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Regional differentiation of cell junctions in the excurrent duct epithelium of the rat testis as revealed by freeze-fracture.

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Regional Differentiation of Cell Junctions in the Excurrent
Duct Epithelium of the Rat Testis as Revealed by
Freeze- Fract ure
Department of Anatomy, School of Medicine, Chiba Uniuersity, Chiba 280, Japan
Intercellular junctions of the excurrent duct system of the
adult rat testis were studied by freeze-fracture. In the terminal segment of the
seminiferous tubule, where there are no spermatogenic cells, the tight junctions of Sertoli cells consist of many parallel strands of particles. The particles
of the junctions predominantly appear on the E face instead of the P face, similar to those of the seminiferous tubules reported previously. From the rete
testis to the ductus deferens, the tight junctional particles or smooth strands
are mainly found on the P face, and the tight junctions show anastomosing networks. In the ductuli efferentes, whose epithelial lining consists of ciliated and
nonciliated cells, the tight junctions between two adjacent nonciliated cells
and between nonciliated and ciliated cells are poorly developed. In the former,
belt-like gap junctions are often associated with segmented tight junctions. In
such area, there are tiny regions, where no junctional elements are observed.
Between two ciliated cells, several strands of the tight junctions can be seen.
The result of the tracer experiment suggests that the barrier of the ductuli efferentes is weak. In the epididymis, the tight junctions are well developed
throughout the duct. Corresponding with the regional variation of the epididymal epithelium, the geometrical organization of the tight junction networks varies along the duct. In the ductus deferens, many strands of the tight
junctions are scattered throughout the lateral cell surface in addition to the
belt-like network of the tight junction in the adluminal area.
The number of the tight junctional strands is presented graphically in the
various segments of the excurrent duct of the testis.
The epithelial lining of the testicular excurrent duct system shows histological variations
along its length. The terminal segment of the
seminiferous tubules is lined by only Sertoli
cells (Perey et al., '61; Dym, '74). The tubuli
recti is lined by the simple cuboidal or columnar epithelium (Dym, '74). In rat, the major
part of the rete testis is a flattened sac a t the
periphery of the testis, and a series of straight
rete tubes pierce the tunica albuginea joining
the ductuli efferentes. According to RoosenRunge ('611, the major part and the tubular
part will be referred t o intratesticular rete
and extratesticular rete, respectively. The
epithelium lining of the rete varies from
squamous to low columnar type in the rat
(Leeson, '62) as well as other species (Dym,
'76; Bustos-Obregon and Holstein, '76).
ANAT. REC. (1978) 191: 503-520.
In many species including rats, the epithelial lining of the ductuli efferentes is comprised of two cell types, ciliated and nonciliated cells (Reid and Cleland, '57; Ladman and
Young, '58; Ladman, '67; Hamilton, '75). Holstein ('69) reported six cell types in human. In
the rat, the nonciliated cells are more numerous than the ciliated cells which are scattered
among the nonciliated cells throughout the
ductuli efferentes.
The epididymis is customarily subdivided
into three regions, the caput, corpus and
cauda, and is lined with pseudostratified epithelium consisting of a t least two cell types,
the principal and basal cells. In the rat, additional types, the halo cell, clear cell and apical
Received No". 28, '77. Accepted Mar. 7, '78.
cell, are reported (Reid and Cleland, '57). The
principal, apical and clear cells reach t h e
lumen, and the latter two type cells a r e situated sporadically among t h e principal cells.
Regional variations of t h e lining epithelium
along t h e epididymal duct are well known.
Reid and Cleland ('57) divided t h e rat epididymis into six zones. Glover and Nicander
('71) have proposed a nomenclature t h a t is t h e
initial, middle and terminal segments. In the
rat, however, they could not identify t h e
boundary between t h e middle and terminal
segments. Hamilton ('75) has reviewed t h e
fine structural features of t h e ductuli efferentes, epididymis and ductus deferens in t h e
Several investigators examined t h e fluid
composition in various segments of t h e system, and found t h a t i t was not only different
from t h a t present in blood plasma or lymph
but also modified along t h e duct (see review
paper by Setchell and Waites, '75). To maintain t h e difference of t h e fluid composition,
the tight junctions of t h e lining epithelia
must serve as a partition between t h e intraand t h e extra-luminal fluids. By freeze-fracture, Claude and Goodenough ('73) have suggested t h a t there is a significant correlation
between t h e number of strands and tightness
of the epithelia. Therefore, we were interested
in examining t h e junctional complexes of t h e
various epithelia along t h e male duct system.
