Mucins of the gastroduodenal junction are secreted by the mucous surface and mucusproducing glandular cells in the stomach, and by goblet cells and Brunner's glands in the duodenum. Developmental studies have demonstrated that Brunner's glands can arise from undifferentiated gastric epithelium and/or intestinal epithelium in the proximal duodenum. The aim of this study was to investigate the carbohydrate composition of mucins from this region and compare it with that of mucins from Brunner's glands to evaluate the probable evolution of mucins from these glands. Toward that end, paraffin sections from 13 mammalian species were stained by classic carbohydrate histochemistry and treated with 13 lectins. In general, the mucous surface cells of the stomach, pyloric glands, duodenal goblet cells, and Brunner's glands secretory epithelium had different lectin-binding patterns. However, the lectin-binding profile of the secretory epithelium of Brunner's glands resembled that of pyloric glands more closely than that of duodenal goblet cells and mucous surface cells of the stomach. Mucins from Brunner's glands and pyloric glands showed a greater terminal carbohydrate residue diversity than those of gastric mucous surface cells or duodenal goblet cells. The lectin-binding profile argues for the evolution of similar mucins from the epithelia of Brunner's glands and pyloric glands. The greater diversity of carbohydrate residues in mucins secreted by Brunner's glands suggests that their mucus is more adaptable. This may explain why Brunner's glands metaplasia rather than goblet cell metaplasia is seen in the mucosa adjacent to chronic intestinal ulcers. Anat Rec Part A 278A:540 -550, 2004. Key words: Brunner's glands; carbohydrate histochemistry; comparative anatomy; gastroduodenal junction; histogenesis; lectin; mucinThe epithelial cells that line the gastrointestinal tract of mammals are protected in part from the somewhat harsh environment of acid, proteolytic enzymes, and abrasives in the lumen by a mucus layer. In the stomach the mucus layer, which is several microns thick, is secreted by both the epithelium lining the gastric mucosa and mucus-producing cells of the gastric glands, while in the duodenum it is secreted by goblet cells and Brunner's glands, which are located in the submucosa. Mucus is a highly hydrated gel that consists of about 95% water, 5% mucins, and minor components, such as electrolytes (Allen, 1981;Neutra and Forstner, 1987). Mucins are high-molecularweight glycoproteins that have gel-forming properties which are highly glycosylated and which in turn are responsible for their protective function.A particular morphological feature of the duodenal submucosa is the presence of Brunner's glands. Brunner's glands are specific to mammals and have been observed in all mammals examined to date (Krause, 1988). They are

Carbohydrates on epithelial cell surfaces play an important role as attachment sites for different microorganisms like bacteria, viruses and protozoa. To obtain more information about the distribution of carbohydrates on the luminal surface along the intestine, lectin histochemical studies on different gut segments of chicks of different age groups were carried out using a panel of 13 lectins with specificities for Man, Glc, Gal, GalNAc, GlcNAc or GlcNAc oligosaccharides and Sia. Furthermore, we tried to find out whether previously reported specificities of certain lectins for M cells (membranous or multifold cells) in the bursa of Fabricius (BF) can be observed also on M cells of the intestine. As a result we were able to demonstrate binding of all lectins employed in these studies in all investigated gut segments. In some cases, the application of the same lectin led to varying staining intensities of the same histological structures in different age-groups (e.g. staining of the brush border with WGA, LEA, MAA or Conarva) or different gut segments (e.g. staining of goblet cells with CMA II, LEA and MPA). Hence, terminal carbohydrate residues of glycoconjugates on the intestinal epithelium vary depending on age and organ site. As glycoconjugates can act as attachment sites for microorganisms, these differences in the distribution of sugar residues may be one explanation for the site-specificity of certain pathogens. Furthermore, the binding of lectins to the follicle-associated epithelium (FAE) of the BF differs from that to the FAE of the intestine again stressing the site specificity of lectin binding. Thus, up to now no universal M-cell marker along the chicken intestine exists.

Cellular interactions within the immune system are in part mediated via the carbohydrate-rich coat of the cell membrane, the glycocalyx, of which the terminal carbohydrate residues are of particular functional importance. Thus, these carbohydrate residues from thymus, bursa of Fabricius, spleen and bone marrow of 2- and 30-day-old chickens were investigated by lectin histochemistry. In the thymus, mannose as well as N-acetyl-glucosamine (glcNAc)-specific lectins labelled macrophages, epithelial reticulum cells and lymphocytes within the cortex. In the bursa of Fabricius, the brush border of the lining epithelium, the macrophages and the endothelium were labelled by mannose-specific lectins. The follicle-associated epithelium was labelled by a broad spectrum of lectins. Epithelial cells that separated the cortex from the medulla and large mononuclear cells in the cortex were only being labelled by N-acetyl-galactosamine (galNAc)-specific and glcNAc-specific lectins, respectively. In the spleen, lymphocytes of the peri-ellipsoid lymphocyte sheaths and macrophages of the red pulp were labelled by lectins of nearly all sugar specificities. In general, glycotopes of these organs were more intensively labelled in the 2-day-old chicken than in the 30-day-old chicken, indicating changes in glycotope expression during post-hatching development. Thus, cells of the avian immune system are as rich and diverse in their lectin binding sites as their mammalian counterparts, indicating that similar carbohydrate lectin interactions between cells and matrices take place in birds as well.