Chapter 29 Histopathology


Brian Wilcock

Like other portions of the gastrointestinal (GI) tract with which it shares embryologic origin, the stomach consists of a tunica muscularis, submucosa, muscularis mucosa, and an innermost mucosa. Almost all of the histologic lesions of clinical significance arise within the mucosa, so endoscopic biopsies are preferred to full thickness biopsies for investigation of almost all gastric diseases. The only exceptions are those that seem restricted to the submucosa or tunica muscularis based on ultrasonographic evaluation (i.e., suspected smooth muscle tumors, pythiosis, and pyloric muscular hypertrophy). The advantages of endoscopic biopsies over full thickness samples have been described elsewhere, but from the perspective of the pathologist the advantages are the greater number of samples that can be taken and the ability to specifically sample abnormal mucosa. The small size and limited depth of the samples are not usually problematic.

Gastric mucosa is similar to that of other portions of the GI tract in that it has a luminal surface of columnar epithelium rich in goblet cells, branched or coiled glands embedded in a fibrovascular lamina propria, and a population of resident leukocytes within the lamina propria. Because the stomach is adapted primarily for secretion rather than absorption, and because its acid environment does not permit much of a resident bacterial population, the leukocyte population is less than is found in the more distal portions of the GI tract. That should make detection of changes in leukocyte numbers easier than elsewhere in the GI tract. That same inhospitable environment also means that there are few infectious causes of gastritis compared with the numerous viral, bacterial, and protozoal diseases of the small and large intestine.

The stomach is divided into several anatomic regions that differ in histologic character and function. Most proximal is the cardia, which surrounds the entry of the esophagus into the stomach, and most distal is the pyloric antrum leading to the sphincter that separates the stomach from the duodenum. In between, and occupying about two-thirds of the stomach, is the gastric body. The gastric fundus is an outpouching of the gastric body at its proximal end where it joins the cardia. This nomenclature is not absolutely standardized, and many authors refer to the combined fundus and body as the “fundic region” and habitually refer to biopsies from this region as being from “fundic mucosa.”1,2 Each of these regions has distinctive histologic appearance, but the transition from one portion to the other is not abrupt and in some biopsies the mucosa has an appearance intermediate between one region and the other.

Almost all gastric biopsies, whether endoscopic or full thickness, are taken from the fundic mucosa and from the pyloric antrum. In all regions, the mucosa is divided into three horizontal zones. In the fundic/body region, the deepest 60% to 80% of the mucosa is occupied by densely packed branched glands consisting of mucus-producing cells and a variety of secretory cells. The glands are connected to the luminal surface by a narrow neck known as the foveola. In contrast to the more distal portions of intestine, the germinal population for all of the gastric epithelium lies at the junction between the glands and the foveolae, a region known as the isthmus, rather than at the base of the glands (Fig. 29-1). These germinal cells migrate upward to replenish the mucus neck cells and surface epithelium, and downward to repopulate the glands as part of normal mucosal turnover and in the event of unexpected epithelial loss. The overall mucosal turnover time is 4 to 5 days—a time that dictates the speed of wound healing, and the rate at which shallow ulcers disappear. The risk of a false-negative biopsy result when investigating episodic gastric disease has not been sufficiently emphasized.

The glands are populated by a mixture of pyramidal, brightly eosinophilic cells that produce hydrochloric acid and cuboidal chief cells with a more basophilic, foamy cytoplasm containing secretory granules of various types including precursors of pepsin and zymogen. Chief cells predominate in the deeper two-thirds of the fundic glands. Mucus-filled columnar cells similar to mucus neck cells persist throughout the full length of the glands. The fourth and final cellular constituent of the epithelium consists of neuroendocrine cells that are inconspicuous in routine histologic preparations.

The glands are separated from the underlying muscularis mucosa by a layer of dense fibrous tissue that varies substantially in thickness. They are separated from one another by a very small amount of fibrovascular lamina propria containing a mixture of lymphocytes, plasma cells, mast cells, and eosinophils. In the normal fundic stomach, the fibrous tissue and the leukocytes are most visible in the superficial 20% of the mucosa. The deeper glands are packed against each other with essentially no intervening lamina propria (Fig. 29-2). Defining the normal reference range for the fibrous tissue and for each cell population is essential to the proper identification and classification of gastric inflammatory disease—a goal that remains elusive.

