List of Abbreviations
Cytotoxic T lymphocyte-associated antigen 4
Common variable immune deficiency
Diverticular disease-associated colitis
Inflammatory bowel disease
Ileal pouch-anal anastomosis
Nonsteroidal antiinflammatory drug
Solitary rectal ulcer syndrome
Sessile serrated polyp/adenoma
Normal Histology of the Bowel
An overarching theme in all avenues of surgical pathology is that, in order to understand the histopathology of a disease, one must know the normal histologic features of the tissue. Before we begin a discussion of the pathology of inflammatory bowel disease (IBD), we will provide a brief introduction on the normal histology of the small and large intestines, as these are the segments of the luminal gastrointestinal (GI) tract most often affected by IBD.
Normal Histology of the Small Intestine
The histology of the small bowel reflects its functional roles, which principally involve digestion and nutrient absorption. As with most portions of the GI tract, the small intestine comprised four major “layers” that are concentrically arranged from the lumen outward as follows: mucosa, submucosa, muscularis propria , and serosa ( Fig. 5.1A ). The mucosa is further subdivided into three compartments: epithelium, lamina propria, and muscularis mucosae .
The mucosa of the small bowel is unique in that the surface epithelium and lamina propria form prominent intraluminal projections called villi, a morphologic modification that maximizes surface area for nutrient absorption. When well oriented on histologic sections, the villi are regularly arranged and relatively evenly spaced, oriented roughly perpendicular to the luminal surface ( Fig. 5.1B and C ). Each villus contains an arteriole with capillary network, veins, and a central lymphatic channel with numerous nerve fibers. This morphology is especially characteristic of the duodenum, where the villi are somewhat shorter and broader than in the distal small bowel. As one progresses into the jejunum and ileum, the villi become longer and more complex with frequent branching. Directly subjacent to the villous epithelium is the crypt compartment; the ratio of the length of the villi to crypt length should be approximately 3:1 to 5:1. Expansion or hyperplasia of the crypt compartment, or blunting of the villi, can occur in the setting of chronic mucosal injury. Either change should reduce the villus-to-crypt ratio. The surface epithelium consists of absorptive cells and goblet cells. The absorptive cells have abundant cytoplasm and with well-formed microvilli. The crypt epithelium is specialized for regeneration, secretion, and immunity, as evidenced by the presence of stem cells, goblet cells, neuroendocrine cells, and Paneth cells. Paneth cells are usually located at the base of the crypt and secrete lysozyme, defensins, and immunoglobulins and are thought to play a role in immunity in association with gut flora. Goblet cells are columnar in shape and have supranuclear mucus droplets. Goblet cells decrease toward the villus tip and increase in frequency distally along the small bowel.
The lamina propria of the small bowel contains loose connective tissue with a robust population of inflammatory cells, including T and B lymphocytes, plasma cells, histiocytes, a few eosinophils, and mast cells. The predominant component consists of plasma cells within the lamina propria. As a matter of routine practice, the presence of plasma cells should be checked in every small intestinal biopsy. While their presence does not exclude an immune deficiency disorder such as common variable immunodeficiency (CVID), up to two-thirds of patients with CVID lack plasma cells in the small intestine and/or colon. Lymphoid aggregates are also a normal component of the small intestine. These become more prominent and frequent in distribution distally, eponymously dubbed “Peyer patches” in the ileum. Lymphocytes are also noted in the epithelium, with a ratio not greater than one lymphocyte per five enterocytes.
Gastric metaplasia and gastric heterotopia are commonly found in the small bowel. Gastric metaplasia is often a normal variant in the duodenal bulb. Away from the bulb, it is most commonly a result of peptic injury (peptic duodenitis), in which case it is typically associated with hyperplasia of intramucosal mucin-secreting Brunner glands and active inflammation. Gastric heterotopia, on the other hand, usually takes the form of a polyp or nodule, and is considered to be a mostly innocuous developmental anomaly. Most cases of gastric heterotopia are incidental findings on endoscopic or histologic examination. Pyloric gland metaplasia (a.k.a. pseudopyloric metaplasia), in which some crypts assume a morphology akin to the crypts in the gastric pylorus, is an indicator of chronic mucosal injury and is often seen in the ileum in IBD. Again, the presence or absence of pyloric gland metaplasia is routinely assessed in small bowel (jejunum and ileum) biopsies, taken from patients when there is a clinical suspicion for IBD, or in pouch biopsies.
