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Intraarterial cushions of the rat uterine arteryA scanning electron microscope evaluation utilizing vascular casts.

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THE ANATOMICAL RECORD 203:19-29 (1982)
lntraarterial Cushions of the Rat Uterine Artery: A
Scanning Electron Microscope Evaluation
Utilizing Vascular Casts
Departments of Obstetrics and Gynecology, Pharmacology, and Anatomy,
University of Iowa, Iowa City, Iowa 52242
The intraarterial cushions present in the rat at the points of
branching of the main uterine artery have been studied by means of scanning electron microscopy. Such studies confirmed the three-dimensional concept of these
structures gained from previous light microscopic serial section reconstructions
as incomplete, raised, asymmetric ridges which encompass the branch orifice. The
examination of methacrylate corrosion casts of the uterine vasculature with the
scanning electron microscope provided a means for evaluating the relative protrusion or retraction of the cushion structures within the vessel lumen, and thus for
assessing their role in regulating uterine blood flow in various physiologic states.
Cushions were studied in this manner at the stages of the estrous cycle, in castrated animals, and in animals receiving pharmacologic doses of an alpha adrenergic agonist, phenylephrine. Evaluation of the relative depth of the impression left upon the vascular casts by cushions permitted the followingconclusions.
The cushions protrudcd maximally (and thus impeded flow most effectively) in
castrated animals and in animals treated with the vasoconstrictor, phenylephrine.
In contrast, the cushions protruded less in animals in proestrus and estrus. These
data suggest that the cushions do respond, either actively, by virtue of the contractile state of the smooth muscle within the cushion, or passively, as a function
of overall vessel geometry, to alpha adrenergic stimulation. The contrast in
cushion protrusion between the castrated state, and proestrus and estrus, suggests that ovarian hormones exert an influence over the functional morphology of
the cushions in a manner which promotes maximal uterine perfusion during those
periods of the estrous cycle which are documented as periods of uterine hyperemia.
These studies thus provide evidence for the dynamic role of intraarterial cushions
in the regulation of uterine blood flow.
Intraarterial cushions, variously termed “intimal cushions,” “subendothelial cushions,” or
“arterial branch pads (polsters),”have been observed within arteries in a variety of organs
(Shanklin and Azzam, 1963; Rosen, 1967; Moffat, 1969; Pogorzelski, 1964; Menschik and
Dovi, 1965; Takayanagi et al., 1972; Moffat,
1952; Moffat and Creasey, 1971). In microscopic section, the structures resemble valves within arteries. However, three-dimensional reconstructions of the cushions (Velican and
Velican, 1977) have confirmed that the cushion
is not a paired structure, as suggested in section, but rather is an elevated, asymmetric ring
of tissue which encompasses the lumen of the
artery at sites where the vessel gives rise to a
collateral branch. The subendothelial cushion
0003-276X/82/2031-0019$03.500 1982 Alan R. Liss. Inc.
contains bundles of longitudinally arranged
smooth muscle, the orientation of which suggests that it could provide sphincterlike control over blood flow to the collateral branch
(Moffat, 1959). The function of the cushion
may be particularly important in vascular beds
which undergo regular changes in blood flow.
In this respect, the uterus is a suitable model
for investigating the role of cushions in the
regulation of blood flow, owing to the cyclical
changes in blood flow which result from the influence of steroid hormones.
Received April 6, 1981; accepted November 10,1981,
Direct correspondence and reprint requests to: Dr.Paul M. Heidger,
Jr., Depertment of Anatomy, University of Iowa, College of Medicine,
Iowa City. Iowa 52242.
In addition to their proposed influence on
blood flow, intraarterial cushions of the uterus
may subserve another function. Numerous
right-angle branches occur along the length of
the rodent’s main uterine artery, which should
give rise to plasma skimming, with consequent
lowering of the hematocrit in the microcirculation. However, Fourman and Moffat (1961)
have suggested that this probably is not the
case in the uterus. These investigators r e
ported that the hemoglobin content of uterine
blood in the arterial branch is greater than in
the parent trunk. They suggested that this
finding may be explained by the presence of
intraarterial cushions within the uterine artery
in that these structures might provide a means
for deflecting red blood cells from the axial
stream of blood into the right-angle branch.