Within the pyloric antrum, the glandular volume is substantially less, occupying less than 50% of the mucosal thickness. The glands are less branched than those of the fundic mucosa and are populated primarily by mucus-producing cells. They are separated from one another by more fibrous lamina propria than in the fundic mucosa, and leukocytes are about twice as numerous as in fundic stomach. Failure to recognize the differences in normal histologic structure between fundic and pyloric mucosa can result in incorrect diagnoses of inflammation, fibrosis, and/or glandular atrophy in samples of pyloric stomach.

Gastric Ulceration

The stomach is surprisingly susceptible to transient shallow ulceration caused by ingested materials ranging from abrasive foods to household chemicals, numerous common garden and woodland plants, an endless variety of clothing items or household decorations, and hairballs. Although one would assume that the abundant mucus lining the stomach would protect it from abrasion, such is not the case. The mucus is probably more important for protection against endogenous acid.

Ulceration caused by mechanical abrasion is usually shallow and transient. Like other portions of the alimentary system, shallow ulcers heal within hours by flattening and sliding of adjacent epithelium followed by replacement of mucus cells generated by a transient increase in mitotic activity within the isthmus zone at the base of the foveolae. The regenerating epithelium is flatter and more basophilic, with little mucus and slight irregularity in orientation of the nucleus (Fig. 29-3). Shallow ulcers heal completely within just a few hours. The adjacent superficial lamina propria will usually have at least some edema and perhaps even hemorrhage, but the leukocytic reaction is usually minimal. A few neutrophils may be found in the very superficial lamina propria or intermingled with some fibrin and mucus filling in the ulcer. The paucity of leukocytes is in part because the injured tissue is sloughed into the intestinal lumen so that there is little chemical stimulus for leukocyte recruitment, and partly because the stomach has almost no resident bacteria to contaminate the wounds. With deeper or more persistent ulceration, there will be more obvious hyperplasia and basophilia within the isthmus region reflecting a greater need for mitotic replacement of injured epithelium. Deeper ulcers that have destroyed the isthmus and portions of the lamina propria will take longer to heal and will require stromal proliferation to build a scaffold for effective epithelial healing, but remodeling is extremely effective and it is rare to see any permanent residual scarring. Regardless of the severity of the original lesion, the usual outcome is histologic normalization (Fig. 29-4).

Causes of ulceration other than mechanical abrasion are numerous and include histamine-induced ulceration in dogs with mast cell tumor,3,4 accidental chemical ingestion, ulcers caused by steroidal and nonsteroidal antiinflammatory (and occasionally other) drugs,57 and exercise-induced ulcers that are best documented in racing sled dogs.8 In general, the histologic character of the ulceration provides no clue as to the pathogenesis. Careful estimation of the age of the lesions (distance of epithelial sliding, restoration of the normal columnar epithelial shape and normal cytoplasmic eosinophilia, and the maturity of subepithelial fibrosis) will often allow the clinician to identify the causative drug or event. Progression to perforation is rare and usually requires continued application of the injurious stimulus (e.g., continued administration of ulcerogenic drugs or persistent histamine production by a mast cell tumor).

Gastric Necrosis Other Than Ulceration

Epithelial necrosis not typically associated with superficial ulceration is seen with uremia and sometimes with mucosal ischemia secondary to thromboembolic disease, submucosal vasculitis, or gastric dilation/volvulus.

