Oesophageal Mucosa: General Features (Fact Sheet 9.1)

The lining of the oesophageal mucosa throughout most of its length comprises non-keratinised stratified squamous epithelium. Distally, there is a variable length of columnar epithelium. Deep to the squamous epithelium there is the connective tissue of the lamina propria, and beneath the lamina propria is the muscularis mucosae Figure 9.1. The squamous epithelium shows ‘maturation’ from the basal (deep) layer to the surface (Figure 9.2). The basal layer consists of cuboidal, round, or ovoid cells, occupies approximately 15% of the thickness of the epithelium, and is up to five cells in depth (Figure 9.2).^1–3 The basal cells contain minimal or no glycogen (which can be demonstrated by a periodic acid–Schiff [PAS] stain if present). The upper limit of the basal layer has been defined as the point where the nuclei are separated from each other by a distance that is equal to the nuclear diameter (Figure 9.2).³ The cells above the basal layer have progressively more horizontal orientation, a more elongated shape, smaller and more pyknotic nuclei, and a decreasing nuclear:cytoplasmic ratio as they become more superficial, and in the highest layers they have a flattened appearance (Figure 9.2).^{1, 3}

Figure 9.2 Normal oesophageal squamous epithelium with papillae that are less than 50% of the height of the epithelium (arrow) and a basal epithelial layer that is no more than five cells thick. There is maturation of epithelial cells towards the surface, where the cells are flatter, the nuclear:cytoplasmic ratio is lower, and the nuclei are elongated rather than round or ovoid.

The rate of squamous epithelial cell turnover is approximately 1 week. If there is gastro-oesophageal reflux, the rate of turnover increases and there is basal cell hyperplasia and elongation of papillae (Figure 9.3) (see later and Chapter 10).² Basal cell hyperplasia is difficult to define. One definition requires basal cells to account for more than half of the lower third of the epithelial layer.²

Figure 9.3 Elongation of papillae and basal cell hyperplasia in a biopsy specimen from a patient with gastro-oesophageal reflux disease.

Glycogenic acanthosis is more frequent in the distal than in the proximal oesophagus and at endoscopy produces pale/white plaques or nodules on the mucosal folds that may coalesce. Histologically, it is often poorly circumscribed. The lesion comprises hyperplastic, enlarged superficial squamous cells showing well-developed and obvious glycogenation with clear cytoplasm, producing a mosaic or ‘basket weave’ appearance (Figure 9.4A). Apart from its possible confusion at endoscopy with candidiasis and mass lesions, this phenomenon is probably a variation of normality. It has no malignant potential.^{21, 4} In fact, a ‘diagnosis’ of glycogenic acanthosis in a histology report may confuse a clinician, resulting in a request for advice about management. Cytoplasmic pallor and glycogenation also have other causes (Figure 9.4B).

(A) Glycogenic acanthosis, showing pallor and enlargement of squamous cells in some areas with a ‘basket-weave’ appearance. This finding is of little significance, apart from its potential endoscopic resemblance to candidiasis and other conditions.

(B) Glycogenation and degenerative change secondary to radiation damage.

Figure 9.4 Glycogenation of the epithelium.

Keratinisation and/or a granular cell layer are abnormal features of oesophageal squamous epithelium. Hyperkeratosis is quite common (Figure 9.5A). In one report, it affected 2% of patients and occurred in or outside the setting of Barrett’s oesophagus. In those without Barrett’s oesophagus, hyperkeratosis was more likely to have multifocality, mid-oesophageal location, endoscopic leucoplakia, and a clinical history of alcohol use, and was associated with a higher risk of squamous neoplasia of the oesophagus and of the head and neck region. These associations did not exist if the patient had Barrett’s oesophagus.⁵ Parakeratosis, lacking a granular cell layer, is very common and of uncertain significance (Figure 9.5B).⁶ It may occur with Candida infection, after radiation therapy, or as a result of other mucosal insults, and may rarely present as a mass lesion or plaque that may mimic neoplasia.^{7, 8} Epidermoid metaplasia is rare, shows hyperkeratosis and a subjacent dense granular cell layer, and may reflect various forms of injury, e.g. smoking, alcohol, drugs, or fungal infection. There is evidence of an association with squamous neoplasia but little evidence that epidermoid metaplasia is actually pre-neoplastic.⁶ Keratohyaline granules and abnormal keratinisation may reflect previous mucosal injury but are not specific (Figure 9.5C).

