resulting tissue to
nuclease treatment so as to remove nucleic acids and associated
phosphorous-containing groups which may bind calcium. Nuclease
treatment effectively stops cell replication and protein synthesis.
In a preferred aspect of this embodiment, the tissue is rendered
essentially acellular, the term "essentially" meaning having at
least 70% fewer cells than the naturally occurring biological
material. The extent of decellularization can be determined
histochemically, for example, by staining the tissue with
hematoxylin and eosin using standard techniques.
Immunohistochemical staining can also be used, for example, to
visualize cell specific markers such as smooth muscle actin and
histocompatibility antigens--an absence of such markers being a
further indication of decellularization.
14. In accordance with the present method, the biological tissue
is, preferably, first washed in a solution of a bioburden reducing
agent, such as an antibiotic. The tissue can then be decellularized
immediately or it can be cryopreserved. Cryopreserved tissue is
thawed prior to decellularization under conditions such that the
cryoprotectant is eliminated and toxicity resulting therefrom
thereby avoided. Appropriate thawing conditions are well known in
the art. The tissue (fresh or thawed cryopreserved) is then placed
in hypotonic solution in order to effect cell lysis. Appropriate
solutions include water or a solution having a solute (eg a salt
such as NaCl) concentration of up to 80 milliosmolar (for example,
a 10-20 or 20-40 mM NaCl solution). Lysis can be effected, for
example, at a temperature in the range of 30.degree.C. to
40.degree.C., preferably 37.degree.C., advantageously in an
atmosphere of 5% CO.sub.2, for example, for about 4 to 24 hours.
The tissue is then transferred to a nuclease solution (eg DNAase-
and/or RNAase-containing) and incubated, for example, at a
temperature in the range of about 30.degree.C. to 40.degree.C.,
preferably 37.degree.C., advantageously in an atmosphere of 5%
CO.sub.2, for example, for about 4 to 24 hours. Subsequently, the
tissue is transferred to a solution that can maintain tissue
structural integrity, for example, a physiologically normal
(isotonic) solution such as a cell culture medium, eg DMEM. Cell
lysis can continue during maintenance of the tissue in the
physiologically normal solution and thus the tissue can be removed
from the lytic/nuclease solutions before 70% decellularization has
been achieved.
15. Tissues that have been decellularized can be terminally
sterilized using any of a variety of sterilants. For example, the
tissue can be subjected to gamma irradiation, ethylene oxide,
peracetic acid, .beta.-propiolactone, povidone-iodine, or UV
irradiation in the presence or absence of photosensitizers.
Appropriate conditions for effecting terminal sterilization are
well known in the art.
16. Biological tissues suitable for use in the present method
include those appropriate for implantation into humans or animals.
Tissues can be human or non-human (eg bovine, porcine or non-human
primate) in origin. As indicated above, the tissues can be fresh or
cryopreserved. In either case, the tissue is decellularized prior
to any fixation. While the present invention is exemplified by
reference to heart valve leaflets, the decellularization method is
applicable to other tissues as well, including tendons, ligaments,
facia, arteries, veins, diaphragm, pericardium, umbilical cords,
dura mater or tympanic membranes.
17. Upon completion of decellularization, the biological tissue can
be processed and/or fabricated as appropriate depending on the
ultimate use of the tissue. Any fixation of the decellularized
tissue can be effected using art-recognized techniques, including
glutaraldehyde fixation. Unfixed tissue, however, can also be used.
Unfixed tissue can be impregnated with any of a variety of agents
including those that stimulate recellularization upon implantation
of the decellularized tissue in vivo. Examples of such agents
include growth factors, adhesion factors, such as
glycosaminoglycans, and soluble extracellular matrix glycoproteins
such as fibronectin, laminin, vitronectin, etc. Other agents that
can be used include those that augment hemocompatability,
thrombomodulators, and antibiotics. Appropriate impregnation
techniques are known in the art. When the tissue is a heart valve,
fabrication with a biological or non-biological stent can be
effected using standard protocols.
18. Bioprostheses produced in accordance with the present invention
can be used as replacements for defective tissues in mammals,
particularly humans. Methods of effecting the replacement of, for
example, heart valves, tendons, ligaments, vessels, etc., are well
known in the art.
