Lab 2 & 3 Epithelium and Glands
Characterize epithelium as a basic tissue.
Sheet of interconnected cells that covers surfaces or lines a cavity.
Cells exhibit polarity.
Introduce morphologic basis for classification of epithelia.
Emphasize value of epithelial morphology in organ identification.
Simple squamous epithelium - e.g., lines blood vessels, body cavities, and thin segment of renal loop.
Simple cuboidal epithelium - e.g., lines ducts and forms secretory units of glands.
Simple columnar epithelium - e.g., lines organs such as intestines and uterus that perform secretory and absorptive functions.
Pseudostratified columnar epithelium - e.g., found in respiratory (trachea, bronchi) and reproductive (uterus, oviduct) organs.
Stratified squamous epithelium - e.g., forms epidermis of skin and lines esophagus.
Stratified cuboidal epithelium - e.g., rare but may be found lining ducts of glands.
Stratified columnar epithelium - e.g., rare but may be found in large ducts.
Transitional epithelium - e.g., capable of considerable distention and is found in urinary bladder.
Correlate border modifications to functional significance.
Microvilli - increase surface area for transport mechanisms.
Glycocalyx - protective layer; site of function of various digestive enzymes.
Cilia - protective; aid in movement of particles and mucus in lumen.
Basal lamina - selective diffusion barrier.
Junctional complexes - adherence of adjacent cells (at apical lateral borders).
Desmosomes - "spot" adherence of adjacent cells.
Gap junctions - channel for movement of ions and small molecules between cells and for electrical contact.
Develop an understanding of the morphological variation of glands and their mode of secretion.
Endocrine - secretion is released into body vasculature (ductless).
Consist of cords or clumps of cells - e.g., adrenal gland, adenohypophysis.
Consist of follicles - e.g., thyroid gland.
Exocrine - secretion released into ducts that carry product to sites of utilization.
Simple tubular glands - e.g., sweat glands, gastric glands.
Simple acinar (alveolar) glands - e.g., sebaceous glands.
Simple tubuloalveolar (tubuloacinar) glands - e.g., buccal glands.
Compound tubular glands - e.g., liver, kidney.
Compound tubuloalveolar (tubuloacinar) glands - e.g., mammary gland, prostate gland, pancreas, parotid gland.
Epithelium: Slide List
16 pig heart
1 canine ovary
82 bovine kidney
84 canine bladder
159 horse duodenum
96 canine trachea
164 canine anus & rectum
135L cow teat
150 canine esophagus
118 cow uterus
140 canine parotid gland
88 thyroid follicles and parathyroid gland
Orientation to the Heart - Glass slide #16
The heart will be studied as an organ at a later time. For now, we are interested in the properties of some of the heart's tissues. This valve leaflet attaches to the interior heart wall between the atrium and the ventricle. The atrium is on the right and the ventricle is on the left. Note that some glass slides are mirror images of this micrograph. The dark red in this image is the heart's specialized muscle tissue. The interior of the heart is lined by specialized epithelium.
The surface layer of the endocardium is a simple squamous epithelium. This surface - and indeed, the entire vascular system - has an identical epithelial covering. More detail is available on the next slide.
The outer surface layer of the heart, the epicardium, is part of a serous cavity called the pericardial sac. The sac contains a small amount of viscous fluid that lubricates the sliding inner surfaces and provides some freedom of movement as the heart contracts. The surface lining of the pericardial sac is a mesothelium - a simple epithelium that may range from squamous to cuboidal. This example from the inner 'visceral' pericardium is predominantly squamous.
The atrio-ventricular sulcus is a groove in the myocardium that is typically filled with a rich vascular supply and nerves surrounded by adipose tissue. In this example a large artery and vein are obvious. Do the inner surfaces of these vessels match the inner walls of the atrium and ventricle? Adipose tissue will be studied later along with other connective tissues.
