THE ANATOMICAL RECORD 199~187-195(1981)
Histology of the Canine Uretha
11. Morphometry of the Male Pelvic Urethra
W. CRAIG CULLEN, THOMAS F. FLETCHER,ANDWILLIAM E. BRADLEY
Department of Veterinary Biology, College of Veterimry Medicine, University
of Minnesota, St. Paul, Minnesota 55108 (W.C.C., T.F.F.)and Neurology
Service, V.A.Hospital, Long Beach, California and Department of Neurology,
University of California, Iruine, California (W.E.B.)
ABSTRACT
Urinary bladders and pelvic urethrae were collected from six
adult and two juvenile male dogs. Within two vesical and six urethral sampling
regions, volume densities were estimated for smooth and striated muscle, connective tissue and elastic fibers, stratum cavernosum, luminal epithelium, and prostate. The neck had significantly less smooth muscle and more connective tissue
than the body of the bladder. In the prostatic urethra, smooth muscle was associated principally with trabeculae surrounding prostate lobules. Smooth muscle was
sparse superficially in the prostatic capsule and practically absent in relation to the
mid-prostatic urethra. Thus there was no mechanism for active closure of the
middle prostatic urethra, and elastic fiber density was correspondingly high in this
region. The smooth muscle sphincter needed to maintain urinary continuence and
prevent semen reflux was primarily the vesical neck. Caudal to the body of the
prostate, striated muscle comprised more than 50% of the urethral wall. Juvenile
and adult postprostatic urethrae were similar except for a decreased quantity of
stratum cavernosum in the pups.
Because of convenient size, availability, and
resemblence to the human urethra, the canine
male urethra has been the subject of a number
of physiologic studies and experimental manipulations. Woodburne ('60, '61) has compared
smooth muscle and elastic fibers in the bladder
and proximal urethra of canine and human
males and has reported similar tissue arrangements in the two species. However, the
entire pelvic urethra of male dogs has not been
studied thoroughly.
The male dog lacks a distinct preprostatic
urethra and has a well-developed prostate
body, as does man, but the dog lacks vesicular
glands (seminal vesicles) (Miller et al., '65).The
dog has disseminated prostate and glands in
the ampulla of the ductus deferens but does not
have bulbo-urethral glands. In addition to prostate, the pelvic urethra of the male dog features
epithelium, stratum cavernosum, connective
tissue with elastic fibers, and smooth and
striated muscle. These tissues were analyzed
morphometrically .
MATERIALS AND METHODS
Urinary bladders and pelvic urethrae were
collected from six young adult and twojuvenile
0003-276X/81/1992~0187$02.00
0 1981 ALAN R. LISS,INC.
mongrel dogs, all intact males. The adults
ranged from 5 to 17 kg in weight. The pups were
two months old and averaged 7.3 kg. Following
pentobarbital anesthesia, four of the adults received whole-body perfusion of saline followed
by 10% buffered formalin or 3% phosphatebuffered glutaraldehyde through a femoral artery. Tissues from the remaining dogs were
fixed by immersion in buffered 10%formalin.
For all dogs, the region between the level of
the ureteral orifices and the cranial extent of
the body of the prostate was regarded as vesical
neck. The region between the cranial and
caudal extents of the prostate body was regarded as prostatic urethra. The region between the prostate body and the bulbus penis
enlargement was regarded as postprostatic
(membranous)urethra. The prostatic and postprostatic urethrae each were subdivided into
three equal-length sampling regions. The vesical neck and the six urethral sampling regions
were each potentially divided into ten equallength segments, and a random number between one and ten determined the site where
each region was blocked for transverse section.
Received May 2 , 1979; accepted Aug 5 , 1980.
188
W. CRAIG CULLEN, THOMAS F. FLETCHER, AND WILLIAM E. BRADLEY
For comparison, a transverse section was collected also from either the right or left midbody of each bladder. Tissues were prepared
and measured as described in the preceding
paper presenting morphometric measurements
of the female canine urethra.
