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Electron microscopic observations on the vascular barrier in the cortex of the thymus of the mouse.

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Electron Microscopic Observations on the Vascular
Barrier in the Cortex of the Thymus of the Mouse'
LEON WEISS
Department of Anatomy, T h e Johns Hopkins University, School of
Medicine, Baltimore, Maryland
ABSTRACT
The thymus of newborn and young adult mice of the DBA strain was
studied under the electron microscope after fixation in osmium tetroxide and embedding in araldite. In most of the animals several intravenous doses of Thorotrast were
administered within 24 or 48 hours of thymectomy.
The fine vessels in the cortex of the thymus appear unusually competent. Their
endothelium is complete, their basement membrane broad, and their wall further increased by adventitial cells and extracellular tissue. The epithelial reticular cell is
the most peripheral adventitial element of these vessels. It extends slender processes
which encircle the vessel and, by similar processes, surrounds individual or clusters of
perivascular lymphocytes. This cell is disposed to mediate influences passing between
cortical lymphocytes and the blood.
By cytoplasmic processes of the endothelium and the adventitial reticular cells a
cellular pathway from the lumen to the outside of these vessels is established. In
smaller vessels, moreover, the adventitial enclosure by reticular cells is incomplete.
After intravenous injection thorium dioxide may be found in the vessel wall and,
to a limited degree, in the surrounding tissue.
These vessels resemble both the fine vessels i n the central nervous system where a
blood-brain barrier is present, and the terminal arterial vessels in the spleen.
The thymus, unlike the spleen and
lymph nodes, contains no germinal centers
and few plasma cells. Its cortex consists
of a uniform blanket of lymphocytes and
interspersed reticular cells. The spleen
and lymph nodes produce significant titers
of antibody after injection of antigen but
the thymus does not (Harris, Rhoads and
Stokes, '48; Thorbecke and Keuning, '53).
Except after injury, moreover, the thymus
clears little dye from the blood (Marshall
and White, '61a, b).
Yet the thymus appears capable of antibody production. It contains lymphocytes.
It reacts anamnestically (Stoner and Hale,
'55). It produces germinal centers, plasma
cells and antibody after antigen is injected
into its substance (Marshall and White,
'61a, b). Marshall and White have, accordingly, postulated the presence of a
barrier between the thymus and the blood,
similar to the blood-brain barrier.
In the mouse, from the junction of cortex and medulla, arterial capillaries run to
the surface, continue as capillaries a short
distance under the capsule, and return as
venous capillaries to medullary veins
(Smith, Conant and Sayer, '39). In this
paper the concept of a vascular barrier is
considered in relation to the structure and
competence of fine vessels in the cortex
of mouse thymus observed by electron
microscopy. Observations upon medullary
vessels will be presented in a subsequent
communication dealing with reticular cells
and the structure of the medulla.
MATERIALS AND METHODS
Thymus and spleen of five newborn and
six young adult mice of the DBA strain
were cut into blocks approximately 2 mm3,
fixed in 1% osmium tetroxide buffered to
pH 7.6 in phosphate buffer, dehydrated in
ethanol and embedded in araldite (Weiss,
'62).
In most mice 0.2 ml thorium dioxide
stabilized by dextrins (Thorotrast obtained
from the Heyden Chemical Company, New
York, N. Y.)was injected into a tail vein
1This work was supported by grant Hema C-5375
(C2) of the United, States Public Health Service.
A portion of thls work was read by title at the
yearly meetings of the Marine Biological Laboratory,
Woods Hole, Massachusetts, August 2 S 3 1 , 1962
(Abstract, Biol. Bull., Vol. 123, No. 2, p. 515, October.
'62). Dr. Sam Clark, Jr., presented a paper on the
hemato-thymic barrier at the Fifth Internaponal Congress for Electron Microscopy, Philadelphia, August
29-September 5, 1962.. (Abstract LL-7, Proceedings of
the Congress, Academlc Press, N.Y:, 62). Dr. Clark's
work and this work each emphasize the role of the
epithelial reticular cell i n the vascular barrier.
413
414
LEON WEISS
15 minutes or 24 hours before thymectomy.
In several animals three doses of Thorotrast were given within 24 hours or within 48 hours of thymectomy.