Porcine Heart - Glass slide #16
The ventricular endocardium is multilayered with a surface that is a sheet of simple squamous epithelium. These 'endothelial cells' have compact pachychromatic nuclei that may be elongated in the direction of blood flow. The plasma membrane surfaces of all endothelial cells are specialized for contact with the various cells found in blood. In some organs and/or regions of the circulatory system the flattened cytoplasm of endothelial cells may have openings called fenestrations. These 'windows' allow diffusion of important molecules into and out of the vascular system.
Topic: Endothelial cells
Black arrows indicate endothelial cell nuclei. Their cytoplasm is a thin, slightly darker line along the surface and is barely visible at this magnification
Dog Ovary - Glass slide #1
Most of the ovary is exposed to the surface of the peritoneal cavity and is covered by mesothelium - a simple epithelium of cells ranging from squamous to cuboidal. Similar epithelia line the entire peritoneal cavity, which is a moist environment allowing the organs to be relatively mobile.
Black arrows indicate the outer edge of the ovary, which is covered by mesothelium.
In some regions of the ovary the mesothelium may be interrupted by follicular cells that accompany oocyte development and remain for some time after an oocyte is expelled from the surface. Try to identify some of these areas on your glass slide.
Bovine Kidney - Glass slide #82
The kidney contains many tubules made of specialized epithelial cells. These collecting duct cells are prime examples of a cuboidal epithelium.
Epithelial Types - Renal papilla - Glass slide #82
Simple Columnar Epith.:
The renal papilla contains many tubules whose functions vary along with their structure. This collecting duct was formed by the merger of several smaller tubules. It is lined by a single layer of columnar cells - a simple columnar epithelium.
Topic: Simple Columnar Epith.
Red arrows indicate columnar cells. Relative to the lumen they are clearly taller than they are wide.
Simple Squamous Epith.:
In this portion of the kidney, simple squamous cells play a prominent role by allowing significant levels of diffusion between the lumen and the surrounding tissues. These examples are very typical of the thin segment of the nephron loop (Loop of Henle).
Topic: Simple Squamous Epith.
Red asterisks indicate the lumens of two small tubules. The purple-stained lining cells are very much wider then they are tall, hence they are squamous. Because the epithelium is only one cell thick, it is a simple squamous type epithelium.
Simple Cuboidal Epith.:
These collecting duct cells are approximately the same hight as width, hence they are cuboidal. In this organ there is a continuum of cell heights. Considering that epithelial heights vary considerably, for classification purposes the aspect ratio of cuboidal epithelial cells may vary somewhat from 1:1.
Topic: Simple Cuboidal Epith.
The cells indicated by red arrows are similar in height and width. Some variation in cell dimensions is to be expected.
Stratified Columnar Epith.:
Near the papillary exit of the kidney the columnar epithelial cells of the collecting duct are often seen in a double layer. This stratified columnar configuration is relatively rare - primarily found in kidney and in ducts of glands.
Topic: Stratified Columnar Epith.
The yellow line indicates the approximate cell boundaries of the two layers of columnar cells.
Transitional epithelium is a pseudostratified type of epithelium with a wide variety of appearances. This example from kidney papilla differs slightly from that found in the urinary bladder, which is more adapted to changing dimensions. (Bladder example on following page.) In pseudostratified epitlelia, all of the cells make some contact with the underlying basement membrane even though it may not be visible in the light microscope. Observe here that the surface cells are somewhat wider than the deeper polyhedral cells. Surface cells cover multiple polyhedral cells. When the epithelium is distended, surface cells are able to 'stretch' by adding to their plasma membranes.
Topic: Transitional Epith.
Surface cells of the transitional epithelium are indicated by red arrows.
Canine Urinary Bladder - Glass slide #84
The upper portion of this composite image is a bladder that was distended at the time of tissue fixation. Note that the connective tissue substrate for the transitional epithelium is considerably thinner with fewer infoldings than that in the relaxed urinary bladder (found on the same slide). On your glass slide, examine the transitional epithelium and find examples of surface cells.
Topic: Distended bladder
Surface cells in the distended epithelium are indicated by red arrows.