OBSERVATIONS
The body and neck of the bladder blended
without sharp demarcation. The neck appeared
to penetrate the body of the prostate gland, so
that a morphologically distinct preprostatic
urethra was not observed. The postprostatic
(membranous) pelvic urethra was distinct, extending from the caudal aspect of the body of
the prostate to the abrupt enlargement produced by the bulbus penis covered by the M.
bulbospongiosus.Except for differences in size,
the urethrae of adult males appeared grossly
and histologically similar. Among the dogs of
different size, however, prostatic glandular
mass was more similar than vesical and urethral mass. Thus it appears that prostatic volume
has varied more conservatively than body size
in canine evolution. Whether fixed by perfusion
or immersion, comparable tissue regions appeared similar, except for greater dilation of
the stratum caverosum with perfusion.
The wall of the vesical body in the male dogs
was similar in histologic appearance to that
described for bitches (preceding paper). A discontinuous muscularis mucosae was evident in
bladders of three of the adult males and both
pups, whereas this structural feature was seen
in only one of the females. In the vesical neck,
small longitudinal muscle fascicles were observed adjacent to the submucosa along with
two thicker longitudinal fascicles associated
with the trigone and urethral crest. Superficial
to this, the main muscle coat consisted of circularly or obliquely oriented muscle fascicles. The
layering of muscle fascicles was irregular and,
typically within a plane of section, individual
fascicles did not completely encircle the neck in
males as they did in females. Thus the principal
musculature of the bladder neck in males consisted of obliquely oriented muscle bundles that
continued on to blend with trabecular muscle in
the prostate. Superficial to the principal musculature, a few longitudinal muscle fascicles
were scattered in the vesical neck and prostatic
capsule.
In the body of the prostate, smooth muscle
was associated principally with trabeculae
separating the glandular lobules (Fig. 1).Only
delicate muscle fascicles were present in the
outer prostatic capsule. In relation to the
urethra, circular smooth muscle formed a thin
sphincter restricted to the cranial sixth of the
prostatic urethra. The middle prostatic urethra
was devoid of smooth muscle, except for occasional, delicate, longitudinal fascicles. In the
caudal prostatic urethra, an increase in submucosal smooth muscle was evident, but the
muscle was either related to prostatic lobules or
oriented longitudinally to the urethral lumen
and did not form a complete sphincter. In the
postprostatic urethra, smooth muscle persisted
mainly as isolated longitudinal fascicles located adjacent to the submucosa.
Striated muscle (M. urethralis) first appeared in the caudal prostatic region as isolated
fascicles in the prostatic capsule and as transversely oriented fascicles coating the urethra
ventrally, where the urethra first emerged
from the prostate body. Interdigitating a t first
with smooth muscle, the striated muscle coat
then developed superficial to smooth muscle.
Striated muscle was quite thick throughout the
postprostatic urethra. The muscle fascicles
were oriented mainly circularly but also obliquely. The two orientations were mixed, and
consistent circular and oblique muscle layers
were not found within the striated muscle coat.
(Fig. 2).
Morphometry
For quantitation, a vesical neck and six
urethral sampling regions were defined. Because sections were taken randomly within
each region in each dog, the entire length of the
pelvic urethra and vesical neck was sampled
over all dogs (Fig. 3). Also the wall of the midbody region of the bladder was sampled and
quantified in terms of volume fraction (Vv).
The body of the bladder averaged 52%
smooth muscle and 46%connective tissue. The
neck of the bladder had significantly less
smooth muscle and more connective tissue than
the body (Table 1).Both regions were similar in
Vv of elastic fibers and epithelium, but the
index of elastic fiber density in submucosa for
the neck was more than twice that of the body of
the bladder. A small volume of disseminated
prostate was found in the vesical neck.
Prostatic glandular epithelium comprised
the most abundant tissue of the prostatic
urethral region, followed by total connective
tissue and smooth muscle. Nearly all of the
smooth muscle was associated with trabeculae
surrounding prostatic lobules rather than arranged for urethral encirclement. The three
sampling regions of prostatic urethra were significantly different in Vv of glandular epithe-
MORPHOMETRY OF THE! CANINE MALE URETHRA
189
Fig. 1. Transverse section through the midprostatic urethra of an adult male dog. Most of the smooth muscle is associated
with trabeculae separating lobules of prostate gland. Very little smooth muscle is in the outer prostate capsule or in the
urethral submucosa. A distinct uterus masculinus is evident in the dorsal submucosa deep to the colliculus seminalis. x 5.4.
