The fine structure of teleost epidermis

© 1968 by Academic Press Inc.

J. tJLTRASTRUCTURERESEARCH21, 213--221 (1968).

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T h e Fine S t r u c t u r e of T e l e o s t E p i d e r m i s 1 II. Mucous Cells RAY C. HENRIKSON2 AND A. GEDEON MATOLTSY

Department of Dermatology, Boston University School of Medicine, and Department of Biology, Boston University, Boston, Massachusetts 02118 Received August 1, 1967 Mucous cells which resemble intestinal goblet cells appear throughout the epidermis of teleosts but vary in number in different body regions and species. The ergastoplasm is highly developed in immature mucous cells. Several systems of Golgi membranes are found and seem to be foci of mucogenesis. Clear vesicles appear near the convex surface of the curved Golgi lamellae and vesicles containing a relatively dense material are seen at the concave surface, adjacent to the developing mucous droplets. Mucogenesis in the epidermis of teleosts appears to involve transport from the ergastoplasm through the Golgi complex to the mucous droplets. At a later stage the droplets fill nearly the entire cell. Mature mucous cells often are opened at the surface of the epidermis indicating a release of their secretion. The filament-containing cell is the most numerous cell type in teleost (bony fish) epidermis. These cells are similar, in m a n y respects, to the filament-laden cells of m a m malian and anuran epidermis and have been discussed in detail in the previous paper (6). The several types of unicellular glands, e.g., mucous cells, club cells, chloride cells, present in teleost epidermis clearly separate it from the epidermis of other vertebrates. The most commonly encountered unicellular glands are the mucus-producing cells which have been studied extensively at the level of light microscopy (1, 2, 4, 5, 7, 9, 10). Mucous cells are found basally in the epidermis and increase considerably in size as they pass through the mid layers. Finally, engorged cells reach the surface, where they discharge their product which presumably is a major component of slime. In this study the fine structure of mucous cells is described as seen at various levels in the epidermis of teleosts, and correlations are made between the fine structure of the cell and its function. 1 This investigation was supported by Research Grant A M 05779, National Institute of Arthritis and Metabolic Diseases, United States Public Health Service. This publication is based on a Doctoral Dissertation submitted by R.C.H. to the Department of Biology, Boston University. 2 Present address: Division of Animal Physiology, C.S.I.R.O,, Prospect, New South Wales,

Australia.

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FIG. 1. Anguilla. The extraordinary thickness of eel epidermis and a variety of cell types are shown in this light micrograph. Mucous cells (M) are relatively large and stain intensely. Immature mucous cells (m) are seen deeper in the epidermis and contain small s~retory droplets. Fully mature mucous cells are noted at the surface releasing their contents (arrows). Relatively large club cells (C) also are present through the epidermis. Deep in the epidermis they are elongate, but they become rounder in the mid-epidermis. They have a characteristic central vacuole and nucleus. The filament-containing cells (barred arrows) are much smaller than the secretory cells. Basally, they are columnar in shape. At the surface they appear flattened and form a cobblestone-like layer that is punctuated only by the orifices of mucous cells. Thick Araldite section. Azure B. x 380. M A T E R I A L S A N D METHODS Epidermis from the following teleosts was studied: Carassius (goldfish), Lebistes (guppy), Corydoras (catfish), and Anguilla (eel). Technical details have been described in an earlier publication (6). OBSERVATIONS By light m i c r o s c o p y it is a p p a r e n t t h a t the epidermis of all teleosts e x a m i n e d contains m u c o u s cells b u t their relative n u m b e r a n d d i s t r i b u t i o n vary f r o m species t o species. The p a r t i c u l a r slimy n a t u r e of the eel reflects the large n u m b e r of m u c o u s cells in its epidermis; also, the d e v e l o p m e n t a l stages of these cells are well s h o w n irt this species. I n the eel (Fig. 1), m u c o u s cells containing distinct granules are f o u n d in the deeper layers of the epidermis. M o r e superficially, the m u c o u s cells are enlarged a n d s e p a r a t e granules c a n n o t be discerned. Such cells do n o t necessarily b e c o m e g o b l e t - s h a p e d a l t h o u g h the nucleus is displaced a l o n g with the r e m a i n i n g c y t o p l a s m