Uremic gastropathy occurs almost exclusively in dogs and its exact pathogenesis remains unknown. The distinctive lesions include mineralization and necrosis of the parietal cells occupying the middle third of the gastric mucosa, mineralization of basement membranes of the glands and of the submucosal blood vessels, and mineralization of degenerate smooth muscle within the muscularis mucosa or even tunica muscularis (Fig. 29-5). Mineralization of submucosal blood vessels may become extensive, involving the full thickness of the vessel wall and accompanied by medial necrosis and endothelial destruction.9

With ischemic injury related to vasculitis or vascular occlusion, the character of the lesion depends on the duration and completeness of obstruction. In gastric volvulus, venous obstruction is complete and the intensely congested gastric wall dies following infarction. The mucosa dies first, undergoing coagulation necrosis that quickly becomes lost amid diffuse hemorrhage. In cats with vasculitis caused by feline infectious peritonitis (FIP) or in dogs with disseminated intravascular coagulation (DIC), the necrosis is often patchy.

Gastric Mucosal Fibrosis and Atrophy

Varying degrees of mucosal fibrosis and glandular atrophy are common in biopsy samples of the gastric mucosa. Such lesions have received almost no attention in the veterinary literature, but are common sources of confusion for pathologists attempting to interpret endoscopic biopsies. Some have referred to this as “atrophic gastritis,” with no proof that the pathogenesis of the fibrosis is linked to previous inflammatory disease. In the only study to specifically address this question, noninflammatory atrophy and/or fibrosis was seen in 128 of 482 vomiting dogs. Similar lesions were also seen in five of 19 clinically healthy dogs.10 It is not clear whether the fibrous tissue represents a true increase in the volume of collagen or simply a condensation of resident collagen subsequent to glandular atrophy. The two lesions (mucosal fibrosis and glandular atrophy) are almost always concurrent.

There remain numerous questions related to gastric atrophy and fibrosis, including these:

Normally, the glands within the fundic mucosa lie against each other with almost no intervening fibrous tissue. There is a small amount of loose connective tissue among the foveolae, and there can be substantial fibrous tissue separating the base of the glands from the muscularis mucosa. The amount of fibrous tissue in this deep location is greater in cats than in dogs. In cats there is also a distinct, very dense band of hyalinized fibrous tissue just superficial to the muscularis mucosa, known as the lamina densa (Fig. 29-6). Within the pyloric antrum there is substantially more fibrous tissue throughout the lamina propria, so any diagnosis of mucosal fibrosis in the pyloric stomach should be viewed with skepticism.

Dogs and cats with chronic gastritis often have persistent mucosal edema that matures into fibrosis. Concurrent inflammatory injury to the gastric glands is followed by regeneration that creates “nests” of hyperplastic glands embedded within the fibrous tissue (see “Atrophic Gastritis” section). It is also common to see the combination of fibrosis and glandular nesting with no evidence of active inflammation and no clinical history of gastric disease (Fig. 29-7). The glands within these nests usually have at least some degree of atrophy of the parietal cell mass with a relative increase in the prominence of the columnar mucus-producing epithelial cells. The combination of glandular atrophy, dysplastic repair, and mucus (intestinal) metaplasia represents a strong risk factor for progression to gastric carcinoma in people,11 an association that has not been proven in dogs.


The gold standard for the histologic diagnosis of gastritis is an increase in mucosal leukocytes accompanied by other evidence of inflammation such as hyperemia, edema, and bystander injury to the adjacent structural components of the mucosa (e.g., epithelial injury or reparative fibrosis). That gold standard is met in a proportion of the cases, but not in all. The lack of uniformity in the criteria used to diagnose gastritis impacts on the credibility of virtually all of the published literature on the prevalence, classification, etiology, and treatment of gastric inflammatory disease. To many, gastritis is defined simply as an increase in the number of leukocytes within the lamina propria beyond the normal range. The most obvious problem with this definition is that we do not know the normal range. Less obvious but even more fundamental is the question of whether an increase in leukocytes within a tissue designed to respond to antigenic challenge by increasing the number and activity of resident leukocytes should be interpreted as “inflammation.” An increase in lymphocytes or plasma cells in a lymph node is usually assumed to reflect an appropriate and purposeful response to antigenic stimulation. In the intestinal lamina propria, the largest lymphoid organ in the body, a similar increase in lymphocytes and plasma cells is generally viewed as pathologic and therefore deserving of therapeutic intervention. The distinction between adaptive and pathologic lymphoid proliferation lies not just in the number of leukocytes, but also in the phenotype and cytokine production profile of the cells. These complexities are beyond routine histopathology and immunohistochemistry, but should not be ignored.