Fig. 2. Transverse section through the middle of the postprostatic urethra of an adult male dog. The submucosa features a
voluminous stratum cavernosum. The thick striated muscle coat has predominantly obliquely oriented muscle bundles;
layering of muscle bundles is not distinct. x 11.
56.6
(3.0)
1.3
(0.1)
0.8
(0.5)
1.0
(0.4)
69.2
(14.1)
0.59
(0.15)
14.3
(6.6)
8.4
(1.3)
1.50
(0.58)
21.1
(4.8)
45.7
(2.3)
1.2
(0.01)
-
-
3.4
(0.6)
-
-
-
0.6
(0.2)
0
1.9
(0.4)
39.8
(3.4)
0
Bladder
neck
1.7
(0.2)
52.0
(2.1)
0
Bladder
M Y
~
~
~
23.5
(2.4)
0.9
(0.1)
67.8
(2.5)
1.3
(0.7)
223.7
(12.1)
0.13
(0.02)
8.8
(5.7)
21.2
(2.5)
0.53
(0.12)
13.9
(1.0)
0.2
(0.1)
7.3
(2.4)
0
Proximal
~
~
9.5
(3.4)
19.7
(3.4)
0.43
(0.15)
18.3
(1.4)
(0.04)
20.0
(1.1)
1.0
(0.1)
75.4
(1.3)
1.4
(0.9)
290.0
(26.8)
0.18
0.2
(0.1)
3.1
(0.8)
0
Prostatic Urethra
Middle
0.29
(0.12)
9.1
(1.0)
(2.6)
0.2
(0.1)
12.1
(2.8)
11.8
(2.7)
21.5
(2.6)
1.1
(0.1)
53.0
(6.1)
1.6
(1.0)
156.5
(36.0)
0.22
(0.06)
16.7
(6.4)
22.0
Distal
(1.0)
0.9
(0.1)
4.9
(1.0)
53.7
(3.2)
39.9
(2.3)
1.4
(0.1)
0.2
(0.2)
0.7
(0.2)
22.0
(3.0)
1.86
(0.39)
10.4
(3.6)
13.1
(1.4)
0.18
(0.02)
8.8
0.8
(0.1)
0.5
(0.2)
58.4
(2.0)
39.9
(1.7)
1.7
(0.1)
0.1
(0.1)
0.7
(0.4)
19.3
(2.5)
2.21
(0.57)
14.5
(5.9)
14.2
(1.7)
0.16
(0.02)
8.0
(0.9)
19.5
(4.9)
11.3
(1.2)
0.15
(0.01)
9.0
(0.8)
(0.46)
0.8
(0.2)
27.5
(7.0)
1.70
0.5
(0.1)
0.3
(0.2)
54.5
(3.4)
44.3
(3.4)
1.6
(0.1)
0
Postl'rostatic Urethra
Proximal
Middle
Distal
>Meanvolume-fraction expressed as percent (standard error shown parenthetically).
2Elastic fiber volume-fraction percent i s included also in total connective tissue.
'Also includes some lymphatic tissue, but excludes stratum cavernosum.
'Mean section area (mmz)~susefulforcomparingregions,butthemeans arecompositesofdogssignificantlydifferentin size.Meanmmlengths (standarderrors)were: 4.8 (0.7)forneck; 8.9(0.6)foreach
prostatic region; and 13.4 (1.8)for each postprostatic repon.
'Sv = endothelial surface area per unit volume of tissue (mmYmm9.
@Index of density of elastic fibers in the submucosa = (mean number of proximal ends of elastic fiberslmm')ilOJ.
Smooth
muscle'
Striated
muscle'
Total connective
tissue'
Elastic
fibers';2
Prostate
gland'
Nerves and
vessels'."