19. Tissue decellularized in accordance with the present invention
is subject to less mineralization (eg calcification) in vivo than
non-treated tissue. Decellularization also results in a tissue that
is reduced in immunogenicity.
20. Certain aspects of the present invention are described in
greater detail in the non-limiting Examples that follow. While the
decellularization methodology of the present invention and that of
U.S. Pat. No. 5,595,571 are distinct, it will be appreciated that
certain details of that disclosure are equally applicable here,
including source of biological tissues, methods of monitoring
extent of decellularization and methods of processing and
fabrication post decellularization. Accordingly, U.S. Pat. No.
5,595,571 is incorporated in its entirety by reference.
EXAMPLE I
21. Decellularization of Leaflets and Whole Values
22. The following solutions are utilized in the protocols that
follow:
23. 1M Tris pH 7.6: To 80 ml deionized water add 11.21 gm Tris,
adjust pH to 7.6 with 1N NaOH and bring volume to 100 ml and store
at 4.degree.C.
24. 1M CaCl.sub.2: To 20 ml deionized water add 2.22 gm CaCl.sub.2
and store at 4.degree.C.
25. 1M MgCl.sub.2: To 10 ml deionized water add 2.033 gm MgCl.sub.2
and store at 4.degree.C.
26. DNAse I Solution: To 4.95 ml sterile water add 5 ml glycerol
(final conc 50%), 20 mg DNAse I (Sigma D5025) (final conc 2 mg/ml),
and 50 .mu.l 1M CaCl.sub.2 (final conc 5 mM). Aliquot 1 ml to
chilled labeled 1.5 ml microfuge tubes and store at
-20.degree.C.
27. RNASe A Solution: To 10 ml sterile water add 100 mg RNAse A,
and mix to dissolve. Aliquot 500 .mu.l of solution to each of 20
prechilled 1.5 ml microfuge tubes and store at -20.degree. C.
28. Nuclease Solution: To 93.66 ml sterile water, add 4.8 ml 1M
Tris pH 7.6 (final 48 mM), 288 .mu.l 1M MgCl.sub.2 (final conc 2.88
mM), 96 .mu.l 1M CaCl.sub.2 (final conc 0.96 mM), filter sterilize
using 0.2 micron filter, add 960 .mu.l 2 mg/ml DNAse I (final conc
19.2 .mu.g/ml) 192 .mu.l 10 mg/ml RNAse A (final conc 19.2
.mu.g/ml).
Decellularization of leaflets
DAY 1
29. A valve is removed from a liquid nitrogen freezer and submerged
in a 37.degree.C. water bath for approximately 15 min. Under
sterile conditions, the valve is removed from the packaging and
placed in a sterile 7 oz. specimen cup with approximately 50 ml of
lactate-ringer 5% dextrose (LRD5) solution for 15 min. at room
temperature. The valve is dissected by making a single cut down the
commisure located between the left and right coronary arteries. The
valve is laid open with the mitral valve leaflet up, the left
coronary leaflet to the left, the right coronary leaflet to the
right, and the non-coronary leaflet in the middle. The leaflets are
dissected free of the valve as close to the conduit wall as
possible and placed in separate labeled 15 ml conical centrifuge
tubes filled with 10 ml LRD5 solution for 10 minutes at room
temperature. The leaflets are moved to second labeled 15 ml conical
centrifuge tubes filled with 10 ml LRD5 solution and allowed to
stand for 10 minutes at room temperature. The leaflets then are
moved to third labeled 15 ml conical centrifuge tubes filled with
10 ml sterile water and placed in an incubator at 37.degree. C. 5%
CO.sub.2 for 2 hours. The leaflets are placed in 6-well culture
plates and weighted down with sterile glass rings. 5 ml nuclease
solution is added to each well and the leaflets incubated overnight
at 37.degree.C. 5% CO.sub.2.
DAY 2
30. The nuclease solution is removed and 5 ml of DMEM is added to
each well and the leaflets are returned to the incubator.
DAY 3-16
31. The medium is changed every other day for two weeks.
32. Alternative procedure for whole valves
33. If valves have been cryopreserved, they are thawed and washed
as above; if valves are fresh, they are washed once in 80 ml of
LRD5 for 15 minutes in a 7 oz sterile specimen cup.