(A) Hyperkeratosis and parakeratosis.

(B) Parakeratosis of the superficial part of the mucosa, characterised by hyperchromatic pyknotic nuclei.

(C) Abnormal keratinisation of epithelial cells of unknown cause.

Figure 9.5 Abnormal squamous epithelial keratinisation.

The region around the GOJ is difficult to classify, partly because of alterations with age and reflux. It includes the GOJ, the lower oesophageal sphincter (LES), and, in some patients, lower oesophageal rings.¹ The LES is an area of increased pressure.¹ Definitions of the GOJ are variable and may take account of the characteristics of the muscle layer, the features of the mucosa, or the physiological findings. Classification has implications for the diagnosis of Barrett’s oesophagus (columnar lined oesophagus). Anatomical definitions of the GOJ include the cardiac incisura (i.e. the acute angle or notch between the left wall of the oesophagus and the greater curvature of the stomach) or the point at which the serosal layer of the stomach reflects onto the diaphragm.¹ The ‘proximal limit of the gastric folds’ is another definition.⁹ Endoscopically, there is an irregular zigzag shaped ‘Z line’ at the proximal limit of the gastric folds, where the squamous mucosa of the oesophagus meets the columnar mucosa of the stomach (Figure 9.6). The Z line varies in position around the circumference and may not represent the ‘true’ GOJ¹ as it lies 10–15 mm above the muscular GOJ. Therefore, the distal 10–15 mm of oesophagus above the muscular GOJ (i.e. above the oesophageal hiatus in the diaphragm) has a lining of columnar epithelium and the length of oesophageal columnar epithelium may increase even in the absence of obvious reflux or Barrett’s oesophagus. The GOJ can also be defined by the diaphragmatic indentation, although the position of the latter is affected by hiatus hernia or by the proximal level of the gastric folds.¹ Physiologically, the distal part of the LES equates broadly to the GOJ. However, LES pressures are often insufficient for manometric measurement to be useful.¹

Figure 9.6 Diagram of Z line and squamocolumnar junction.

In an apparently normal oesophagus, endoscopy may reveal lower oesophageal ‘rings’. The most common is Schatzki’s ring, typically located at the squamocolumnar Z line.^{1, 10} The upper surface of Schatzki’s ring has a lining of squamous mucosa while the lower surface has a lining of columnar mucosa, and between them there is a core of fibrous tissue and muscularis mucosae.¹ Schatzki’s ring may occur in as many as 6%–14% of subjects, may be associated with intermittent dysphagia, and may be more common in those with hiatus hernia (particularly of sliding type), reflux oesophagitis, and oesophageal webs.^{1, 10} Other oesophageal rings also occur.¹ Eosinophilic oesophagitis may produce endoscopically visible oesophageal rings, and according to some reports has an association with Schatzki’s ring.¹⁰

Inflammatory Cells in the Oesophageal Mucosa

A few lymphocytes are typically present in the squamous epithelium, and they are more numerous distally than proximally. They often lie between epithelial cells and, as a result, are slightly elongated and irregular with a ‘squashed’ or ‘squiggle’ appearance (Figure 9.7). Large numbers may prompt a search for other evidence of inflammation and particularly for other types of inflammatory cell. The upper limit of normal in one study, in biopsies from more than 30 mm above the GOJ, was 20 lymphocytes per high-power field (hpf).²

Figure 9.7 Lymphocytes in the normal squamous epithelium may have an abnormal shape because of their interposition between closely packed epithelial cells, leading to the term ‘squiggle cells’. Up to 20 lymphocytes per hpf is acceptable in the oesophageal squamous epithelium.