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Ft~. 2. Anguilla. An immature mucous cell (m) is shown in the mid-epidermis surrounded by filament-containing .:ells and a club cell (C). The nucleus is located in the center of the mucous cell and the nuclear envelope is covered by a mat of fine fibrils. The abundant ergastoplasm is concentrically arranged and contrasts sharply with the very few rough-surfaced elements found in filament-containing cells. Mucous droplets (*) are formed in close proximity to a Golgi complex; this area of the cell is seen more highly magnified in Fig. 4. x 15,000.

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FIG. 3. Anguilla. The relationship between mucous droplets (*) and the agranular membranes of the Golgi apparatus is observed in this field. Those surfaces of the ergastoplasmic cisterns facing the Golgi membranes usually are smooth (arrows) whereas the opposite surfaces of the same cisterns are studded with ribosomes. Filaments are found in these cells although they are much less numerous than those in filament-containing cells. × 30,000.

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FIG. 4. Anguilla. A Golgi apparatus is in close proximity to several young but coalesced mucous droplets (*). At the convex surface of the Golgi complex clear vesicles are seen (arrows) whereas at the concave side, next to the mucus, are vesicles containing a material that resembles mucus (barred arrows). × 35,000.

against the cell margin. Any one histologic section shows several mucous cells releasing their contents over the skin surface. Effete mucous cells also are seen; such cells are less reactive with the PAS and Azure B stains and appear collapsed to various degrees. In the extremely thin epidermis of the guppy (ref. 6, Fig. 1) a single mucous cell might extend from the dermoepidermal junction to the surface. The distribution of mucous cells is demonstrated remarkably well by the PAS staining of whole mount caudal fins of the goldfish. In such preparations it is apparent that mucous cells are sparse in the thin, distal part of the tail and much more numerous proximally. Electron microscopic studies of eel skin demonstrate that mucous cells in the initial stages of mucogenesis are situated deep in the epidermis surrounded by filamentcontaining cells (Fig. 2). While mucous cells and filament-containing cells often interdigitate, desmosomes are rarely found between them. During early stages in the differentiation of a mucous cell its nonlobulated nucleus is centrally located. A moder-

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FIG. 5. Corydoras. M u c o u s droplets nearly completely fill this m a t u r e m u c o u s cell; s o m e organeUes are wedged between the droplets, b u t m o s t are compressed into a p r o t o p l a s m i c r i m at the cell perimeter. Surface filament-containing cells are t h r u s t a p a r t (arrows) by the apex of the m u c o u s cell. x 22,000. FIG. 6. Anguilla. A r u p t u r e d m u c o u s cell is s h o w n ; the secretion flows o u t of the cell onto t h e surface of t h e teleo9t F i l a m e n t - c o n t a i n i n g cells a n d a p o r t i o n of a club cell (C) are also seen. x 11,000.

