Elastic Fibers in the Rat Exorbital Lacrimal Gland Duct System

Mortimer Lorber

In the rat, each paired subcutaneous exorbital lacrimal gland overlies the retromandibular area. The fibrous cord containing its ducts passes over the masticatory muscles to the temporal canthus. Thus, the lacrimal system must accomodate both jaw and lid movements. To see whether elastic fibers exist to modulate alterations in tension caused by such movements, light and electron micrographs were made of its duct system. The innermost elastic fibers are connected by elaunin fibrils and oxytalan microfibrils to the lamina densa, particularly near the hemidesmosomes. The innermost elastic fibers appear to be longitudinal and about 0.2 nm from the lamina densa. Circumferential fibers exist about 0.8-3.2 nm from that structure. More peripheral fibers of both orientations also exist. Light microscopy of the extraglandular duct demonstrated circumferential fibers near the basement membrane and longitudinal and angular elastic fibers amid the collagenous layers. Some of the longitudinal fibers assume a loose cross-weave. Intraglandularly, as duct size diminishes elastic fibers progressively decrease in number and size until the smallest ducts have none. Thus, an elastic gradient exists. It is believed that recoil of the angular elastic fibers aids distension of the large and medium ducts when secretion is great and that recoil of the circumferential ones permits those duct diameters to diminish when secretion does. The longitudinal elastic fibers would allow all but the smallest ducts to recoil from the stretching of much of the exorbital lacrimal duct system accompanying blinking and other facial movements. Invest Ophthalmol Vis Sci 30:2002-2011,1989

Rat exorbital lacrimal glands are paired, shield- organ itself that might modulate dimensional shaped, subcutaneous structures overlying the front changes was performed. Their presence was antici- of the parotid gland, the retromandibular area and pated because in the embryo the lacrimal gland de- the rear portion of the body of the mandible. Venable velops as an evagination of the conjunctiva.4 The lat- and Griffin1 noted that they are oriented perpendicu- ter is known to contain elastic tissue,5'6 so it should larly to the zygomatic arch, resting on the superficial not be surprising if the epithelial outpouching that muscles of mastication. They lie slightly below and becomes the duct and eventually the gland carries just in front of each ear.2 Because these serous, tu- with it some of the collagen and elastic fibers of the bulo-acinar glands3 are the largest components of the connective tissue surrounding the bud. This presum- rodent lacrimal system, presumably they are the ably explains not only the present observations but major contributors to the secretions moistening the those made by Sundwall in 19167 that elastic fibers surface of the ipsilateral eye. Each has a gross struc- exist in the lacrimal gland of the ox. In 1897, Fuma- ture appearing to be its duct that extends between the galli8 described an intricate network of what he upper part of the organ near its anterior border and termed elastic fibers throughout the human lacrimal the conjunctiva near the temporal canthus. gland. Although the internal elastic membrane of ar- Because of the slight displacement of the exorbital teries was stained, his illustrations and description lacrimal gland as the mandible moves and the need of indicate that he often deemed reticulin fibers to be its duct to alter its length with eyelid movements, that elastic fibers in addition to those meriting such classi- is, stretching as the lids blink and recoiling as they fication. open, a search for elastic fibers in the duct and the Materials and Methods

From the Department of Physiology and Biophysics, George- One or both exorbital lacrimal glands were re- town University School of Medicine, Washington, DC. moved from six adult rats of both sexes following Submitted for publication: November 14, 1988; accepted March anesthesia with intraperitoneally administered so- 13, 1989. 9 Reprint requests: Mortimer Lorber, DMD, MD, Department of dium pentobarbital. For light microscopic studies, Physiology and Biophysics, Georgetown University School of Med- eight glands, often with a portion of their gross duct, icine, 3900 Reservoir Road, N.W., Washington, DC 20007. were excised from five animals and fixed in phos-

