Electron microscope studies on the mast
cells and blood and lymphatic capillaries
of the human corneal limbus
Takeo Iwamoto* and George K. Smelser
Presumably normal human corneal limbal tissue from five eyes, enucleated because of malignant
tumors, was used in this study. The anterior area of the limbus, the only place where blood
and lymphatic vessels and mast cells were found in the normal cornea, was studied toith the
electron microscope, and the fine structure described and discussed. The majority of the mast
cells found in this area contained granules with a variable fine structure. Their constituents
could be resolved into several main components: a finely particulate, a round, and a rodshaped component. It is thought that the round and rod-shaped sections are the same structure
cut in different planes. Only a few mast cells contained granules of uniform structure, in xohich
case they were composed mainly of the fine particulate material, the particles of which were
sometimes arranged in an orderly fashion, giving a quasilamellar appearance. Most of the
limbal blood capillaries had a thick-walled endothelium. Some, however, possessed localized
thin-walled areas with or without fenestrations. Such fenestrations were not patent pores, but
toere closed by a thin membranous diaphragm. The basement lamina, variable in thickness,
generally consisted of one to several layers of alternating dense and less dense material. The
lymphatic capillaries tuere characterized by their poorly developed basement membrane, thin
endothelial cells, except near nuclei, absence of fenestrations, corpuscles within the lumen, and
pericytes.
trition, and pathology. To date, few studies
have been made of this area.1"'1 It is variable in width, approximately 1 mm., beginning at the termination of Bowman's
membrane and extending laterally toward
the sclera. The blood and lymphatic vessels associated with the cornea are found
at the corneoscleral junction almost exclusively in the above area. Although the
cornea receives most of its nutrition
through its anterior and posterior 'surface,
its limbal area receives some nutritional
support from these vessels which may be
of particular importance because such substances as proteins are more available from
them than from the aqueous humor or
tears. The limbal lymphatics are the drainage channels from the cornea from which
.he corneoscleral limbus possesses characteristics different from those of the central portion with respect to structure, mi-.
From the Department of Ophthalmology, College
of Physicians and Surgeons, Columbia University, New York, N. Y.
This investigation was supported by Research
Grants NB 00492-11 and NB 01202-09, and
Training Grant 5 TI NB 5324-04 from the
National Institute of Neurological Diseases and
Blindness, National Institutes of Health, United
States Public Health Service, Contract NONR
266 (71), Office of Naval Research, and a grantin-aid from the Alfred P. Sloan Foundation.
This investigation was conducted under a Fight
for Sight Postdoctoral Research Fellowship of
the National Council to Combat Blindness, Inc.,
New York, N. Y.
815
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816 hoamoto and Smelser
Figs. 1-4.
ic opposite page.
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Investigative Ophthalmology
October 1965
Volume 4
Number 5
Electron microscope studies of human comeal limbus 817
antigens, from grafts, for example, are presumed to be conducted most directly to
immunologically competent cells in lymph
nodes. Mast cells, the function of which
is entirely speculative, are found in this
tissue. They are absent from the normal
central cornea, appearing there only when
it becomes vascular.5' ° The cytology of
these cells and their fine structure have
been investigated in various tissues, including those of the limbus.1' 3 In general,
however, the fine structure of the mast
cells has not yet been clearly established.
These considerations, plus the paucity of
information concerning the fine structure
of limbal vessels, led to the present study
of this area in human eyes.
Materials and methods
Presumably normal comeal limbal tissue, from
five human eyeballs enucleated because of malignant tumors of the orbit, retina, or palpebral
conjunctiva, were used in this study.* Immediately after the enucleation, they were fixed with 1
per cent osmic acid for 2 hours, and, following
dehydration with graded alcohols, embedded in
Epon, as reported previously.7
For identification of the mast cells, correlated
thin and thick serial sections were made using the
LKB-Ultrotome. The thick (1 to 2 fi) sections,
0
The authors greatly appreciate the cooperation of Drs.
Algernon Reese, Ira Jones, and Robert Ellsworth who
made this surgical material available.
mounted on glass slides, were stained for light
microscopy in a solution consisting of 1 part of
Giemsa solution (Fisher Scientific Company) and
1 part phosphate buffer (1/10M, pH 7.2) 30-60
minutes at 60° C , or were stained by heating a
few drops of this staining solution directly on the
slide which was then rinsed with distilled water,
dried, and mounted in balsam. There was no
appreciable precipitate.