Normal Gastric Mucosal Cellularity

Only three studies address the critical question of the reference range for leukocyte types in the normal canine gastric mucosa. The first reports findings from examination of endoscopic biopsies from 20 clinically normal young adult dogs (Table 29-1).12 These dogs had no history of vomiting or diarrhea for at least 2 months prior to entering the study, and were all fed a standardized diet and had the same husbandry conditions. Leukocytes were identified with light microscopy and immunohistochemistry, and were counted separately in both superficial and deep lamina propria in “mucosal units” of 250 µm2 (corresponding to approximately one-half of a ×40 microscopic field). Within the superficial lamina propria of the fundic mucosa, this corresponds to a rectangle of tissue incorporating three adjacent foveolae and the associated lamina propria, and extending vertically to the level of the first parietal cell. Within the pyloric antrum, the “mucosal unit” corresponds to an area of lamina propria bordered by two adjacent foveolae, with its deep border capturing the most superficial portion of the gastric glands.

Table 29-1 Leukocyte Numbers in the Superficial Gastric Mucosa of Healthy Dogs

Cell Type Superficial Fundic* Superficial Pyloric
Lymphocytes (CD3 and CD79) 8.4 (1.5-27.0) 22.9 (11.5-32.0)§
Plasma cells 1.6 (0-5.8) 6.8 (0.5-15.5)§
Eosinophils 0.5 (0-2.0) 2.7 (0-6.0)§
Total leukocytes** 10.4 (3.6-26.2) 32.6 (20.0-46.5)§

* Number of cells within a rectangle of lamina propria encompassed horizontally by three foveolae, and vertically to the level of the first parietal cell.

Number of cells in a square of lamina propria encompassed by two adjacent foveolae with the deep border at the foveolar–glandular junction.

Mean (range) of three cell counts in each of 20 dogs.

§ Mean (range) of two cell counts in each of 8 dogs.

** Sum of CD3+ and CD79+ lymphocytes, plasma cells, and eosinophils, but excluding mast cells and unidentified cells.

Source: Adapted with permission from Southorn EP. An improved approach to the histologic assessment of canine chronic gastritis. DVSc Thesis, University of Guelph, Ontario, Canada, 2004.

The other two studies were not specifically designed to provide data about normal gastric mucosal cellularity or other features relevant to the diagnosis of gastritis. Nonetheless, both provide a series of pictorial templates of normal and diseased stomachs in an attempt to reduce interobserver variation in interpretation of gastric histopathology. Neither study provided any information about how the photographic templates identified as “normal” were selected.

The first of these studies was based on 18 client-owned dogs with GI disease and eight dogs euthanized for nonenteric disease. None of the dogs had been treated with antibiotics, corticosteroids, or antacids in the months prior to sampling. Lesions within the fundic mucosa alone were graded by two pathologists using pictorial templates adapted from the human Sydney system that depicts various severities of atrophy, fibrosis, and cellular infiltration. The results from the eight “normal” dogs were not separately identified when tabulating the results (presented as the mean grading results from all 26 dogs).13

The second and much larger study provided not only a series of pictorial templates, but also a brief text to guide the grading of GI biopsies, including those from stomach. It included pictorial templates for leukocyte accumulations, fibrosis, ulceration, and other parameters within the fundic and antral mucosa.14

In both of these latter studies in which the pictorial templates were specifically designed to reduce interobserver variation, the results showed poor correlation among grading scores assigned by different pathologists. Painful though it is, we must collectively confront the reality that it is essentially impossible to compare results from different studies claiming to evaluate causation or therapy of “gastritis.” In most published studies of gastritis in either dogs or cats, the descriptions of leukocyte numbers or the published photographs of lesions claimed to represent mild and even moderate gastritis would be considered “normal” using criteria in the three studies listed previously. This is particularly true of the many studies attempting to establish the pathogenicity of Helicobacter spp. for dogs and cats in which any histologic changes were described as mild increases in superficial mucosal cellularity.1523