Blocks were cut in a Porter-Blum microtome, mounted on celloidin-coated grids
on which carbon had been evaporated,
stained with both uranyl acetate (Watson,
’58) and lead (Karnovsky, ’61), or with
lead alone, and studied under a Siemens
Elmskop I.
OBSERVATIONS
Fine cortical vessels have an outside
diameter of 4-8 ~1 and a luminal size of
1-3 LI in largest dimension. Larger vessels are near the medulla; smaller ones
lie beneath the capsule. The vascular wall,
even in the smallest vessels, is usually
lamellated, layers of cytoplasm alternating
with layers of extracellular tissue (figs. 4
and 5).
Endothelium. The endothelium forms
a complete cellular lining without apertures (figs. 2, 3, 4, 5, 6, 9 and 12). In any
transverse section portions of two or
three endothelial cells are present. Their
cell junctions are simple approximations
which, occasionally, are interdigitated. In
some cases densities in the cell membrane
at cell junctions, suggestive of desmosomes, are present. These are similar to
densities observed in arterial vessels in
the spleen (see, especially, fig. 6, Weiss,
’62). No intercellular cement is present
in endothelial cell junctions. Endothelial
cells present an irregular luminal contour.
Slender, finger-like processes, larger bulges
and irregularities are present (see, especially, fig. 2). However, when these fine
vessels contain a leukocyte, these irregularities are absent, and the luminal surface of the endothelium is smooth. Frequently cytoplasmic processes extend from
the basal endothelial surface into the underlying basement membrane (see, especially, fig. 5).
Endothelial cells contain moderate to
considerable numbers of small cytoplasmic
vacuoles bounded by smooth membrane,
some in touch with the plasma membrane.
Some vacuoles contain dense granular
material but most are optically empty.
Moderate numbers of small mitochondria
with simple cristal patterns are present.
The Golgi complex is small. The concentration of particulate RNP is usually low.
In a few cells filaments of the cytoskeletal
system, approximately 75 A in thickness,
are present (Weiss, ’62) (fig. IS).
Basement membranes and associated
extracellular tissue. The basement membrane may be very broad. It is continuous
with the extracellular tissue surrounding
adventitial elements (see below). In
smaller vessels this extracellular mural
complex contains ground substance and
no fibers. In larger vessels the more peripheral portion of the complex is variably
rich in collagenous fibers.
The basement membrane has a lucent
medial zone (fig. 2 ) . The peripheral zones
at the surfaces in touch with endothelial
and adventitial cells are moderately dense.
This zonation is typical of the associated
extracellular connective tissue as well.
An occasional finding is that a basal
endothelial projection extends through the
basement membrane and associated extracellular tissue and touches the adventitial reticular cell (fig. 7). Thus there is a
limited area where the vessel has no extracellular enclosure.
The extracellular connective tissue is
deeply stained with the PAS reaction.
Adventitial cells. The term “adventitial
cells” refers to cells in the wall of a vessel
exclusive of endothelium and muscle.
In the smallest vessels the basement
membrane is limited, and the vessel is
enclosed, by reticular cells, apparently of
epithelial origin. In all but the smallest
vessels, and sometimes in these, slender
irregular cytoplasmic plates are present in
the wall in the extracellular material between endothelial and the last adventitial
cells (see, especially, figs. 4 and 5). These
plates are continuous with the abluminal
endothelial projections, either directly or
by cell junction. They are also continuous
with corresponding projections from the
limiting reticular cell (see next paragraph).
In some cases, indeed, they closely resemble the cytoplasm of reticular cells and
are phagocytic. These slender cytoplasmic
structures, surrounded by extracellular
material, confer a lamellated nature upon
the vessel, wherein layers of cytoplasm
alternate with layers of extracellular tissue. They constitute a circuitous narrow
VASCULAR BARRIER IN THYMIC CORTEX
cytoplasmic path through extracellular tissue, connecting endothelium with the limiting cells of the vessel wall.
The most peripheral element of the
vessel wall is an epithelial reticular cell.