Mean section
area4
Sv stratum
cavernosumj
Vv stratum
cavernosum'
Index of elastic
fiber density6
Mean epithelial
area (mm2)
Mean lumen
perimeter (mm)
Epithelium1
Tissue
TABLE 1 . Tissue constituents rn regions of the bladder and pelvic urethra for six adult male dogs
c
0
(D
191
MORPHOMETRY OF THE CANINE MALE URETHRA
lium and smooth muscle, and the caudal prostatic urethra had equal volumes of smooth and
striated muscle. The three sampling regions
were not significantly different in Vv of total
connective tissue, Vv of elastic fibers, or index
of elastic fiber density in the submucosa.
Striated muscle was the dominant tissue
constituent of the postprostatic urethra, occupying over 50% of the urethral wall in all
three sampling regions. Total connectivetissue
occurred next in abundance, comprising approximately 40%0of the urethral wall. The cranial third of the postprostatic urethra was significantly different from the distal two-thirds
in having a greater amount of smooth muscle.
A trace amount of disseminated prostate was
present in the proximal half of the postprostatic
urethra.
Stratum cavernosum, quantified by surface
area of endothelium per unit volume of tissue
(Sv) or by absolute endothelial surface area,
was best developed in the postprostatic urethra. The regions of prostatic urethra and vesi-
cal neck, which were not significantly different
from one another in absolute endothelial surface area for stratum cavernosum, averaged
only half the endothelial surface area of the
postprostatic urethra. Although Vv of stratum
cavernosum also was measured, Vv was highly
variable generally and particularly as a consequence of perfusion. Furthermore, the Vv
statistic probably incurred bias, since a fixative-induced contracture of striated muscle
seemed t o impede vascular distension of
stratum cavernosum in the postprostatic urethra.
Vv of epithelium was not significantly different between the body and the neck of the bladder. For the urethra, epithelial Vv was not a
meaningful parameter because of the massive
prostate, and epithelial area per section, estimated by number of points falling on epithelium, was regarded as a better parameter for
comparison. In terms of area per section,
epithelium declined through the prostatic
urethra and remained low in the postprostatic
0
0
7
600 /
0
0
- 50-
8
0
0
0
-
0
.-C0 4 0 - 8
0
0
0
0
0
0
(D
t
0
0
0
0
0
30-
0
0
0
0
OO
0
0
0
0
8 0
0
0
0
w
0
0
i.. .
*.
-*.
5
0.
Body
I
Neck
Bladder
.
I
0
I
c
.*
2 1 3
Prostatic Urethra
ma
.*
.. . . . .
0 8
I
0.
2
4
3
Postprostatic Urethra
Fig. 3. The volume-fraction (Vv)of total connection tissue (0)
and of elastic fibers ( 0 )is plotted for each of the
six adult males with respect to location within each samplingregion. Notice that by random sampling within each
region, the entire length of the urethra was examined over all dogs. For total connective tissue, the bladder neck
was significantly different from the body of the bladder and from the postatic and postprostatic urethral regions.
Despite the mass of prostate gland, elastic fiber Vv was not greatly reduced in the prostatic urethra because of an
increased density of elastic fibers in the submucosa of this region.
192
W. CRAIG CULLEN, THOMAS F. FLETCHER, AND WILLIAM E. BRADLEY
urethra. In general, epithelial area was proportional to lumen perimeter except in the
midprostatic urethra, where epithelium decreased despite an increase in lumen perimeter
(Fig. 4).Thus the epithelial layer was appreciably thinner in this region. But this could be
artifactual, owing to the lack of urethral
sphincter muscle allowing the lumen to balloon
preferentially in the midprostatic region.
Pelvic urethrae of the two pups were smaller
than those of the adult males, but the pup prostates were disproportionately small. Vv of prostatic glandular epithelium averaged only 25%
of the prostatic urethral region for the pups
compared to 65% for adult males. Consequently, Vv's of smooth muscle, connective tissue, and urethral epithelium were proportionately greater in pup compared to adult
prostatic urethrae (Table 2). Proportions of the
various tissues in the postprostatic urethra and
in the neck and body of the bladder were quite
similar in pups and adults, with one exception.