The submucosa of the small bowel consists of delicate connective tissue without significant inflammation. Variably sized arteries, veins, capillaries, and lymphatics are present. Ganglion cells and nerve trunks (Meissner’s plexus) are present but without evidence of inflammation or hypertrophy. In the duodenum, Brunner’s glands are a normal finding in the submucosa. There should be no submucosal fibrosis in the normal small bowel.
The muscularis propria of the small bowel consists of two layers of smooth muscle fibers, the inner circular and outer longitudinal layers, with myenteric ganglion cells and interstitial cells of Cajal (Auerbach’s plexus) situated between these two layers. The thickness of the muscularis propria is relatively uniform throughout the length of the normal small bowel. Inflammation, fibrosis, or degeneration should be absent in the muscularis propria of the normal small bowel.
The subserosa and serosa of the small bowel consists of a thin layer of delicate connective tissue covered by a layer of inconspicuous mesothelium. There should be no inflammation, fibrosis, or mesothelial hyperplasia in the subserosa and serosa of the normal small bowel. The mesentery of the small bowel has lobulated yellow fat and that fat should not go beyond the mesenteric attachment.
Normal Histology of the Large Intestine
Like the small bowel, the colon has four layers that are concentrically arranged from the lumen outward as follows: mucosa, submucosa, muscularis propria , and serosa. The mucosa is also further subdivided into three compartments: epithelium, lamina propria, and muscularis mucosae .
In contrast to the small intestine, the colon functions primarily in water absorption, propulsion of fecal matter, and antigen sampling. The mucosa of the colon has regularly arranged crypts that extend from the surface epithelium to the muscularis mucosae , where they “anchor” on the muscularis mucosae . The crypts are lined up in parallel to each other and perpendicular to the longitudinal axis of the colonic lumen, imparting the so-called “test tubes in a rack” appearance ( Fig. 5.2 ). Some mild crypt architectural distortion can occur adjacent to normal lymphoid follicles or aggregates. Crypts can also appear mildly distorted in the normal anatomic mucosal folds of the colon, known as innominate grooves. The crypts in the sigmoid colon and rectum often have a hyperplastic appearance, and crypts at these sites can be dilated or distorted by increased surrounding fibromuscular stroma, perhaps secondary to mucosal prolapse. On biopsy material where the mucosa was evulsed without the underlying muscularis mucosae to anchor it, crypts can also appear unusually oriented. The surface epithelium consists of low columnar to cuboidal absorptive cells and goblet cells, containing abundant cytoplasm and mucus droplets, respectively. The surface epithelium rests on a thin basement membrane. The crypts open into surface epithelium or innominate grooves. The crypts represent a proliferative compartment and usually contain stem cells, endocrine cells, and basally located Paneth cells (in the right colon).
The lamina propria of the colon usually contains loose connective tissue with uniform capillaries and lymphatics just above the muscularis mucosae . Mononuclear inflammatory cells are normally present in the lamina propria of the colon but with some variation in different areas of the colon. The cecum and ascending colon tend to have greater mononuclear cell infiltrates, eosinophils, lymphoid follicles, and lymphoplasmacytic aggregates, as this is an antigen-rich environment. The inflammatory cells are mostly confined to the lamina propria and should not obliterate or expand the lamina propria. The left colon, on the other hand, tends to have sparser lymphoplasmacytic infiltrates, increased muciphages in the lamina propria, and increased goblet cells within the epithelium. A few T lymphocytes (less than 20 per 100 surface epithelial cells) can be seen in the normal colon. Surface intraepithelial lymphocytes numbering greater than 20 per 100 surface epithelial cells, known as surface intraepithelial lymphocytosis, is abnormal. Some individuals, especially young patients, may have intramucosal lymphoid aggregates that disrupt the muscularis mucosae , causing slight crypt distortion. In addition, the epithelium overlying lymphoid aggregates often contains abundant intraepithelial lymphocytes; thus these areas should be not be used to evaluate surface intraepithelial lymphocytosis. Neutrophils should not involve normal epithelium, although significant bowel preparation can produce focal active inflammation.
In adults, Paneth cells are normally present in the small intestine, cecum, appendix, ascending colon, and transverse colon. Infants and children may have Paneth cells in the descending colon and rectum. The presence of Paneth cells in histologically normal rectums in children is strongly associated with idiopathic constipation but does not predict the development of IBD. However, the appearance of metaplastic Paneth cells in areas of the GI tract in which they should not normally exist, as in the descending colon or rectum in adults, is an important feature of chronic mucosal injury.