Supporting this hypothesis was their observation that the hematocrit was decreased at rightangle branches of the intestinal artery, where
cushions are absent. Aside from their proposed
role in the control of blood flow and hematocrit,
the function of intraarterial cushions within
the uterine vasculature is not well understood.
In the present study, we have attempted to
correlate the configuration, extent of projection, and position of intraarterial cushions in
the arterial wall with the degree of uterine
blood flow (low, moderate, or high) to determine if cushions may function in regulating
the size and configuration of the branch orifice.
Detecting such changes by light microscopy
would require serial sections taken from numerous branch sites of the uterine artery, an
approach that is both technically difficult and
time consuming. The three-dimensional shape
of intraarterial cushions may be studied directly utilizing scanning electron microscopy
(Yohro and Burnstock, 1973); however, it is
technically difficult to expose the cushion and
view its shape in its entirety. In order to circumvent these problems we have used scanning electron microscopy to examine the cushion imprints left on corrosion casts of the uterine vasculature. This technique involves infusing a low-viscosity casting medium into the
uterine vasculature and allowing it to polymerize. Digestion of the tissue leaves a replica, or
mold, of the entire circulatory system which
can be viewed with the scanning electron
microscope (Kardon and Kessel, 1979; Kessel
and Kardon, 1979; Kardon and Kessel, 1980;
Murakami, 1978).The shape of the cushion can
be evaluated by viewing the impression left by
the cushion upon the cast. In the present
study, we have used this technique to assess
the shape of uterine intraarterial cushions in
rats during different stages of the estrous cycle, in pregnant rats, in castrated rats, and in
rats preinfused with the alpha agonist, phenylephrine, with the objective of correlating
changes in cushion structure with changes in
uterine hemodynamics.
Female Sprague-Dawley rats (Bio-Labs,
Madison, WI) weighing between 175 and 200
gm were housed six to eight per cage under
conditions of constant temperature and
humidity; a 12-hour lightldark cycle was maintained, with the lights being turned on at 0600
hours. Animals received Purina Formulab
Chow and water, ad libitum.
Direct SEM observation of intraarterial
Sites of branching of the uterine artery were
dissected from Sprague-Dawley rats following
perfusion fixation with 3% glutaraldehyde in
cacodylate buffer. These specimens were dehydrated, dried by the critical-point method, and
coated for routine scanning electron microscopy.
Estrous cycle group
To confirm that rats were undergoing a normal estrous cycle, vaginal smears were taken
between 0800 and 0900 hours and graded according to the criteria of Long and Evans
(1922).Only those rats completing at least two
consecutive estrous cycles were used for the
study. Two rats from each stage of the cycle
(diestrus, proestrus, estrus, and metestrus)
were anesthetized with 50 mglkg pentobarbital midmorning and prepared for vessel
Pregnancy group
Two female Wistar inbred rats (University of
Iowa, Iowa City, IA) that were in the 20th day
of pregnancy were anesthetized and prepared
for vessel casting.
Castrated group
In this group, four rats were bilaterally
ovariectomized under ether anesthesia and
sacrificed at 2 and 4 weeks postovariectomy
and prepared for vessel casting.
Phenylephnne-treated group
Two castrated rats were given a maintenance dose of 0.1 pglkg estradiol benzoate sub-
solution and casting medium into the vascular
system. The flow was set at the maximal flow
rate during the infusion of Ringer solution.
This flow (40 mllminute) produced an intraarterial infusion pressure of 75 mmHg. During
the infusion of Ringer solution, the polymerization of the casting medium was initiated by the
addition to the casting mixture of 18 drops of
promoter from the Batson’s corrosion kit.