34. After the valve is washed, it is transferred to a 7 oz sterile
specimen cup containing about 80 ml of sterile H.sub.2O and placed
in the 37.degree.C. 5% CO.sub.2 incubator for 4 hours.
35. The valve is removed to a 7 oz sterile specimen cup containing
about 80 ml nuclease solution and returned to the incubator
overnight.
DAY 2
36. The valve is removed to a 7 oz sterile specimen cup containing
about 80 ml (ALT+) solution (containing netilmicin, 54 .mu.g/ml;,
lincomycin, 131 g/ml; cefotaxime, 145 .mu.g/ml; vancomycin, 109
.mu.g/ml; rifampin, 65 .mu.g/ml; fluconazole, 100 .mu.g/ml; and
amphotericin B, 84 .mu.g/ml).
DAY 3-16
37. The medium is changed every other day for two weeks using ALT+
solution for the first week and DMEM for the second.
38. The foregoing procedures are open culture procedures. Thus the
specimen cup lids are loosened when placed in the incubator.
EXAMPLE II
Experimental details
39. Porcine heart valves. Porcine hearts were obtained from market
weight pigs (>120 kg). After rinsing in sterile phosphate
buffered saline, the hearts were field dissected (apex removed) and
shipped at 4.degree.C. in sterile PBS. All hearts arrived within 24
hr of animal slaughter. Aortic and pulmonary valves were dissected
as roots. These tissues were subjected to a bioburden reduction
step of incubation in a mixture of antibiotics and antimycotics for
48 hr at 48.degree.C. The disinfected tissues were either
cryopreserved (10% (v/v) DMSO and 10% (v/v) fetal bovine serum,
-1.degree. C./min) or were decellularized by a procedure involving
treatment with hypotonic medium followed by digestion with a
mixture of deoxyribonuclease I and ribonuclease A. After 12 days,
the decellularized valves were either cryopreserved as above or
chemically fixed in 0.35% (w/v) glutaraldehyde at 2 mmHg in
phosphate buffered saline (pH 7.4) for a total of 7 days; the low
pressure fixation ensures maintenance of the natural crimp of the
collagen matrix. The fixed tissues were not cryopreserved, but were
stored in 0.35% glutaraldehyde solution.
40. Prior to any examination (calcification, biomechanics,
histology), the cryopreserved tissues were thawed rapidly to
prevent ice-recrystallization by immersion of the packaged tissue
in a 37.degree.C. water bath. Cryopreservation medium was eluted
from the thawed valves with 500 ml of lactated-Ringers solution
containing 5% dextrose. The glutaraldehyde-fixed tissues were
washed three times each with 200 ml of normal saline.
41. In vivo static calcification. Calcification of treated tissues
was assessed in vivo by subdermal implantation in rats. Weanling
male, Sprague-Dawley rats were obtained from Charles Rivers
Laboratories. After one week equilibration, animals averaged
136.+-.18 g in weight. The heart valves were dissected to provide
aortic and pulmonary leaflets and vascular conduit sections, each
0.5 cm square. With the rats under ketamine and xylazine (10 mg/kg
and 5 mg/kg, respectively, IP) anesthesia, and following
preparation of a sterile field, 2 cm diameter pouches were formed
in the dorsal subcubitae, four per animal, and sections of tissues
inserted. Incisions were closed with stainless steel staples. The
rats were allowed to recover and were then permitted free access to
food and water. Tissue samples were recovered at 1, 2, and 4 months
post-implantation for determination of calcium content.
42. Method for calcium determination in tissue samples. Recovered
tissues were washed in sterile calcium and magnesium-free phosphate
buffered saline, three times 10 ml each. Wet weight was measured,
and after mincing, the pieces were dried overnight in a centrifugal
evaporator (Savant Speed-Vac). After recording dry weight the
tissues were digested in 10 ml of 25% (v/v) HNO.sub.3 for at least
24 hr at 70.degree.C. An aliquot of the digest solution was diluted
10-fold in 0.2 N HCl containing 1% (w/v) lanthanum nitrate.
Finally, calcium content was measured using a Perkin-Elmer 300
atomic absorption spectrometer calibrated with a certified calcium
standard from SPEX Plasma Standards (Cat. PLCA2-3Y. Response in
this system was linear between 0.2-20 .mu.g/ml.