The junctions of t h e seminiferous tubules
(Gilula e t al., '76; Nagano and Suzuki, '76a,b)
and of t h e epididymis (Friend and Gilula, '72)
have already been reported by freeze-fracture.
Friend and Gilula ('72) have documented t h e
extremely well developed tight junctions of
t h e epididymis, but they did not mention t h e
regional differences along t h e epididymal
recti [27, 571 ' ; (ii) t h e extratesticular rete [3,
121; (iii) t h e distal zone of t h e ductuli efferentes [15, 1521; (iv) t h e initial segment of the
caput epididymidis (zone 1 of Reid and Cleland, '57) [13,311;(v) t h e next zone (zone 2) t o
t h e initial segment, and this region will be refered t o the caput epididymidis in this paper
[13, 591; (vi) t h e corpus epididymidis (zone 5)
[31, 561; (vii) t h e cauda epididymidis (zone
6A) L25, 701; (viii) t h e straight portion of the
ductus deferens [16, 261. Small pieces of the
tissues were treated with buffered 30% glycerol for five to ten hours. Subsequently, they
were placed on a copper plate of t h e specimen
holder and frozen with liquid Freon 22 cooled
by liquid nitrogen. Freeze-fracturing a n d
replication were made in a brass block type
machine (Hitachi, HFZ-1) in
Torr as described previously (Nagano and Suzuki, '76b).
In some replicas, made on the tissue pieces
of t h e intratesticular rete, we found t h e epithelium consists solely of t h e Sertoli cells, and
i t was considered as t h e terminal portion of
t h e seminiferous tubule (Dym, '76). Between
t h e two cells, t h e tight junctions are not situated adluminally, but appear almost throughout t h e lateral cell surface. They consist of numerous parallel rows of ridges on t h e P face
and shallow grooves on t h e E face. There are a
few anastomoses of t h e rows (fig. 2). The junctional particles a r e located preferentially in
Fifteen adult male rats of t h e SpragueDawley strain were fixed by intravascular
perfusion through t h e abdominal aorta (Forssman e t al., '77). The fixative consists of 2.5%
glutaraldehyde buffered with 0.05 M cacodylate at pH 7.4. After completion of t h e perfusion, t h e testis and t h e excurrent duct system
were removed. Under a binocular microscope,
t h e following eight different regions of t h e
duct system were isolated: (i) t h e intratesticular rete testis with t h e terminal portion of t h e seminiferous tubules and tubuli
L a n t h a n u m infusion
In two adult rats of t h e same strain, a fixative containing collidine buffered 2.5% glutaraldehyde and 2% lanthanum nitrate at pH 7.4
adjusted with sodium hydroxide solution was
perfused through t h e abdominal aorta (Dym
and Fawcett, '70). The distal zone of the ductuli efferentes were selected and cut into
small pieces. The tissues were postfixed with
1%osmium tetroxide buffered with collidine
at pH 7.4, and processed for t h e Epon embedding as usual except t h a t 2% lanthanum nit r a t e was added up to t h e dehydration solutions.
The replicas and thin sections were examined in t h e Hitachi 11-D electron microscope
at 75 or 100kv.
The terminal portion of the
seminiferous tubule
I The numbers in the brackets indicate the number of replicas and
cells with tight junctions examined In each region, respectively.
C.-Non C.
Ductuli sfferentes
Fig. 1 Histogram of t h e mean number and standard deviation of the strands of the tight junctions in
various parts of the duct system. Junctions from more t h a n ten different cells of each part of t h e duct were
enumerated. The number of the strands in the tight junction depth was counted a t three different places per
cell eliminating the specialized area of 3-cell joining area. The tubuli recti were included in t h e rete testis.
