2 Dana Alrafaiah Dareen Abu Shalbak Mohammad Almuhtaseb 1 P a g e
Esophagus: A muscular tube that is 25 cm long, but if measured from the incisors it would be 45cm long. Extends from C6 of cervical vertebra, from the lower border of cricoid cartilage and ends in the cardia of the stomach. It contains a mucosa, submucosa, Muscularis Externa, serosa or adventitia, and according to the type of Muscularis Externa we can divide the esophagus into three sections: 1) Upper third: striated muscles. 2) middle third: mixed; striated and smooth muscles. 3) lower third: smooth muscles. Layers: 1) Lining epithelium of the mucosa: non-keratinized stratified squamous epithelium. 2) Lamina propria: loose connective tissue that contains lymphatics, capillaries and glands known as Cardiac glands of the esophagus, these glands are common in the lower third of the esophagus. 3) Muscularis mucosa: two to three ribbons of smooth muscles. 4) Submucosa: contains glands known as esophageal glands proper. 5) Muscularis externa. NOTE: in the GI there are only two organs that have glands in their submucosa; the esophagus and the duodenum, whose glands are knows as Brunner glands. 6) Adventitia or serosa: First, adventitia is connective tissue, where serousa is simple squamous epithelium that can also be called mesothelium. The esophagus above the diaphragm contains adventitia, and below it serousa. 2 P a g e
Stomach: Composed of four parts: cardia, fundus, body and pylorus. The cardia is 1.5-3 cm long, it is a continuation of the esophagus. It also works as a sphincter, making it a physiologic sphincter not an anatomical one. What s the difference? An anatomical sphincter has a thickening in the inner smooth muscle whereas a physiologic one doesn t. But the physiologic sphincter is still capable of constricting and reducing its diameter and so doing the same function as the anatomical one. The pylorus is divided into antrum, pyloric canal and pyloric sphincter; which is an anatomical sphincter with thickening in the inner circular smooth muscle. The function of the stomach: is to convert the bolus into chyme, which is a viscous mass. The bolus spends 2-4 hours here in which it is digested then evacuated through the pyloric sphincter. The stomach has the same layers as other organs in the GI tract: mucousa, submucosa, muscularis externa and serousa; as it is covered by peritoneum. However, the stomach is the only organ in the GI tract whose Muscularis Externa layer is actually composed of three layers: the inner most oblique layer (the extra one), followed by inner circular and outer longitudinal layers. Mucosa: The lining epithelium here is simple columnar without goblet cells. There are glands known as Gastric glands with the same type of epithelium. Those glands contain five types of cells: 1) mucous cells which secrete mucous (most important) 2) parietal cells which secrete HCL. 3) enteroendocrine which secrete Gastrin. 4) stem cells for mitosis and replacement of tissue. 5) chief cells which secrete pepsinogen. 3 P a g e
One common question is, why doesn t the stomach digest itself since it produces HCL and pepsinogen? The answer is because it also secretes mucous, which is composed of 95% water, lipids and glycoproteins. This causes it to be a hydrophobic molecule that provides the mucosa of the stomach with a protective gel, and also allows it to maintain the ph around 7. The cardia Marks the ending of the esophagus and the beginning of the stomach and it works as a sphincter. It contains pits and glands of the same thickness. The ducts of the gland are called pits which connect the glands to the surface. The glands here are known as Cardiac glands, that contain the above mentioned cells (stem cells, enteroendocrine, mucous cells, and few partial) expect chief cells, in addition to lysozymes which act as antibacterial agents against foreign bacteria. NOTE: when we examine the lower part of the lamina propria of the esophagus, we find that it contains the same cardiac glands that are present in the cardia. This is to allow a gradual change in the tissue section. Fundus and Body The body and the fundus have a similar histology. The pits are short and wide and form 1\5 of the thickness of the tissue, whereas the glands form 4\5. This is because most of the digestion and secretion happens in the fundus and the body. The glands contain all 5 types of cells and they reach the surface quickly; due to the short pits, further facilitating the secretion and the digestion process. Each type is present in a specific place; mucous and few stem cells are present in the isthmus, parietal cells that secrete HCL are present in the neck, and the base contains chief cells (which release pepsinogen, appear in a dark color), few enteroendocrine and stem cells. The stem cells are present in more than one place but the highest count is in the neck, due to high rate of mitosis as from their it could head upwards or downwards for the replacement of the dead cells. 4 P a g e
Those are parietal cells, which can also be called Oxyntic cells. An active parietal cell contains prominent intracellular canaliculi in its cytoplasm. However, once they disappear and vesicular-tubular structures appear, it indicates that the cell is now a resting cell. Notice also how those cells appear faint under the microscope. Mucous cells are acidophilic, the nucleus is central and rounded, and sometimes the cell can be binucleated. Notice how mucous cells appear to have vacuolation within them under the microscope; this is due to the mucus dissolving during the preparation of the slides Chief cells are also known as zymogenic cells, they are basophilic, contain mitochondria, zymogenic granules and rough ER. They re responsible for the secretion of pepsinogen, which is converted to pepsin in the acidic media of the stomach. They also release lipase. 5 P a g e