It extends processes which enclose a portion of the circumference of the vessel,
forming a boundary to the extracellular
connective tissue and becoming, in the
process, the most peripheral vascular element. By the participation of two or three
reticular cells the perimeter of the vessel
is enclosed. In the larger vessel this enclosure is complete. In the smaller ones,
there are gaps in the reticular cell cover
(see, especially, figs. 4 and 5). These
reticular cells extend processes away from
the vessel and these enclose perivascular
lymphoctyes (figs. 2 and 8).
Reticular cells. The nucleus of reticular cells is large and often polygonal. It is
larger and less dense than nuclei of
lymphocytes and endothelial cells, with
it:, chromatin often marginated; occasionally nucleoli are present (fig. 1). The
cjtoplasm of reticular cells is abundant
and rich in structure. Mitochondria are
large, their cristae few in number, their
ground material moderately dense. Particulate RNP is quite concentrated in
places, and scanty or absent elsewhere.
Under the light microscope basophilia is
subject to similar variation. The Golgi
complex is usually small. Vacuoles,
bounded by smooth membranes and optically empty, are common. Vacuoles about
0.1; u in diameter but varying in size, containing a granular or vesicular material
art: conspicuous and highly characteristic
of reticular cells (fig. 1). Reticular cells
mily also contain granules or related linear
stIuctures greatly varying in size. A charac eristic of these granules is a very dense
pe "ipheral component surrounding a large
inner granular moderately dense material
(we, especially, fig. 8).
rhe cytoplasm of reticular cells may be
swdlen. The organelles are set further
aprrt, and the cytoplasm appears empty
(fizs. 3, 4, 6, and 11). In such cells the
plasma membrane often appears broken.
This finding appears different from the
clear ectoplasmic cytoplasm at the periphery of reticular cells and in their processes
as in figure 11.
415
The cell surface of reticular cells is
noteworthy. Desmosomes are frequently
present between contiguous reticular cells,
but not in a predictable pattern (fig. 2).
No desmosomes were observed between
reticular cells and lymphocyes. The cell
surface may be prolonged into great numbers of vesicular or knobby proturberances
which indent neighboring cells (fig. 13).
The membranes of contiguous reticular
cells may follow one another in a whorled
pattern. These findings are especially
marked in Hassall's corpuscles but may be
seen between other reticular cells.
Lymphocytes. Almost all lymphocytes
in the cortex are small, oval cells whose
contour may be modified by surrounding
cells. The nucleus is dense, the cytoplasm
contains few membranes or vacuoles and
slight to moderate concentrations of particulate RNP. Activity of the cell surface
is seldom marked. Mitoses are frequently
found.
Capsule. The capsule, and septae, are
structures of dense connective tissue rich
in collagenous fibers and ground substance.
Plasma cells are present directly beneath
the capsule and septae, seldom elsewhere.
Extracellular space. Lymphocytes and
reticular cells are close together in the
cortex, the process of the latter running
between the former, with the result that
there is very little extracellular space in
the cortex. In places there may be a gap
about the size of a lymphocyte but in most
places there is virtually no extracellular
space. The most consistent extracellular
compartment is the complex in the blood
vessel wall of which the basement membrane is a part.
Thorotrast. After a single injection of
Thorotrast, thorium dioxide is present in
the plasma and in the extracellular complex of the vascular wall (fig. 12). It is
found in the endothelium (fig. 10). Even
after three injections it is seldom seen in
adventitial cells.
DISCUSSION
The fine vessels in the thymic cortex
appear unusually competent. Their endothelium is without apertures, their basement membrane broad, and their wall further increased by adventitial cells and extr acellular tissue.
416
LEON WEISS
Some thymic vessels have luminal diameters of about 1 N, Although erythrocytes
may occasionally be found squeezed into
them, such vessels may carry plasma preferentially. Their large-angle origin from
arteries in the cortico-medullary region
would facilitate plasma skimming, a
mechanism considered for the spleen
(Weiss, ’62).