For a comparison of the staining of osmiumfixed, Epon-embedded mast cells and those prepared in the more usual histologic manner, the
choroid and limbal corneas of rats, where numerous mast cells are found,0 were fixed with either
10 per cent formalin or 1 per cent osmic acid,
embedded in paraffin, and stained with Giemsa or
toluidine blue (pH 8.5).
The thin sections were preheated with lead
citrates or uranyl acetate and lead citrate (double
staining) and studied with a Siemens Elmiskop 1.
The higher power election micrographs were taken
at an original magnification of x30,000.
Observations
The blood vessels in the comeal limbus
are seen almost exclusively in the anterior
comeal layers. This area, the peripheral
portion of which merges with the subconjunctiva, is triangular, or wedge shaped,
and intei-posed between the epithelium and
comeal stroma of the limbus with its apex
at the end of the Bowman's membrane
when seen in a radial section (Fig. 1).
The collagen fibrils constituting this area
are arranged more irregularly and loosely
Figs. 1 and 2. Light micrographs stained with Giemsa. All the others (Figs. 3 to 23) are
election micrographs. The line in each Fig. indicates 1 At.
Fig. 1. Light micrograph of a human comeal limbus, showing the anterior area. Arrow shows
the end of Bowman's membrane. E, epithelium; S, corneal stroma of the limbus; V, blood
vessel. Fixed with osmium and embedded in Epon. (Giemsa. xl20.)
Fig. 2. The area surrounded by the dotted line in this light micrograph corresponds to Fig. 3.
M, mast cell; the mast cell granules, stained reddish purple with Giemsa, are seen as black
dots. Figs. 2 and 3 are taken from the adjacent serial thin-and-thick sections.
Fig. 3. An electron micrograph of corneal limbal tissue corresponding to that shown in the
light micrograph, Fig. 2; E, epithelium; V, blood vessel; M, mast cell. The dotted area is
magnified in Fig. 4.
Fig. 4. Higher magnification of the area encircled by a dotted line in Fig. 3, showing the fine
structure of the mast cell. Each granule is encircled by a single limiting membrane. The fine
structure of the granules (a, h, c, d) is variable. However, the granule constituents may be
resolved into three main components, i.e., finely granular, round, and rod-shaped components
(see Fig. 9); P, cytoplasmic protrusion. Fine dots similar to RNP granules are seen in the
cytoplasm.
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Investigative Ophthalmology
October 1965
818 Iwamoto and Smelser
than those of the central corneal stroma.
The posterior part of this area appeal's
to make a gradual transition with the
deeper limbal corneal stroma. The following observations were made on the anterior area of the presumably normal human corneal limbus, referred to simply as
"corneal limbus" or "limbus" in this paper.
Results
Mast cells. There are many mast cells
in the limbus of the human cornea, frequently within several microns of blood
vessels. They were identified by comparison of the electron micrographs of thin
sections with light micrographs of the
immediately adjacent tliick section stained
with Giemsa (Figs. 2 and 3). The granules of the mast cells in the tliick sections
were stained dark reddish purple and the
cytoplasm was stained lightly. In the control rats the granules stained similarly with
both Giemsa and toluidine blue, but were
darker following osmium than following
formalin fixation. The comparison of formalin- and osmium-fixed thick sections with
respect to the staining of the mast cell
granules provided confidence that the cells
studied in the electron micrographs were
in fact mast cells.
The mast cells in the limbus are variably round, oval, or elongated, with cytoplasmic protrusions of their surface (Figs.
3 to 5, 8). The nucleus is central.
The cytoplasm was usually almost completely filled with granules, mitochondria
(Figs. 5, 6, 8, m), and Golgi apparatus
(Figs. 5. 6, g), with some rough-surfaced
endoplasmic reticulum (Fig. 6 er) in the
remaining spaces. Many fine granules indistinguishable from RNP particles (Fig.
4) and occasionally lipid droplets (Fig. 5
Z) were also seen. A group of mitochondria
was sometimes observed associated with
the Golgi apparatus (Figs. 5 and 6).
Fibrillar structures were occasionally observed, particularly in the cytoplasm of an
elongated cell (Fig. 7).
