Histology of the Alimentary Canal II: Stomach – Anus Lecture Notes PDF

DEPARTMENT OF ANATOMY
ANA 221 LECTURE PRESENTATION
ON
HISTOLOGY OF THE ALIMENTARY CANAL II: STOMACH – ANUS
Course Lecturer: G.O. Oluwatunase 1  OUTLINE Stomach – Introduction – Mucous membrane...

DEPARTMENT OF ANATOMY
ANA 221 LECTURE PRESENTATION
ON
HISTOLOGY OF THE ALIMENTARY CANAL II: STOMACH – ANUS
Course Lecturer: G.O. Oluwatunase 1  OUTLINE Stomach – Introduction – Mucous membrane – Lining epithelium – Lamina propria – Muscularis mucosae Small intestine – Mucous membrane – Epithelia lining – Distinguishing features of duodenum, jejunum, & ileum Large intestine – The colon – Vermiform appendix The rectum Anal canal Clinical correlates Reference 2 STOMACH 3  INTRODUCTION The wall of the stomach has four basic layers : – a mucous membrane, – a submucosa, – a muscularis externa, – a serous layer. 4  THE MUCOUS MEMBRANE As seen with the naked eye the mucous membrane shows numerous folds (or rugae) that disappear when the stomach is distended. is made up of – A lining epithelium. – A layer of connective tissue, the lamina propria, that supports the epithelium. – A thin layer of smooth muscle called the muscularis mucosae. 5  LINING EPITHELIUM The lining epithelium is columnar and mucous secreting. The apical parts of the lining cells are filled by mucin that is usually removed during processing of tissues so that the cells look empty (or vacuolated). Mucous secreted by cells of the lining epithelium protects the gastric mucosa against acid and enzymes produced by the mucosa itself. [The mucous cells lining the surface are also believed to produce blood group factors]. 6  M= Mucosa SM= submucosa ME= Muscularis externa V= Blood vessels S= Serosa 7  LINING EPITHELIUM cont’d At numerous places the lining epithelium dips into the lamina propria to form the walls of depressions called gastric pits. These pits extend for a variable distance into the thickness of the mucosa. Deep to the gastric pits the mucous membrane is packed with numerous gastric glands. These glands are of three types: main gastric, cardiac 8 and pyloric  1= Gastric pits 2= Gastric glands Source: IB Singh – textbook of human histology, 6th edition 9  THE MAIN GASTRIC GLANDS The main gastric glands are present over most of the stomach, but not in the pyloric region and in a small area near the cardiac end. The main gastric glands are simple or branched tubular glands that lie at right angles to the mucosal surface. The glands open into gastric pits, each pit receiving the openings of several glands. 10  THE MAIN GASTRIC GLANDS cont’d The following varieties of cells are present in the epithelium lining the glands:  The most numerous cells are called chief cells, peptic cells, or zymogen cells. – They are particularly numerous in the basal parts of the glands. – The cells are cuboidal or low columnar, cytoplasm is basophilic. – With special methods the chief cells are seen to contain prominent secretory granules in the apical parts of their cytoplasm. – The granules contain pepsinogen that is a precursor of pepsin. 11  THE MAIN GASTRIC GLANDS cont’d  The oxyntic or parietal cells are large, ovoid or polyhedral, with a large central nucleus. – present singly, amongst the peptic cells. – more numerous in the upper half of the gland than in its lower half. – called parietal cells as they lie against the basement membrane, and often bulge outwards (into the lamina propria) creating a beaded appearance. – With the light microscope they appear to be buried amongst the chief cells. – The EM shows, however, that each parietal cell has a narrow apical part that reaches the lumen of the 12 gland  THE MAIN GASTRIC GLANDS cont’d – The cell membrane of this apical region shows several invaginations into the cytoplasm, producing tortuous intracellular canaliculi that communicate with the glandular lumen. – The walls of the canaliculi bear microvilli that project into the canaliculi. – The cytoplasm (in the intervals between the canaliculi) is packed with mitochondria. – The mitochondria are responsible for the granular appearance and eosinophilia of the cytoplasm (seen with the light microscope). – Secretory granules are not present. 13  THE MAIN GASTRIC GLANDS cont’d – Oxyntic cells are responsible for the secretion of hydrochloric acid. – They also produce an intrinsic factor (a glucoprotein) that combines with vitamin B12 (present in ingested food and constituting an extrinsic factor) to form a complex necessary for normal formation of erythrocytes. 14  THE MAIN GASTRIC GLANDS cont’d  Near the upper end (or ‘neck’) of the glands there are mucous secreting cells that are called mucous neck cells. – These are large cells with a clear cytoplasm. – The nucleus is flattened and is pushed to the base of the cell by accumulated mucous. – The supranuclear part of the cell contains prominent granules. – The chemical structure of the mucous secreted by these cells is different from that secreted by mucous cells lining the surface of the gastric mucosa. 