The presence of abluminal cytoplasmic
projections of the endothelium and adluminal cytoplasmic projections of the adventitial reticular cells create a cytoplasmic
pathway from the lumen of the vessel to
the outside of the vessel. This path is
usually long, narrow and winding, through
the extracellular connective tissue, but
occasionally, as illustrated in figure 7, it
is quite direct. The implications of the
presence of this cytoplasmic path across
the vessel wall includes the likelihood that
some substances, barred by extracellular
tissue, may cross the wall. Now in the
peripheral cortical vessels the reticular cell
enclosure (vide infra) is incomplete. This
may indicate that the lymphocytes in the
peripheral cortex of the thymus are most
apt to receive humoral influences. LeBlond and Sainte-Marie (’60) have postulated a movement of cortical lymphoctyes
to the outside. Perhaps these lymphocytes
are initially stimulated in the thymus and
then move to the spleen to respond further
(Fichtelius, ’60).
The reticular cell is at once the most
peripheral constituent of the vascular
wall, and the cell which by its cytoplasmic
processes, segregates and encloses perivascular lymphocytes. Phagocytosis is one
of its properties (Lowenthal and Smith,
’52; Metcalf and Ishidate, ’62). It is likely
that this reticular cell is of epithelial origin
because of its well developed desmosomes
and the whorled and otherwise distinctive
modifications of its plasma membrane.
The epithelial reticular cells have been
characterized as “foamy cells” (Lowenthal
and Smith, ’52) or “PAS cells” (Metcalf
and Ishidate, ’62) on the basis of the appearance of the cytoplasm. While cytoplasmic bodies of many sorts may be
present, a highly characteristic and common vacuole is one approximately 0.20.4
in diameter, bounded by smooth
membrane and containing very small ves-
icles or granules. It resembles vacuoles
in reticular cells of the spleen. These vacuoles appear to account for the light microscopal picture of “foamy cells” or “PAS
cells.” It is likely that they are phagocytic
vacuoles, their uniformity of appearance
due to a uniformity of fare, i.e., lymphocytes. Other bodies, highly distinctive for
these cells, also stained in the periodic acid
reaction- but less common than the
phagocytic vacuoles - are dense spherical
or vermiform structures bounded by membrane, having a very dense peripheral component, and often displaying myelin figures. They resemble FOB-Kurloff bodies
of guinea pigs (Weiss, ’58) and appear
part of a complex secretory cycle. The
appearance of these structures under experimental conditions will be presented in
a subsequent paper. It is interesting to
speculate about their relationship to the
stimulating and retarding factors extracted
from the thymus by Szent-Gyorgyi, Hegyeli
and McLaughlin (’62).
The fine thymic vessels bear comparison to terminal arterial vessels in the
spleen of rabbits and dogs (Weiss, ’62).
In each case, vessels of capillary size
possess abluminal endothelial projections,
lamellated walls and phagocytic adventitial or vicinal cells. Structures in endothelial cell membranes reminiscent of
desmosomes are also present both in
splenic and thymic vessels. In the spleen
significant portions of red pulp are undoubtedly spared exposure to blood-bourne
substances cleared by sheaths. It appears
likely that thymic lymphocytes may be
similarly spared exposure to substances in
blood by action of adventitial reticular
cells. But in the spleen these vessels open
into broad responsive vascular spaces. In
the thymus, blood traverses these vessels
to be delivered into further competent and
relatively unresponsive vessels. These
findings underly the observations that the
spleen responds vigorously to blood-bourne
stimuli and the thymus does not. Yet the
unresponsiveness of the thymus may not
be laid entirely to its isolation from the
blood. It is likely the lymphocytes in the
thymus are less responsive to antigen than
lymphocytes in spleen and lymph nodes.
The matter is currently reviewed by Arnason, JankoviC and Wakesman (’62).
.
VASCULAR BARRIER IN THYMIC CORTEX
’
’
In both sheathed capillaries and thymic
vessels, thorium dioxide is found in the
basement membrane and associated extravascular tissue. The passage of colloid
through the vessel wall is consistent with
the capacity of the thymus to produce
some antibody after subcutaneous or intraabdominal injection of antigen (Stoner
and Hale, ’55).
The vascular barrier in the thymus has
been compared to the blood-brain barrier
by Marshall and White (’61a, b). The
comparison may be extended to their appearance under the electron microscope.