Enteroendocrine cells(ultra-structure): release serotonin and gastrin. Gastrin leads to stimulation of gastric acid secretions and gastric mucosal growth. Pylorus The pits(ducts) are narrow and long. This is because the pylorus has no rule in digestion, as it only neutralizes the acidity of the digested material through alkaline secretions. The glands are simple (can be branched too), tubular and coiled. The cells present here are mainly mucous cells for neutralization, with few enteroendocrine and stem cells (no parietal or chief cells). The pylorus also has lymphatic nodules; in all parts before it the lymphocytes were scattered throughout the lamina propria, but in the pylorus, they aggregate into nodules. This aids in the immunity and protection against any foreign body. NOTE: The epithelium of both the surface and the pits is simple columnar without goblet cells. NOTE: unlike other parts of the stomach, the pylorus has no inner oblique muscle. 6 P a g e
Small Intestines The small intestines are divided into three parts: duodenum, which is retroperitoneal expect for the first and last inches, jejunum and ilium, both of which are intraperitoneal, with long mesentery that allows us to move them as we want in the abdomen. Jejunum and ilium are both 6 meters long, while the duodenum is 10 inches. Again, the small intestines have the same layers as the rest of the GI tract: mucosa, submucosa, Muscularis Externa and serosa. And they re responsible for completing the digestion process as well as carrying out absorption. Here we find finger-like projections in the mucosa knowns as villi; they re 0.5-1.5 mm long, and their function is to increase the surface area to enhance absorption. In the duodenum those projections are actually leaf-like projections, and gradually become finger-like as we near the ilium. The epithelium here consists of simple columnar and goblet cell unlike the stomach, which its without goblet cells. In its lamina propria, between the villi there re glands known as crypts glands or glands of Lieberkühn; they are simple (could be branched) tubular glands with goblet cells and other types as well; stem, enteroendocrine and Paneth s cells. Paneth cells are also present which release lysozymes to control the normal flora of the small intestines. The lamina propria also contains blood capillaries, loose connective tissue and blind lymphatic vessels which are used for fat absorption as they contain and carry chylomicrons. blind lymphatic vessels: Are special for the lamina propria of the villi, they contain special smooth muscle, lymphocytes, fibrocytes and macrophages. Plicae circulares or Kerckreing s valves are invagination s(folds) formed by the submucosa into the mucosa, their function is to further increase the surface area of absorption in addition to the villi. The lining epithelium of both is simple columnar with goblet cells. Absorptive cells or enterocytes are tall columnar cells, each with an oval nucleus in the basal half of the cell. On their apex we have what is known as the striated brush border, which is composed of densely packed microvilli. Each absorptive cell contains at least 3000 microvilli. They are especially prominent in the duodenum. microvillus: has a diameter of 0.1 mm. Contains villin, actin and fibrin filaments. 7 P a g e
Microvilli will also aid in increasing the surface area, alongside the villi and plicae circulares. Together, they are responsible for a 600-fold increase in the intestinal surface, resulting in a total area of 200 m2. Goblet cells: Present between absorptive cells and aids in absorption. Paneth s cells: Acidophilic and present in the basement of intestinal glands. They release lysozymes which provide protection against foreign bodies and control the intestinal flora. Microfolds-cells (M-cells): Present at the surface of the ilium and Peyer s patches. Their function is to engulf foreign bodies at the surface and pass it through its cytoplasm and open basement membrane to underlying macrophages and lymphoid. Macrophage will kill this foreign body while lymphocytes will produce antibodies that will deal with it. Peyer s patches: lymphoid tissue that is special for the ilium and is part of the GALT. What is GALT? - The GI tract is exposed to many invasive organisms and so requires enhanced immunity. This is provided through GALT or Gut Associated Lymphoid Tissue, which contains -in addition to Peyer s patches- IgAs which are the first line of defense, lymphocytes, antibody-secreting plasma cells, macrophages, M-cells. NOTE: another mechanism of protection is provided by the tight junctions between cells that form barriers against any invaders. 8 P a g e
Questions: What are the characteristics of M-cells? - Part of GALT, contain macrophages and lymphocytes inside the cytoplasm, have an open basement membrane. What are the characteristics of the duodenum? - Villi is leaf-like projections, contain Lieberkühn glands in its lamina propria, the submucosa contains Brunner s glands (responsible for alkaline secretions for neutralization of passing material). NOTE: the duodenum is the most common site for peptic ulcer formation, especially the first inch despite the presence of Brunner s glands. Lacteal: lymphatic capillaries with smooth muscle. Characteristics of the ilium? - Peyer s patches (lymphoid tissue), which are present antimesentric and have M-cells formed against it. Characteristic of the Jejunum? - Plicae circulares is common. Myenteric plexus Found between the layers of Muscularis Externa (between the internal circular and outer longitudinal muscle), contains nerve cells that have a large nucleus. We have mainly parasympathetic innervation here; the preganglionic comes from the vagus nerve and the postganglionic neuron goes directly to its target and so its axon is short. Still, we do have sympathetic innervation here too. NOTE: a similar plexus is found in the submucosa, however their its called Meissner's plexus. The function of the parasympathetic is to provide sensorimotor innervation for the glands and motility for the smooth muscles; which makes it responsible for the peristaltic movement of the small intestines. The sympathetic is mainly for the blood vessels and for the contraction of the pyloric sphincter of the stomach. However, upon evacuation, the parasympathetic is what causes it to relax. 9 P a g e
10 P a g e Don t forget to refer to the slides.