43. Biomechanics testing. Aortic and pulmonary leaflets were die
cut in the circumferential dimension to provide "dog-bone"-shaped
specimens, 0.5 cm wide at midsubstance. Thickness of each sample
was derived from the average of three measurements taken with a low
mass pin attached to a conductance circuit and digital caliper.
Leaflets were mounted in specially designed clamps with a standard
gauge length of 1 cm. All testing was carried out with the tissue
in Hank's balanced salt solution maintained at 37.+-.2.degree. C.
Each specimen was preconditioned to a load of 150 g until
successive load-elongation curves were superimposable (-20 cycles).
The following measurements were then taken: 1) low-load elongation
to derive stress-strain relationships while imposing up to 150 g
load on the tissue at an extension rate of 10 mm/min, a rate which
reflects previously reported studies of leaflet biomechanics
(Leesson-Dietrich et al, J. Heart Valve Disease 4:88 (1995)); 2)
examination of viscoelastic properties of the specimens in a
stress-relaxation study (tissue elongated to a load of 150 g and
following residual loads for up to 1000 sec)--both the % of initial
load remaining at these time points as well as the rate of
stress-relaxation (i.e., the slope of the percent stress remaining
versus time) were determined; and 3) ultimate uniaxial tensile
testing to tissue failure. At least 8 specimens of each tissue type
were examined.
44. Histochemistry. Samples of fresh and explanted tissues were
immersed in 10% sucrose solution for 4-18 hr at 4.degree.C. After
brief fixation in 10% formalin, the pieces were placed in molds and
frozen in OCT using a liquid nitrogen bath. Cryosections, 6-10
.mu.m thick, were cut using an IEC cryostat (Needham Heights,
Mass.). Sections were then stained either with hematoxylin and
eosin or stained specifically for calcium according to the method
of von Kossa (Theory and Practice of Histological Techniques,
edited by Bancroft and Stephens, Churchill Livingstone, Edinburgh
(1990)). Sections were viewed and photographed using a Nikon
Optiphot microscope.
45. Statistics. Statistical differences in the group means of
biomechanical parameters was assessed by independent t-tests. A p
value of 0.05 was chosen as the level of significant differences.
Calcium data were analyzed according to ANOVA testing carried out
with the statistical program for the IBM-PC, SPSS-PC.
Results
46. Biomechanics. Low load testing--extensibility and low modulus.
The biomechanical properties of strips of aortic and pulmonary
porcine heart valve leaflets were compared between
fresh-cryopreserved and decellularized-cryopreserved tissues. Fresh
aortic and pulmonary leaflets were found to have significant
differences in extensibility; pulmonary leaflets had extension
2.3-fold greater than aortic leaflets (p<0.01)). However, the
elastic modulus of these tissues were not different
pre-decellularization (10.6.+-.1.1 vs. 9.15.+-.0.64, p=0.255, FIG.
1). With decellularization, the extensibility of the two leaflet
type became indistinguishable (30.4.+-.2.5 vs. 30.2.+-.3.3,
p=0.981). The elastic modulus of the aortic leaflets was unchanged
by decellularization (p=ns (not significant)), as compared to the
fresh tissue). In contrast, pulmonary leaflet tissues was markedly
stiffened by decellularization, with the elastic modulus rising by
660%, (p=<0.05). As a result, the elastic modulus of
decellularized pulmonary tissue was 550% greater than that of the
decellularized aortic leaflet.
47. Stress-relaxation testing. The initial (10 sec) and the final
(1,000 sec) rates of stress-relaxation for fresh aortic and
pulmonary leaflets were comparable and not statistically different
(p=0.103 and p=0.115, respectively, FIG. 2). For decellularized
tissues, only the initial rate of stress-relaxation or aortic
leaflets was obtained; this was no different from the fresh tissue
value. The increased stiffening of the pulmonary leaflets with
decellularization which was observed with low-load testing was
reflected by a higher final level of stress remaining (increase
from 64.1.+-.2.18% to 81.5.+-.2.5%). The relaxation slope for the
pulmonary leaflets were reciprocally changed by decellularization,
decreasing from 9.8.+-.0.8 in the fresh tissue to 4.7.+-.1.5 in the
treated tissue.