Histologic Classification of Gastritis

The previously mentioned problems notwithstanding, there is no doubt that genuine gastritis does exist and is quite common. Most cases are part of more generalized GI inflammatory disease,24 but some are purely or predominantly gastric. There is no evidence that histologic classification based on the distribution or type of leukocytes has relevance to pathogenesis, treatment, or prognosis, nor is there evidence to the contrary. Thus it is appropriate to use (at least for the moment) a classification system that reflects histologic changes, with the expectation that at least some of these differences will eventually prove to have etiologic, therapeutic, or prognostic significance.

Superficial Mucosal Gastritis

Most examples of gastritis in dogs and cats involve leukocyte infiltration of the superficial 20% of the mucosa. In almost all cases, the surface epithelium remains normal. In most cases, lymphocytes and plasma cells predominate, but there are almost always at least some eosinophils. Neutrophils are rarely present, but their presence almost always means nearby or recent ulceration. There may be a concurrent increase in intraepithelial lymphocytes, and in cats there can be an increase in intraepithelial large granular lymphocytes (formally known as globular leukocytes) that are of unknown significance. Edema accompanies the leukocytes and there is often activation of resident fibroblasts and blood vessels (Figs. 29-8 and 29-9).

It has been traditional to describe the predominant cell type (lymphocytic, plasmacytic, eosinophilic) as part of the classification system, but there is no evidence that this is useful.7,14,24 First, there is no uniformity between laboratories in how well eosinophils, mast cells, or even plasma cells are stained. Second, pathologists differ in defining what proportion of cells is required before that cell type is considered “predominant.” Some will diagnose a lesion as “eosinophilic” if they believe that the eosinophils are the most numerous cells entering the tissue on an hourly basis, even if they are outnumbered by lymphocytes or plasma cells that have accumulated over weeks and months. Others use a more traditional approach and name the lesion based on the cell type that is most numerous within the tissue at the time of assessment. That means that almost any chronic gastritis will be classified as “lymphoplasmacytic.” Until everyone uses the same convention for naming such lesions, subclassification based on the predominant cell type will remain unreliable. It makes more sense to simply list each identifiable cell type as a percentage of the total infiltrate, so that such information can later be analyzed to see if it influences therapeutic response or other clinical parameters.

Transmucosal Gastritis

Approximately 30% of samples identified as having gastritis by conservative histologic criteria have an increase in leukocytes with variable degrees of edema and/or fibrosis throughout the full thickness of the lamina propria, causing separation of the gastric glands from each other and from the underlying muscularis mucosa (Figs. 29-10 and 29-11). There usually is no obvious necrosis within the glands or along the surface. Lymphocytes and plasma cells predominate, but eosinophils are often conspicuous and sometimes even predominant.

Even though it is unusual to see actual necrosis, there often is some glandular atrophy so that the glands appear in isolated nests separated by a mixture of leukocytes, edema, and fibrosis. This is probably the precursor for the lesion usually referred to as atrophic gastritis, an extremely frequent but controversial microscopic entity that usually has no clinical counterpart.

Etiologic Classification of Gastritis

The majority of cases classified as gastritis by histologic criteria have no identified cause. The list of potential causes includes infectious agents, chemical irritants, toxic plants, and immune-mediated diseases.7 Most of the chemical and plant irritants cause transient gastric ulceration rather than true gastritis, and the inclusion of various allergic or other immune-mediated pathogeneses is based on assumptions rather than proof. Because the stomach is not a site of any significant protein absorption, assumptions of immune-mediated disease may be more appropriate for small intestine than stomach.

