fibrous trabeculae in the liver of alligator (alligator mississippiensis)
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ANNA~OfANATOMY===================
Fibrous trabeculae in the liver of alligator (Alligator mississippiensis)
W. A. Beresford
Department of Anatomy - HSN, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA
Summary. In the mature alligator, fibrous trabeculae run from the portal areas and capsule through the hepatic parenchyma. The extent of these trabeculae becomes clear only after staining for collagen with, for example, Fast green or Picrosirius red. The trabeculae are less well developed in young caiman. The alligator's liver might use the trabeculae to withstand thrashing of the body.
Key words: Liver - Trabeculae - Alligator - Picrosirius -Polarization
Introduction
The substantial fibrous trabeculae running through the substance of the mature alligator's liver have not been reported until now. Pettit (1904), in his brief abstract on alligator liver, wrote only of masses of connective tissue enclosing the branches of the portal vein and the other elements of the portal triad. Those examining the crocodilian liver microscopically have mostly been interested in particular chemical or ultrastructural features (Hack and Helmy 1964; Arvy and Bonichon 1958; Storch et al. 1989; Henninger and Beresford 1990), and have not used the collagen-selective staining that makes the trabeculae visible.
Materials and methods
Large pieces of Ii ver were removed immediately from eleven mature alligators, legally shot, and were fixed in 10% formol saline. After fixing for several weeks, smaller subcapsular and central pieces were postfixed in Bouin's solution, dehydrated through alcohols, and imbedded in paraffin for microtomy at 8 /Lm thickness. Sections were stained with Gomori's trichrome (with Fast green for collagen) or Picrosirius Red S. Sections stained with the latter technique were viewed and photographed using polarized transmitted-light micros-
Ann. Anal. (1993) 175: 357-359
Gustav Fischer Verlag Jena
copy (Junqueira et al. 1978). Pieces of liver from four juvenile spectacled caiman (Caiman crocodilus), killed with excess pentobarbital for neuroanatomical study, were treated similarly.
Results and Discussion
The alligator liver is reinforced on three scales: 1. the fine perisinusoidal network staining respectively green or orange with trichrome or Picrosirius Red, accompanying the contorted tubules ofhepatocytes; 2. the thick connective tissue of the capsule and portal areas; 3. trabeculae of collagen, intermediate in scale and connecting the other two categories of support. Smooth muscle, visible with trichrome staining, is present as sparse fibres or bundles in the branches of the portal vein, but not in the trabeculae.
The trabeculae are of various widths and have a clear identity (Figs. 1 and 2), distinct from the connective tissue of small portal areas. They are crudely round or oval in crosssection and are clearly not septa - sheet-like partitions. Unlike some of the trabeculae of the mammalian spleen, the hepatic trabeculae do not form the walls of veins. They traverse the parenchyma linearly without affecting the tubular pattern, although the fine peri sinusoidal fibres sometimes connect with them, and occasionally a ductule runs alongside. In caiman, only three of the four livers displayed trabeculae and these were thin. The hepatocytes (Fig. 1) of both species are small compared with those of chelonians, and are arranged to form branching tubules: a configuration encountered in other ectothermic vertebrates (Beresford and Henninger 1986).
Are the trabeculae characteristic of crocodilians? Storch et al. (1989) mention neither trabeculae nor portal structures in their study of the liver of West African crocodile, but they concentrate on parenchymal cellular ultrastructure. That trabeculae are inconsistently present in these caiman and may reflect the immaturity of the animals. Steiner and Ratcliffe
(1968) found that the intra-hepatic septa of Suidae are slow to form. Studies oflivers from mature specimens of more crocodilian species are needed, if hypotheses accounting for the presence of trabeculae are to be plausible. One such is that the thra-shing movements for subduing large prey impose exceptional loads on the liver. However, in seeking corrobative measurements, the insertion of devices within the liver might provoke a fibrosis or otherwise disturb the architecture.
The extensive, comparative reviews of hepatic connective tissue by Castaldi (1920) and Aterman (1981) deal with the fine supporting framework of the sinusoids and the tubules or plates of hepatocytes. However, the trabeculae seen here are coarser, have an independent orientation, and possibly owe their existence to a fibroblastic population shared with the capsule and portal areas. A very few mammalian species are noted for septa partially or completely separating hepatic
lobules (Kostorz 1936; Steiner and Ratcliffe 1968; Ekataksin and Wake 1991). In kangaroo, elephant, and several species of ungulate, the interlobular connective tissue exists as strands extending from the portal areas, but the closest parallel to the trabeculae in alligators is the additional, intralobular, fibrous strands in elephant (Kostorz 1936) and camel (Turner 1877).
The diseased or poisoned mammalian liver lays down excess connective tissue, but usually in the form of crude partitions or septa. Occasional reports refer to a more strand-like deposition (James et al. 1989). A clinical interest in trabeculae is that they should disturb blood flow less than do septa. If the cells responsible for cirrhosis (fibroblasts, fat-storing cells, or modified fat-storing cells) could be identified and targeted, they might be induced to make trabeculae rather than septa.
Fig. I. Alligator liver, trichromestained. Two substantial trabeculae cut partly lengthwise are accompanied by two others (arrows) barely visible in cross-section. The dark structures are pigment cells. Bile canaliculi run centrally in the tubules composed of small hepatocytes. x 190.
Fig. 2. Alligator liver, Picrosirius red-stained. The polarizing filters are not completely at 90° in order to leave the parenchyma visible. Standing out brightly are many variously sized trabeculae in longitudinal and cross-section. The fine lines around the tubules are the perisinusoidal connective tissue. x 73.
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Acknowledgements. I thank Mr. N. Kinler, Louisiana Department of Game and Wildlife, and Dr. M. Pritz, University of California, Irvine, for the livers.
References
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Accepted January 21, 1993
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