A few eosinophils may be present in otherwise normal squamous epithelium in patients who have no evidence of reflux. The maximum was 5 eosinophils per hpf in one report.² The mean number of eosinophils in normal oesophageal mucosa in another study was 0.07 per mm² with a range of 0 to 2.52.¹¹ In practice, eosinophils are usually absent or very sparse. The presence of even small numbers of eosinophils should prompt a search for other histological evidence of inflammation and/or gastro-oesophageal reflux.

Intraepithelial neutrophils, unless very sparse (maximum 1 per hpf), are abnormal and are indicative of oesophageal inflammation.² They should prompt a search for other evidence of inflammatory conditions and for microorganisms, particularly Candida.

Table 9.1 summarises the features of the inflammatory cell population of the normal oesophageal mucosa.

Sparse melanocytes may be present in the oesophageal squamous epithelium and may occasionally form aggregates but are usually not identifiable on haematoxylin and eosin (H&E) staining. Endocrine cells are also infrequent and are difficult to recognise without the application of immunohistochemical stains. Langerhans cells are also present and very difficult to see.

Oesophageal Lamina Propria and Muscularis Mucosae

The lamina propria beneath the epithelium consists of loose connective tissue, which may contain vascular channels and a variety of inflammatory cells, e.g. mast cells, plasma cells, and eosinophils. Lymphoid aggregates are sometimes present in the lamina propria, may be prominent, may include germinal centres (lymphoid follicles), and do not necessarily indicate oesophagitis. The lamina propria, particularly in the distal oesophageal mucosa but also elsewhere, may contain aggregates of glands lined by gastric-type columnar cells that secrete neutral mucins (Figure 9.1 and 9.8A). They open into the oesophageal lumen via transepithelial ducts lined by gastric foveolar-type columnar cells.¹ Guidance suggests that the presence of these glands and their ducts can help confirm oesophageal rather than gastric location, but in practice the distinction is often very difficult (Figure 9.8B). Another suggestion is that submucosal glands may be useful for the distinction of true submucosa from lamina propria in the oesophagus.¹² However, the possibility that apparently ‘submucosal’ glands are actually lamina propria mucinous glands is always worth considering, because the cytological and architectural differences between mucosal and submucosal glands may be minor and may be compromised by inflammation and other factors.

Figure 9.8(A) Lamina propria glands, usually arranged in well-circumscribed aggregates and lined by mucin-producing gastric-type epithelium.

(B) A duct lined by columnar epithelial cells (arrow), passing upwards from gastric-type glands in the lamina propria. Differentiation from cardia-type mucosa may be difficult.

Papillae are finger-like projections of lamina propria that extend into the overlying epithelium and, in normal circumstances, occupy no more than half of the thickness of the epithelium. Measurement of papillary height should be from the basement membrane of the non-papillary epithelium to the basement membrane of the most superficial part of the papilla (Figure 9.2).³ The epithelium overlying the top of the papilla may adopt a cone-shaped arrangement similar in shape to that of the papilla, but this epithelium is not included in the measurement of papillary length (Figure 9.9). Features characteristic of gastro-oesophageal reflux, though not specific, include basal cell hyperplasia and elongation of papillae (Figure 9.3). Given the frequency of reflux in the population, a minor degree of papillary elongation is common. In addition, many people with no other clinical evidence of reflux have papillary elongation and/or basal cell hyperplasia in the distal 30 mm of the oesophagus.^{1, 2} Accordingly, biopsies from this part of the oesophagus may not be reliable for supporting a diagnosis of reflux and should be interpreted with caution.² Furthermore, a poorly orientated biopsy may appear to have elongated papillae (Figure 9.10). In general, symptomatic patients with reflux show more severe histological changes than those who are asymptomatic.

Figure 9.9 Condensation of suprapapillary epithelium in normal mucosa. This phenomenon is not included in the measurement of papillary height.

Figure 9.10 A poorly orientated oesophageal squamous mucosal biopsy giving a false impression of papillary elongation.