The mast cell granules were round or
oval in shape, with diameters ranging from
0.3 to 0.8 p. (majority 0.5 to 0.6 p). Each
was encircled by a single limiting membrane (Figs. 4 to 8) in which a trilaminar
structure similar to that of a unit membrane9 could be demonstrated at high magnification (Fig. 9, in). An apparently
empty space was frequently seen between
the limiting membrane and the contents
of the mast cell granule. Occasionally, a
double membranous structure 100 to 200 A
in total thickness was seen around the
granule surface (Fig. 9, arrow). The density and the fine structure of the granules
varied. However, their constituents could
be resolved into three main components or
possibly four (1, finely participate, 2,
round, 3, rod-shaped, and possibly 4, fine
particles arranged in a lamellar fashion).
The finely particulate component constitutes the matrix of each granule (Fig. 9,
(1)) in which a round body composed of
similar material is sometimes distinguished
(Fig. 9, r). The second, a round component about 0.1 /x in diameter, consists
of a peripheral zone composed of 2 to 5
alternate dense (about 35 A) and less
dense (about 65 A) concentric lamellae.
The central core is composed of less dense
material (Fig. 9 (2)). In higher magnification, a still finer substructure with
about 60 A periodicity was revealed traversing the concentric lamellae nearly radially (Fig. 9 (2)p). The third, a rodshaped component, about 0.1 JX in width,
also had a peripheral zone on both sides
parallel to the long axis, and a less dense
central zone (Fig. 9 (3)). Each peripheral
zone consisted of 2 to 5 dense, straight,
parallel lamellae, about 35 A in thickness,
separated by a less dense layer, about 65 A
thick. These lamellae run quite parallel
throughout the long axis of the rod. Finer
lamellae with about 60 A periodicity were
also disclosed with high magnification
(Fig. 9 (3) c, a, b), especially in the
central zone. These were oriented parallel
or obliquely to the long axis or formed an
interwoven lattice pattern. These finer
lamellae were observed mostly within the
two round or rod-shaped components, but
Text continued on p. 825.
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Electron microscope studies of human corneal limbus 819
Volume 4
Number 5
•4 J
g
Fig. 5. Kor legend see page S25.
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820 hoamoto and Smelser
Investigative Ophthalmology
October 1965
• • •
t.
8
Figs. 6-8. For legends see page 825.
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Volume 4
Number 5
Electron microscope studies of human corneal limbus 821
V
V
\
4
(i)
a
9
Fig. 9. For legend see page 825.
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822 hoamoto and Smelser
Investigative Ophthalmology
October 1965
bl
er
10
Figs. 10, 11. For irtifiiil.s set- pa<4»- <S25.
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Volume 4
Number 5
Electron microscope studies of human corneal limbus 823
bl
L
12
Figs. 12, 13. For lengends see page 825.
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Inoastigative Ophthalmology
October 1965
824 hoamoto and Smeher
f.
mv
g
m
4
bl
bi
r 15
Figs. 14, 15. For legends see opposite page.
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Volume 4
Number 5
Electron microscope studies of human corneal limbus 825
were also occasionally seen within the
granular component first mentioned. Each
mast cell granule was composed of either
one of the above described components,
or of their combination. The round or rodshaped components sometimes appeared as
fragments or debris, often located in the
periphery of a granule, otherwise com-
posed of the fine particulate component.
Thus, the apparent fine structure of these
granules is variable. Most of the mast cells
encountered in the limbus contained various types of granules within a single section of a cell (Figs. 4, 5, 7). Occasionally,
however, the granules of some mast cells
were uniform. In such cells, they consisted
Fig. 5. A frequently seen type of mast cell in the human corneal limbus; p, cytoplasmic protrusion; n, nucleus; g, Golgi apparatus; m, mitochondria; I, lipid droplet. The other round
structures are mast cell granules, the internal structure of which varies. The area outlined by
the dotted line is magnified in Fig. 9.
Fig. 6. A part of a mast cell in the human limbus; n, nucleus; g, Golgi apparatus; m, mitochondria; er, rough-surfaced endoplasmic reticulum; mg, mast cell granule. An apparent space
is seen between the limiting membrane and the content of the granule, which consists only
of rod-shaped components in this particular granule.
Fig. 7. A part of an elongated mast cell; mg, mast cell granule. Fibrils are seen in the cytoplasm (arrow).