Following 15 seconds of mixing, a 60-ml disposable syringe was filled with the medium.
Vascular casts
The syringe was then connected to a three-way
stopcock after the infusion of 40 ml of Ringer
The detailed procedures followed in preparsolution and the medium infused at a rate of
ing vascular casts for evaluation with the
approximately 2 ml/min.; this produced an inscanning electron microscope have been
tra-arterial infusion pressure within the physireported in earlier publications (Kardon and
Kessel, 1979; Kessel and Kardon, 1979). In or- ologic range (75-100 mmHg). The infusion was
stopped when the casting medium began to
der to insure a viscosity of the casting medium
low enough to fill all divisions of the micro- polymerize as evidenced by an increase in infusion pressure at constant flow. Typically, 16
vasculature, the following modification of Batml of casting medium was introduced into the
son’s corrosion compound was employed. The
casting medium was prepared by combining vascular system. Polymerization of the casting
medium to hardness occurred in approximatethe following chemicals: 10 ml of methyl methacrylate monomer (Aldrich Chemical), 10 ml of
ly 20 minutes.
The uterus was then excised and placed in a
monomer base, 5 ml of catalyst, and dye. The
Petri dish containing Tyrode-Ringer solution.
monomer base, catalyst, and dye are constitFor light microscopy, the two horns of the
uents of Batson’s No. 17 corrosion compound
kit (Polysciences).The components were mixed uterus were dissected free; one horn was prewith a magnetic stirring bar in a beaker cov- served in formaldehyde, dehydrated, and emered with Parafilm; all mixing of components
bedded in Epon. One-micrometer serial secof the casting medium and their infusion were tions were cut and stained with toluidine blue.
performed under a well-ventilated hood owing The second horn was digested and processed
to the hazardous nature of the fumes.
with the remaining tissues for scanning electron microscopy of the casts. The cast uteri
Using a dissecting microscope, a cannula of
polyethylene tubing (PE50) was placed retro- were macerated in 6.0 M potassium hydroxide
grade to flow within the right external iliac ar- at 60°C. The solution was changed at least
tery of each anesthetized rat. Lidocaine (1%) once a day and the casts were rinsed with diswas applied to the outer surface of the artery to tilled water between changes. A total macerafacilitate cannulation. The tubing, which was tion time of 1-2 days was usually required to
prefilled with heparinized TyrodeRinger solu- free the casts of all tissue. Following maceration, was connected to a pressure transducer tion, casts of the main uterine artery with its
and recorder to monitor arterial pressure dur- primary branches were usually dissected free
ing the infusion of casting medium. The left ex- from the rest of the uterine cast, although
ternal iliac artery was then cannulated as some specimens were left intact to facilitate
above with polyethylene tubing for the infu- study of the entire uterine vascular bed. The
sion of the casting medium. After cannulations casts were then dehydrated in ethanol, dried in
CO, by the critical-point method, and mounted
were completed, a 60-ml syringe was filled with
40 ml of heparinized Tyrode-Ringer solution on aluminum specimen holders using adhesive
and connected via a three-way stopcock to the copper tape. Specimens were rendered electron
infusion cannula which previously had been conductive by sputter coating with gold (apfilled with heparinized Tyrode-Ringer solution. proximately 300 A thick) and subsequently
At the onset of infusion of casting medium into viewed in a JEOL 35C scanning electron microscope operated at an accelerating voltage of
the iliac artery, the aorta was ligated below the
16 KV. The specimens were coded so that they
origin of the renal arteries. The inferior vena
cava was then cut to provide a route for could be randomly viewed, photographed, and
drainage. A Harvard constant flow infusion evaluated in an unbiased manner with respect
pump was used to introduce the Tyrode-Ringer to the shape and prominence of the imprints of
cutaneously on the seventh day postcastration. On the 14th day postcastration, they
were anesthetized and perfused with 40 ml of
Tyrode-Ringer solution; thereafter, they were
perfused with 40 ml of Tyrode-Ringer containing phenylephrine (1 mg/ml). The phenylephrine solution was infused at the same rate as
was the Tyrode-Ringer flush and the intravascular pressure was allowed to increase as the
vasoconstriction proceeded.