2002

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Fig. 1. Extraglandular portion of rat lacrimal duct stained for elastic tissue with orcein. (a) Cross-section of the cord which grossly appears to be the duct midway in its length. Two dissimilar ducts with precipitated luminal secretion are evident. At least five arterioles and one large venule (V) are present. Bar = 100 /im. (b) Higher magnification of bottom of the upper duct in (a). An occasional circumferential elastic fiber (arrows) immediately underlies the basement membrane. Numerous transected longitudinal ones exist more peripherally in several layers. Bar = 10 iim.

phate-buffered neutral formalin, dehydrated and em- lacrimal gland indicates that at least part of the struc- bedded in paraffin. Sectioning was done at 5 nm fol- ture contains two ducts of dissimilar size and shape lowed by hydration and staining with a Weigert's res- (Fig. la). orcin-fuchsin stain or a modified Tanzer's stain of Most elastin lies near the duct epithelium. Immedi- 0.4% orcein in acidified ethanol with Van Gieson ately subepithelially, a layer of delicate circumferen- counterstaining. tial elastic fibersabou t 0.2-0.5 urn in diameter is seen For electron microscopy, part of the gross extra- (Fig. lb). Many larger, longitudinal elastic fibers, up glandular duct of one adult rat of each sex was fixed to 1.2 nm in diameter, exist more peripherally in a for 1 hr in 5% glutaraldehyde and 4% paraformalde- number of layers (Figs, lb, 2a). Some angular elastic hyde in 0.2 M phosphate buffer, pH 7.4. The tissue fibers that extend diagonally or somewhat perpendic- was postfixed for one hour in 1% osmium tetroxide in ularly are more peripheral (Fig. 2b). They are about 0.2 M s-collidine buffer at pH 7.4, a modified Kar- 0.4-0.8 /jm in diameter. Because they do not encom- novsky procedure.10 En bloc staining with 2% pass the duct and are often not truly perpendicular to aqueous uranyl acetate was followed by dehydration. the basement membrane, they have not been termed Transversely and longitudinally oriented pieces were radial fibers. then embedded in epoxy resin. Survey sections (1 There is also a loose weave of delicate elastic fibers, nm) were stained with 0.1% toluidine blue in 2% so- about 0.2-0.4 ^m in diameter, that is occasionally dium borate. Selected areas were sectioned at 0.08 seen close to the base of the epithelium (Fig. 2c). fim, placed on copper grids and exposed to 2% uranyl These connect with the surrounding thicker and acetate in 25% ethanol for 15 min followed by 0.04 M more numerous longitudinal elastic fibers (Fig. 2c, d). lead citrate for one to 2 min." The thin sections were Ducts in which a loose elastic weave is evident may examined using a JEOL transmission electron micro- be more relaxed than those that do not display that scope (Tokyo, Japan). architecture. Relaxation is indicated by unusually Light microscopic measurements of dimensions great undulation of collagen fibers (Fig. 2c). Whether and distances were made using an ocular reticle that this diminished tension existed in situ or developed had been calibrated by an objective micrometer. subsequently is not known. These studies accord with the ARVO Resolution Within the gland, between the largest ducts whose on the Use of Animals in Research. lumens have a maximum dimension of about 45-75 urn and the smallest ducts, the elastic fibers progres- Results sively diminish in number and often in thickness. Light Microscopy The main duct and its largest branches have a rela- Histological examination of what is grossly a single tively few, delicate circumferential fibers, often about cord appearing to be the duct of the rat exorbital 0.3-0.4 fim in diameter, usually appearing in a single