Instances of gastritis with a known cause are almost always related to specific infections. These represent a very small proportion of all examples of gastritis, and for most of these agents infection is not usually linked to clinical signs of gastritis or to significant histologic lesions. Parasitic agents include the nematodes Physaloptera, Gnathostoma, and Cylicospirura in dogs and cats, and Ollulanus in cats alone. Infrequent but devastating examples of infection with the fungus Pythium insidiosum cause mucosal and submucosal suppurating granulomas in the stomach and elsewhere in the GI tract of dogs. The stomach is an occasional victim in the course of systemic fungal diseases (e.g., blastomycosis, cryptococcosis). Finally, the crusade to establish Helicobacter as a cause for clinically significant gastritis or gastric ulceration in dogs or cats has triggered more descriptions of alleged gastric lesions than has any other proposed agent of gastric disease.


Helicobacter spp. are normal inhabitants of the surface and foveolae of dogs and cats. In dogs, most infections are with Helicobacter bizzozeronii or Helicobacter heilmannii, while in cats H. heilmannii and Helicobacter felis predominate. These large spiral bacteria are easily seen in routine endoscopic biopsies, and their detection is enhanced by the use of silver stains. More elaborate and sensitive detection methods (e.g., polymerase chain reaction [PCR] or tissue urease activity) are research tools rarely if ever used in a diagnostic setting. In the past 15 years, there have been at least nine studies specifically examining the relationship between Helicobacter infection, development of histologic lesions, and clinical disease. None of these studies has proven any causal relationship between infection and disease.13,1520 All but one, however, have claimed that infection is associated with an increase in gastric mucosal leukocytes, fibrosis, and lymphoid follicles. The problem is that the increased leukocytes claimed to represent “gastritis” invariably fall within the normal range for gastric mucosal cellularity as established by Southorn and as subsequently adopted by the World Small Animal Veterinary Association (WSAVA) Gastrointestinal Standardization Group.12,14 Most of those studies had no uninfected control against which to measure the claims for increased cellularity. Even if one were to accept the hypothesis that colonization by Helicobacter causes a small increase in mucosal leukocytes (including lymphofollicular hyperplasia), the most plausible explanation is that the Helicobacter simply represent an increased antigenic challenge to the resident mucosal leukocytes that respond in an entirely appropriate fashion, analogous to what occurs when gnotobiotic animals are first introduced to a “normal” bacterial flora.31

Gastric Proliferative Disease

Proliferative changes within the stomach include focal adenomatous hyperplasia in the form of gastric polyps, more diffuse but rare canine hypertrophic gastropathy, and genuine neoplasms.

Focal Adenomatous Hyperplasia

Focal adenomatous hyperplasia occurs in two forms. The more prevalent is focal pyloric mucosal hypertrophy in which the foveolae of the pyloric antrum become greatly exaggerated and form villus-like protrusions covered by mature and otherwise normal epithelium.32 The lamina propria sometimes contains an increase in eosinophils, but it is unknown whether those examples with eosinophils have a different pathogenesis than those without. It is unknown whether the eosinophils are involved in the development of the lesion, are the result of the lesion, or are unrelated.

The prevalence of these lesions is directly related to the frequency of endoscopy and many are clinically silent. They become significant only if they grow large enough and if they are located where they may cause pyloric obstruction. The diagnosis is difficult to confirm in endoscopic samples. If orientation and depth are both perfect, then the diagnosis is based on seeing the biopsy filled with superficial and foveolar epithelium with no glands because the increased thickness of the foveolar region occupies the entire capacity of the biopsy forceps (Fig. 29-12).

Less prevalent than pyloric mucosal hypertrophy is focal adenomatous hyperplasia creating polyps in the gastric body or fundus. These are about three times more prevalent in cats than in dogs. In contrast to the proliferation of mucus-laden superficial epithelium typical of pyloric mucosal hypertrophy, these gastric polyps are formed primarily by papillary proliferation of hyperchromatic glandular epithelium forming branching and sometimes cystic tubules. Most of the epithelium is hyperchromatic and primitive columnar epithelium with no mucus production, but with occasional maturation into parietal and chief cells. They differ from papillary adenocarcinomas by not showing any invasion across the basement membrane and into the lamina propria or submucosa. The lack of invasion is the only reliable criterion distinguishing gastric polyp from a well differentiated papillary adenocarcinoma. There is no proof that these lesions progress to neoplasia.

