Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Internal Organs
Digestive System
Development of the Digestive Tract

The digestive tract—the phylogenetically oldest system of Internal Organs—develops primarily from the endoderm. However, the endoderm forms only its primary, functionally principal part, namely the inner lining. The initial and a small terminal portion of this tract are formed by the invaginating ectoderm. In the embryo, the digestive organs originate as a longitudinal endodermal groove that invaginates toward the notochord (Fig. 4.1). By the fusion of the ventral margins of this groove during the 4th week of embryonic development, the primitive gut tube is formed, blindly closed at both ends. At its cranial end, it abuts the bottom of the oral pit, which is a deep invagination of the ectoderm (Fig. 4.24). Soon, the membrane between the oral pit and the cranial end of the gut, consisting of ectodermal and endodermal layers, ruptures; the Oral Cavity and Pharynx begin to develop. Somewhat later, the caudal end of the tube ruptures into the ectodermal anal pit, which forms the terminal part of the rectum and the anus. The primitive gut tube in the embryo is divided into the cranial and trunk guts. The middle part of the gut is connected to the yolk sac, and an allantoic diverticulum is clearly visible in its posterior section.

Class="center">

Fig. 4.20. Bile canaliculus bounded by three hepatocytes. Electron micrograph x13000:

1 — tight junction; 2 — desmosomes; 3 — rough Endoplasmic reticulum; 4 — lysosome; 5 — Cell/35.html">Mitochondria; 6 — smooth endoplasmic reticulum; 7 — lumen of the bile canaliculus

Bile secreted by hepatocytes into the bile canaliculi flows through them toward the bile duct located in the portal tract. Each bile duct collects bile from canaliculi occupying a specific position within the classic hepatic lobules (Fig. 4.21, A). This area is roughly triangular in shape and is called the "portal lobule" (Atl. Fig. 190).

Liver Cells perform a vast number of Functions related to maintaining metabolic processes in the body. In this regard, the Blood supply to hepatocytes is of great importance. To facilitate the understanding of this concept, the term "hepatic acinus" was introduced. An acinus includes 1/6 of each of two adjacent lobules (Fig. 4.21, B) and is diamond-shaped. As blood flows through the sinusoids, it delivers oxygen and nutrients to the hepatocytes of the hepatic cords while removing carbon dioxide and Metabolic waste products from them. Therefore, one might assume that cells lying near the central Veins OF THE lobules receive fewer of these substances from the blood than cells located near the portal tracts. However, before entering the sinusoids, blood from the hepatic artery and portal vein flows through a network of vessels of progressively decreasing diameter. These vessels penetrate the liver parenchyma and open into the sinusoids. Thus, hepatocytes located near these vessels (zone I in Fig. 4.21, B) receive more substances from the blood than those further away (zones II and III). The part of the acinus located near the central vein receives the most depleted blood. This difference in blood supply causes metabolic processes in these Zones of the acinus to differ slightly from one another. The cells in these zones react differently to dietary nutrient deficiencies or to certain toxins, with cells lying near the central veins being more vulnerable.

Fig. 4.21. Portal lobule (A) and hepatic acinus (B) of the liver (diagrams after Ham and Cormack):

1 — portal tract; 2 — BOUNDARIES OF THE classic lobule; 3 — portal lobule (triangular); 4 — central vein; 5 — acinus (diamond-shaped); 6 — network of Blood Vessels between lobules; 7 — zones of hepatocytes receiving blood of different composition (I, II, III)

Substances brought to the liver with the blood pass through the wall of the sinusoidal capillaries and are taken up by hepatocytes (Fig. 4.22). Between the sinusoidal wall and The surface of the hepatocytes lies the slit-like space of Disse, which is filled with Blood Plasma. In the postnatal period, Blood Cells are not found here.

Fig. 4.22. Diagram of the relationship between hepatocytes and sinusoidal capillaries in hepatic cords:

1 — hepatocyte Nucleus; 2 — Golgi complex; 3 — space of Disse; 4 — endothelial cells; 5 — smooth endoplasmic reticulum; 6 — Lysosomes; 7 — bile canaliculus; 8 — rough endoplasmic reticulum; 9 — Kupffer cells

Numerous microvilli of hepatocytes project into this space. The sinusoidal wall is formed by a single layer of two cell types. These are primarily thin endothelial cells. Between them lie larger Kupffer cells. They develop from blood monocytes and function as macrophages. In the Cytoplasm of Kupffer cells, all Organelles characteristic of macrophages can be distinguished: phagosomes, secondary lysosomes, and Enzymes are frequently observed. The Cell surface facing the sinusoidal lumen is covered with A large number of microvilli. These cells clear the blood of foreign particles, fibrin, and activated blood clotting factors. They participate in the phagocytosis of erythrocytes, and the METABOLISM OF BILE pigments, Hemoglobin, and Steroid Hormones.