Fig. 1. Scanning electron micrograph of endothelial surfaceof uterine artery a t point of branching. Theorificeof the
collateral is bounded by an asymmetric, elliptical ridge, the
intraarterial cushion. Note that the region of asymmetry indicated by the arrow is tapered; the point of the taper is directed retrograde to arterial blood flow. The arrowhead indicates the site of a protruding endothelial cell nucleus.
X 500.
Fig. 2. Scanning electron micrograph of vascular cast of
uterine artery and collateral, from region similar to that indicated by arrow in Figure 3. The indentation in the cast a t
the base of the collateral was formed by the intraarterid
cushion. Comparision of Figures 1 and 2 facilitates the iden-
tification of corresponding structures in scanning electron
microscopic preparations of cushions in situ, and in vascular
casts. The m o w s in each indicate the tapered “quill point”
portion; the arrowheads indicate the bulges produced by endothelial cell nuclei (Fig. I), and their “negative image” preserved in the cast (Fig. 2). X 250.
Fig. 3. Scanning electron micrograph of cast of uterine vasculature. The uterine artery (A)and vein (V)lie a t the base of
the preparation. Collaterals arise from the artery (arrow)
and give rise to the profuse myornetrial vascular bed (MVB).
X 16.
cushions preserved within the vascular casts.
Morphology of casts
Direct visualization of the intraarterial cushion with scanning electron microscopy was accomplished using specimens fixed in situ by
vascular perfusion. In those fortuitous preparations in which the point of branching within
the artery was exposed during the processing
procedures, en face views of the cushion structure were afforded in such preparations (Fig.
l),an elliptical ridge protruded into the lumen
of the main artery, encircling the opening of
the side branch. However, the ridge was incomplete at the proximal side of the branch point.
Thus, the cushion assumed the overall configuration of a horseshoe, with the open end of the
structure directed retrograde to blood flow.
Such specimens also depicted the bulging of
endothelial nuclei into the lumen of the vessel.
By means of vascular casts, a more extensive overview of the complex ramifications of
the uterine vasculature was obtained (Figs.
2,3).As depicted in Figure 3, arterial, capillary,
as well as venous channels were cast using the
technical procedures described. Casts of arteries were easily distinguished from those of
adjacent veins by their smaller diameter, nature of branching, round cross-sectional a p
pearance, and characteristic fusiform surface
depressions (Fig. 2). These latter features correspond to the impressions left by the endothelid cell nuclei which projected into the vessel
Casts of the main uterine artery revealed
that the vessel lumen was expanded at the origin of each branch. Furthermore, the base of
each branch was surrounded by a groove
formed by the raised intraarterial cushion (Fig.
2). Two configurations of the cushion impressions were observed. In the first, the cushion
impressions in the casting medium correlated
well with the structure observed by direct
SEM of the vessel. These were pyriform in
shape with the tapering point directed retrograde to the flow of blood (Figs. 2 , 4 , 5 , 9 , 10).
The second configuration was that of a sphincteric ring surrounding the orifice of the arterial
branch and lacking a tapering point (Figs. 6,7).
Both of these configurations varied in the extent to which they projected into the vessel
Light microscopic examination of 1-pm sections of uterine arteries filled with casting m e
dium corroborated the observations made using scanning electron microscopy with respect
to the degree of projection and positioning of
the intraarterial cushion. In cross section, the
edges of the cushion resembled valves. In some
cases, the cushion ridge projected from the
branch site inward, toward the lumen of the
main uterine artery. Such cushions would produce an impression on the surface of the main
uterine artery cast (Fig 4 and inset). In other
specimens, the ridge of the cushion projected
perpendicularly into the lumen of the branch
vessel (Fig. 6 and inset). Cushions of this configuration would produce an impression at the
branch site of the cast resembling a constricting ring. Thus, the prominence of the cushion
ridge, as well as the angle and extent to which
it projects from the branch site, would appear
to determine the configuration of impression
left upon a vascular cast.