As the neuroglia invests blood vessels in
the brain, the epithelial reticular cell invests the vessels in the thymus. The apparent variation in hydration in thymic
reticular cells with a consequent variation
in volume, suggests a role in regulating
fluid transport across the vascular wall
and, along with this, control of the volume
of the extracellular space. Since the reticular cells comprising Hassall’s corpuscles
best show the cellular swelling and concentration of such substances as antibody
(White and Marshall, ’62),control of volume of the extravascular space and passage of substance to various parts of the
organ may be a function of Hassall’s corpu scles.
Vascular specializations in the lymphatic
system are pronounced. Lymph nodes
are open to influences of surrounding tissues through afferent lymphatics and with
the blood through post-capillary venules
(Schulze, ’25). The responsive tissue of
the spleen is in the blood stream (Weiss,
’63). The thymus lacks afferent lymphatics and its fine cortical vessels are
unusually competent. It is more severely
isolated from corporal influences than the
remaining lymphatic structures.
ACKNOWLEDGMENTS
I wish to thank Miss Linda Windsor for
preparing the sections for electron microscopy and Mrs. Jessie Price for help in preparing the plates.
LITERATURE CITED
Arnason, B. G., B. D. Jankovic and B. H. Wakesman 1962 A survey of the thymus and its
relation to lymphocytes and immune reactions.
Blood, 20: 617-628.
417
Fichtelius, K. E. 1960 Haemopoiesis.
Cell
production and its regulation. Ciba Foundation Symposium, Ed.: G. E. W. Wolstenholme
and Maeve O’Conner. Little, Brown and Co.,
Boston.
Harris, T. N., J. Rhoads and J. Stokes, Jr. 1948
A study of the role of the thymus and spleen
in the formation of antibodies in the rabbit.
J. Immunol., 38: 27-32.
Karnovsky, M. J. 1961 Simple methods for
“staining with lead” at high pH in electron
microscopy. J. Biophys. & Biochem. Cytol.,
11: 729-732.
LeBlond, C. P., and G. Sainte-Marie 1960 Models
for lymphocyte and plasmacyte formation.
Ciba Foundation Symposium, Ed.: G. E. W.
Wolstenholme and Maeve O’Connor. Little,
Brown and Co., Boston, p. 152.
Lowenthal, L. A., and C. Smith 1952 Studies
on the thymus of the mammal. IV. Lipid-laden
foamy cells in the involuting thymus of the
mouse. Anat. Rec., 112: 1-16.
Marshall, A. H. E., and R. G. White 1961a
Experimental thymic lesions resembling those
of myasthenia gravis. Lancet, 1 : 1030.
1961b The immunological reactivity
of the thymus. Brit. J. Exp. Path., 42: 379385.
Metcalf, D., and M. Ishidate 1962 PAS-positive
reticulum cells i n the thymus cortex of high
and low leukemia strains of mice. Australian
J. Exp. Biol. and Med. Sci., 40: 57-72.
Schulze, W. 1925 Untersuch ungen iiber die
Capillaren und postcapellaren Venen lymphatischer Organe. Zeitschrift fur Anatomie und
Entwicklungsgeschichte, 76: 421462.
Smith, C., B. D. Conant and E. G. Sayer 1939
The vascular pattern of the mouse thymus.
Anat. Rec., Supplement, 73: 47 (Abstract).
Stoner, R. D., and W. M. Hale 1955 Antibody
production by thymus and Peyer’s patches
intraocular transplants. J. Immunol., 75: 203208.
Szent-Gyorgyi, A., A. Hegyeli, and J. A. McLaughlin 1962 Constituents of the thymus gland
and their relation to growth, fertility, muscle
and cancer. J. Nat. Acad. Sci., 48: 1439-1442.
l‘horbecke, G. J., and F. J. Keuning 1953 Antibody formation in vitro by haemopoietic organs after subcutaneous and intravenous immunization. J. Immunol., 70: 129-133.
Watson, M. L. 1958 Staining of tissue sections
for electron microscopy i n heavy metals. J.
Biochem. & Biophys. Cytol., 4: 475-478.
Weiss, L. 1958 Aspects of the reticuloendothelial system studied with the light microscope and the electron microscope. Annals of
the N. Y. Acad. of Sci., 73: 131-138.
1962 The structure of fine splenic arterial vessels in relation to hernoconcentration
and red cell destruction. Amer. J. Anat., 111:
131-179.