Compared to the muscularis mucosae of the stomach and intestine, the muscularis mucosae of the oesophagus is more deeply located and thicker (particularly in the distal oesophagus) and is longitudinal rather than a mixture of longitudinal and circular (Figure 9.1). It is usually not present in mucosal biopsies unless there is disruption of this layer by pathological conditions. If muscularis mucosae is present in the biopsy, its appearances can help distinguish oesophageal biopsies from gastric.¹

Oesophageal Submucosa

The oesophageal submucosa comprises loose connective tissue within which there are submucosal glands, nerves, and blood vessels. Submucosal glands can occur at any level but are more prominent proximally and distally than in the mid oesophagus. They are small glands arranged in acini surrounded by a basement membrane and myoepithelium and composed of cells with pale or clear cytoplasm that are similar to those seen in salivary glands (Figure 9.11A). The constituent cells include mucous/mucinous cells, serous cells, and oncocytes.^{1, 13} Mucinous cells are the most numerous and have clear/pale mucinous cytoplasm and small basal nuclei. Serous cells are less numerous and have pale pink cytoplasm. Oncocytes are sparse and have bright pink cytoplasm that may reflect the presence of many mitochondria.¹³ Ducts extend from these glands vertically through the overlying lamina propria and epithelium to the surface. Chronic inflammatory cells may be present adjacent to the glands and in close proximity to the ducts (Figure 9.11B).¹ The cells lining the ducts are cuboidal at the base (Figure 9.11C) and become squamous more superficially in the mucosa (Figure 9.11D) and can be distinguished on this basis from the ducts associated with lamina propria glands (that are lined by columnar epithelium). In practice, submucosal glands and their ducts are a reliable landmark for oesophageal rather than gastric location when there is difficulty determining the anatomical source of a biopsy,¹³ and are more reliable than lamina propria glands when making the distinction between oesophagus and stomach. Although the submucosa is rarely included in mucosal biopsies, the ducts originating from submucosal glands and lined by squamous epithelium superficially are worth seeking when the site of origin of a biopsy is not obvious (Figure 9.11D; Practice Points 9.1).

(A) Submucosal glands (arrow).

(B) Low-power view of submucosal glands and a duct. Chronic inflammatory cells are closely apposed to the duct.

(C) Deeper part of submucosal gland duct lined by cuboidal and columnar cells.

(D) Superficial part of a submucosal gland duct, lined by squamous cells and distinct from the surrounding metaplastic intestinal-type columnar epithelium.

Figure 9.11 Oesophageal submucosal glands and ducts.

Gastro-oesophageal Junction

The area of the GOJ is difficult to classify, for several reasons.

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  Approximately 1–4 mm of the proximal stomach has a lining of columnar mucosa that can be termed cardia mucosa. Opinions differ as to whether the cardia is a normal entity or a metaplastic area.

  
  
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  The cardia mucosa can extend distally into the stomach and proximally into the oesophagus for a variable distance. Extension distally is more common with increasing age. Extension proximally may be secondary, at least in part, to chronic gastro-oesophageal reflux.

  
  
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  The anatomical GOJ equates to the proximal lining of the native gastric folds,⁹ but in most adults the squamocolumnar Z line (junction) moves proximally as a result of gastro-oesophageal reflux.⁹ The columnar mucosa in this area has features of cardia mucosa, with or without parietal cells. Some authors believe that the normal cardia contains oxyntic glands (with parietal and chief cells) and is identical to the corpus mucosa, whereas others assert that the normal cardia comprises mucinous epithelium with mucous glands and/or mixed mucous/oxyntic glands and that this appearance does not indicate metaplasia.⁹ The latter view prevails currently, and regards the process of change in the distal oesophagus as one of increasing length of the cardiac mucosa with age.⁹

  
  
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  Consequently, a short segment of columnar mucosa at the distal end of the oesophagus is common but is difficult to categorise. In any individual it might represent an extension of the gastric cardia (which itself may be normal or metaplastic), or metaplastic epithelium that is not necessarily Barrett’s epithelium, or a form of columnar-lined oesophagus or Barrett’s oesophagus – equivalent to ‘ultra-short segment’ / ‘short segment’ Barrett’s oesophagus. In any event there is often an increase in the length of the ‘cardia’ that produces this appearance.¹⁴