15  THE MAIN GASTRIC GLANDS cont’d  Near the basal parts of the gastric glands there are endocrine cells that contain membrane bound neurosecretory granules. – As the granules stain with silver salts these have, in the past, been called argentaffin cells. – These cells are flattened. – They do not reach the lumen, but lie between the chief cells and the basement membrane. – These cells probably secrete the hormone gastrin. 16  P= Parietal cell IC= Intracellular canaliculi C= Chief cells EC= Enteroendocrin e cell 17  CARDIAC GLAND Confined to a small area near the opening of the oesophagus. Mucosa is relatively thin. Gastric pits are shallow. The cardiac glands are either simple tubular, or compound tubulo- alveolar. They are mucous secreting. An occasional oxyntic or peptic cell may be present. 18  PYLORIC GLAND In the pyloric region of the stomach the gastric pits are deep and occupy two thirds of the depth of the mucosa. The pyloric glands that open into these pits are short and occupy the deeper one-third of the mucosa. They are simple or branched tubular glands that are coiled. The glands are lined by mucous secreting cells. Occasional oxyntic and argentaffin cells may be present. Some pyloric glands may pass through the muscularis mucosae to enter the submucosa. In addition to other substances, pyloric glands secrete the hormone gastrin. 19 20  LAMINA PROPRIA As seen above the mucous membrane of the stomach is packed with glands. The connective tissue of the lamina propria is, therefore, scanty. It contains the usual connective tissue cells. Occasional aggregations of lymphoid tissue are present in it. 21  THE MUSCULARIS MUCOSAE The muscularis mucosae of the stomach is well developed. Apart from the usual circular (inner) and longitudinal (outer) layers an additional circular layer may be present outside the longitudinal layer. 22  THE MUSCULARIS EXTERNA The muscularis externa of the stomach is well developed. Three layers, oblique, circular and longitudinal (from inside out) are usually described. The appearance of the layers in sections is, however, highly variable depending upon the part of the stomach sectioned. The circular fibres are greatly thickened at the pylorus where they form the pyloric sphincter. There is no corresponding thickening at the cardiac end. 23 SMALL INTESTINE 24  INTRODUCTION The small intestine is a tube about five meters long. It is divided into three parts. The wall of the small intestine is made up of the four layers 25 26  THE MUCOUS MEMBRANE The surface area of the mucous membrane of the small intestine is extensive. This is achieved by virtue of the following. – The considerable length of the intestine. – The presence of numerous circular fold in the mucosa. – The presence of numerous finger-like processes, or villi, that project from the surface of the mucosa into the lumen. – The presence of numerous depressions or crypts that invade the lamina propria. – The presence of microvilli on the luminal surfaces of the cells lining the mucosa. 27  CIRCULAR FOLDS The circular folds are also called the valves of Kerkring. – Each fold is made up of all layers of the mucosa (lining epithelium, lamina propria and muscularis mucosae). – The submucosa also extends into the folds. – The folds are large and readily seen with the naked eye. – They are absent in the first one or two inches of the duodenum. – They are prominent in the rest of the duodenum, and in the whole of the jejunum. – The folds gradually become fewer and less marked in the ileum. 28 29  CIRCULAR FOLDS cont’d The terminal parts of the ileum have no such folds. Apart from adding considerably to the surface area of the mucous membrane, the circular folds tend to slow down the passage of contents through the small intestine thus facilitating absorption. 30  THE VILLI The villi are, typically, finger-like projections consisting of a core of reticular tissue covered by a surface epithelium. The connective tissue core contains numerous blood capillaries forming a plexus. The endothelium lining the capillaries is fenestrated thus allowing rapid absorption of nutrients into the blood. Each villus contains a central lymphatic vessel called a lacteal. Distally, the lacteal ends blindly near the tip of the villus; and proximally it ends in a plexus of lymphatic vessels present in the lamina propria. Occasionally, the lacteal may be double. 