The stratum cavernosum was much better developed in adults, presumably a reflection of
sexual maturity. However, the pups displayed
the adult pattern of postprostatic cavernosum
exceeding prostatic stratum cavernosum.
30
DISCUSSION
The morphometric method used in this study
of normal urethrae could be adapted to examine
dysfunctional canine urethrae, or could be used
to compare urethrae among species. In sampling to compare urethrae, one should avoid
urethral regions of abrupt transition where tissue variability is high. For the male canine
urethra, the middle third of the prostatic
urethra and the caudal two-thirds of the postprostatic urethra are optimal sampling regions
for comparing urethrae because of minimal
transition of tissue proportions.
Some of the morphologic arrangements we
observed had implied functional correlates.
The principal distribution and orientation of
prostatic smooth muscle around lobules of the
gland implied that the muscle functions to increase intralobular pressure. Muscle in the
prostatic capsule was skimpy, indicating no
significant mechanism for active compression
of the overall gland and the included urethra.
The observations that smooth muscle encircled
urethral submucosa only at the cranial extent
of the prostatic urethra and that the midprostatic urethra was particularly deficient in
70
T
m
25
-E
0.
rc
1.5
2o
E
v
L
2
15
.-i?
L
e,
Q
10
5
0
1
B f : c l k T Prostatic
1 2 Urethra
1
3
2
Postprostatic Urethra
3
0
53.3
(0.8)
1.3
(0.2)
0
1.0
(0.1)
35.3
(3.2)
0.14
(0.003)
0.2
(0.1)
48.0
(2.2)
1.3
(0.03)
0
1.o
-
-
-
(0)
1.8
(0.1)
44.0
(0.9)
0
Bladder
neck
1.8
(0.2)
49.3
(2.1)
0
Bladder
M Y
58.3
(0.2)
1.0
(0.2)
23.0
(1.3)
1.7
(1.1)
33.8
(3.6)
0.018
(0.001)
0.3
(0)
0
1.4
(0.6)
16.4
(0.8)
Proximal
0.014
(0.002)
0.4
(0.3)
(1.6)
53.8
(1.8)
1.2
(0.1)
33.7
(3.1)
0.6
(0.2)
54.2
0.9
(0.3)
11.0
(1.3)
0
Prostatic urethra
Middle
(0)
0
0.8
(0.2)
(0.004)
0.2
(0.1)
20.1
(0.3)
0.031
(0.1)
(0.1)
0.8
(0.5)
4.0
(1.6)
53.1
(1.5)
41.8
(3.4)
1.3
0.8
(0.2)
16.1
(2.2)
0.030
(0.008)
0.8
(0.2)
0.9
(0.2)
1.3
(0.3)
56.5
(4.9)
40.6
(4.3)
1.5
(0.2)
0
0.8
(0.3)
16.6
(0.7)
0.041
(0.006)
1.4
(0.2)
0
(0.1)
52.1
(1.9)
46.5
(2.3)
1.7
(0)
0.1
(0)
0.7
PostProstatic urethra
Proximal
Middle
Distal
17.8
(1.4)
0.5
(0.1)
39.0
(2.2)
0.014
(0.001)
0.2
15.2
(1.1)
12.8
(2.2)
53.2
(2.0)
1.3
(0)
0.3
Distal
‘Mean volume-fraction expressed as percent (standard error).
*Elastic fiber volume-fraction is included also in total connective tissue.
3Excludes stratum cavernosum.
‘Sv = endotheIialsurfaceareaperunit volumeoftissue(mms/mm’l.Meanmm lengths(standarderrors1 were:5.0(1.0)forneck;3.7(O)foreach
prostat1cregion;and lO.O(O1foreachpostprostaticregjon.
Smooth
muscle‘
Striated
muscle’
Total connective
tissue’
Elastic
fibers‘J
Prostate
gland’
Nerves and
vessels’.3
Mean section
area (mm?