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Fig. 2. Extraglandular portion of rat lacrimal duct to demonstrate elastic tissue, (a), (b) and (d) are stained with Weigert's, (c) is stained with orcein. (a) Wall of transected duct showing a few delicate, dark, circumferential elastic fibers (arrows) between the epithelium and the innermost circumferential collagen fibers. Most elastic fibers are transected longitudinally as are the large collagen fibersi n the center. Bar = 5 f*m. (b) The transected duct has some circumferential fibers (arrow) both beneath the epithelial cells and at a distance from them (arrowhead). Transected longitudinal elastic fibers and more peripheral, thicker, angular elastic fibers(A ) are near the upper left of the duct. Bar = 10 Mm. (c) Longitudinal section grazing the base of the epithelium to reveal a loose cross-weave (arrowheads) of elastic fibersthroug h which the bases of the epithelial cells are seen. The undulating collagen suggests the tissue is relaxed. Many longitudinal elastic fibers (arrows) exist, particularly beneath the duct where the thick collagen fibers stain prominently. Some of those elastic fibers may connect with those in the cross-weave. To the left of the left arrow are several transected circumferential elastic fibers. Bar = 10 /urn. (d) About the longitudinally sectioned duct whose lumen (L) is visible very long elastic fibers are asymmetrically arranged, ie, those above the duct are fewer and often thicker than those beneath it. Some transected circumferential fibers (arrowheads) are present. Bar = 10 nm.

layer immediately beneath the basement membrane ^m thick are present in moderate numbers. They are underlying the pseudostratified columnar epithelium usually in one layer relatively near the basement (Fig. 3a). The surrounding longitudinal elastic fibers membrane but additional layers exist in the sur- are thicker, about 0.6-0.8 ^m, and more numerous. rounding collagen (Fig. 4a). Some close to the epithe- They are usually arranged in two to four layers lium assume a loose weave (Fig. 4b) resembling that among thick longitudinal collagen fibers in an 11-22 in the extraglandular portion of the duct (Fig. 2c). lim zone (Fig. 3b). That weave disappears around small ducts. Likewise, The medium ducts, whose lumens are about 20-40 no angular or circumferential elastic fibers and no or fxm in diameter, not only have circumferential and very few longitudinal ones are seen around the small longitudinal elastic fibers but may have some angular ducts, whether interlobular (Fig. 5a) or intralobular collagen and small numbers of fine angular as well as (Fig. 5b). No elastic fibers are seen in the smallest cross-woven elastic fibers (Fig. 4a, b). Typically, cir- ducts. cumferential elastic fibers are seen near the basement In general, elastic fibersappea r thin, rather straight membrane. Longitudinal elastic fibers about 0.4-0.9 and unbranched. Longitudinal elastic fibers are the

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Fig. 3. The largest intraglandular ducts, (a) The first portion of the longitudinally sectioned duct bends on entry. Several delicate vertical (arrowhead) and transected (arrows) circumferential elastic fibers lie close to the epithelial cells. The short vertical fibers are believed to be circumferential elastic fibers in the frontal plane of the figure. Longitudinal elastic fibers are more prominent, particularly at the bend of the duct. Angular collagenous fibersexis t at the lower left and above the duct. Clumps of desquamated epithelial cells are in the lumen. Orcein. Bar = 10 *im. (b) Longitudinal section of large duct. Longitudinal elastic fibers are present particularly above the duct. A few vertical circumferential elastic fibers (arrowheads) are seen as well as a few transected ones (arrows). On the right, a suggestion of an elastic weave exists through which the epithelium is seen. Weigert's stain. Bar = 10 (im.

most abundant and most constant. They, and those arterioles where an internal elastic membrane exists. in the cross-weave, are best seen when the ducts are Its elastic tissue is more like that in another ramifying sectioned longitudinally. However, circumferential low pressure system, the venous system, but the fibers and angular fibers are most evident in transected are not as close as in venules. ducts. By light microscopy, circumferential elastic The fibers that stain with elastic stains in the ab- fibers and those in the cross-weave are the finestones . sence of prior oxidation, as at present, are either elas- The lacrimal duct system's elastic component is tic fibers or delicate elaunin fibrils. By contrast, the not nearly as extensive or as highly organized as in even finer oxytalan microfibrils are said to lack elas-

Fig. 4. Medium intraglandular ducts, (a) is stained by Weigert's and (b) is stained by orcein. (a) Longitudinal section with predominantly longitudinal elastic fibers. Vertical circumferential elastic fibers (arrow) are seen at about five micron intervals. The longitudinal collagen fibers undulate more above than below the duct. A small area of elastic fiber cross-weave is exposed (arrowhead). Bar = 10 jim. (b) Longitudinally sectioned duct with small numbers of similarly oriented elastic fibers (arrow) and a sparse cross-weave of elastic fibers (arrowheads). Angular collagenous and a few angular elastic fibers (*) are perpendicular to the duct wall. Several acinar cells are at the lower left. Bar = 10 urn.