Changes with estrous cycle and pregnancy
Intraarterial cushions were observed within
the microcirculation of the uterine horn at
branch sites of small arteries and arterioles in
each of the hormonal states investigated. However, only those cushions at the branch site of
the main uterine artery were considered in this
investigation. The cushion impressions were
evaluated as to whether they appeared deep,
indicative of decreased orifice diameter, or
shallow, indicating an enlarged orifice. Within
the uterine arteries of each experimental
group, variability existed as to the depth of
cushion impression. The proportions of deep
and shallow impressions observed in each
group are summarized in Table 1. As shown by
the table and Figures 6,7, and 8, the frequency
of shallow cushion impressions was increased
in casts from animals in states of increased
uterine blood flow-proestrus, estrus, and
Castrated animals
Casts of vessels from this group of animals
generally revealed deeply indented cushion impressions (Table 1, and Fig. 9). The impressions left upon the casts by endothelial cell
nuclei resembled closely those observed in
estrus animals (cf., Fig. 7).
Phenylephrine-treated animals
As shown in Figure 10, deeply indented
cushion impressions were observed in all arterial branches studied in this group. Also, the
endothelial cell nuclei produced deep irregular
creases on the vascular cast, rather than the
oval-shaped depressions observed in nontreated animals. Marked constrictions were
TABLE 1. The percentage of shallow and deep impressions made by intraarterial cushions upon uterine vascular casts
in various animal Emups
No. of cushion
impressions observed
Animal groups
Cycling animals
Pregnant animals
Castrated animals
observed along casts of the uterine artery and
vein (Fig. 11).At certain sites along casts of
these vessels, areas were identified where casting medium had passed from the lumen of the
vessel into its wall, producing thin semicircular bands which surrounded the vessel casts
(Fig. 11).This was not observed in casts of the
main uterine artery or vein from any of the
other experimental groups.
Scanning electron microscopy of corrosion
casts has provided evidence for changes in the
shape and orientation of intraarterial cushions
under varying conditions of blood flow. Our
data support that in states of relatively high
Figs. 4-7. All figures are scanning electron micrographs of
vascular casts of uterine arteries a t the point of branchingof
a collateral exhibiting an arterial cushion.
Fig. 4. Specimen from animal in diestrus. Note the deep, elliptical impression left by the arterial cushion. Such impressions result from the protrusion of the cushion ridge toward
the lumen of the main vessel, as suggested in light microscopic preparations (inset). X 280. Inset: 0.5-pm Eponembedded section, toluidine blue stain. X 195.
Fig. 5. Specimen from animal in metestrus. As in diestrus
(Fig. 4). a prominent cushion impression lies at the base of
the collateral. X 275.
Fig. 6. Specimen from animal in proestrus. Note the a b
sence of the elliptical cushion impression seen in Figures 4
and 5. The narrow stricture a t the base of the collateral r e
flects that the cushion ridge was directed perpendicularly into the lumen of the branch vessel. Both profiles such as this,
and shallow profiles as seen in Figure 7, were characteristic
of specimens examined from animals in proestrus. X 200.
Inset: 0.5ym Epon-embedded section, toluidine blue stain.
X 195.
Fig. 7. Specimen from animal in estrus. Note the shallow
cushion impression a t the base of the collateral vessel.
X 340.