The submucosa of the colon consists of delicate connective tissue without significant inflammation. Ganglion cells and nerve trunk are presents, but without inflammation or hypertrophy. In addition, variably sized arteries, veins, capillaries, and lymphatics are present. There should be no submucosal fibrosis in the normal colon.
The muscularis propria of the colon consists of two layers of smooth muscle fibers, the inner circular and outer longitudinal layers, with myenteric ganglion cells and interstitial cells of Cajal (Auerbach’s plexus) situated between these two layers. The thickness of the muscularis propria is relatively uniform throughout the colon. Inflammation, fibrosis, or degeneration should be absent in the muscularis propria of the colon.
The serosa of the colon consists of a thin layer of delicate connective tissue covered by an inconspicuous mesothelium. There should be no inflammation, fibrosis, or mesothelial hyperplasia in the serosa of the normal colon. The rectum is variably covered by serosa; the distal third of the rectum is not covered by serosa at all. The mesentery of the colon has lobulated yellow fat and that fat should not go beyond the mesenteric attachment.
Overview of Histopathology of IBD
Efforts to categorize disease-defining features of ulcerative colitis (UC) and Crohn’s disease (CD) have resulted in an academically and, to a great degree, practically useful dichotomous classification scheme based on gross and microscopic findings (see Table 5.1 for gross features of UC and CD, Table 5.2 for microscopic features of UC and CD). This scheme provides a solid starting point from which to approach the histopathologic diagnosis of IBD and to differentiate between UC and CD. Of course, in actual practice, surgical pathology cases do not always “read the book.” These classifications should be prudently applied by the surgical pathologist, always in the context of clinical, radiographic, endoscopic, and intraoperative findings. There can be overlapping features between UC and CD, such as the presence of deep or fissuring ulceration, or granulomas associated with ruptured crypts versus discrete epithelioid granulomas. Mucosal biopsy specimens sometimes are not adequate for the definitive classification of a disease process as UC or CD, as many of the histologic features of CD involve deeper portions of the bowel wall and are not accessible to mucosal sampling by biopsy. Furthermore, there are numerous differential diagnoses to consider on an initial encounter for IBD. Even resection specimens can have overlapping features of both UC and CD. In these situations, the term “indeterminate colitis (IC)” is favored, an interim diagnosis that will be discussed later in this section.
|Gross Features||Ulcerative Colitis||Crohn’s Disease|
|Small bowel involvement||No||Yes|
|Microscopic Features||Ulcerative Colitis||Crohn’s Disease|
|Disease extent||Diffuse, continuous||Segmental, patchy|
|Rectal involvement||Universal (sometimes spared in children)||Variable (often spared)|
|Disease pattern||More severe distally||Variable (patchy)|
|Fissures, sinuses, fistulas||No||Yes|
|Granuloma(s)||No (mucin granulomas may be present)||Epithelioid|
When evaluating biopsies or resection specimens, the surgical pathologist should always bear in mind a few salient points with regard to normal bowel versus that with chronic injury. Normal intestinal mucosa should not have significant architectural changes, such as crypt distortion or branching. Normal intestinal mucosa should have some degree of an inflammatory milieu in the lamina propria, the amount of which is location dependent. A significant increase in chronic inflammation, especially in a basal distribution, is abnormal, and active (neutrophilic) inflammation is always abnormal unless it is focal and potentially the result of rigorous bowel preparation. The differential diagnosis for active inflammation in the absence of chronicity (i.e., an active colitis pattern) includes infection, medication-induced mucosal injury, ischemia, and significant bowel preparation. However, up to 15.6% cases with focal active colitis were ultimately diagnosed as IBD in one study. Finally, pyloric gland metaplasia (often in the small bowel) and Paneth cell metaplasia (often in the left-sided colon or rectum) signify chronic mucosal injury.