Fig. 8. One of relatively few mast cells in the human limbus in which most of the granules
(mg) are composed mainly of a fine, granular material. In some of the granules, diffuse,
apparent lamellae of granules spaced at about 100 A intervals are resolvable with higher
magnification. These lamellae are barely seen (arrow) at this relatively low magnification; m,
mitochondria; p, cytoplasmic protrusion.
Fig. 9. Higher magnification of the area outlined by dots in Fig. 5, showing the detailed fine
structure of the mast cell granules. The limiting membrane has a trilaminar structure (m);
( I ) , finely particulate component; (2), round component. This consists of a peripheral area
(p) of concentric, grosser lamellae (about 100 A interval), and a less dense central area (c).
Finer lamellae (about 60 A periodicity) are seen traversing the grosser lamellae. (3), rodshaped component; this also consists of peripheral areas (p) of grosser lamellae and a less
dense central area (c). In the latter, finer lamellae are seen, shown more clearly at a, b; r,
round body.
Fig. 10. A thick-walled blood capillary in the human limbus; E, endothelium with nucleus
n; L, lumen; R, erythrocyte; P, pericyte; the rough-surfaced endoplasmic reticulum is prominent; hi, basement lamina; p, pseudopodial projection.
Fig. 11. A thick-walled blood capillary with a localized thin area without fenestration (arrow),
P, pericyte; E, endothelium; hi, basement lamina; g, Golgi apparatus.
Fig. 12. One of a few capillaries in the human corneal limbus which has fenestrations (arrow);
E, endothelium; hi, basement lamina; L, lumen; P, pericyte.
Fig. 13. Higher magnification of a fenestrated area of a blood capillary in the human limbus.
A membranous diaphragm (arrow) is seen closing the fenestration; hi, basement lamina; this
particular area consists of only one layer of dense material (see Fig. 16); L, lumen; R, erythrocyte; E, endothelial cell.
Fig. 14. An endothelial. cell of a blood capillary in the human limbus, showing cell organdies';
in, mitochondria; er, rough-surfaced endoplasmic reticulum; g, Golgi apparatus; c, centriole;
r, free RNP granules; mo, multivesicular body. Fine filaments are also seen; L, lumen; hi,
basement lamina.
Fig. 15. An endothelial cell of a blood capillary, showing filaments (/) in the cytoplasm; n,
nucleus. Pinocytotic vesicles (v) are also abundant; hi, basement lamina; L, lumen.
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• HT;"
826 hoamoto and Smelser
In vestigatioe Ophthalmology
October 1965
bl
•I
x
*'
17
Figs. 16, 17. For legend see page 829.
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Volume 4
Number 5
Electron microscope studies of human corneal limhus 827
•
18
Figs. IS, 19. l\)Y legend sec page 829.
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1
Investigative Ophthalmology
October 1965
828 hoamoto and Smelser
L
21
t
22
£*.-.^.~'
23
Figs. 20-23. For legend see opposite page.
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Volume 4
Number 5
Electron microscope studies of human corneal limbus 829
mainly of the granular component containing a few fractions of the others (Fig. 8).
In addition, in some of such granules, the
constituent finely particulate material was
arranged in parallel layers extending across
the entire granule, giving the appearance
of lamellae, separated by a space of about
100 A. These could be considered as a
fourth component.
A schematic representation of the structure of the mast cells and of their granules
is shown in Fig. 24.
Blood capillaries. Capillaries in the human corneal limbus consisted of an endothelium and a basement lamina in which
pericytes were frequently found. The endothelial cells were usually fairly thick; especially near the nucleus (Fig. 10). Some
capillaries, however, possessed localized
thin-walled areas, occasionally as thin as
0.06 /.<., without pores or fenestrations (Figs.