C-Non C, tight junctions between ciliated and nonciliated cells. Non C-Non C, between two nonciliated cells.
the center of the grooves on the E face rather
than on the ridges of the P face. Narrow furrows are seen on the top of each ridge. They
are probably left behind by the displaced particles. The linearly arranged particles in the
center of the grooves in the E face are smaller
than sporadically arranged ones on the narrow
furrows in the P face. It is probably due t o the
fact that the particles in the grooves cannot be
entirely seen.
The rete testis with the tubuli recti
In the present observations by freeze-fracturing i t was difficult to distinguish between
the tubuli recti and the rete testis. Therefore,
both may be included. The tight junctions of
the rete testis with the tubuli recti are situated near the lumen, and appear as a network
of frequently branching and anastornosing
ridges on the P face with complementary
grooves on the E face. The junctional elements
are composed of the smoothly surfaced strands
with some discontinuities. The particles in the
grooves are few on the E face (fig. 3). Some
basal strands are terminated as free endings
(fig. 3: arrow). The number of the strands in
the apical-basal direction varies from 3 t o 25,
and the mean number with standard deviation
is 8.8 5.0 (fig. 1). The large deviation is due
t o the regional variation within the rete
testis. The tight junctional meshwork in the
extratesticular rete is more extensive than
that in the intratesticular rete.
The ductuli efferentes
Between two nonciliated cells, the most
striking feature is that the belt-like gap junctions (fig. 4) are associated with poorly developed tight junctions. The number of the
tight junction strands is only 1.6 f 1.7. Fundamentally, the gap junction particles occupy
the compartmental space of the tight junction
meshwork, but the tight junctional elements
are frequently lacking. Therefore, the tight
junctional domain is almost completely replaced by the gap junctions in many places.
On the P face, the rows of the tight junction
particles are often obscured with the particles
of the gap junctions, while on the E face, tight
junction grooves with particles can be recognized easily (fig. 4: inset). The gap junction
particles are closely packed in hexagonal
arrangement, but in some areas, they lie scattered. Furthermore, in the belt-like gap junctions, particle-free regions are seen. Therefore, in some regions of the adluminal side,
there is no junctional element, as far as examined in single replica (fig. 5 ) .The macula type
of the gap junctions is also found a t the lateral surface below the tight junctional level
(fig. 4).
The lanthanum injected appears t o reach
the luminal surface between the nonciliated
cells (fig. 6).
Between ciliated and nonciliated cells, the
1.0. In
number of the tight junctions is 2.3
those cell types which are clearly identified as
ciliated and nonciliated cells, the gap junctions are not associated with the tight junction network (fig. 7 ) .
As the ciliated cells appear to be scattered
among the nonciliated cells, the junctions between two ciliated cells are infrequently observed. Out of 64 junctions in the ductuli efferentes whose cell types had been identified,
only three junctions between the ciliated cells
were found (fig. 8 ) . The tight junction meshworks are well developed, and their pattern
shows branching and anastomosing. The gap
junctions associated with the tight junction
meshwork could not be observed.
In figure 9 showing a cilium, small gap junctions in the poorly developed tight junction
meshwork are encountered. Although the associated tight junctions are similar t o those
between the ciliated and nonciliated cells, the
partner cell type cannot be identified in the
figure 9.
The epididymis
Throughout the epididymis, the tight junctions are formed by a large number of strands
showing geometrical variation along the duct.
Free ends of the strands to the basal side are
frequently observed throughout t h e epididymis. A t the initial segment of the caput
epididymidis (zone 11, the width of the tight
junctions is the largest proportionally to the
height of the epithelium (fig. 10). The branching and anastomosing of the strands are frequently observed in the luminal side. The
meshworks of the strands become loose in the
basal part of the junction. The orientation of
the strands is relatively ambiguous compared
with that of other parts in the epididymis.
Particle aggregations are recognized in the
basal part of the junction (fig. 10). They may
be corresponded with the desmosomes reported by thin sectioning (Hamilton, '75). On
the P face, the strands are mainly formed of
discrete rows of particles, and the short segments of smoothly outlined strands blended in
them. On the E face, many complemental particles are attached on the grooves. The number of the tight junction strands is 12.7 -+ 2.8.