% Shallow
% Deep
uterine blood flow (proestrus, estrus, pregnancy)the casts exhibited shallow cushion impressions, reflecting less protrusion of the cushion
into the vessel lumen. In contrast, in states of
low uterine blood flow (metestrus, diestrus,
castrate, and phenylephrine-treated rats),
intraarterial cushions tended to protrude further into the vessel lumen, making deep impressions upon the casts. Thus, a correlation
appears to exist between blood flow and the degree to which the cushions project into the vessel lumen. By projecting into the branch vessel
lumen, and thereby acting as a sphincter, the
ridges of the cushion could effectively decrease
the diameter of the opening from the main
uterine artery into the collateral branch. Since
the resistance to blood flow through vessels is
directly proportional to the fourth power of the
radius (Burton, 1965), small changes in the
opening of each branch site of the main uterine
artery could significantly influence blood flow.
This concept of arterial cushions functioning
in the regulation of blood flow gains support
from the work of Harvey and Owen (1976),who
measured changes in uterine blood flow during
the estrous cycle in the rat, and who
documented flows which are in concert with
the above observations on uterine arterial
A high percentage of shallow cushion impressions would be intuitively expected in a
high blood flow state, such as pregnancy. However, our study detected an almost equal percentage of shallow and deep cushion impressions in arterial specimens from pregnant
animals. We feel that one plausible explanation of this result is that regional perfusion of
the pregnant uterus most likely represents a
dynamic process in which blood flow to different segments of the uterine horn may change
significantly over time. Indeed, Markee (1929)
reported direct observations of dynamic
regional changes in blood flow to segments of
the uterus over time. I t is unfortunate that
such dynamics cannot be appreciated in casts
of the uterine artery, and that resolution of the
questions concerning the control of regional
perfusion of the uterine vascular bed must
await further investigation with other
The degree to which a cushion projects into
the vessel lumen may be determined by the
contractile state of the smooth muscle within
the cushion. At this time it is not known what
factors may influence the smooth muscle at
this location. Although direct innervation of
the cushion smooth muscle has been found to
be lacking, nerve fibers which show catecholamine fluorescence have been observed in the
vessel adventitia (Falck et al., 1974).Alpha adrenergic receptors are apparently present in
this area of the vessel since cushions project
further into the vessel lumen after administration of the alpha adrenergic agonist, phenylephrine. These observations suggest that
either the smooth muscle cells of the cushion
possess adrenergic receptors that respond
directly to the drug or that the change in cushion shape is an indirect effect resulting from
changes in vessel geometry brought about by
vasoconstriction. The presence of alpha adrenergic receptors on cushion smooth muscle
cells lacking neuronal innervation may indicate that the cells could be responsive to bloodborne substances that can act on the receptors
(e.g., circulating catecholamines, catechol estrogens). Changes in vessel geometry did result from phenylephrine treatment, as eviFigs. 8-10 are scanning electron micrographs of vascular
casts of uterine arteries a t points of branching of collaterals.
Fig, 8. Specimen from pregnant animal. Note the shallow
cushion impression a t the base of the branch vessel. X 360.
Fig. 9. Specimen from a castrated animal. In contrast to
the pregnant, proestrus. and estrus animals, the cushions
from castrated animals were often deep and asymmetric, as
depicted here. X 385.
Fig. 10. Specimen from phenylephrinetreated animal.
Note the deep, asymmetric cushion a t the base of the collateral, and the elongated impressions left by the endothe
l i d cell nuclei (arrow). X 335.
Fig. 11. Specimen from phenylephrinetreated animal. A
region of marked vasoconstriction filled with casting
medium is seen in the lower arterial cast. Narrow bands of
casting material, circumferentially disposed upon the surface of the vascular casts (arrows). were frequently observed
in specimens from this treatment group. X 155.
denced in casts by the decreased diameter of
both the main uterine artery and its branches.
Thus, overall vascular constriction may have
been a contributing factor to the enhanced projection of the cushion into the arterial lumen.
Cushion position was also affected by the
steroid milieu of the vessel. In the group of castrated animals studied, the cushions left a deep
impression upon vascular casts, and the
branch sites along the main uterine artery appeared to be very constricted. This finding was
interesting not only because of the direct corre
lation with decreased uterine blood flow that
occurs in this state, but also because this represented the state of the cushion in the absence
of ovarian hormones.