Sv stratum
cavernosum%
Vv stratum
cavernosum’
Epithelium’
Tissue
TABLE 2. Tissue constituents of bladder and pelvic urethra for twojuvenile male dogs
z
8
E
A
-.
a
6
c
0
*
i
8
<
%
m
8z
-
8
z
194
W. CRAIG CULLEN. THOMAS F. FLETCHER, AND WILLIAM E. BRADLEY
urethral related smooth muscle, indicated that
closure of the lumen of the prostatic urethra is
passive. The abundant elastic fibers found in
the submucosa of the prostatic urethra implied
that elastic fibers hold the lumen closed, with
closure facilitated by an apparently firm colliculus seminalis that indents the urethral
lumen dorsally.
With regard to urinary continence, canine
males and females had somewhat different arrangements. In females, the vesical neck was
augmented by a long smooth muscle sphincter
extending throughout the cranial half of the
urethra. This gradually was replaced by a
striated sphincter caudally. The entire urethra
contained a uniform amount of elastic fibers. In
males, the bladder neck appeared to be the primary smooth muscle sphincter; it was augmented only by a brief extension of the sphincter into the cranial sixth of the prostatic
urethra. Caudal to this there was no smooth
muscle sphincter. The postprostatic pelvic
urethra featured a long, thick, striated sphincter, occupying in males more than twice the
relative volume of the striated sphincter in
females. The striated muscle was relatively
just as well developed in the pups, which were
too young to have possibly exercised the muscle
in squirting urine for territorial marking as
adult males do. Except for the submucosa of the
prostatic urethra where elastic fibers were concentrated, elastic fibers were less numerous in
the male than in the female urethra and bladder. The numerical deficiency of submucosal
elastic fibers in the male was moderated by a
larger mean fiber diameter in the male (0.8pm)
than in the female (0.6 pm). However, the volume density of elastic fibers was still significantly less in males than in females.
The perimeter of the urethral lumen was not
significantly different along the length of the
female urethra. In males, luminal perimeter in
the midprostatic region and, to a lesser extent,
the cranial prostatic region was significantly
larger than that observed more caudally in the
pelvic urethra. In both sexes, epithelial area
per section was proportional to lumen perimeter, except in the midprostatic region where
epithelium appeared thin in our specimens.
Whether this thin epithelium is a consistent
structural feature or merely a consequence of
lumen distension in our specimens was not
clear.
Male-female urethral comparisons were
complicated by presence of the prostate and
greater development of striated muscle in the
male. In the vesical neck, the higher muscle
connective tissue ratio for males vs. femalesdid
not prove to be significant statistically. In the
vesical body, muscle and connective tissue were
not significantly different between the sexes,
but epithelium was relatively decreased in the
males.
Investigating the mechanics of the canine
pelvic urethra during erection and ejaculation
would pose an extraordinary experimental
challenge. Based on histology of the organ, we
hypothesize the following sequence of events:
With erection the urethral stratum cavernosum expands, reducing the urethral lumen
and firming the urethral wall for conveyanceof
a relatively small volume of ejaculate under
relatively high pressure. Contraction of smooth
muscle in the vesical neck and the most cranial
prostatic urethra maintains urinary continence and prevents ejaculate reflux. Contraction of smooth muscle surrounding prostatic
lobules forces prostatic fluid into the elastically
closed midprostatic urethral lumen. Simultaneous contraction of the postprostatic
striated muscle traps fluid in the midprostatic
urethra, which is ballooned by entering fluid.
Peristalsis of each ductus deferens adds spermatozoa to the ejaculate pool. At this point,
relaxation of striated muscle allows expulsion
of the ejaculate pool into the postprostatic
urethra, powered by elastic recoil of the ballooned prostatic urethral wall. A striated peristaltic wave conveys ejaculate into the penile
urethra. Then, striated muscle tetanus initiates another cycle of ejaculate pooling during
continued contraction of smooth muscle in the
prostate and vesical neck.
Woodburne ('60, '61) reported that deep longitudinal muscle of the vesical neck-urethra
was thick and, along with a rich supply of elastic fibers, it constituted a myoelastic tissue. We
were not impressed by the amount of longitudinal muscle, which formed only delicate fascicles adjacent to the submucosa in our specimens. Woodburne also reported that there was
no sphincter at the canine (and human) vesical
neck, because muscle bundles extending from
the vesical body to the neck arched past rather
than around the vesico-urethral junction. Because of this arrangement and because stimulation of muscle bundles at the vesical body
opened the urethral orifice, Woodburne concluded that neck musculature does not serve as
a sphincter and that elastic tissue must maintain continence. Woodburne had discountedthe
concept that sympathetic innervation operates
to constrict an internal urethral sphincter.