At the caput epididymidis (zone 21, the
number of the tight junctional strands is
larger (15.6 f 3.7) but the width is narrower
than in the initial segment. As shown in
figure 11, the strands tend t o be oriented parallel t o the luminal surface.
In the corpus epididymidis (zone 51, the
tight junctional belt is narrower than the two
proximal parts. The number of the strands is
1.6). The
the lowest in the epididymis (8.3
branching and anastomosing of the strands
are observed frequently. The parallel orientation of the strands to the luminal surface is
less obvious than in the caput (fig. 12).
In the cauda epididymidis (zone 6A), the
belt has similar width to the corpus, and the
number of the strands within the depth is 9.9
2 1.9. As shown in figure 13, the most remarkable feature of the junctions is the parallel orientation of the strands. In comparison
with the more proximal part of the epididymis,
anastomosis and branching of the strands are
less frequently observed.
The ductus deferens
A t the adluminal side, the distribution of
the strands is moderate in density similar to
that of cauda. The belt of the tight junctions is
about 0.6 pm in width and many tight junctions extend toward the base (fig. 14). Some
strands appear randomly while others run
parallel to each other in regular spacing in the
lateral side of the cell (inset of fig. 14).
At the terminal segment of the seminiferous tubule, the tight junctions are endowed with the characteristics of the typical
Sertoli junctions described previously (Gilula
et al., '76; Nagano and Suzuki, '76b) in spite of
the absence of germ cells. The normal appearance of the Sertoli cell junctions in testicular
feminization syndrome (Nagano and Suzuki,
'76a1, in the germ cell-free testis induced by
busulfan (Gilula et al., '761, and in the genetically germ cell-free testis (Nagano et al., '77)
has been reported. The present work shows the
typical Sertoli cell junctions in physiological
condition a t the terminal segment of the seminiferous tubule devoid of the germ cells,
indicating that the germ cells are not essential for the junction formation of the Sertoli
cells. The junctional particles preferentially
left on the E face are not restricted in the Sertoli cells, but in the stria vascularis of the
inner ear (Reale et al., '75) and in the vascular endothelium (Simionescu et al., '75, '76).
Simionescu and Simionescu ('77) have interpreted behavior of the cleavage plane that
"the interactions between paired junctional
particles appear t o be generally stronger than
the interactions between each particle and the
underlying protoplasmic structures." In the
rat seminiferous tubules, the junctions of the
Sertoli cells show a resistance t o hypertonic
solutions (Gilula et al., '76). This result and
the tracer experiment (Dym and Fawcett, '70)
suggest t h a t the Sertoli junctions provide a
morphological evidence to act as a blood-testis
Dym ('76) has reported that the intercellular tracer compounds are blocked from entering the lumen of the rete testis by the juxtaluminal tight junction. He suggests t h a t a
blood-testis barrier exists a t the level of the
rete epithelium. He has also observed that the
junctions of the rete consist of one or two occluding junctions in the apical-basal direction
by thin sectioning. In the present study, a t
least 3 in number and 8.8 of the mean of the
strands are observed by freeze-fracture. The
discrepancy of the results obtained by freezefracture and sectioning has also been noted in
the vascular endothelium (Simionescu et al.,
'751, and still remains as an unsolved problem.
In the ductuli efferentes, the tracer reached
the luminal surface particularly between two
nonciliated cells. The number of the strands in
the juxtaluminal surface of the nonciliated
cell is the smallest in the male duct system.
