chapter 1 and 2 for test 1

Most of the pancreas is composed of acinar cells. Acinar cells are polarized cells clustered around a central lumen which is the entrance to a duct (i.e., an acinus). Compare the nuclei in acinar cells those within blood vessels and other structures.

Acinar cells have circular nuclei while other cell types have nuclei with elongated shapes. Red blood cells within the blood vessels do not have nuclei.

The kidney contains many different types of epithelia. However, the focus here is only on a simple squamous epithelium

Find one of the round structures (~250 µm diameter) known as renal corpuscles. Each contains a glomerulus (a tuft of capillaries) surrounded by Bowman's capsule. The interior of the capsule is lined by a simple squamous epithelium that rests on a thick basement membrane. The only part of these cells that can be seen are their nuclei bulging into the interior.

It's relatively easy to see mitochondria in the cells lining the small intestine. Examine the surfaces of villi within the cross section of the intestine

Mitochondria can be distinguished as darkly stained, thread-like structures in the apical cytoplasm of some cells. They appear as dark grey lines that run parallel to the long axis of the cell and perpendicular to the cell surface. The Golgi apparatus is visible as an unstained region adjacent to the nucleus. These cells are polarized with their basal surface resting on the underlying connective tissue while the apical surface faces the lumen.

This liver is from a fed animal. At higher magnification, examine the staining of individual hepatocytes.

PAS staining appears as course magenta-colored particules throughout the cytoplasm of most cells. Almost all the cells are intensely stained. Nuclei are dark blue to purple because the section was counter stained with hematoxylin. This indicates that the hepatocytes of a fed animal contain considerably more glycogen than a fasted animal.

The liver is composed primarily of hepatocytes. They store large quantities of glucose (as glycogen) after a meal and release it when fasting. Periodic acid-Schiff (PAS) reagent can be used to demonstrate the amount of glucose stored as glycogen. Examine the staining of individual hepatocytes from this fasted animal.

PAS staining appears as scattered red to magenta particles in the cytoplasm. The intensity of staining varies between cells from light to no staining. Nuclei are dark blue to purple because the section was counter stained with hematoxylin.

Transitional epithelium (urothelium) is a specialized stratified epithelium found in the lower urinary tract. It rapidly adapts to distention and contraction by changing from a taller to thinner epithelium. Umbrella cells are highly dynamic cells at the luminal surface. Relaxed (non-stretched) The transitional epithelium has several layers of cells and large, dome-shaped umbrella cells on its surface. (They are called umbrella cells because they cover several underlying epithelia cells.) Extended (stretched) The transitional epithelium has become thinner. The umbrella cells have become elongated and flattened.

Relaxed (non-stretched)

Epithelia are classified by the shape of the surface cells and whether it has single or multiple layers of cells. Shape of surface cells: Squamous - flattened cells whose width is greater than their heightNuclei are often taller than the flattened cells and bulge into the lumenDifficult to see their thin cytoplasm Cuboidal - cells appear as high as they are wideSpherical nuclei are centrally positioned Columnar - cells are taller than they are wideOval nuclei located near the base of the cells Number of layers: Simple - a single layer of cellsAll cells contact the basement membrane (purple) Stratified - two or more layers of cellOnly the lowest layer of cells (yellow) contact the basement membrane (purple)

Specialized epithelia: Pseudostratified ColumnarSingle layer of cellsNuclei appear being at two or more levelsNot all cells reach the free surfaceAll cells contact the basement membrane (purple) Transitional - used where the epithelium is stretchedMultiple layer of cellsOnly the lowest layer of cells (yellow) contact the basement membrane (purple)Morphology varies depending on whether it is relaxed or stretchedRelaxed - surface layer of large, dome-shaped cellsStretched - surface layer of flattened cells

Epididymis In 1898, Camillo Golgi described an "internal reticular apparatus" in cells stained using Golgi's method (silver stain). Many years later, electron microscopy confirmed the discovery of this organelle. It's relatively easy to see the Golgi apparatus in the cells lining the epididymis, the organ of the male reproductive tract in which sperm mature. Examine the septa composed of two layers of cells facing adjacent cross sections of the duct separated by a small amount of connective tissue.

The Golgi apparatus is located between the nucleus and the apical pole of the cell (i.e., supranuclear). It's reticular structure is stained black. These cells are polarized with their basal surface resting on the underlying connective tissue while the apical surface faces the lumen

Most of the pancreas is composed of acinar cells. They are polarized cells clustered around a central lumen that is the entrance to a duct (i.e., an acinus). Unfortunately, the staining of the pancreas is highly variable (either under or over stained). Search for a region in which only the Golgi apparatus is stained.