48. Ultimate tensile testing--failure load, maximum stress, and
elastic modulus (FIG. 3). In fresh tissues the aortic leaflets
failed a twice the load as did the pulmonary valve tissue
(p<0.001). However, there was no statistical difference maximum
stress at failure of the aortic and pulmonary leaflets (8.0.+-.1.2
MPa vs. 6.0.+-.0.9, p=0.202). As well, the moduli of the fresh
leaflets were not statistically different (p=0.333).
49. Decellularized aortic leaflets failed at the same load and
maximum stress as did the fresh tissue. The failure load of
pulmonary leaflets rose slightly but not significantly, but there
was almost a tripling of the stress at failure.
50. The stiffening of pulmonary leaflets observed with load testing
was again reflected when the tissue was loaded to failure. The
modules of pulmonary leaflets taken to failure increased 2.6-fold
after decellularization; in contrast, the elastic modules of the
decellularized aortic leaflets declined slightly (45.5.+-.6.2 MPa
vs. 38.3.+-.5.2 Mpa).
51. Tissue calcification. The kinetics of calcification of porcine
heart valve tissues at 1, 2, and 4 months of implantation are
presented in FIG. 4. Glutaraldehyde-fixed porcine pulmonary heart
valve tissues appeared especially prone to calcify in the subdermal
rat model. The pulmonary leaflets and vascular conduit calcified
more rapidly than their aortic valve counterparts, the fixed
pulmonary leaflets calcifying most rapidly of all tissues examined.
Furthermore, glutaraldehyde-fixed pulmonary leaflets attained the
highest tissue content of calcium over the four months of
subcutaneous implantation. In general, the fixed vascular conduits
calcified more slowly than the leaflets from the same valve type
and the final calcium content was significantly lower (p<0.05
for both aortic and pulmonary valves) at 4 months.
52. The impact of depopulation on heart valve calcification seen as
a slowing of the calcification of fixed or non-fixed tissue
(pulmonary leaflet) or a plateauing of calcification after two
months of implantation (aortic leaflet, aortic conduit, pulmonary
artery). The plateau phenomenon was seen in either the unfixed
tissues or in those which were decellularized prior to
glutaraldehyde fixation. No statistically significant difference in
the calcification of aortic conduit was found among the treatment
groups over the 4 months of implantation. Calcification of
decellularized aortic conduit proceeded more quickly than fixed
tissue for the first 2 months of implant, and then leveled off
while fixed conduit calcium content continued to rise. An
attenuating effect on the increase in pulmonary artery calcium
content was also observed relative to either fixed tissue
group.
53. Aortic and pulmonary leaflets had somewhat different responses
to decellularization. Decellularization of aortic leaflets with
subsequent fixation resulted in lower calcium content (73.+-.17 mg
Ca.sup.2+/g tissue) than aortic leaflets which were not fixed
(121.+-.8 mg/g, p<0.05). Although tissue was not available from
the 4 month time point, in pulmonary leaflets, the decellularized
tissue per se tended to have lower calcium content (152.+-.5 vs.
101.+-.34 mg/g at 2 months of implantation).
54. Histologic examinations. Areas of decellularized porcine aortic
leaflet at 1 month can be shown free of endogenous cells within the
tissue matrix as well as having no deposits. Since measured tissue
calcium in this group was 60.+-.14 mg/g, calcific deposits were
found only in localized areas. When examined further using von
Kossa's stain, such areas were evident. Within these areas calcium
deposits appeared in association with nonspecific structures. In
contrast, the early calcification of nondecellularized
glutaraidehdye-fixed tissues was always associated with cell
nuclei. The increasing extent of involvement of the leaflet tissue
with time of implant is evident from a 1, 2, and 4 month sequence.
The midsubstance of the leaflets calcified early, while the margins
calcified later. In either the aortic or pulmonary valve vascular
components, calcified areas typically remained at the periphery of
the implant, and only infrequently did tissues show evidence of
mineralization of the midsubstance of the implant.
55. All documents cited above are hereby incorporated in their
entirety by reference.
56. One skilled in the art will appreciate from a reading of this
disclosure that various changes in form and detail can be made
without departing from the true scope of the invention.
* * * * *