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Fig. 5. Small intraglandular ducts stained with orcein. (a) Relatively few longitudinal elastic fibers (arrows) lie near the epithelium of the longitudinally sectioned interlobular duct. The transected metarteriole above the right end of the duct and the venule at its upper left have circumferential elastic fibers (arrowheads). Bar = 10 nm. (b) A longitudinally sectioned intralobular duct has some adjacent longitudinal elastic fibers (arrows). Bar = 10 »m.

tin because they are not visualized by these routine brils, about 0.015-0.03 Mm in diameter. These have procedures for staining elastic tissue12 but are seen by tiny, gray elastin cores surrounded by microfibrils electron microscopy. (Figs. 8b, 9a). In turn, they ramify as black, oxytalan microfibrils, about 0.008-0.010 Mm in diameter, ap- Electron Microscopy pearing to have no cores except at very high magnification (Fig. 9b, c), where the microfibrils are seen to Electron micrographs of the extraglandular portion be tubular with a lumen diameter of about 0.004- of the exorbital lacrimal duct system demonstrate 0.006 Mm. The zone they occupy between the elaunin elastic fibers whose cores are surrounded by dark mi- and the lamina densa is about 0.10-0.20 Mm wide. crofibrils as well as having some internal ones13 (Fig. 6a). The innermost elastic fibers are about 0.2 /im Other filaments, about 0.005-0.009 Mm in diame- from the lamina densa. They are longitudinally ori- ter, cross the lamina rara interna from the lamina ented, have gray cores, and are about 0.25-0.3 nm densa to the basal surface of the epithelial cells (Figs. thick (Fig. 6a, b). Examination of other electron mi- 8b, 9a). These delicate structures are particularly crographs reveals the typical range of diameters to be concentrated opposite the hemidesmosomes on the 0.18-0.64 Mm. basal surface of those cells. The widths of the lamina densa and of the lamina rara change reciprocally. More peripherally, about 0.8-3.2 ^m from the That of the lamina densa is about 0.03 Mm in the lamina densa, are circumferential elastic fibers about interval between hemidesmosomes but increases to 0.3-1.1 nm diameter (Fig. 6b). Beyond them are lon- about 0.05 Mm opposite a hemidesmosome. By con- gitudinal and often other circumferential elastic trast, the lamina rara is about 0.07 Mm wide and a fibers. In the central region of the extraglandular gray color between hemidesmosomes but narrows to duct, the fibers are oriented longitudinally in several 0.04 Mm and is much darker opposite one (Fig. 9a). layers about 1.0-2.6 Mm apart (Fig. 7). In contrast to That the peripheral fibrils of an elastic fiber are the innermost fine elastic layer, the peripheral ones tubular is evident in high magnification views of such are not joined to the basal lamina and have pale, fibers near epithelial cells (Fig. 9b). The fibrils have a amorphous cores (Fig. 6a, b). The distances of all diameter of 0.010-0.020 Mm. The lumen diameter is layers from the basal lamina vary as the latter undu- about 0.004-0.008 Mm. The interfibril distance is ap- lates. Thus, the closest elastic fibers may be between proximately the same as the fibril diameter (Fig. 9c). 0.05 and 0.4 Mm from the basal lamina (Figs. 8a, b, 9a). Some elastic fibers in adjacent layers are con- Discussion nected by fibrils (Fig. 8a). Connections between the innermost elastic fibers In the exorbital lacrimal gland duct system, the and the basal lamina occur particularly where the grouping of the elastic fibers into several layers with undulating lamina densa is very close to the elastic differing orientations resembles the arrangement in fibers. The connection is by two types of fine fila- blood vessels.14 However, in the latter many of the ments. Nearest the elastic fibers are the elaunin fi- numerous fibers are organized into elastic mem-