1963 The structure of the intermediate vascular pathways in the spleen of rabbits.
Ibid., (In Press).
White, R. G., and A. H. E. Marshall 1962 The
autoimmune response in myasthenia gravis.
Lancet. no. 7247, ii: 120-123.
PLATE 1
EXPLANATION OF FIGURE
1
418
This field illustrates several differences between reticular cells (R.C.)
and lymphocytes (Ly). The reticular cells have large, open nuclei
and abundant cytoplasm. The cytoplasm contains many vacuoles
(V) which have some dense granular material in a clear matrix.
These are probably phagocytic vacuoles. The lymphocytes are small
cells, their chromatin i n dense blocks, their cytoplasm scanty. Lymphocytes may have moderate concentrations of RNP and mitochondria but usually have little membranous cytoplasmic material.
The preparation has been stained with uranium and lead. x 30,000.
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 1
419
PLATE 2
EXPLANATION O F FIGURE
BASEMENT ME
2
420
This field illustrates the relationships of a
blood vessel, epithelial
reticular cells and extracellular tissue. A n arterial capillary, about
6 p in diameter, from the deeper cortex, lies i n the right upper quadrant. A portion of a red cell is in its lumen. The endothelium form
a complete layer. Note the increased densities resembling desmosomes at endothelial cell junctions. Note, too, the variety of luminal
endothelial projections and vacuoles within the endothelium. The
basement membrane is well developed with a medial lucent layer.
Collagenous fibers are present, particularly in the left upper segment
of the basement membrane. Note the cellular processes lying in the
extracellular tissue of the vessel wall. The large process containing
many granules and vacuoles, lying primarily in the left lower quadrant of the wall resembles the cytoplasm of epithelial reticular cells
and may represent a cytoplasmic prolongation of a reticular cell.
Reticular cells form the outer limit of the vessel wall. They also
enclose perivascular lymphocytes by cytoplasmic process extending
away from the vessel. Note, particularly, the three lymphocytes in
the lower half of the field. The reticular cells possess desmosomes,
(arrows in the tracing), large vacuoles, and a fine fibrous cytoplasmic component, in addition to mitochondria, RNP, and endoplasmic
reticulum.
The preparation is stained with uranyl acetate and lead. x 20,000.
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 2
421
PLATE 3
EXPLANATION O F FlGURE
Er
3
422
A capillary is cut longitudinally. The endothelium is very attenuated
and the lumen, except a t the upper part of the plate is reduced to a
slit. See the accompanying tracing. Note the large ovoid adventitial
cell with the trunk-like process extending toward the top of the plate.
The basement membrane, particularly at the lower margin of the
plate is very clear. The basement membrane and associated extracellular tissue is moderately rich in fibers. Note the relatively
“empty” appearance of the cytoplasm of adventitial reticular cells
on the right.
This preparation is stained with uranium and lead. X 20,000.
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 3
PLATE 4
EXPLANATION OF FIGURES
Figures 4 and 5 are stained only with lead.
424
4
Note the lamellated wall in this subcapsular capillary. A reticular
cell ( R ) sends slender processes which constitute the most peripheral
adventitial element. Note (arrows) gaps in the reticular cell cover.
Note, too, the “empty” cytoplasm, “C,” on upper and lower aspects
of this vessel. Lymphocytes (Ly) surround the vessel. X 23,000.
5
This vessel is also subcapsular. Note the abluminal endothelial projection ( P ) , the adventitial reticular cell ( R ) and the gaps in that
cell’s envelopment of the vessel. X 23,000.
VASCULAR BARRIER I N THYMIC CORTEX
Leon Weiss
PLATE 4
425
PLATE 5
EXPLANATION O F FIGURES
426
6
Note the marked development of vesicles in the endothelium of this
subcapular vessel. The basement membrane contains collagenous
fibers. There are fibers of the cytoskeletal system in the cytoplasm
of a n adventitial reticular cell at “F.” Note the “watery” cytoplasm
( a t “C”) which appears a part of that reticular cell. X 34,000.
7
In this vessel two abluminal endothelial projections cross the basement membrane and have junctions with an adventitial reticular
cell. Note the gaps (arrows). x 50,000.