Gastric Neoplasia

Although a wide variety of neoplasms are reported within the stomach,35,36 the majority fall into one of three groups: gastric carcinoma, lymphoma, and smooth muscle tumors.

Gastric Carcinoma

Mucus-producing, scirrhous tubular adenocarcinoma is the most common tumor of the canine stomach, but gastric carcinomas are almost nonexistent in cats. In dogs, they usually occur as a regional plaque-like thickening with ulceration in the distal body or within the pyloric antrum. Ulcerated tumors may be confused both macroscopically and microscopically with chronic ulcers that have stimulated abundant reactive fibrosis and dysplastic epithelial repair.

The earliest recognizable histologic lesion is focal obliteration of mucosal glandular architecture by proliferating polygonal epithelial cells that violate the basement membrane of the glands/crypts to proliferate as individualized cells within the lamina propria. These cells, which may not be easily recognized as epithelial cells, may be found only deep within the lamina propria. They may be difficult to capture in endoscopic biopsies especially if the tumor is arising in pyloric mucosa (the thickness of the pyloric mucosa makes it difficult to capture the deep third of the mucosa). There will be times in which full-thickness samples are required to confirm the diagnosis, but that is the exception rather than the rule. The tumor cells invade the submucosa and then the tunica muscularis. In most cases, the initial invasion is in the form of slender cords or tubules of mucin-producing epithelium within or surrounding lymphatics. They penetrate the serosa and exfoliate into the peritoneal cavity to seed the omentum and mesentery. In most cases the tumor cells are well differentiated and form easily recognizable tubules, but some examples comprise only lakes of mucin within the tunica muscularis, or nodules of reactive fibrous tissue on the mesentery or omentum. In such cases, the use of a PAS stain will accentuate the mucus within the cytoplasm of the anaplastic epithelial cells that will be around the mucus or buried within the fibrous tissue.

It is traditional to subclassify gastric carcinomas based on histologic pattern (e.g. papillary, tubular, scirrhous),35 but most examples exhibit substantial regional variation that includes more than one subtype. In addition, there is no proven prognostic or therapeutic significance to these patterns so it is difficult to justify subclassification.


The identification, classification, and treatment of alimentary lymphoma in dogs and cats is a “work in progress,” with dramatic changes in our understanding arising from more widespread use of immunohistochemistry and clonality testing. Most of what was written prior to 2005 is more or less irrelevant and even misleading, especially with respect to cats. Alimentary lymphoma in cats is very different from that in dogs, although the differences are perhaps less obvious in gastric lymphoma than in examples affecting small intestine.

Feline Gastric Lymphoma

Lymphoma in the stomach of cats occurs in two different forms. The first and more prevalent is solitary large B-cell lymphoma that occurs as a space-occupying mass. The second is diffuse infiltrative small T-cell lymphoma that occurs in the stomach only as part of more generalized alimentary lymphoma that affects primarily the small intestine. There are a few cases that conform to neither of those descriptions, including a few large granular cell lymphomas and “mixed” or “histiocytic” lymphomas with intermingled eosinophils, T cells, and B cells.

Small cell lymphocytic (usually T-cell) lymphoma in the stomach is initially detected as an excessive accumulation of monotypic, hyperchromatic small lymphocytes within the superficial lamina propria and the epithelium. The accumulation is usually diffuse and may result in obliteration of the distinction between the lamina propria and the epithelium. As the disease progresses, tumor cells invade deeply into the lamina propria, obliterating glandular architecture, and spread throughout submucosa and tunica muscularis to completely replace the gastric wall. At that stage, the diagnosis is simple, but it is more challenging when dealing with early disease in endoscopic biopsies. The question is always the same: is this severe lymphocytic gastritis or early lymphoma? With inflammatory bowel disease, the increased cells are usually a mixture of lymphocytes and plasma cells, and there is no obliteration of the distinction between epithelium and lamina propria.