31  THE VILLI cont’d Some muscle fibres derived from the muscularis mucosae extend into the villus core. In some situations the villi, instead of being finger-like, are flattened and leaf-like, while in some other situations they are in the form of ridges. The villi are greatest and most numerous (for a given area) in the duodenum. They progressively decrease in size, and in number, in proceeding caudally along the small intestine. It has been estimated that the presence of villi increases the surface area of the epithelial lining of the small intestine about eight times. 32 33 34  THE CRYPTS The crypts (of Lieberkuhn) are tubular invaginations of the epithelium into the lamina propria. They are really simple tubular intestinal glands that are lined by epithelium. The epithelium is supported on the outside by a basement membrane. 35  IC= Intestinal crypts LP= Lamina propria V= Villi 36  THE EPITHELIAL LINING Epithelium covering the villi, and areas of the mucosal surface intervening between them, consists predominantly of columnar cells that are specialised for absorption. These are called enterocytes. Scattered amongst the columnar cells there are mucous secreting goblet cells. The cells lining the crypts are predominantly undifferentiated. These cells multiply to give rise to absorptive columnar cells and to goblet cells. Near the bases of the crypts there are Paneth cells that secrete enzymes. 37 Endocrine cells are also present  ABSORPTIVE COLUMNAR CELLS Each cell has an oval nucleus located in its lower part. When seen with the light microscope the luminal surface of the cell appears to be thickened and to have striations in it, perpendicular to the surface. With the EM this striated border is seen to be made up of microvilli arranged in a very regular manner. The presence of microvilli greatly increases the absorptive surface of the cell. 38 39  GOBLET CELL Each goblet cell has an expanded upper part that is distended with mucin granules. The nucleus is flattened and is situated near the base of the cell. Goblet cells are mucous secreting cells. In consonance with their secretory function these cells have a prominent Golgi complex and abundant rough endoplasmic reticulum. 40  GOBLET CELL cont’d The luminal surface of the cell bears some irregular microvilli. In haematoxylin and eosin stained preparations, the mucin content of goblet cells appears to be unstained. It stains brightly with the PAS technique. Mucous cells increase in number as we pass down the small intestine, being few in the duodenum and most numerous in the terminal ileum. 41 42  ZYMOGEN CELLS (PANETH CELLS) These cells are found only in the deeper parts of intestinal crypts. They contain prominent eosinophilic secretory granules With the EM Paneth cells are seen to contain considerable rough endoplasmic reticulum. Other organelles and some irregular microvilli are present. The cells are rich in zinc. The function of zymogen cells is not well known. They are known to produce lysozyme that destroys bacteria. They may also produce other enzymes. 43 44  M= Paneth cells 45  ENDOCRINE CELLS Also called enterochromaffin cells. Cells containing membrane bound vesicles filled with neuroactive substances are present in the epithelial lining of the small intestine. Most numerous near the lower ends of crypts. As the granules in them stain with silver salts these cells have, in the past, been termed argentaffin cells. With the introduction of immunohistochemical techniques it has now been demonstrated that these cells are of various functional types, and contain many amines having an endocrine function. 46  DISTINGUISHING FEATURES OF
DUODENUM, JEJUNUM, & ILEUM Sections through the small intestine are readily distinguished from those of other parts of the gut because of the presence of villi. The duodenum is easily distinguished from the jejunum or ileum because of the presence in it of glands in the submucosa. (No glands are present in the submucosa of the jejunum or ileum). These duodenal glands (of Brunner) are compound tubulo- alveolar glands. Their ducts pass through the muscularis mucosae to open into the intestinal crypts (of Lieberkuhn). 47  DISTINGUISHING FEATURES OF
DUODENUM, JEJUNUM, & ILEUM cont’d The proximal part of the jejunum shows significant differences in structure from the terminal part of the ileum. There being no hard and fast line of distinction between the jejunum and the ileum. As compared to the ileum the jejunum has the following features:  A larger diameter.  A thicker wall.  Larger and more numerous circular folds.  Larger villi.  Fewer solitary lymphatic follicles. Aggreated lymphatic follicles are absent in the proximal jejunum, and small in the distal jejunum.  Greater vascularity. 