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Fig. 6. Electron micrographs of the longitudinally sectioned extraglandular portion of a lacrimal duct in the vicinity of its basal lamina, (a) A longitudinal elastic fiber with many fibrils and with a grayish core (e) is connected to the overlying lamina densa (*) by more delicate fibrils and microfibrils. Peripheral to this fiber is a group of three transected circumferential elastic fibers. These are larger and have pale elastin cores (E). The fibrils of the elastic fibers have diameters of about 0.05-0.10 (im and many exhibit a 0.013 ^m «$ interval beading (arrowhead). Similar prominences 6a are between the numerous transected collagen fibrilsa s well as between and around the transected elastic fibers. A few lie within their cores. Bar = 0.13 Mm- (b) A lower magnification of the preceding. Alternating longitudinal (arrows) and groups of circumferential (c) elastic fibers exist at various distances from the basal lamina of the epithelial cell, a portion of whose nucleus is seen at the upper left. A fibroblast process is at the lower right. Bar = 0.74 fim.

branes. These do not exist in the lacrimal duct these differences may have physiological importance system. is believed for three reasons: The architecture of this system's elastic compo- (1) Angular and circumferential elastic fibers pre- nents, together with that of the collagen fibers in sumably aid changes in duct diameter. The angular whose midst they exist, arouses interest as to the pos- elastic fibers would be tensed not only normally but sible functions of the variously oriented fibers. That particularly when the ducts toward which they are

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Fig. 7. Electron micro- graph of a longitudinally sectioned extraglandular lacrimal duct at low magnification showing some very fine longitudinal elastic fibers next to the lamina densa along the upper border of the figure. Layers of larger longitudinal elastic fibers lie in undulating, predominantly longitudinal, collagen within 12 ^m of the epithelial cell. The two narrow fibers marked (*) are probably one longitudinal fiber whose central portion is out of the plane of the section. Dark, organelle- containing fibroblast processes contact the two large elastic fibers toward the epithelium as well as the very long one farthest from it. Bar = I A3 urn.

oriented narrow. Their subsequent recoil would facil- similarly to those elastic fibers.Fo r example, as a duct itate and maintain distension of the large and me- constricts both types of angular fibers would elongate dium ducts when lacrimal secretion is increased, per- and become more taut and both types of circumfer- mitting them to serve as reservoirs. Concomitantly, ential fibers would shorten and become more lax. the circumferential elastic fibers would be tensed as Had they not done so, the lengths of all fibers would they stretch to accomodate the larger lumen size. The have been disproportionate to the smaller lumen, if ensuing recoil of these circumferential elastic fibers, such change in lumen size could even have occurred accompanied by tensing of the angular fibers, would independent of fiberalterations . aid the return of those ducts to their resting diameters (2) The longitudinal fibers would primarily pro- when secretion diminishes. It is believed that the cor- vide resiliency to the duct system. This would be due respondingly oriented collagen fibers would function not only to the longitudinal elastic fibers15 but to the W

Fig. 8. Electron micrographs of the extraglandular portion of a lacrimal duct at high magnification, (a) Transverse section showing a band of fibrils connecting a small elastic fiber with a more peripheral, larger one that is in contact with several fibroblast processes. The former is connected by microfibrils (arrowhead) to the basal lamina near a hemidesmosome (arrow). Another transected longitudinal collagen fiber is at the lower right. Both fibers have gray cores with faint outlines of tubular fibrils. Bar = 0.25 Mm. (b) Longitudinal section showing part of one elastic fiber with a pale gray elastin core (e) surrounded by fibrils which extend between the adjacent white collagen fibers. Some of the dark fibrils are tubular with a gray amorphous core {arrowheads). These are elaunin fibrils. From those near the lamina densa (arrow) tinier oxytalan microfibrils extend to that structure. Other microfibrils are seen traversing the lamina rara to complete the linkage between the innermost elastic fibers and the duct cell. Bar = 0.20 ftm.