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ately dense fibrillar layer forms a halo around the nucleus. Only a few mitochondria are seen in a typical section and several systems of Golgi membranes, vesicles, and flattened sacs are observed. Concentrically arranged, ribosome-studded membranes of the rough-surfaced endoplasmic reticulum are abundant (Fig. 2). The cisterns formed by these membranes contain a flocculent material (Fig. 3). When ergastoplasmic cisterns and agranular membranes are apposed, the cisterns usually lack ribosomes on that surface closer to the smooth membranes (Fig. 3, arrows). Closely associated with the Golgi membranes are the developing mucous droplets, which appear incompletely bounded by membranes. Flattened Golgi sacs are sometimes apposed to the droplet. Vesicles at the concave surface of the Golgi lamellae usually contain material which appears similar to that forming the larger droplets; vesicles at the convex surface are empty (Fig. 4), suggesting a unidirectional transport. As more mucus is formed the cell enlarges considerably. Deep in the cell, individual mucous packets retain their limiting membrane, but near the cell apex the intracellular partitions are often, but not always, lost and the secretion coalesces. By this relatively late stage in differentiation, almost all the cell organeUes are compressed against the cell membrane, where they form a thin husk around the enormous blob of mucus. As the engorged cell nears the surface of the epidermis, its wedge-like apical extension forces apart the surrounding filament-containing cells (Fig. 5). The plasma membrane of the mucous cell, as it emerges between the filament-containing cells, is usually intact. The plasma membranes of other mucous cells are ruptured and their contents are released over the surface of the fish (Fig. 6). DISCUSSION A unique feature of the epidermis of teleosts is its rich population of several different types of unicellular glands; the most commonly encountered type is the mucous cell. In these cells a close spatial relationship exists between the Golgi saccules and vesicles, and the developing mucous droplets. This relationship suggests a transport mechanism operating from other membranous organelles, through the Golgi zone, to the mucous droplets. Neutra and Leblond (8) comment upon the role of the Golgi membranes in the synthesis of the carbohydrate component of mucus, and Freeman (3) proposes a mechanism relating both the rough-surfaced endoplasmic reticulum and Golgi membranes to the synthesis of mucus. Freeman postulates that the proliferated rough-surfaced endoplasmic reticulum of jejunal goblet cells synthesizes a protein moiety which is transported to the Golgi apparatus where it is combined with acid mucopolysaccharides and glycoproteins. Evidence correlating the ergastoplasm with transport in mucogenesis is also seen in teleost epidermal mucous cells where the parts of the ergastoplasm which face the agranular membranes are often

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smooth surfaced (Fig. 3). Perhaps the vesicles at the convex aspect of the Golgi zone are derived f r o m these parts of the rough-surfaced endoplasmic reticulum and contain an osmiophobic protein moiety. The content of these vesicles might then be transported through the Golgi apparatus, where it is combined with a carbohydrate to form the osmiophilic mucous droplet (Fig. 4). Mature mucous cells packed with drople, ts are found with one pole near the surface of the epidermis. At a slightly later stage the mucous cell is exposed to the exterior whereupon it ruptures and releases its secretion over the surface of the skin. Since the epidermis of teleosts is not keratinized, it seems likely that the protective role of the epidermis is related more to the deposition of a mucous layer over its surface than to a layer of keratinized cells, as seen in nearly all other classes of vertebrates. The authors express their gratitude to Drs. Louise M. Luckenbill and Paul F. Parakkal for their critical review of the manuscript and many enlightening discussions.

REFERENCES 1. AUST,S., Zool. Jahrb. (Anat.) 62, 1 (1936). 2. BERTIN, L., in GRASSI~,P.-P. (Ed.), Trait6 de Zoologic, Vol. 13 (1), p. 433. Masson, Paris, 1958. 3. FREEMAN,J. A., Anat. Record. 154, 121 (1966). 4. GRAUPNER,H. and FISCHER, I., Z. Mikroskop. Anat. Forsch. 33, 91 (1933). 5. HARDER, W., in WUNDSCH, H. H. (Ed.), Handbuch der Binnenfischerei Mitteleuropas, Vol. IIA, p. 241. Schweizerbart'sche, Stuttgart, 1964. 6. HENRIKSON,R. C. and MATOLTSY,A. G., J. Ultrastruet. Res. 21, 194-212. 7. KANN, S., Z. Zellforseh. Mikroskop. Anat. 4, 482 (1926). 8. NEUTRA, M. and LEBLOND,C. P., J. Cell Biol. 30, 119 (1966). 9. RABL, H., in BOLK et al. (Eds.), Handbuch der vergleichenden Anatomic der Wirbeltiere, Vol. I, p. 284. Urban & Schwarzenberg, Berlin and Vienna, 1931. 10. REID, E. W., Phil. Trans. Roy. Soe. London B185, 319 (1894).