  
  
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  By definition, the presence of intestinal-type epithelium, i.e. goblet cells, in the cardia area or the most distal part of the oesophagus indicates intestinal metaplasia (IM). However, this could represent either IM on a background of gastritis that involves the cardia (‘cardia IM’), or oesophageal columnar metaplasia, i.e. columnar lined oesophagus/Barrett’s oesophagus (‘ultra-short segment’ Barrett’s oesophagus) (see Chapter 10). Indeed, some authors equate ultra-short segment Barrett’s oesophagus with cardia IM, while others discourage the use of the terms ultra-short segment and

  
  
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  short-segment Barrett’s oesophagus. In clinical practice, the implications of true cardia IM may differ from those of IM of the distal oesophagus and they should ideally be distinguished from one another if this is possible.^{2, 9}

  
  
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  In biopsy material, the cardia mucosa is similar to the antral gastric mucosa and distinction is often difficult without knowledge of the anatomical site of origin. Comparing the cardia to the antrum, the foveolae are longer, there may be a few dilated or cystic glands, and there is usually more stroma between foveolae and glands (Figure 9.16). A characteristic finding of cardia mucosa is the presence of fairly well circumscribed aggregates of glands in the deep part of the mucosa around which there are smooth muscle fibres passing upwards from the muscularis mucosae and separating the glandular aggregates into lobules. However, this feature is not always apparent in a biopsy specimen.

Another consideration is the presence of inflammation in the cardia mucosa at the GOJ. In some studies, inflammation of the cardia mucosa immediately below the Z line in the absence of inflammation of the remaining stomach is a more sensitive marker of reflux than histological abnormalities of the squamous mucosa above the Z line.² Indeed, the cardia mucosa in ‘GOJ’ biopsies is often inflamed. In this circumstance, it is difficult without close clinicopathological correlation to know whether this inflammation reflects gastritis (either isolated carditis or a wider gastritis) or reflux.

In clinical practice, normality of the GOJ columnar mucosa is often difficult to confirm unequivocally, whether endoscopically or histologically. In other words, exclusion of metaplasia is rarely possible. Conversely, there may be unequivocal endoscopic and/or histological evidence of abnormality. For example, there is no doubt that an endoscopic squamocolumnar junction several centimetres above the diaphragm is abnormal and that metaplasia (or possibly heterotopia) has occurred, even if histology shows gastric-type mucosa only. Similarly, histological intestinal metaplasia is always abnormal, even if endoscopy shows no abnormality.

Overall, the diagnosis and interpretation of subtle abnormalities in the distal oesophagus and at the GOJ is very difficult. Furthermore, pathology is interpretable only if there is accurate and precise endoscopic information (which is not always the case). In addition, the histopathologist can only report what is visible and should not overinterpret biopsies from this area (Practice Points 9.2). Please see Chapter 10 for more detail about columnar-lined oesophagus/Barrett’s oesophagus.

Heterotopia and Metaplasia in the Oesophagus

An ‘inlet patch’ refers to ectopic or heterotopic gastric mucosa (Figure 9.12A) in the proximal oesophagus, usually within 30 mm of the upper oesophageal sphincter. Foci of ectopic mucosa are typically small (<5 mm in diameter) but may exceed 50 mm in maximum dimension and are sometimes multiple. The epithelium may be of specialised (fundic/body) type or, less often, non-specialised type resembling cardia mucosa. Foci of intestinal metaplasia occur rarely within an inlet patch, e.g. 4% of cases.¹⁵ The prevalence of an inlet patch is uncertain, with figures ranging from 0.2% to 14.5%. This probably depends partly on the endoscopist’s experience.¹⁵ Parietal cells may secrete acid and cause damage to adjacent oesophageal, laryngeal, or pharyngeal mucosa with associated symptoms, e.g. symptoms of reflux. Chronic inflammation, ulceration, and even strictures can occur. Histologically, inflammation is common while intestinal metaplasia and dysplasia are rare. There may be associations with Barrett’s oesophagus and various other complications, including neoplasia. However, reports often conflict.^15–17

(A) Gastric-type mucosa in the upper oesophagus, often termed an inlet patch. Without knowledge of the biopsy site, the appearance risk misinterpretation as Barrett’s oesophagus or as oesophagogastric junction.

