Welcome to the lab class in histology of organs and systems. Introducing lab class (in spanish)
The aim of these lab class is to provide students with the educational resources necessary to acquire the basic practical skills of each subject, i.e. to recognize, locate and describe the types of cells, tissues and structures of each organ.
Each of these practical sessions are structured around three main elements:
- Definition of learning objectives.
- HandBook: Description of structures, tissues and cell types of the system itself.
- Exercises: location of structures / cell types in virtual slides

LEARNING OBJECTIVES
- Identify the lobular structure of the major salivary glands.
- Identify the secretory units and ducts of the salivary glands.
- Differentiate between mucous , serous and mixed acini.
- Identify demilunes of Von Ebner and myoepithelial cells.
- Identify the Glisson capsule of the liver.
- Identify the lobes of the liver and connective septa between them.
- Identify the hepatic portal area (triads) and their ducts.
- Identify the center-lobular vein and the hepatic sinusoids.
- Identify hepatocytes and bile ducts.
- Identify the sinusoidal endothelium, the space of Disse and Kupffer cells.
- Identify the structure of the gallbladder, to differentiate the different layers that make up the wall: mucosa, muscular and adventitia.
- Identify the tissues that make up the wall of the gallbladder.
- Identify the acinar structure of exocrine pancreas.
- Characterize the cell types that make up the pancreatic acini, acinar cells and acinar central cells.
- Identify the pancreatic ducts and its different sections.

HANDBOOK
GENERAL ISSUES
The digestive glands are organs attached to the digestive system located outside the gastrointestinal tract, these glands drain their secretion products in different portions of the digestive duct.
These secretory products are used in the digestive system for different functions: lubrication, aids digestion, protection and helps absorption.
The digestive glands that are considered in this class are: salivary glands, liver and exocrine pancreas. Each of these glands secrete a number of substances that meet each of them, a particular function.

SALIVARY GLANDS
The salivary glands are found in the oral cavity and secrete saliva.
Saliva is composed of different substances that perform different tasks in the digestive function. The human being produces about 0.13 to 0.25 gal. of saliva per day containing:
- Mucus and water (major components). Have the function of lubrication and moistening of the oral tissues during swallowing and phonation, and the dissolution of the particles of food, for the taste buds can get flavoring substances.
- Lysozyme, lactoferrin and IgA, which play a protective role. Lysozyme attacks the bacterial walls, the lactoferrin is a powerful iron chelator (iron is needed by the bacteria for growing), the immunoglobulin A (IgA) that attacks viruses and bacteria.
- Lingual amylase and lipase, which plays a digestive function. Amylase (also called ptyalin) is an enzyme that begins to degrade starch, lipase breaks down lipids.
This saliva is secreted by three major salivary glands (pairs) (parotid, submandibular and sublingual), as well as numerous minor salivary glands located on the lips, tongue, cheeks and palate.

Major salivary glands. General structure of the salivary glands (submandibular glands)
They are surrounded with a connective tissue capsule that emits walls separating different lobes. In these connective walls we can find the interlobular excretory ducts. The lobes are composed of numerous secretory units and masses of connective tissue in which are located different intralobular ducts. General structure of the salivary glands (submandibular glands) Salivary glands, serous acini
Histologically, the secretory units consist of compound acinar, tubulo-acinar or compound tubulo-acinar glands from which we can distinguish three types:
- Serous. They are easy to distinguish because the cytoplasm of the cells stained strongly with conventional techniques such as hematoxylin-eosin. Salivary glands, serous acini Mixed seromucous salivary glands, structure of glandular acini
- Mucous. The cytoplasm of the cells stained poorly with conventional techniques such as hematoxylin-eosin and appear white under the microscope. Mixed seromucous salivary glands, structure of glandular acini
- Mixed (seromucous). They consist of a mucous tubule surrounded by a group of serous cells in a half moon crescent shape that is known as Von-Ebner's demilunes. Mixed seromucous salivary glands, structure of glandular acini
Surrounding each of these glands (regardless of type) are frequently seen contractile myoepithelial cells that help evacuate the secretion product. Mixed seromucous salivary glands, structure of glandular acini
The three large glands have a different composition in the type of secretory units is concerned:
- The parotid glands (which represent 25% of the production of saliva) are formed by serous production units and they are compound acinar type.
- The submandibular glands (which produce 70% of saliva), its production is mixed, mainly serous, they are compound acinar, and tubulo-acinar type.
- The sublingual glands are mixed glands, mainly mucous, they are mainly compound tubulo-acinar type.
The wall of the excretory duct becomes more complex as ducts fuse to form larger ducts. So, the tubes that emerge from the secretory unit (intercalated ducts) are formed by a simple cuboidal epithelium (as well as the following levels), while large interlobular ducts and main duct are formed by a stratified columnar epithelium (cubic-columnar). The transition from one extreme to another is gradual and we can find tubes formed by simple cuboidal epithelium, simple columnar, pseudostratified columnar and stratified columnar.