48 LARGE INTESTINE 49  THE COLON The mucous membrane of the colon shows numerous crescent-shaped folds. There are no villi. The mucosa shows numerous closely arranged tubular glands or crypts similar to those in the small intestine. The mucosal surface, and the glands, are lined by an epithelium made up predominantly of columnar cells with a striated border. Their main function is to absorb excess water and electrolytes from intestinal contents. 50  THE COLON cont’d Many columnar cells secrete mucous and antibodies (IgA). The antibodies provide protection against pathogenic organisms. Numerous goblet cells are present, their number increasing in proceeding caudally. The mucous secreted by them serves as a lubricant that facilitates the passage of semisolid contents through the colon. Paneth cells are not present. Some endocrine cells, and some stem cells, are seen. 51 52  THE COLON cont’d The epithelium overlying solitary lymphatic follicles (present in the lamina propria) contains M-cells similar to those described in the small intestine. Scattered cells bearing tufts of long microvilli are also seen. They are probably sensory cells. The submucosa often contains fat cells. Some cells that contain PAS-positive granules, termed muciphages, are also present. These are most numerous in the rectum. 53  M= Mucosa S= Submucosa M= Muscularis externa 54  THE COLON cont’d The serous layer is missing over the posterior aspect of the ascending and descending colon. In many situations the peritoneum forms small pouch -like processes that are filled with fat. These yellow masses are called the appendices epiploicae. 55  THE VERMIFORM APPENDIX The structure of the vermiform appendix resembles that of the colon with the following differences.  The appendix is the narrowest part of the gut.  The crypts are poorly formed.  The longitudinal muscle coat is complete and equally thick all round. Taenia coli are not present.  The submucosa contains abundant lymphoid tissue that may completely fill the submucosa. The lymphoid tissue is not present at birth. It gradually increases and is best seen in children about 10 years old. Subsequently, there is progressive reduction in quantity of lymphoid tissue. 56 57 THE RECTUM 58  THE RECTUM The structure of the rectum is similar to that of the colon except for the following:  A continuous coat of longitudinal muscle is present. There are no taenia.  Peritoneum covers the front and sides of the upper one-third of the rectum; and only the front of the middle third. The rest of the rectum is devoid of a serous covering.  There are no appendices epiploicae. 59 ANAL CANAL 60  ANAL CANAL The anal canal is about 4 cm long. The upper 3 cm are lined by mucous membrane, and the lower 1 cm by skin. The area lined by mucous membrane can be further divided into an upper part (15 mm) and a lower part (15 mm). The mucous membrane of the upper 15 mm of the canal is lined by columnar epithelium. The mucous membrane of this part shows six to twelve longitudinal folds that are called the anal columns. 61  ANAL CANAL cont’d The lower ends of the anal columns are united to each other by short transverse folds called the anal valves. The anal valves together form a transverse line that runs all round the anal canal: this is the pectinate line. The mucous membrane of the next 15 mm of the rectum is lined by non-keratinised stratified squamous epithelium. This region does not have anal columns. 62  ANAL CANAL cont’d The mucosa has a bluish appearance because of the presence of a dense venous plexus between it and the muscle coat. This region is called the pecten or transitional zone. The lower limit of the pecten forms the white line (of Hilton). The lowest 8 to 10 mm of the anal canal are lined by true skin in which hair follicles, sebaceous glands and sweat glands are present. 63  ANAL CANAL cont’d Above each anal valve there is a depression called the anal sinus. Atypical (apocrine) sweat glands open into each sinus. They are called the anal (or circumanal) glands. The anal canal is surrounded by circular and longitudinal layers of muscle continuous with those of the rectum. The circular muscle is thickened to form the internal anal sphincter. 64  ANAL CANAL cont’d Outside the layer of smooth muscle, there is the external anal sphincter that is made up of striated muscle. For further details of the anal musculature see a book on gross anatomy. Prominent venous plexuses are present in the submucosa of the anal canal. The internal haemorrhoidal plexus lies above the level of the pectinate line, while the external haemorrhoidal plexus lies near the lower end of the canal. 65  CLINICAL CORRELATES Peptic ulcer disease Adenoma (polyps) Cancers Carcinoids Appendicitis Haemorrhoids 66  REFERENCE Anthony.L.M,2010,Junqueira’s Basic Histology,12th Edition, pgs 121-139. Barbara Y, James S.L, John W.H, 2006, Wheater’s Functional Histology, A text and colour Atlas,5th Edition, pgs 189-206. IB Singh - Textbook of Human Histology, 6th Edition 67