Chronicity is generally defined as the presence of at least two of the three following features and applies to both UC and CD: (1) architectural distortion (including shortening of the crypts with lack of crypt extension to the muscularis mucosae ), (2) a band-like basal lymphoplasmacytic infiltrate, and (3) Paneth cell or pyloric gland metaplasia ( Fig. 5.3 ). In the small bowel, unequivocal villous atrophy is a feature of chronicity. Marked mononuclear cell infiltrates in the lamina propria can be seen adjacent to or within an ulcer, in fulminant colitis, and in chronic mucosal injury. Therefore it may be prudent to search for other evidence of chronicity when heavy plasma cell-rich lamina propria infiltrates are encountered in a biopsy taken immediately adjacent to an ulcer. Some pathologists accept sheets of mononuclear cells in the lamina propria as a feature of chronicity. However, it should be kept in mind that the right colon can have prominent lamina propria mononuclear inflammatory cells in normal states. As mentioned previously, Paneth cells in the descending colon and rectum is often a normal finding in children. In one study involving 245 biopsies from pediatric patients, Pezhouh et al. demonstrated that only 1 out of 42 cases with rectal Paneth cells in a background of otherwise unremarkable mucosa was subsequently diagnosed with CD. Paneth cells in the rectum in children have a strong association with idiopathic constipation. On the other hand, the presence of just one metaplastic Paneth cell distal to the splenic flexure in adults is a marker of chronicity. Pyloric gland metaplasia in the ileum or colon indicates chronic mucosal injury, and while it is not specific for CD, it is more common in CD than in UC. Some other features of chronicity, including hyperplasia of the muscularis mucosae and submucosal fibrosis, may be seen in biopsy specimens from both UC and CD. Somewhat more CD-specific histologic markers are epithelioid granulomas.
Histologic active inflammation, also applicable to both UC and CD, is defined as neutrophilic or eosinophilic infiltrates involving the epithelium, crypt abscesses, regenerative or degenerative epithelial changes, necrosis, erosions, and/or ulceration. The term “active colitis” is preferred over “acute colitis” because the former is more representative of a histologic pattern, while the latter implies a clinical presentation. The degree of histologic active inflammation can be characterized as mild, moderate, or severe, depending on activity involving <50% of crypts in a biopsy, >50% of crypts, and the presence of ulceration, respectively ( Table 5.3 ). The term “fulminant colitis” is more often used in the context of UC, and the criteria that separates fulminant colitis from severe colitis is surface ulceration involving greater than 50% of the mucosa. The extent of active inflammation can also be qualified; for example, we use diagnostic phrases such as “focal active chronic colitis” and “patchy active chronic colitis” when the activity involves only one crypt or only a few crypts (focal) in different areas of a biopsy specimen or only a few fragments of colonic biopsy (patchy). It is important for surgical pathologists to report focal active colitis, especially in a patient without an established history of IBD. Shetty et al. found that up to 16% of cases of focal active colitis were ultimately diagnosed as IBD. Most turned out to be CD, and the average period of time that elapsed between the diagnosis of focal active colitis and the diagnosis of IBD varied from 18 months to 6 years. Diagnosing focal and patchy active colitis also has important implications, as we will discuss later, for pediatric patients.
|Diagnosis||Typical Clinical Scenario||Criteria|
|Architectural Distortion||Increased Chronic Inflammation||Neutrophilic Inflammation||Ulceration|
|Chronic quiescent colitis||Clinical remission||Present||Absent||Absent||Absent|
|Chronic inactive colitis||Treated ulcerative colitis||Present||Present||Absent||Absent|
|Chronic active colitis, mild||Symptomatic diarrhea, no systemic toxicity||Present||Present||Present, involving <50% of crypts||Absent|
|Chronic active colitis, moderate||Symptomatic diarrhea, minimal systemic toxicity||Present||Present||Present, involving >50% of crypts||Absent|
|Chronic active colitis, severe||Symptomatic diarrhea, systemic toxicity, increased erythrocyte sedimentation rate||Present||Present||Present, involving >50% of crypts||Present, involving <50% of colonic mucosa|
|Chronic active colitis, fulminant||Symptomatic diarrhea, systemic toxicity, with or without megacolon or perforation||Present||Present||Present, involving >50% of crypts||Present, involving >50% of colonic mucosa|
Chronic colitis without evidence of active inflammation is termed “chronic inactive colitis”; this is often seen in treated IBD. While some authors consider “chronic inactive colitis” to be synonymous with “chronic quiescent colitis,” the latter is technically used to designate colonic mucosa with only architectural distortion but without either chronic or active inflammation, in a patient who is apparently in disease remission.