11, 17). Other capillaries, few in number,
also had localized thin-walled areas, but
clearly showed "fenestrations" (Figs. 12,
13). These were not patent pores, for they
were closed by a thin membranous diaphragm (Fig. 13, arrow). These are, in
fact, focal attenuations of the endothelial
cells; however, the term "fenestration" is
commonly used. Mitochondria, rough-surfaced endoplasmic reticulum, free RNP
granules, smooth-surfaced endoplasmic reticulum in vesicular form, as well as pinocytotic vesicles (Figs. 14, 15), were seen
especially in the thicker parts of the
endothelial cells. In addition, Golgi apparatus, centrioles, "multivesicular body," a
dense round body, an oval or rod-shaped
structure containing bundles of fibrils, and
frequently fine filaments 50 to 100 A, appearing as dots in cross section, were also
found (Figs. 14 to 17). The intercellular
borders were more or less tortuous, parallel,
and composed of adjacent cell membranes,
Fig. 16. A wall of a blood capillary in the human limbus, with a relatively thick basement
lamina. The basement lamina (bl) consists of several layers of dense material (d) separated
by less dense interzones (i). The former usually run nearly parallel to the outer contour of
the endothelium. A few collagen fibrils ( c ) are seen within the basement lamina. E, endothelium; the filaments are seen as dots in their cross sections ( / ) ; L, lumen; R, erythrocyte;
P, pericyte.
Fig. 17. A wall of a blood capillary in the human limbus. The dense zone of the basement
lamina is seen as a complicated network at x. Arrow: thin area of the endothelium, without
pores. A narrower, denser zone (o) is seen in the intercellular border (f) of the endothelium.
Dense round bodies ( r ) are seen in the endothelial cytoplasm.
Fig. 18. A lymphatic capillary in the human limbus. The lumen ( L ) contains no blood corpuscles. The endothelium ( E ) is flat and thin. No pericyte is seen. The basement membrane
is not as well developed as in the blood capillaries.
Fig. 19. Endothelial cells of a lymphatic capillary in the human limbus; n, nucleus; g, Golgi
apparatus; c, centriole; m, mitochondria. An intercellular junction (arrow) is very short. No
basement membrane is seen around the endothelium:
Figs. 20-23. Endothelial cells of lymphatic capillaries in the human limbus. In Fig. 20, a
basement membrane (bin) is clearly seen, but not in the other figures. Pinocytotic vesicles
( « ) are seen in the cytoplasm.
Fig. 2 1 . Mitochondria ( m ) , rough-surfaced endoplasmic reticulum (er), and filaments (/) are
seen. A basement membrane is not discernible.
Fig. 22. A tortuous intercellular border (i) is shown in which a narrower, denser area (arrow)
is also seen near the lumen.
Fig. 23. Another intercellular border (t) is shown in which a localized widening (arrow) of
the intercellular space is seen. Filaments (/) are abundant in the endothelial cytoplasm; L,
lumen. There is no basement membrane.
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Investigative Ophthalmology
October 1965
830 Iwamoto and Smelser
generally separated by 150 to 200 A. A
localized area, with a narrower gap and
denser cell membranes, associated with
slight densification of the adjacent cytoplasm was frequently seen near the luminal
side (Fig. 17, o), but this did not occur as
regularly as in the corneal endothelium.10
Microvilli or pseudopodial projections
(Figs. 10, 17, p) were observed on the
luminal surface. The endothelium was covered externally by a basement lamina.
The basement lamina of the limbal capillaries generally consisted of one to several dense layers, each of which was 0.03
to 0.1 JX thick, and nearly parallel to the
outer contour of the endothelial cells (Figs.
10 to 16, hi, 16, d). A less dense zone
filled the spaces between the dense layers
(Figs. 10 to 16, bl, 16, i). Occasionally, the
dense zone was seen as a complicated network (Fig. 17, %). Both zones appeared
to consist of a fine, filamentous material
condensed in the dense zone and loosely
arranged in the less dense interzone. Occasionally, collagen fibrils were seen embedded within the basement lamina (Fig.
16, c).
Pericytes were frequently observed
within the basement lamina. They were
generally located some distance from tlie
endothelial cells, with one or two dense
zones of the basement lamina between
them and the endothelium (Figs. 10 to 12,
16 P) although, occasionally, they lay very
close to the endothelial cell. At least one
dense zone of the basement membrane
surrounded the outer surface of tlie pericytes. Endoplasmic reticulum, free RNP
granules, mitochondria, Golgi apparatus,
centrioles, pinocytotic vesicles, and fine
filaments were seen in their cytoplasm, but
the filaments, in general, appeared less
prominent than in the endothelial cells.
The rough-surfaced endoplasmic reticulum
was often well developed (Fig. 10, p). Pinocytotic vesicles frequently were more
prominent on the outer than on the inner
surface of the pericytes, as had also been
noticed in retinal capillaries.11
Lymphatic capillaries. Lymphatic cap-
illaries were also observed in the limbus.