Minor salivary glands. Lip salivary glands, structure of glandular unit
They are located in the lamina propria of mucosa of the above organs, and drain the saliva to the oral cavity through short excretory tubes.
Each of these glands may be serous, mucous, or mixed. Organize themselves into small lobules separated by connective tissue that merges with the connective tissue of the lamina propria where they are located. Lip salivary glands, structure of glandular unit

LIVER
The liver is the largest gland in the body and takes up a considerable portion of the abdomen.
The liver performs many functions, although not all are related to digestion. The most relevant are:
- The liver produces and secretes bile. Bile is made up of different substances, product of the metabolism of different compounds (some toxic). Thus we find in the bile, the bile salts (sodium and potassium salts of fatty acids derived from the metabolism of cholesterol), cholesterol itself, bilirubin, products of the metabolism of medications, drugs, pesticides and alcohol. Obviously, the bile is a route of excretion of substances, which in turn is used as an aid to digestive process. The bile secreted by the liver is stored in the gallbladder, and from there released into the small intestine (duodenum), where it is released to mix with the chyme helping its degradation.
- The liver plays an important role in the metabolism of carbohydrates. Captures glucose from blood (after digestion) and converts it into glycogen. When the body needs glucose, glycogen is degraded to glucose and this is released into the bloodstream. This process is regulated by the insulin-glucagon hormonal system.
- The liver is also involved in lipid metabolism (some of its metabolites are incorporated into the bile).
- The liver is involved in the deamination of amino acids, releasing their nitrogen groups to blood, in the form of urea to be eliminated by the kidneys.
- The liver synthesizes several proteins such as albumin, prothrombin, fibrinogen, angiotensinogen, etc ...
All these functions (and some more) are performed by the predominant cell type in the liver: hepatocytes.
From the histological point of view the liver is surrounded by a layer of connective tissue rich in collagen and elastic fibers, known as Glisson capsule.
This capsule surrounds the hepatic parenchyma which is organized as a polygonal shaped lobes (mainly hexagonal), which are separated from each other (fully or partially, depending on species) by connective tissue septa, which links to the capsule of Glisson These lobes are called hepatic lobes (classic). General structure of the liver Liver, structure of the hepatic lobule In the connective septum, at the height of the vertices of the lobe, are located-the portal areas (also known as portal triads). Liver, structure of the hepatic lobule In the portal areas we can see, at least, one artery, one vein and one bile duct. Liver, Portal system (portal triad) These three tubes can be easily differentiated by their wall: - The artery wall consists of endothelium (simple squamous epithelium), and smooth muscle - the wall of the vein is composed only of endothelium and - the bile duct wall is formed by cuboidal epithelium. These tubes are in the midst of a matrix of connective tissue. Liver, Portal system (portal triad)
In the center of the lobe is located center-lobular vein. Liver: Hepatic lobule central vein The hepatocytes are arranged in anastomosing cords that converge in this central vein. Liver: Hepatic lobule central vein Among these cords of hepatocytes can be located the blood sinusoids. Liver: Hepatic lobule central vein Hepatic sinusoids
The hepatocytes are polygonal shaped cells (cubic), Liver: Hepatocytes, sinusoids and Kupffer cells that are often binucleated. Liver, Ultrastructure of hepatocytes They show rounded nuclei with loose chromatin and the nucleolus is often seen. In the cytoplasm there is an abundance of organelles among which mitochondria, RER, SER. We can see, also, glycogen clusters as well as lipid droplets. Liver, Ultrastructure of hepatocytes This profusion of organelles indicates that these cells have a high metabolic activity.
The hepatocytes are arranged adjacent to each other forming anastomosing cords. Between adjacent hepatocytes are bile canaliculi. Liver: Hepatocytes, sinusoids and Kupffer cells These tubules are formed exclusively by depressions in the membranes of both hepatocytes and are not covered by any other cell. Liver: Hepatocytes, sinusoids and Kupffer cells The hepatocytes secrete the bile into these bile canaliculi. The bile canaliculi cross the liver lobe from the area of lobular central vein to the portal area in which drain into the bile ducts. Thus the flow of bile shows a centrifugal direction, ie from the center of the lobe up to the portal area, in the lobe vertices. Morfo-functional structure of the liver