Since that time, several investigators have
MORPHOMETRY OF THE CANINE MALE URETHRA
demonstrated sympathetic induced localized
contraction of the vesical neck-proximal
urethra i n dogs (Awad and Downie, ’76;
Kleeman, ’70; Koraitim et al., ’77; Raz et al.,
’72; Rohner et al., ’71).
We would agree with Woodburne’s observation that muscle bundles tend to arch rather
than encircle the vesical neck; however, when
viewed in transverse section, most of the fascicles a t least partially surround and some even
completely encircle the neck. Of particular significance are the muscle bundles that arch
around the neck and continue into the body of
the prostate. It seems likely that localized contraction of these muscle bundles, anchored cranially a t the distended bladder and caudally at
the prostate, is sufficient to squeeze the vesical
neck closed for continence. In addition to
physiologic evidence for a n active sphincter at
the vesical neck, our finding that elastic fibers
are reduced in the neck compared with the prostatic urethra argues for an active proximal
sphincter to prevent ejaculate reflux. The primary sphincter must reside in the vesical neck
since the prostatic urethral sphincter is so abbreviated.
Woodburne’sobservation that stimulation of
muscle bundles at the vesical body produces
opening of the vesical neck is not pertinent to
continence since the latter is related physiologically to localized contraction of musculature a t
the neck. However, the observation does reinforce appreciation for the dual mechanical role
of vesical neck muscle cells: During active continence they must shorten; during voiding they
must elongate mechanically to allow distention
of the lumen of the vesical neck. This dual role
of muscle cells pertains throughout the bladder, but muscle cells of the vesical neck must
function out of phase to those in the rest of the
195
bladder, and this relates to the great dependency of the bladder-urethra on properly
coordinated innvervation. Thus, in the male
dog, the internal “urethral” sphincter resides
in the vesical neck, except for a minor component encircling submucosa of the proximal prostatic urethra. The sphincter is a functional
entity generated by reciprocal innervation.
Awad and Downie (’76)studied the effects of
adrenergic drugs and hypogastric nerve stimulation on fluoroscopically visualized urethrae
of male dogs. Consistent with our findings, they
demonstrated that the lumen of the midprostatic urethra was more dilated than the rest of the
urethra and that sympathetic stimulation obliterated the lumen only of the vesical neck and
most cranial prostatic urethra. They also reported that norepinephrine in sufficient dosage
could produce complete occlusion of the lumen
of the prostatic urethra, and we presume this
resulted from elastic recoil following complete
blockage of perfusion fluid a t the vesical neck.
LITERATURE CITED
Awad, S.A., and J.W. Downie (1976) The effect of adrenegic
drugs and hypogastric nerve stimulation on the canine
urethra. Invest. Urol., 13:298-301.
Kleeman, F.J. (1970)The physiology of the internal urinary
sphincter. J. Urol., 104t549-554.
Koraitim, M., W. Schaffer, H. Melchoir, and W. Lutzeyer
(1977) Urogenitodynamics of the male bladder neck. 1.
During continence. Invest. Urol., 14r496-497.
Miller, M.E., G.C. Christensen, and H.E. Evans (1965)Anatomy of the Dog. W.B. Saunders Co., Philadelphia.
Raz, S.,M. Zeigler, and M. Caine (1972) Isometric studieson
canine urethral musculature. Invest. Urol., 9:443-446.
Rohner, T.J., D.M. Raezer, A.J. Wein, and H.W. Schoenberg
(1971) Contractile responses of dog bladder neck muscle to
adrenergic drugs. J. Urol., 105r657-661.
Woodburne, R.T. (1960) Structure and function of the urinary bladder. J. Urol., 84:79-95.
Woodburne, R.T. (1961) The sphincter mechanism of the
urinary bladder and urethra. Anat. Rec., 141:ll-20.
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