They differed from blood capillaries in
several respects. The basement membrane
was generally not well developed, or appeared to be absent in places (Figs. 18 to
23). When present, it consisted of fine filaments (Fig. 20, bm). No pericytes were
seen in the material studied. The endothelium consisted of a single continuous
layer of cells, usually thin and flat, except
near nuclei. The endotlielial cells were
frequently very thin, but contained no
pores or fenestrations. Blood corpuscles
were seen very regularly in the blood capillaries, but not in any of the sections of
lymphatic capillaries studied.
The endothelial intercellular border appears to be similar to that of blood capillaries; the double lines, made of adjacent
cell membranes, frequently followed a tortuous course (Fig. 22, i), and were more
closely approximated in areas where adjacent cytoplasmic densification was also
seen (Fig. 22, arrow). Sometimes the length
of the interface between two endothelial
cells was veiy short, however (Fig. 19,
arrow). A localized widening of the intercellular gap was occasionally seen (Fig.
23, arrow) which seldom occurred in blood
capillaries. However, there was no definite discontinuity between the cells. The
cytoplasm of the lymphatic endothelial
cells contained mitochondria, rough-surfaced endoplasmic reticulum, free RNP
granules, Golgi apparatus, centrioles, pinocytotic vesicles, and fine filaments (Figs.
19 to 21, 23) as did the endothelial cells of
blood capillaries.
Discussion
Blood capillaries. Blood vessels which
could supply nutrition to the cornea were
confined to the anterior area of the limbus.
Classification of capillaries based on their
fine structure has been reported.12'13 Many
of those seen in the corneal limbus have a
rather thick endothelium, and, therefore,
may belong to the "muscle type" in Fawcett's classification.13 However, some capillaries had thin walls, in localized areas,
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Volume 4
Number 5
Electron microscope studies of human corneal limbos S31
Fig. 24. A schematic representation of human limbal mast cells and their granules; (A) The
most frequently found type, and (B) the type only occasionally found. Nucleus (n), cytoplasmic
process (p), Golgi apparatus (g), and mitochondria (m). Cytoplasmic organelles found less frequently, e.g., ribosome-like particles, lipid droplets, filaments, and endopla.smic reticulum have
been omitted. The upper two circles are three dimensional representations of mast cell granule
structure. (1) A representation of the fine structure of the participate component which forms
part or all of the granules in both cell1 types (A and B). It forms the matrix (arrows in upper
circles) of granules which also contain the round, rod-shaped, and fragmented components.
Note the fine reticulum in this material. (2) Represents the structure of the round component
seen in some granules of A or as fragments in B. The fine lines crossing the concentric lamelles
which are about 100 A apart are about 60 A apart, and are not necessarily arranged radially. It
is believed that this is the appearnce, seen in cross sections, of the cylindrical entity as indicated by the arrows in the upper left circle. (3,) A schema of a longitudinal section of the
cylindrical entity. The fine lines are about 60 A apart, and the heavy, parallel lamelles are
similar in density, thickness, and spacing (about 100 A) to the concentric ones shown in (2).
The magnification of (2) and (3) is greater than that of (1).
occasionally containing fenestrations. Such
capillaries may be classified as the "visceral type" (Fawcett). Each of the fenestrations in the limbal vessels, however, was
closed by a membranous diaphragm, as is
also seen in nonocular tissues,1J'1G as well
as in the ciliary process17'1S and the choroid.17 It is not clear whether the two or
three types of limbal capillaries are simply
manifestations of different functional states,
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Investigative Ophthalmology
October 1965
832 hoamoto and Smelser
or whether they are, in fact, different types,
such as arterial or venous capillaries. In
any case, the thick-walled capillaries with
localized thin areas without fenestration
may be an intermediate form. It is tempting to suppose that diffusion from a capillary is facilitated by the thin areas, or
those showing fenestrations. The basement
lamina, composed of areas of variable
density and thickness, may act not only as
a barrier, or ultrafilter, concerned with the
passage of material, but also as a mechanical "buffer zone" between the outer, presumably firm, collagen wall and the inner
endothelial wall. Possibly this may allow
the capillary freer movement, although it
is not known whether the filaments seen
in the endothelium are related to active
contraction of the endothelial wall.