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longitudinal collagen fibers whose normal laxity16 because of their sparse collagen and essentially absent would permit them to extend as the duct stretches elastin. Thus, the transmission of forces to those areas during blinking. This would occur because the tem- would be minimal, sheltering them from such varia- poral canthus near which the duct attaches moves tions in their physical environment. This would con- forward then when the lids close in blinking and trast greatly with the larger structures of the rat lacri- moves backward when they open. As the lids open, mal duct system in which both types of fibers are the longitudinal elastic and corresponding collage- abundant. nous fibers, both of which had been stretched by the The dimensional changes in which the elastic fibers preceding lid closure, would shorten. The straight participate must in some fashion be transmitted to elastic fibers would rapidly recoil and the accom- the duct epithelium and to the neighboring collagen. panying collagen fibers would regain their usual de- These will be considered separately. Where elastic gree of undulation. As a result, both the extraglandu- tissue is subjacent to the duct epithelium a physical lar and the larger intraglandular portions of the duct connection exists by means of elaunin fibrils and system would shorten. Length changes would also be oxytalan microfibrils. The latter are connected to the expected when other tensile forces, such as those en- basal plasma membrane, particularly at hemidesmo- gendered by vigorous scratching of the facial skin or somes20 where the cytoplasmic tonofilaments also in- by mandibular movements, act on these superficial sert.21 This microstructural framework may provide lacrimal structures which rest on the muscles of mas- the mechanism by which the elastic fibers' tension tication. and movements could affect the basement mem- (3) As the extraglandular duct is stretched, the brane and its adjacent duct epithelium, including the long elastic fibers that form a loose weave near the width of its lateral intercellular folds.22 All would be base of its epithelial cells and those of its larger in- expected to vary as duct length and diameter change. traglandular branches would become more longitu- Thus, this mechanism might affect the movement of dinal, thereby enhancing support of the epithelium, water and of ions such as potassium and chloride23 to reenforcing the duct wall and presumably constrict- influence the composition of the lacrimal secretion ing the lumen somewhat as the weave tightens. This and permit both the ductal epithelium and the un- would contrast with the looser cross-weaving of elas- derlying connective tissue to move harmoniously in tic fibers noted in relaxed tissues.17 response to displacing or distending forces. That the longitudinal elastic fibers have immature The connection of the elastic fibers with collagen morphology compared to the circumferential ones is may be by their most peripheral microfibrils pene- suggested by the smaller and darker cores of the trating the matrix between adjacent collagen fibrils former (Figs. 6a, b, 8a, 9b, c). Such moderately elec- (Figs. 6a, 8b). Even collagen fibrils appearing to be at tron-dense areas are said to correspond to young, un- some distance from an elastic fiber in the plane of crosslinked elastin.18 Although some lighter core section are bounded by similar-appearing structures areas are present within the longitudinal fibers (Figs. (Figs. 6a, 8b). Haust25 stated that microfibrils form a 8b, 9a), even they are not pale. continuum common to both collagen and elastic tis- A gradient of elastic fibers exists within the duct sue. He likewise noted that the microfibrils have a system of the rat exorbital lacrimal gland. Their num- periodicity of 0.004-0.014 nm. At present, a periodic- ber and size tend to be maximal in the extraglandular ity of about 0.013 nm has been found (Fig. 6a). This portion of the duct and progressively diminish within linkage between the elastic and collagen fibers may be the organ. Additionally, the number of layers of elas- the mechanism by which forces exerted on one type tic tissue decreases. These reductions are accompa- of fiber can be transmitted to the other so that coordi- nied by the elimination of fiberso f a particular orien- nate movements occur. tation from various parts of the system, leaving those In man, the conjunctiva from which the lacrimal that presumably would be physiologically more nec- organ arises26 contains elastic tissue27 that presum- essary. Because the longitudinal ones are the most ably affects conjunctival extensibility.6 Likewise, the numerous and because they comprise the last re- presence of such tissue in the lacrimal duct system maining elastic fibers in the smaller ducts, the ability may affect its response to forces. A similar relation- of these particular fibers to shorten the duct system ship of elastic fibers to the basement membrane be- may be the chief function of its elastic components. neath epithelium, as has presently been shown, also The biophysical property of connective tissue exists in the skin. termed compliance ("stretchability"),19 which both Cotta-Pereira et al28 noted the presence of elaunin collagen and the elastic tissue in the gland aid, would fibrils superficial to the dermal elastic fibers of be markedly diminished about the most distant radi- human skin. The former ramify as oxytalan microfi- cles of the duct system (the smallest ducts and acini) brils that anchor the basal lamina and are, as is