Large-cell (lymphoblastic) lymphoma is a more traditional lymphoma in the sense that it creates an unmistakable “mass” with early obliteration of mucosal architecture and invasion across the gastric wall. Perhaps because this disease is typically focal and limited to the stomach rather than being part of a more diffuse GI disease, the lesion is usually advanced at the time of initial diagnosis; consequently, histologic confirmation via endoscopic or ultrasound-guided core biopsies is not usually problematic. Most examples seem to arise from the diffuse lymphoid population in the superficial third of the gastric mucosa. The tumor spreads rapidly to destroy mucosal, submucosal, and muscular architecture, creating a diffuse sheet of monotypic large lymphocytes with nuclei at least twice the diameter of a red blood cell. Epitheliotropic behavior is rarely observed. In a series of 26 sequential examples studied in preparation for this chapter, all were confirmed as B-cell lymphomas. In a recent published study, nine lymphomas limited to the stomach were all immunoblastic B-cell lymphomas.37

Large granular lymphoma is a rapidly progressive transmural lymphoma with a unique histologic and cytologic appearance. The cells are pleomorphic large lymphocytes with distinctive large red cytoplasmic granules. The nuclei are often cleaved or even convoluted. The malignant cells are often accompanied by other cell types, including small benign lymphocytes, eosinophils, and fibroblasts. Most published examples have arisen in distal small intestine, but there are a few that arise in the stomach. At least in the small intestine, almost all are CD3+CD8+ T-cell tumors that probably arise from intraepithelial lymphocytes. Historically, these tumors were mistaken for mixed transmural granulomatous inflammation or mast cell tumors.

Leiomyoma and Leiomyosarcoma

Any published estimates of the prevalence of smooth muscle tumors must be viewed with skepticism. In part this is because smooth muscle tumors are usually small and affect tunica muscularis only, so the majority do not cause clinical signs and cannot be detected endoscopically. Estimates of prevalence therefore differ in studies based on clinical signs as contrasted with postmortem studies.42,43 Further confusion arises from the fact that many of those tumors initially classified as leiomyosarcomas based on traditional histopathology have been reclassified as gastrointestinal stromal tumors (GISTs) on the basis of immunohistochemistry (positive for CD117, negative for smooth muscle actin).44,45 Those papers addressed small intestinal, cecal, and colonic tumors rather than specifically looking at those in stomach, and in addition specifically addressed smooth muscle tumors initially diagnosed as poorly differentiated leiomyosarcomas rather than more common, mature leiomyomas. It is therefore a mistake to assume that all tumors previously diagnosed as well-differentiated smooth muscle tumors are at risk of having that diagnosis “overturned” by immunohistochemistry.

Smooth muscle tumors usually occur as focal, solitary, discrete masses arising within the tunica muscularis. They represent a continuum from benign to malignant, with no precise histologic division between leiomyoma and leiomyosarcoma. Those with discrete, purely expansile growth and formed of histologically and cytologically mature smooth muscle cells are called leiomyomas. At the other end of the spectrum are primitive stromal tumors with invasive growth, increased mitosis, and only vague resemblance to smooth muscle cells that have traditionally been classified as leiomyosarcomas. Many examples lie in the middle of that continuum, and pathologists are not uniform in how they classify these. Leiomyomas are formed by cells virtually indistinguishable from normal smooth muscle, except that they grow in interlacing bundles to create a disorganized nodule clearly different from the surrounding, laminar arrangement of the tunica muscularis. These tumors may protrude outward through the serosa or inwardly to cause bulging of the mucosa with subsequent ulceration.

The poorly differentiated tumors are comprised of pleomorphic spindle cells with the usual criteria of malignancy, including regional invasion. Any decision to refer to these as smooth muscle tumors rather than GISTs is subjective and should be confirmed with immunohistochemistry.

There is poor correlation between histologic appearance and biologic behavior. As a group, the smooth muscle tumors have a relatively good postoperative prognosis with prolonged postoperative tumor-free intervals and survival times regardless of histologic appearance.45 For those tumors that behave aggressively, the initial histologic appearance (i.e., leiomyoma vs. leiomyosarcoma) is not predictive.


Michael Day

Jul 10, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Histopathology
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