- Blood supply of the hepatic lobe
The blood reaching the hepatic lobe does as much from the vein as from the artery of the portal area. The vein contributes between 75 to 80%, while the artery provides only 20 to 25% of the blood. This dual blood supply has a clear physiological reason, as we said, the liver performs a number of functions related to the nutrients absorbed by the digestive system, these nutrients are transported by the venous blood coming from the intestine. The arterial blood supply is tied to the supply of oxygen.
The blood entering the liver lobe (both arterial and venous) flows through the sinusoids up to the central vein, where it is drained, so the blood flow in the hepatic lobule follows a centripetal direction. Morfo-functional structure of the liver
The sinusoids are among the cords of hepatocytes. Capillaries are lined by a discontinuous endothelium, where the exchange of substances is very easy. Liver: Hepatocytes, sinusoids and Kupffer cells Liver, Ultrastructure of hepatocytes
Between the endothelium and hepatocytes can see a space, which is called space of Disse. Liver: Hepatocytes, sinusoids and Kupffer cells Liver, Ultrastructure of hepatocytes In this space we can distinguish few collagen fibers produced by cells of mesenchymal origin, known as Ito cells (not shown in the pictures). Another cell type to be considered are the Kupffer cells. Liver: Hepatocytes, sinusoids and Kupffer cells Liver, Ultrastructure of hepatocytes These cells are macrophage-like and are located in the light of the sinusoid, although they can locate in the space of Disse, as they are mobile cells.

The described hepatic lobe is the classical classification of the liver structure using exclusively morphological criteria. Nowadays functional criteria are also used for other classifications, which are not exclusive either between themselves or with traditional classification.
- The first of these classifications uses the criterion that the bile flow. Thus the portal lobe is defined as the portion of liver tissue that drains into a unique bile duct of a portal area. The portal lobe contains parenchyma of three contiguous hepatic lobes (classical) and its limits would be the triangle that connects the center-lobular veins of these adjacent hepatic lobes which shows in its center the portal area located in the common vertex. Flows in the liver
- The other classification takes into account the blood flow, and defines the structure known as the liver acinus. The liver acinus can be defined as a part of the liver parenchyma of two adjacent lobes irrigated by arteries and veins of the portal areas located at both ends of the connective septum separating them. The shape of the acinus is rhomboidal and its vertices would be in center-lobular veins of both lobes and the two portal areas located at the ends of the connective septum separating the two lobes. In this liver acinus we can define three concentric areas that correspond to areas with different oxygen levels, the first and more oxygenated is adjacent to the connective wall and the last (with a lower blood oxygen level) is the furthest from the connective septum. Flows in the liver

GALLBLADDER
The two main hepatic ducts drain the bile to the gallbladder. The function of the gallbladder is to store bile and excrete it into the duodenum in time to the passage of chyme. During storage, the gallbladder concentrates bile by absorbing water and electrolytes.
The gallbladder wall is composed of three layers - mucosa (lining epithelium + lamina propria of connective tissue), - Muscular and - adventitia or serosa (areolar connective tissue). General structure of the Gallbladder
To increase absorptive surface the mucosa has a characteristic anastomosed folds. General structure of the Gallbladder Gallbladder, structure of the mucosa The lining epithelium is simple columnar type. Gallbladder, structure of the mucosa

PANCREAS
The pancreas is a gland with elongated appearance located adjacent to the stomach.
The pancreas is a double gland, exocrine and endocrine at the same time. Each of these functions is carried out by different structures. So exocrine function is carried out by the pancreatic acini, while endocrine function is performed by the islets of Langerhaans.
Histologically, the pancreas is surrounded by a connective tissue capsule from which emerge septa, which separate different lobes. General structure of the Pancreas in each lobe can be found both exocrine structures (acini, which are the most numerous), such as endocrine ones (islets of Langerhaans, which are rare and not found in all lobes). Pancreas, Pancreatic ducts