The Golgi apparatus is located between the nucleus and the apical pole of the cell (i.e., supranuclear). It's reticular structure is stained black. These cells are polarized with their basal surface resting on the underlying connective tissue while the apical surface faces the lumen.

The liver is composed primarily of hepatocytes. Examine individual hepatocytes at high magnification.

The cytoplasm is filled with so many mitochondria that it is difficult to distinguish them individually. The mitochondria are thread-like or granular depending on their orientation. Although the nucleus does not stain, the nucleolus is prominently stained. Some of the cells have more than one nucleolus.

The surface epithelium is composed of polarized, column shaped cells with an oval nucleus found in the basal portion of these cells. Polarized cells have a basal surface that rests on supportive connective tissue and an apical free surface that faces the opposite side. The free surface always faces a lumen, such as, the interior of the gut or a duct.

The surface epithelium

Toluidine blue is a basic dye that binds nucleic acids but preferentially stains RNA. At higher magnification in the cerebellum, examine Purkinje cells stained with toluidine blue.

Their large nuclei contain lightly stained euchromatin. Their nucleoli are intensely stained because they contain negatively charged RNA involved in ribosome assembly. Nissl (i.e., chromophil) substance appears dark blue due to the staining of ribosomal RNA, giving the cytoplasm a mottled appearance. The presence of Nissl substance suggests these cells synthesize large amounts of protein.

The cerebellar cortex is composed of the outer, hypocellular molecular layer and an inner, densely populated granular cell layer. At the interface of these two layers are large neurons known as Purkinje cells. At higher magnification, examine Purkinje cells stained with hematoxylin and eosin (H&E).

Their nuclei are much larger compared to those in the inner, densely populated granular layer. Their cell bodies and dendrites are intensely acidophilic. This indicates they contain large amounts of proteins. Only the nuclei of neurons in the inner, densely populated granular layer are visible.

This pancreas section was incubated with the enzyme deoxyribose (which hydrolyzes DNA) before Feulgen staining.

This pancreas section was incubated with the enzyme deoxyribose (which hydrolyzes DNA) before Feulgen staining.

This section of the small intestine is poorly preserved with most of the epithelium detached from the underlying connective tissue. The surface area of the small intestine is increased by finger-like projections called villi. Examine a villi in which the epithelium is still attached to the underlying tissue. The epithelium is a single layer of cells that are taller than they are wide. However, this is difficult to see because this section is cut tangential to the surface of the villi. It passes through adjacent cells resulting in multiple nuclei appearing at different levels.

epithelim This simple columnar epithelium contains: Surface Absorptive Cells (or enterocytes) - their apical surface has tightly packed microvilli to increase their surface area. This is known as a brush border. The dark line underneath the brush border is the terminal web in which the microvilli are anchored. Goblet Cells - scattered cells that secrete mucus. The basophilic secretion granules form circular clusters near the apical surface.

The esophagus is a muscular tube through which food passes from the pharynx to the stomach. Cuboidal epithelium has a limited distribution. It is most often found in the ducts from exocrine glands. These ducts range from simple cuboidal, simple columnar, to stratified cuboidal epithelia. Simple cuboidal epithelium - often found in small ducts. Simple columnar epithelium - found in intermediate size ducts. Stratified cuboidal epithelium - usually found in both small and large ducts. It most often has only two layers of cuboidal cells.

esophagus

The ovary and kidney contain examples of simple cuboidal epithelia. Ovary - the outer surface of the ovary is covered with a simple cuboidal epithelium. It is very fragile and you may have to search for a region where it is intact.' Kidney - the area around renal corpuscles usually contains many cross-sections of tubules with a simple cuboidal epithelium.

ovary

The largest cells in the human body are motor neurons in the spinal cord and mature ovum in the ovary. At higher magnification, find motor neurons cut in cross section through its nucleus with or without processes (axons or dendrites).

spinal cord

The trachea is a tube that connects the larynx to the lungs allowing the passage of air. It is lined with a pseudostratified columnar epithelium. As its name implies, this epithelium appears to be stratified (i.e., has multiple layers of cells) but in fact all of the cells are attached to the basement membrane. Therefore, it is a simple epithelium. The stratified appearance is due to the nuclei of individual cells being present at different levels.

trachea Features of this pseudostratified epithelium: Cilia - extend 5 to 7 µm from the surface of the epithelial cells. The dark line at their base is from their basal bodies. Goblet Cells - scattered cells that secrete mucus. They are difficult to identify in this specimen, but a thick layer of mucus covers some regions of the epithelium. Basement Membrane - the epithelium is separated from the underlying connective tissue by a thick basement membrane. It is seen as an acidophilic band beneath the epithelium. Since it lines the respiratory tract, a pseudostratified ciliated, columnar epithelium with goblet cells is referred to as the "respiratory epithelium".