The lower portion of this composite image is a bladder that was relaxed at the time of tissue fixation. Note that the connective tissue substrate for the transitional epithelium is considerably thicker with deeper infoldings than that in the distended urinary bladder (found on the same slide). On your glass slide, examine the transitional epithelium and find examples of surface cells. Though not visible at the light level, surface cells contain plaques of membrane that are internalized when relaxed. Thus, the cell profiles may not rise much above the surface in a relaxed state.
Topic: Relaxed bladder
Surface cells in the relaxed epithelium are indicated by blue arrows.
Horse Duodenum - Glass slide #159
The inner surface of the horse jejunum is at the top of the zooming image. The simple columnar epithelium on the surface has a single row of nucle and all cells reach the surface. It is specialized for absorbtion: it is polarized and the apical membrane contains many microvilli. Because individual microvilli can not be resolved at the light microscopic level, they are often referred to as a 'brush border'.
Topic: Simple Columnar
Red arrows = position of the basement membrane. Blue arrows = brush border.
Many polarized epithelia have junctional complexes that serve important functions. They sequester the diffusible components of the apical membrane and they impart strength or stability to the surface of the epithelium. In this specimen, junctional complexes are sometimes visible as dark spots at the outer 'corners' of the cells. Again, note the prominent brush border (containing microvilli) on the apical surface of these simple columnar epithelial cells.
Topic: Junctional Complex
Red arrows indicate the cell membranes in which denser junctional complexes are obvious. All of the cells contain them but they must be oriented correctly to be visible.
Brush border and junctional complexes in polarized epithelium
(A) Tight junctions (zonula occludens) block fluid flow at the apical membrane. Zonula adherens junctions provide reinforcement, in the form of adhesion belts between epithelial cells. The beltlike junction encircles each of the interacting cells. Its most obvious feature is a contractile bundle of actin filaments running along the cytoplasmic surface of the junctional plasma membrane. (B) Some of the molecules that form an adherens junction. When strained and aligned with the optical axis of the microscope, these proteins are visible as a small spot known as a terminal bar. The actin filaments are joined from cell to cell by transmembrane adhesion proteins called cadherins. The cadherins form homodimers in the plasma membrane of each interacting cell. The extracellular domain of one cadherin dimer binds to the extracellular domain of an identical cadherin dimer on the adjacent cell. The intracellular tails of the cadherins bind to anchor proteins that tie them to actin filaments.
Canine Trachea - Glass slide #96
The tracheal wall is supported by rigid cartilage rings, and the inner epithelium is very specialized to serve the airway. It is an example of pseudostratified columnar epithelium. While all of the cells contact the basement membrane, the positions of the nuclei give the false appearance of stratified layers. In airways this epithelium is typically ciliated, easily seen in this example.
Topic: Respiratory epithelium
The surface cells (green arrows) are visibly columnar in shape. Multiple layers of nuclei are found between the airway and the basement membrane (red arrows). Respiratory epithelium often includes prominent cilia (blue arrows), which are motile cell extensions that propel the surface layer away towards the exterior. Also present are Goblet cells, mucus-secreting cells indicated by black arrows.
EM of Cilium/Flagellum Cross Section
(A) Electron micrograph of the flagellum of a green-alga cell in cross section, illustrating the 9 + 2 arrangement of microtubules. (B) The various projections from the microtubules link the microtubules together and occur at regular intervals along the length of the axoneme.
Canine perianal zone - Glass slide #164
This complex slide of dog rectum/anus will be studied in more detail later. For now we are interested in only one tissue, the stratified squamous epithelium. Two types are present along the same surface.
Blue arrows indicate the region of this tissue section that contain the epithelium of interest.
The perianal zone is covered by relatively thin skin whose epithelial surface communicates with the anus. The perianal epithelium is stratified squamous in type, and contains a typical stratum corneum made mostly of proteins called keratins. In this example the stratum corneum stains dark red. The source of the stratum corneum is dead, desquamated cells of the stratified squamous epithelium.
Topic: Stratified squamous
Blue arrows indicate the red stratum corneum. Red arrows approximately overly the basement membrane.
The anus contains a mucosa - a surface or compartment in a moist environment. The anal epithelium is stratified squamous in type. Note that there is no stratum corneum outside of the surface squamous cells. This epithelium is described as 'non-cornified' (or non-keratinized').