- Exocrine Pancreas
The function of the exocrine pancreas is the secretion, up to the duodenum, of substances needed to complete digestion.
These substances are of two types:
- On the one hand, the pancreas secretes bicarbonate (HCO3-), which serves to neutralize the acidic pH of chyme, due to hydrochloric acid secreted in the stomach.
- On the other hand, the pancreas secretes many enzymes: Proteases (trypsin, chymotrypsin, procarboxypeptidase), lipases (pancreatic lipase, cholesterolase), nuclease (RNase and DNase) and amylolytic (amylase). Obviously, these enzymes are tasked to degrade the food slurry up to its basic nutrients.
For the secretion of these two types of functions, pancreatic acini show two cell types:
- The acinar cells. Pancreas, Cellular types on exocrine pancreatic acini Pancreas, Acinar cells in pancreatic acini
They are large cell with polyhedral shape, with round nuclei in basal position and numerous zymogen granules located in the apical area. Pancreas, Acinar cells in pancreatic acini These cells are located in the outermost part of the acinus. These cells are responsible for the secretion of enzymes, some of which (eg trypsin) are secreted as precursors (trypsinogen) to avoid self-digestion.
- Center-acinar cells. Pancreas, Cellular types on exocrine pancreatic acini
They are flat shaped cells, with oval nucleus and cytoplasm poorly stained with hematoxylin-eosin. Pancreas, Cellular types on exocrine pancreatic acini They are located, as its name suggests, the central part of the acinus. These cells are responsible for the secretion of bicarbonate.
Form each acinus deparst a tube formed by flat cells, which are morphologically identical to the center-acinar cells, and like them, also produce bicarbonate. Pancreas, Pancreatic ducts & Acini These ducts drain into larger ducts, which are composed of cuboidal cells, which have no secretory function. These ducts merge in a tree way, Pancreas, Pancreatic ducts finally forming a great pancreatic excretory duct that drains the beginning of the duodenum.
The secretion of enzymes is regulated by nerve fibers, but mainly by a hormone, cholecystokinin. Cholecystokinin is secreted by entero-endocrine cells from the small intestine which detect a protein-rich and/or lipid-rich chyme.
Bicarbonate secretion is regulated by the hormone secretin, which is synthesized by other intestinal entero-endocrine cells, which react with an acid chyme.

EXERCISES
- Locate serose and mixed acini of the salivary gland Salivary gland (submandibular) H-E 1,5 um
- Locate mucous secretion cells Salivary gland PAS- H 1,5 um
- Locate Von Ebner's demilunes and myoepithelial cells Salivary gland (submandibular) H-E 1,5 um
- Locate different types of excretory ducts of the salivary glands Salivary gland (submandibular) H-E 1,5 um Salivary gland PAS- H 1,5 um
- Locate hepatic lobes and connective septa (Classic) Liver H-E 7 um Liver Masson's trichrome 7 um
- Locate the center-lobular veins with different staining techniques Liver H-E 7 um Liver H-E 7 um Liver Masson's trichrome 7 um Liver Lectin 10 um
- Locate-hepatic portal areas (triads) and the tubes that compose them, with different staining techniques. Liver H-E 1,5 um Liver H-E 7 um
- Locate hepatocytes and sinusoids with different staining techniques. Liver H-E 1,5 um Liver Masson's trichrome 7 um Liver H-E 7 um Liver H-E 7 um Liver Lectin 10 um
- Locate bile canaliculi. Liver H-E 1,5 um
- Locate Kupffer cells with different staining techniques. Liver H-E 1,5 um Liver Masson's trichrome 7 um Liver Lectin 10 um
- Locate the mucosa of the gallbladder and its folds Gallbladder H-E 1,5 um
- Find the exocrine and endocrine portions of the pancreas. Pancreas H-E 7 um Pancreas H-E 1,5 um
- Locate central-acinar and acinar cells in the pancreatic acini. Pancreas H-E 7 um Pancreas H-E 1,5 um
- Locate different excretory ducts of the exocrine pancreas. Pancreas H-E 1,5 um