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Downloaded from iovs.arvojournals.org on 10/02/2021 No. 9 RAT EXORBITAL LACRIMAL DUCT ELASTIC FIBERS / Lorber 2011

Fig. 9. Electron micrographs of the extraglandular portion of a lacrimal duct at very high magnification, (a) Longitudinal section showing part of one longitudinal elastic fiber with a pale gray elastin core (E) surrounded by fibrils.I n some of the latter a tiny, gray core (arrowheads) is evident. These are elaunin fibrils. From them, oxytalan microfibrils (*) extend to the lamina densa, particularly opposite hemidesmosomes (arrows). Traversing the lamina rara (**) are many microfilaments, especially at the hemidesmosomes, to complete the linkage between the innermost elastic fibers and the duct cells. A larger elastic fiber (F) and three fibroblast processes occupy the right border of the figure. Bar = 0.094 fim. (b) Cross-section of two adjacent longitudinal elastic fibers showing the tubularity of the peripheral fibrils and how they encompass two intervening transected collagen fibers (asterisks). Both oxytalan (arrowhead) and elaunin (arrow) are tubular. Bar = 0.08 urn. (c) Longitudinal section showing part of one longitudinal elastic fiberwhos e associated longitudinal fibrils are tubular. Its elastin core is barely evident left of center. The fibrils extending toward the black hemidesmosomes (H) are not clearly seen. Many of the inner collagen fibrils are circumferential and have been transected. Bar = 0.064 /im.