The functional significance of the number of
the strands has been discussed in connection
with the permeability of various epithelia by
Claude and Goodenough ('73). According to
their classification, the nonciliated cell junc-
tions belong to the leaky type. MartinezPalomo and Erlig ('751, however, have suggested t h a t t h e number itself may not
necessarily be related to the tightness, since
cells with large number of strands are not
always tight in permeability. Although our
observations were made from a single replica,
the absence of the junctional elements shown
in figure 5 may give another evidence for the
weak barrier a t the ductuli efferentes. Focal
interruptions of the tight junctions have also
been reported in the mesothelium (Simionescu
and Simionescu, '77). Crabo ('65) calculated
t h a t almost 99%of the fluid leaving the testis
was resorbed a t the proximal part of the excretory duct in the bull. The nonciliated cells
of the ductuli efferentes have been known to
uptake the particulated matter injected into
the rete (Montorzi and Burgos, '67). As a more
effective way of fluid transport through epithelial layer, solute may be actively pumped
by the cells into the intercellular space resulting in a passive influx of water. Diamond ('74)
has speculated that if the junctions were
leaky, more salt could cross the junctions into
the intercellular spaces. The weak intercellular barrier of the ductuli efferentes may facilitate the fluid movement.
The rete testis was believed t o be the
weakest barrier along the duct system. Waksman ('59) and Johnson ('70) in the isoimmunization experiment suggested that the
rete testis was the primary site of the antibody invasion. The present observations, however, suggest that, morphologically, the ductuli efferentes are the most permeable in the
duct system, though experimental approval is
necessary to determine the real invasion site
of the antibody.
The belt-like gap junctions between the
nonciliated cells may indicate a functional
syncytium of the epithelium, since macula
type of the gap junctions is the site of the communicating channels (Staehelin, '74). The
functional significance of the belt-like gap
junction, however, has not been interpreted.
Such gap junctions with small number of discontinuous, short strands of the tight junction
are frequently observed, and have not been reported previously a t least along the duct system of the testis, but in the epidermal cells,
the junctions consisting of small gap junctions abutting against short tight junctional
strands have also been demonstrated (Elias
and Friend, '76).
Along the epididymal duct, geometrical or-
ganization of the tight junction meshworks
varies in accordance with the regional difference of the epithelium in the following
points. First, the width of the tight junctional
domain becomes narrower from the proximal
to the distal portion of the epididymis. Second,
more discontinuities of the strands appear in
the more proximal duct. Third, the network is
more frequently cross-linked in the more proximal duct, while the parallel orientation of the
strands is more conspicuous toward the distal
duct. The arrangement of the tight junctions
of the initial segment (fig. 10) and the cauda
(fig. 13) shows opposite extremities of the variations. These variations may reflect the different stress which distends the lumen around
the circumference, as was presented in the alimentary tract of the Xenopus (Hull and
Staehelin, ’76). Our observations on the preand postnatal development of the tight junctions in the caput epididymal epithelium also
support the correlation between the stress
condition and the geometrical organization of
the meshwork, since the pattern changes from
more anastomosing type t o parallel type coinciding with the accumulation of the luminal
fluid (Suzuki and Nagano, ’78).
It has been reported t h a t the proliferation
of the tight junctions throughout the lateral
surface of the cell appears after the treatment
with pronase in the pancreatic beta cell (Orci
et al., ‘731, phalloidine in the hepatocyte
(Montesano et al., ’76) and Vitamin-A in the
cultured skin (Elias and Friend, ’76).The scattered tight junctions in the lateral cell surface
of the ductus deferens are similar to those
above. The strands in the lateral side are not
likely t o be related t o the cell undergoing
autolysis or division, since these features are
frequently observed, and mitosis is rarely
found in the epithelium of the ductus deferens. One might speculate from the geometric appearance of these cells that the dispersed elements of the strands may not serve
as an effective sealing device, but they may be
related to the passive shape change of the epithelial cells by the pressure of the luminal
contents and the contraction of the thick muscle coat in the ductus deferens.
A part of this work was initiated by F. S. a t
the Department of Anatomy, Harvard Medical School. We thank Doctor Don W. Fawcett
for giving the research opportunity. The work
is supported by grants of the Japanese Ministry of Education.
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In freeze-fracture electron micrographs, the shadow direction is indicated by the
arrow at t h e lower right comer of each micrograph. The lumen side is mounted up in
all figures, unless otherwise mentioned.
2 Tight junctions of adjacent Sertoli cells in the terminal p r t i o n of t h e seminiferous
tubule. They are almost parallel to each other. Linear arrangements of t h e small
particles (small arrow) in the center of t h e grooves are observed on the E face (El.