The endothelial Cells of the sinusoidal wall have numerous pores in their cytoplasm (Fig. 4.23). The basement membrane is absent. Blood plasma components up to 100 nm in size penetrate through these pores. Due to the free passage of fluid from the sinusoidal lumen into the space of Disse, equal pressure is exerted on the endothelial cells from both inside and outside, allowing the sinusoid to maintain its shape. The sinusoidal wall is also supported by the processes of lipid-storing cells (lipocytes or Ito cells). These cells reside near the sinusoids among the hepatocytes and are capable of synthesizing Collagen. For this reason, lipocytes may be involved in The Development of liver cirrhosis. Furthermore, throughout the liver parenchyma, and around the sinusoids in particular, There is a large number of reticular fibers that perform a supportive function.

Fig. 4.23. Sinusoid and space of Disse (scanning electron micrograph) (after Ham and Cormack):

1 — hepatocyte; 2 — microvilli on the hepatocyte surface facing the space of Disse; 3 — fenestrated endothelium of the sinusoid

As previously mentioned, the surface of hepatocytes facing the sinusoidal lumen is covered with microvilli. They significantly increase the cell surface area required for absorption of substances from the bloodstream and for secretion. The other secretory surface of the hepatocyte faces the bile canaliculus.

Hepatocyte functions are diverse. In the presence of Insulin, they are capable of taking up excess glucose from the bloodstream and storing it in the cytoplasm as Glycogen. This process is stimulated by the adrenal cortex hormone hydrocortisone. In this case, glycogen is synthesized from Proteins and Polypeptides. When blood glucose is low, glycogen is broken down, and glucose is secreted into the blood. The cytoplasm of hepatocytes contains a large number of mitochondria, lysosomes, a well-developed smooth and rough endoplasmic reticulum, and Microbodies containing fatty acid metabolism enzymes. Hepatocytes remove excess Lipoproteins from the blood plasma entering the space of Disse. They also synthesize Blood Plasma Proteins: albumins, fibrinogen, and globulins (except IMMUNOGLOBULINS), and metabolize drugs and chemical substances absorbed in the intestine, as well as alcohol and steroid hormones.

The liver produces a large amount of protein-rich Lymph. Lymphatic vessels are detected only in the portal tracts; they are not found within the tissue of the hepatic lobules.

Bile secreted by hepatocytes into the lumen of the bile canaliculus is collected into small bile ductules located along the boundaries of the lobules. These ductules merge into larger ducts (Atl. Fig. 191). The walls of the ducts are formed by simple cuboidal epithelium surrounded by a basement membrane. As previously mentioned, these ducts merge to form the hepatic ducts. Bile is secreted continuously (up to 1.2 l per day), but in the intervals between periods of intestinal Digestion, it is directed not into the intestine, but through the cystic duct branching off from the hepatic duct, into the Gallbladder.

The gallbladder consists of a fundus (slightly projecting from under the inferior margin of the right lobe of the liver), a body, and a narrowed portion—the neck, facing the porta hepatis (Atl. Fig. 192). The gallbladder serves as a temporary reservoir for bile (capacity 60 cm³). Here, the bile concentrates due to the absorption of Water by the gallbladder walls. With the onset of intestinal digestion, bile flows through the cystic duct into the common bile duct. The latter is formed by the union of the cystic duct with the hepatic duct and opens into the duodenum at an elevation—the papilla (Atl. Figs. 184, 185). Often, the common bile duct merges with the pancreatic duct. At the site of fusion, a dilation is formed—the ampulla of the duct. The duct is equipped with two sphincters formed by smooth Muscle. One of them lies in the region of the papilla, and the other is in the wall of the bile duct. Contraction of the second sphincter blocks the passage of bile into the duodenum. It is then diverted through the cystic duct and accumulates in the gallbladder.

The gallbladder is lined with a mucosa that forms folds. These folds flatten out when the gallbladder is distended. The mucosal epithelium is formed by simple columnar absorptive cells. Their surface is covered with microvilli. The epithelium rests on a thin lamina propria of Connective Tissue, beneath which lies a poorly developed muscularis externa. The latter is formed by longitudinal and circular smooth muscle cells with numerous elastic fibers. Externally, the gallbladder is covered by connective tissue that merges with the liver.

Bile produced by the liver emulsifies dietary fats and activates the fat-splitting pancreatic enzyme, but contains no enzymes itself.

Review Questions

1. Name the Divisions of the digestive tract and their functions.

2. The oral cavity and its Structure. Walls of the oral cavity. STRUCTURE AND FUNCTIONS of the Tongue.

3. Salivary Glands: structure, functions, and development.

4. Teeth. Structure of deciduous and permanent teeth. Tooth replacement. Prenatal tooth development.

5. The pharynx. Nasopharynx and oropharynx. STRUCTURE OF THE pharyngeal wall.

6. Describe The Role of oral and pharyngeal structures in the mechanical Processing of food. Swallowing and sucking.

7. General structural patterns of the walls of hollow Organs of the digestive tract.

8. The Esophagus: topography, divisions, and structural features.

9. The Stomach: topography, divisions, and structural features.

10. The Small Intestine: topography and divisions.

11. The Large Intestine: topography, divisions, and structural features.

12. The Pancreas: its topography and structural features.

13. The liver: its topography, lobes, functions, structural features, and blood supply. Structure of the classic hepatic lobule, portal lobule, and acinus.

14. The biliary System of the liver. Structure of the gallbladder.

15. Prenatal Development of the Digestive System divisions. Age-related Changes in the digestive tract.