Lymphatic capillaries. There are rather
few reports on the fine structure of lymphatic vessels.10~2c; The lymphatic capillaries
in the corneal limbus appear to be similar
in structure to small lymphatics seen in
other tissues, with respect to: the indistinct
basement membrane,19*20 absent or rare
pericytes,—'23 thin endothelium,19'22j 23> 25
no fenestrae,21' 23> 2fi the cell organelles,23' -4
and the apparently loose intercellular connection."-0' -lj 23 However, no clear opening
or discontinuity between the adjacent endothelial cells, as seen by others,20' 23' 2f>
was observed in our material.
Mast cells. Several investigators have reported on the fine structure of the mast
cells, including their granules, in both
normal and pathological conditions in various tissues of animals and man. There have
been recent reviews by Smith27 and
Bloom.2S Our observations permit a classification of the fine structure of the granules into three main components as described above, among which the first component, i.e., the finely granular or perhaps
reticulated structures had been observed
in the mast cells of the other tissues.27'33
Some lamellar structures, including concentric or scrolled lamellae, similar to our
second component, have also been observed in other tissues,2Si 30"32> 31> 35 as well
as in the limbus.1 However, the descriptions of these were concerned only with
the grosser lamellae occurring at about
100 A intervals, but not the finer lamellae
of about 60 A periodicity reported here.
The third, or rod-shaped, component was
described as short cylinders by Jakus1 in
limbal and by Feeney and Hogan35 in choroidal mast cell granules. The round and
the rod-shaped components may be seen
in the same individual granule. The size,
number, and location of the grosser lamellae and the existence of the less dense
central zones lead us to the belief that the
round components are simply cross sections of the rod-shaped ones, as was also
presumed earlier.1'35 Therefore, mast cell
granules appear to consist of a finely granular, or reticulated, matrix in which cylindrical bodies are frequently embedded.
Such bodies may be arranged regularly or
haphazardly (Figs. 6, mg, 4, a, b, c, d),
and may appear as fragments or debris.
A somewhat similar but much larger cylindrical body composed of many lamellae
has also been reported in eosinophilic
granules of granulocytes.30 This, however,
is large and occupies nearly the entire
granule rather than comprising only small
component units of a granule as in mast
cells. One unusual fine structure of the
granules may possibly be classified as still
another component. It appears as a lamellar arrangement of particles, extending
across an entire granule at about 100 A
intervals. It has been observed only in a
relatively few limbal mast cells, notably
when the finely particulate component was
packed densely. A somewhat similar lamellar structure, distributed rather diffusely,
has been observed in the granules of the
blood basophils,37 as well as in other mast
cell granules,3S in which, however, neither
the rod-shaped nor round components
were seen.
This fourth component could be only
a variation of the heterogeneously arranged
particulate first component, and result from
its more dense packing. It also may be
possible that these apparent lamellae could
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Volume 4
Number 5
Electron microscope studies of human corneal limbus 833
eventually be organized to form the lamellae seen in the round and rod-shaped
components.
Hibbs and associates31-'A2reported two
types of mast cells in other human tissues.
The mast cells of one type were spindle
shaped and the granules appeared to be homogeneous or to consist of particles. Those
of the second type were round or oval,
and each granule consisted of lamellar
groups, some in the form of scrolls, in
close association with participate material.
With regard to the characteristics of the
granules, the majority of the mast cells
encountered in the limbus appear to be
similar to this second type of Hibbs and
colleagues,82 and the occasional cells we
observed, containing granules with a rather
uniform pattern, may be similar to their
first type. In our opinion, the apparently
different patterns of granules are merely
variations in the combination of two main
elements (participate material and the cylindrical entity). Possibly those granules
composed of particles arranged in an orderly fashion (quasilamellar) could be considered as an additional component. Thus,
this analysis may be consistent with the
conclusion of Hibbs and co-workers32 that
their two types of mast cells were only
variations of one. Contrary to their findings,
no correlation of the type of granule and
the shape of the limbal mast cells was
found in this study.
The finer lamellae with 60 A periodicity
resemble a crystal lattice observed in protein. 3I)> l0 In the mast cells, these lamellae
are seen most frequently within the presumed cylindrical bodies. They may be
concerned with the chemical component
of the granules, or may represent a basic
structural organization or framework of
the granules.
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