Topic: Stratified squamous
>Blue arrows indicate the outer squamous cell layer. Red arrows approximately overly the basement membrane.
Cow Teat Wall - Glass slide #135L
Compare this epithelium with that on slide 164, the dog perianal zone. The very much thicker epithelium found on this cow teat demonstrates the adaptability of skin in adjusting to its environmental demands. The thicker epithelium and undulating basement membrane impart abrasion resistance.
Topic: Strat. Squamous
Green lines = junction between stratum corneum and the outer live layer of squamous keratinocytes. Blue lines = the approximate position of the basement membrane.
Simple Tubular Glands in Bovine Uterus - Glass slide #118
Glands enter lumen:
The connective tissue beneath the lumenal epithelium of the uterus contains many glands. They are categorized as simple because they do not branch. These glands develop on demand to support zygote implantation. As they grow they become coiled, which is evident in this slide.
Topic: Glands enter lumen
Red arrows mark epithelial infoldings that are the entry points where long coiled glands release their secretions onto the uterine lining.
These simple tubular glands have no ducts. The secretory epithelium lines the entire tubular infolding. Note the abundance of wavy profiles that indicate the glands are coiled.
Topic: Coiled tubes
Red arrows indicate typical coiled tubular glands.
Gland Development and Classification
Development of glands, by type. Exocrine glands have a duct system that carries secretions to a specific location for release. The gland and ducts differentiate from the surface epithelium of the developing organ. Endocrine glands do not have ducts, even though they also originate from the epithelial cells of their organ. The difference is that the cells that would make up a duct disappear, after which the gland cells associate closely with capillaries inside the organ.
These images display the basic differences among exocrine glands. Compound glands have branched ducts, whereas ducts of simple glands do not branch. The secretory units may vary in size and shape. Acini may be nearly round or flask-like. Tubular glands may be relatively short, and combinations of these are often observed.
Variations in Acini:
Five variations in the structure of acini. Cells may be mucous or serous or mixed. In seromucous acini, the serous cells form demilunes (half moon shapes). In addition, some glands have mypepithelial cells surrounding their acini. Contraction of the myoepithelial cells aids in the propulsion of secretions into the duct systems.
Canine Parotid Gland - Glass slide #140
The parotid is a compound tubuloacinar gland. Because the duct system is branching and complex, we expect to see several kinds of ducts in addition to the acini that generate the parotid secretions. In this example the acini are surrounded by very little extracellular space and most are poorly defined.
Topic: Intralobular ducts
The green circle indicates one acinus - note that it is exclusively serous. The blue arrows indicate an intralobular duct called a striated duct. Striated ducts stain acidophilically. The red arrow indicates the smallest intralobular duct, called an intercalated duct because it originates within acini.
This interlobular duct is embedded in the collagen that separates one lobe from another. This example appears to contain both simple columnar epithelium and stratified cuboidal epithelium (2 layers). The latter is mostly found in glandular ducts.
Topic: Interlobular ducts
Blue arrows indicate the simple columnar epithelium of an interlobular duct. The parotid contains significant amounts of connective tissue (purple staining) in which larger ducts travel towards the main duct. Red arrows = an arteriole - note the endothelial cell lining in cross section.
Canine Thyroid & Parathyroid - Glass slide #88
Although the thyroid and parathyroid appear very different, they share an organizing principle that is critical to all endocrine glands: the secretory cells are intimately positioned near the capillaries. Examine both organs to confirm this. More information on the thyroid is found on the next slide.
PTH = Parathyroid gland. TH = Thyroid gland.
Even at low magnification it is possible to distinguish thyroid folicles. More detail on the next page.
PTH = Parathyroid gland. TH = Thyroid gland.
Canine Thyroid Gland - Glass slide #88
Note that cuboidal folicular cells are closely arranged around capillaries. This is typical of all the endocrine organs. The follicles store the substrate for thyroid hormones.
Topic: Thyroid capillaries
Green 'F' = thyroid folicles containing colloid. Blue arrows = capillaries.
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