The above fields were stained with lead.
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 5
427
PLATE 6
EXPLANATION O F FIGURE
8
428
This is a capillary, approximately 5 p i n diameter, i n the outer third
of the cortex. Some Thorotrast lies o n its lumen. The endothelium
is markedly vesicular and contains some particles of RNP. The
vessel is limited by reticular cells. Along its right margin the reticular cell contains several vacuoles considered phagocytic in type. The
reticular cell along the vessel's lower margin contains some dense
granules which may be of a secretory nature. Note how the reticular
cell along the upper margin extends a slender cytoplasmic process
which lies i n the vessel wall (arrow). Lymphocytes enveloped
singly or in groups by reticular cells, surround the vessel.
The preparation is stained with lead. X 28,000.
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 6
429
PLATE 7
EXPLANATION OF FIGURE
9
A venous capillary just before its junction with a vein is presenl
here. Particles of T h o n are present in its lumen. Note the reticula]
cell with the large vacuoles along its lower border. It extends slen
der process away from the vessel and encloses a lymphocyte (Ly:
on the lower margin. O n the opposite aspect of the vessel a portior
of a reticular cell constitutes the most peripheral adventitial cell and
on its other surface, lies against perivascular lymphocytes (ly) ai
the upper right corner. The nucleus and cytoplasm of a n adventitiai
cell (Ad) lying between the endothelium and the above mentionec
reticular cell is also present here. Note the “watery” or empty char
acter of perivascular cytoplasm at the left margin (arrow). The
junction with the vein is on the right, out of the field.
This preparation is stained only with lead. X 20,000.
430
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 7
431
PLATE 8
EXPLANATION OF FIGURES
10 In this capillary, T h o n is present i n the lumen and in vesicles in the
vessel wall. Compare, especially, with figure 12.
The preparation is stained with lead. x 42,000.
11
432
Most of this field consists of cytoplasm of a reticular cell. Note, in
particular, the clear ectoplasmic process continuous with endoplasm.
Note, too, the three vacuoles containing small vesicles against a
clear ground. Portions of two lymphocytes are present along the
lower margin.
This preparation is stained with lead and uranium. x 23,000.
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 8
433
PLATE 9
EXPLANATION OF FIGURE
12 In this field the section reveals two cross-sections of capillaries.
Perhaps this is near a bifurcation. The lumen is marked “L.” Note
the vesicular endothelium, the endothelial cell junctions, the abluminal endothelial projections.
The basement membrane is particularly broad here. It is quite
clear and contains some ground substance and very long, slender
cytoplasmic processes. The latter originate as projections from endothelium or adventitial reticular cells. Compare the lamellated pattern
seen here in longitudinal section with that in figures 4, 5 and 6
seen in cross section. Note the slender cytoplasmic layer of reticular
cells limiting the vessel, separating it from lymphocytes.
Thorotrast, given 24 hours before thymectomy, is present primarily
in the extracellular tissues. Compare with figure 10.
The preparation is stained with lead. x 21,000.
434
VASCULAR BARRIER IN
THYMIC CORTEX
PLATE 9
Leon Weiss
435
PLATE 10
EXPLANATION OF FIGURE
13
436
This portion of the thymus is juxtamedullary cortex of a newborn
mouse. In the right upper corner, note the reticulocyte filling the
lumen of a capillary. Again, the lamellated pattern of cytoplasm and
extracellular material is evident in the vascular wall.
Note especially the formation of reticular cells about the vessel.
The cell surface of a given cell is thrown in vesicular processes
which indents the rarified cytoplasm of another. The continuity of
a rarefied cytoplasm with nucleus and perikaryon of a reticular cell
may be followed in the topmost reticular cell. At the arrows is the
change from dense to rarefied cytoplasm. These are changes which
are marked in Hassall's corpuscles but are also evident at the junction of reticular cells elsewhere. Another characteristic formation is
a whorled cytoplasmic junction, present near the center of the plate.
The preparation is stained with lead. X 21,000.
VASCULAR BARRIER IN THYMIC CORTEX
Leon Weiss
PLATE 10
437
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vascular, observations, microscopy, mouse, electro, corte, barriers, thymus
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