(B) Pancreatic metaplasia with ducts and a few acinar-type cells.

(C) Sebaceous metaplasia with well-developed sebaceous units beneath the epithelium, consisting of clear cells and resembling their cutaneous counterpart.

(C courtesy of Dr Laura Lamps, University of Michigan, USA.)

Figure 9.12 Heterotopia and metaplasia.

Ectopic gastric mucosa can also occur in the more distal parts of the oesophagus but is less common at these locations and may require distinction from the more common findings of Barrett’s oesophagus or extension of cardia-type mucosa into the oesophagus.

Ectopic or heterotopic pancreatic tissue can occur in the distal oesophagus, usually in the submucosa.

Ectopic thyroid tissue is also described.¹ Pancreatic metaplasia sometimes occurs in the distal oesophagus and cardia and is characterised by acinar-type cells (Figure 9.12B).

Sebaceous hyperplasia (also known as sebaceous metaplasia or as ectopic sebaceous glands) is a rare entity characterised by aggregates or lobules of sebaceous-type cells with clear or microvesicular cytoplasm lying beneath or within the squamous epithelium (Figure 9.12C). The pathogenesis is uncertain. The phenomenon could represent metaplasia of the squamous epithelium, metaplasia of glands, or a developmental disorder. Lesions range from less than 1 mm to 20 mm in diameter, but are usually small, and may be single or multiple (sometimes more than 100).¹⁸ They may be related to reflux but they produce no symptoms and have no implications for future management.

Anatomy of the Stomach

The stomach includes the cardia, fundus, body, and antrum/antropyloric region (Figure 9.13). There are two curvatures. The greater curvature is convex and extends from the cardiac incisura to the inferior border of the duodenum, and the lesser curvature is shorter and concave and extends from the right border of the oesophagus to the superior border of the duodenum.

Figure 9.13(A) Anatomy of the stomach.

(B) Structure of a gastric unit.

The cardia is a short segment of stomach immediately distal to the oesophagus, distinguished from the adjacent stomach by its histology (see earlier). The fundus is the part of the stomach that protrudes superiorly above the horizontal plane of the left oesophago-gastric junction. The remainder of the proximal stomach is the body or corpus. The fundus and body are histologically similar. Distally, the body is continuous with the antrum. The most distal part of the antrum is the pylorus, or pyloric sphincter, where the muscularis propria is considerably thicker than elsewhere in the stomach.

The boundary between body/fundus and antrum is difficult to define anatomically or histologically. The incisura angularis is an indentation on the lesser curvature that is a rough indicator of the body-antrum boundary, but it is not always obvious and it is unreliable as a marker. In addition, the junction between antrum and body may move proximally within an individual with age or may change as a result of gastric inflammatory conditions. Histology is usually more reliable than macroscopic appearances for distinguishing body/fundus mucosa from antral mucosa but there are transitional areas and some overlap.

Gastric Mucosa: General Features

Histologically, classification of the gastric mucosa is according to site and gland morphology. There are three types: cardia (a small minority; see earlier), fundus/body (approximately 75%–80%), and antrum (approximately 20%–25%). Oxyntic mucosa is another name for fundic/body mucosa, because oxyntic cell is a synonym for parietal cell. Distinction of the different types of mucosa depends mainly on the properties of the glandular layer, including its thickness (as a proportion of the total mucosal thickness), constituent cells, and architecture.

Throughout the stomach, the mucosa comprises several layers (Figure 9.13B). From lumen to base, broadly speaking, these are

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  Surface epithelium

  
  
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  Foveolae (also known as pits or crypts)

  
  
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  Mucous neck region or isthmic region

  
  
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  Glands

Together these four components form a structure from lumen to gland base that can be termed a gastric ‘unit’ (but certainly not a gastric ‘gland’).