Our study documented the presence of cushion structures at the bifurcation of small uterine arteries and arterioles, as well as at the
right-angle branching points of the main uterine artery. We originally attempted to study
the casts of the smaller arterial branches within the uterine horn. However, we could not consistently sample the smaller casted vessels because they were frequently obscured by dense
capillary plexuses. Attempts to isolate these
casted vessels by dissection could not be performed consistently. Only by studying vessels
in which we could consistently observe the
cushion impressions could an accurate assessment of cushion shape and position be made.
The present investigation, therefore, was
limited to the study of the cushions surrounding larger arterial vessels, the site upon
which previous investigations of uterine arterial cushions have also focused.
The correlative light microscopic findings
have indicated that the intimal cushions vary
not only in the extent of their projection into
the vessel lumen, but also with respect to their
angle of projection. This finding should not be
attributed to perfusion artifact since all animals were perfused with casting medium
within a physiologic range of pressure and the
hardened casting medium prevented subse
quent deformation of the cushions. A consideration of differences in the angle of projection of
the cushion ridge appears to hold a plausible
explanation for why some cushion impressions
on vascular casts appear to result from the
presence of a pyriform ridge projecting into
the lumen of the main uterine artery, and
others from a constricting ring of tissue surrounding the junction between the collateral
branch and the main uterine artery. Dif-
ferences in configuration of the cushion impression may be influenced by the angle of vessel branching from the main uterine artery.
The existence of a number of differently
oriented bands of smooth muscle within the
cushion could account for movement of the
cushion in more than one plane, thus contributing to changes in its configuration.
Phenylephrine administration yielded two
unusual findings apart from the effect on cushion shape, i.e., the appearance of semicircular
bands of casting material surrounding cast
vessels and the change in shape of the endothe
l i d cell impressions. The strips of casting
material observed to surround casts of large
arteries and veins in the phenylephrine-treated
group may reflect sites of increased permeability of the endothelium which allowed
passage of the casting medium into the subendothelial space. Capillarylike structures, interpreted to represent increased permeability of
the vessel wall, have been observed to surround casts of arteriolar divisions in a number
of organs (Anderson and Anderson, 1978;
Hodde, 1977; Reynolds and Kardon, 1981).
However, such structures have not been described previously in association with casts of
larger vessels such as we have observed in the
phenylephrinetreated vessels. In order to verify that these strips of casting medium were
not merely extravasated by high perfusion
pressure, two additional animals were perfused at a decreased flow rate such that intravascular pressure did not exceed the physiological level of 75-100 mmHg; vessel casts
revealed that the strips were present regardless of perfusion pressure employed. However,
cushion impressions observed in this latter
group of animals were of both the deep and
shallow varieties. The changes in endothelid
cell impression, which were supported by correlative light microscopic studies conducted,
may reflect a direct effect of the drug on endothelial cells, or an indirect effect involving alterations in the configuration of the underlying vessel wall. These endothelial cell
changes and the apparent alteration in perme
ability of the endothelium following phenylephrine administration warrant further exploration.
The results of these experiments have suggested a possible role for intraarterial cushions
in the regulation of uterine blood flow. The
techniques employed in this study have permitted an assessment of the dynamics of both
shape and extent of projection of intraarterial
cushions, which appear to be related to the hor-
monal state of the animal. I t is hoped that future investigations into specific factors which
may influence the shape of intraarterial
cushions, as well as investigations into the
quantitative effect that these structures have
on blood flow, will contribute to the better understanding of the precise role of intraarterial
cushions in uterine physiology.
The authors wish to thank Linda Radde for
her assistance in casting; Susan Wiltse and
Jack Burke for preparation of one-pm sections;
and Paul Reimann for photographic assistance. These studies were supported by a grant
from NIH (HD06380).
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artery, cast, cushion, vascular, microscopy, evaluation, scanning, electro, rat, utilizing, intraarterial, uterine
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