elaunin, fibrotubular (Fig. 9b). Thus, the most im- high osmolarity for use in electron microscopy. J Cell Biol mature fibers are near the epidermis and the most 27:137A, 1965. 11. Reynolds ES: The use of lead citrate at high pH as an electron- mature ones are deeper in the dermis. The arrange- opaque stain in electron microscopy. J Cell Biol 17:208, 1963. ment of lacrimal duct epithelium and its underlying 12. Alexander RA, Grierson I, and Garner A: Oxytalan fibers in elastic fibers is comparable except for the latter being Fuchs' endothelial dystrophy. Arch Ophthalmol 99:1622, far fewer than in skin. Other similarities are recogniz- 1981. able; for example, the positioning of the superficial 13. Kobayasi T: Electron microscopy of the elastic fibers and the dermal membrane in normal human skin. Acta Dermatol elastic fibers close to the basement membrane has Venereol 48:303, 1968. been said to aid the attachment of the epidermis to 29 14. Kelly DE, Wood RL, and Enders AC: Bailey's Textbook of the corium and the basement membrane itself has Microscopic Anatomy, 18th edition. Baltimore, Williams & been cited as offering "elastic support" to its attached Wilkins, 1984, pp. 403-407. cells.30 The presently observed microstructural link- 15. Gosline JM and French CJ: Dynamic mechanical properties of age appears to be the mechanism by which such an- elastin. Biopolymers 18:2091, 1979. 16. Ham AW and Cormack DH: Histology, 8th edition. Philadel- chorage and support would also exist in the lacrimal phia, J. B. Lippincott, 1979, p. 211. system and suggests that this mechanism may be a 17. Minns RJ and Steven FS: Scanning electron microscopy of widespread one for uniting epithelia and their under- stretched elastin fibres. Micron 5:127, 1974. lying elastic tissue. 18. Jaques A and Serafini-Fracassini A: Morphogenesis of the elastic fiber: An immunoelectronmicroscopy investigation. J Ultrastruct Res 92:201, 1985. Key words: basement membrane, elastic fibers, elaunin, 19. Viidik A, Danielsen CC, and Oxlund H: On fundamental and exorbital lacrimal gland, oxytalan phenomenological models, structure and mechanical properties of collagen, elastin and glycosaminoglycan complexes. Acknowledgments Biorheology 19:437, 1982. 20. Staehelin LA and Hull BE: Junctions between living cells. Sci The author wishes to thank Donald C. Hay and the per- Am 238:140, May 1978. sonnel of the Department of Pathology, particularly Eileen 21. Krawczyk WS and Wilgram GF: Hemidesmosome morpho- Rusnock of the Electron Microscopy Laboratory, as well as genesis during epidermal wound healing. J Ultrastruct Res Cassie Arthur and the staff of the Bethesda Dermatopa- 45:93, 1973. thology Laboratory for the histological preparations. 22. Scott BL and Pease DC: Electron microscopy of the salivary and lacrimal glands of the rat. Am J Anat 104:115, 1959. References 23. Alexander JH, van Lennep EW, and Young JA: Water and electrolyte secretion by the exorbital lacrimal gland of the rat 1. Venable JH and Grafflin L: Gross anatomy of the orbital studied by micropuncture and catheterization techniques. glands in the albino rat. J Mammalogy 21:66, 1940. Pflugers Arch 337:299, 1972. 2. Greene EC: Anatomy of the Rat. New York, Hafner, 1935, p. 24. Bock P: Staining of elastin and pseudo-elastica ("elastic fiber 86. microfibrils", type III and type IV collagen) with paraldehyde- 3. Walker R: Age changes in the rat's exorbital lacrimal gland. fuchsin. Mikroskopie 33:332, 1977. AnatRec 132:49, 1958. 25. Haust MD: Fine fibrils of extracellular space (microfibrils). 4. Loewenthal N: Driisenstudien II: Die Gl. infraorbitalis und Am J Pathol 47:1113, 1965. eine besondere der Parotis anliegende Druse bei der weissen 26. Ask F: Studien iiber die Entwickelung des Driisenapparates der Ratte. Arch Mikr Anat 56:535, 1900. Bindenhaut beim Menschen. Anat Hefte 40:489, 1910. 5. Leeson CR, Leeson TS, and Paparo A A: Textbook of Histol- 27. Garner A and Alexander RA: Histochemistry of elastic and ogy, 5th edition. Philadelphia, W. B. Saunders, 1985, p. 551. related fibres in the human eye in health and disease. Histo- 6. Alexander RA and Garner A: Elastic and precursor fibres in chem J 18:405, 1986. the normal human eye. Exp Eye Res 36:305, 1983. 28. Cotta-Pereira G, Rodrigo FG, and Bittencourt-Sampaio S: 7. Sundwall J: The lachrymal gland. Am J Anat 20:147, 1916. Oxytalan, elaunin, and elastic fibers in the human skin. J In- 8. Fumagalli A: II tessuto elastico nella glandola lagrimale vest Dermatol 66:143, 1976. dell'uomo. Monit Zool Ital 8:167, 1897. 29. Medawar PB: The micro-anatomy of the mammalian epi- 9. Pearse AGE: Histochemistry Theoretical and Applied, 4th dermis. Quart J Microscop Sci 94:481, 1953. edition. Vol. 2. Edinburgh, Churchill Livingstone, 1985, p. 30. Kefalides NA: Structure and biosynthesis of basement mem- 592. branes. International Review of Connective Tissue Research 10. Karnovsky MJ: A formaldehyde-glutaraldehyde fixative of 6:63, 1973.

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