On the P face (PI, narrow furrows (F) can be seen a t the top of t h e ridges, and large
particles (large arrow) are observed. The lumen is situated at the left side of this
figure. X 71,000.
3 Showing t h e tight junction network of t h e extratesticular rete testis. Most of the
junctional elements can be observed as strands with some discontinuities on the P
face (PI. On the E face (El, t h e complementary grooves are observed. Many branchings and anastomoses of the strands are noted. The arrow points to a free ending. L,
luminal side. X 63,000.
Fumie Suzuki and Toshio Nagano
The junctional area of the adjacent nonciliated cells of the ductuli efferentes. The
belt-like gap junction encircles the adluminal surface of the cells. The particle aggregation and corresponding pits are noted. The segmented tight junctions (arrows)
can be seen a t the border of t h e belt-like gap junctions. A macula type of the gap
junction (GI is situated below the adluminal region. Inset showing the tight junctional grooves (arrowheads) in the E face. The gap junctions are accompanied with
the tight junctions. Mv, microvilli; L,lumen. X 52,000; X 90,000.
5 Higher magnification of the belt-like gap junction between adjacent nonciliated
cells of the ductuli efferentes. In the gap junctions, a particle-free lane devoid of
tight junction is indicated by pair of arrows. A large arrow shows a segmented tight
junction. Note much more particles on the P face of the microvilli (Mv) than on the
cytoplasmic P face. x 100,000.
6 The lanthanum can be traced between the nonciliated cells, and reaches toward the
lumen (L). Without counter staining. X 75,000.
Fumie Suzuki and Toshio Nagano
7 The tight junction between the ciliated (left) and nonciliated (right) cells in the
ductuli efferentes. The junction consists of only one strand in about a half length of
the belt. The cell type of the left can be identified by the particles of the ciliary
necklace indicated by arrows. The right cell can be recognized as a nonciliated cell
by the presence of the apical canaliculi ( A d , which are characteristic in this cell
type. E, E face; P, P face. x 41,000.
8 The tight junction between two ciliated cells in the ductuli efferentes. The meshworks of the tight junctions are remarkable in number. On the P face (PI, continuous and some discontinuous strands of the tight junctions can be seen. On t h e E
face (El, some particles are observed in the grooves. The cilia (C) in t h e lower right
cell and ciliary necklace (arrow) as well as ciliary rootlets (circles) in the upper left
cell indicate that the junction is situated between two ciliated cells. X 44,000.
Fumie Suzuki and Toshio Nagano
9 Segmental tight junction and small gap junctions can be seen in the ciliated cell of
the ductus efferentes. Regions devoid of both junctions are indicated by the
arrows. The type of partner cell could not be identified. C, cilium. X 73,000.
10 The initial segment of the epididymis. The tight junction belt is wide. The mesh-
works are finer in the luminal side than in the basal side. Desmosome (*) can be
identified in the basal compartments of the meshworks. On the P face (PI, the
strands are discrete rows of particles blending smoothly outlined strands (small
arrows). Many strands terminate freely in the basal side (large arrows). On the E
face (El, the particles can be seen in the grooves. X 43,000.
The tight junction belt with parallel arrangement of the strands can be seen in the
caput epididymidis. The arrows point the free endings of the strands. P, P face; E,
E face. x 36,000.
12 The tight junction with anastomoses is in the corpus epididymidis. The parallel
orientation of the strands is not obvious in this region. The arrows indicate the
free endings. X 49,000.
Fumie Suzuki and Toshio Nagano
13 Most of the strands are parallel to each other with small number of branching8
and anastomoses in the cauda epididymis. Each strand shows smooth outline.
Free endings (arrows) are seen in the network. X 60,000.
14 The meshworks can be seen near the luminal side of the ductus deferens. Many
junction strands extend toward the base. They are scattered randomly, but are partially arranged in parallel (arrows). Inset is higher magnification of the scattered
junctional elements in the lateral surface of the cells. X 28,000; X 53,000.
Fumie Suzuki and Toshio Nagano
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epithelium, regional, differentiation, excurrent, junction, testis, revealed, rat, fractured, freeze, cells, duct
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