On intestinal specimens resected from patients with IBD, deep layers of the bowel wall may show abnormalities, especially in patients with CD, which is discussed in the following.
Histopathology of Ulcerative Colitis
The histologic hallmark of UC is a diffuse superficial chronic active colitis. The mucosa is affected, while deeper layers of the bowel wall are spared ( Fig. 5.4 ). The degree and severity of active inflammation is dependent on disease activity and treatment and is histologically classified according to the extent of crypt involvement and the presence or absence of ulceration ( Table 5.3 ). The Montreal classification of UC is a clinical classification scheme that emphasizes the importance of disease extent and severity at diagnosis: E1 is inflammation limited to the rectum (proctitis), E2 is inflammation limited to the splenic flexure (left-sided colitis), and E3 is pancolitis.
UC tends to involve the colon in a diffuse and continuous, rather than segmental or patchy, distribution. Conventional wisdom asserts that the rectum is universally involved and that UC preferentially affects the left side of the colon, with proximal extension as the disease progresses. However, it has been established that UC can present with patchy disease and relative rectal sparing, especially in the following circumstances: on initial presentation in children, in the setting of fulminant colitis, and in response to therapy. Whether the patient was treated with topical or systemic therapy, posttreatment biopsies are notoriously unreliable for differentiating UC from CD. The initial presentation of IBD in children may manifest with only focal or patchy active inflammation. In left-sided UC, there may be focal involvement of the appendix or cecum with normal intervening colonic mucosa, referred to as an “appendiceal skip lesion” or “cecal patch.” Care should be taken not to interpret this as CD. Patchy right-sided colonic inflammation in the setting of left-sided UC does not portend significant differences in the natural history of disease when compared with only left-sided UC. Children also tend to present more often with pancolitis and a lesser degree of diffuse crypt architectural distortion, whereas adults are more likely to present with a classic left-sided colitis and significant chronic changes.
As its name implies, UC manifests as ulceration in severe disease. An ulcer is an area of markedly eroded mucosa with fibrin deposition, inflammatory cell infiltration, and granulation tissue formation. Ulceration exceeding 50% of the examined mucosa is referred to as fulminant colitis. Diffuse ulceration leaves polypoid portions of nonulcerated mucosa in its wake; these are inflammatory pseudopolyps, not true neoplasms, but rather islands of relatively intact mucosa surrounded by ulcer. Filiform pseudopolyps take on an exaggerated, prominent, vermiform appearance.
Unusual Features of Ulcerative Colitis
Backwash ileitis in UC : UC is traditionally thought to involve only the colorectum, whereas CD can involve any region of the GI tract. However, inflammation and ulceration in the ileum is a well-established phenomenon in UC, thought to be the result of retrograde flow of cecal contents past the ileocecal valve, inciting inflammation and mucosal injury in the ileum. This finding was described as early as 1936 by Crohn and Rosenak and in 1949 by McCready et al.; early authors used the term “backwash ileitis” to describe the purported mechanism of ileal involvement. Features of backwash ileitis can include active inflammation (cryptitis, crypt abscesses, erosions, ulceration) and chronic changes (villous atrophy, crypt regeneration, increased mononuclear lamina propria inflammation, pyloric gland metaplasia) ( Fig. 5.5C and D ). The reported incidence of ileal involvement in UC in the literature of the 1930s and 1940s varied from 1.3% to 39%. In a contemporary study, Haskell et al. found active ileitis in 17% of 200 UC resection specimens (Haskell et al., 2005). In most cases, backwash ileitis occurs in the setting of pancolitis, and the degree of activity and chronicity in the ileum tends to be proportional to (but generally less than) the degree of activity and injury in the cecum. There is often continuous involvement of the distal ileum from the ileocecal valve to a more proximal portion of the ileum, or, in severe cases, even the jejunum. However, few patients with ileal involvement in UC have only mild or moderate colonic activity, more severe colonic activity distally, or no cecal involvement at all. Rare cases of ileal erosions and ulcerations without cecal involvement have been reported, suggesting that retrograde flow (backwash) may not be the sole pathogenetic mechanism of ileitis in UC. However, backwash ileitis in UC should not have granuloma, fissuring ulcers, sinus tracts, or transmural aggregates. It is worthy to note that inflammatory changes in the ileum appear to have no effect on the prevalence of pouch complications, dysplasia, or cancer in ileal pouch-anal anastomosis (IPAA) in patients with UC.