ANAT D502 - Basic Histology

Epithelial Tissue Pre-Lab

 Revised 9.6.12

Objectives:

1. Be able to classify epithelial tissues by:

-cell shape
-cell stratification
-cell surface specializations

2. Define cell polarity (apical vs. basal).
3. Distinguish between endothelium and mesothelium.
4. Distinguish endocrine from exocrine glands.
5. Distinguish unicellular and multicellular glands.
6. Recognize that multicellular glands are classified as either simple (single unit) or compound (multiple, branched units); both types can be subdivided by the shape of their constituent units.

Slides:

s10 mesentery
s19 heart (atrium and ventricle)
s30 abdominal skin
s45 submandibular gland
s48 trachea and esophagus
s63 liver and gallbladder
s69 urinary bladder
s86 thyroid/parathyroid gland
s89 adrenal [TA suprarenal] gland
s90 kidney (monkey)
s93 lung (human)

s95 jejunum
s98 eyelid monkey
s99 colon
demonstration slides: M69 contracted & distended urinary bladder

Index of images (there are a lot!!!)


Recall that there are four types of tissues:  epithelial, connective, muscle and nerve.  Epithelial tissues consists of sheets of conjoined cells that (1) line surfaces and (2) form glands.  As such, epithelial tissue covers the body, both inside and out, and affords a barrier property to the body. To accomplish this, the cells making up the tissue are typically tightly linked together and to the underlying tissue using cell - cell and cell - substrate specializations. The epithelial cell shape and stratification are related to its function. For example, the skin is specialized to act as a "physical" barrier to the outside world. This barrier is both structural (multi-layered) and chemical in nature. Another example is the cells that make up the tubular elements of the kidney. These cells are usually a single layer, are cuboidal to columnar in shape and have obvious polarity. These cells primarily present a chemical barrier and are involved in directional transport of water and solutes. As you learn about epithelia, think about what the cell's primary function is and relate this back to its structure.

I. Types of epithelial linings

Epithelial linings are classified by (1) the number of cell layers between their basement membrane and the surface (simple (1 layer), pseudostratified, and stratified (2+ layers)), (2) the shape of the outermost layer of cells (squamous, cuboidal, columnar), and, if present, (3) surface specializations (e.g., ciliated, keratinized). When naming the type of epithelia, be sure to include the first two criteria in the name, adding the third only if it is present.   One key in identifying epithelia is the shape of the nucleus of the outermost cell which often mimics the shape of the cell.

1. Simple squamous

Simple squamous epithelia are noted by the flattened appearance (squamous) of their cells in transverse-section, and these epithelia are only one cell layer thick (simple). Found in many places, this type of epithelia also lines the circulatory systems and serous cavities of the body.  Endothelium is the simple squamous epithelium that lines the lumen of both the cardiovascular and lymphatic circulatory systems.  Mesothelium is the simple squamous epithelium that lines the serous body cavities derived from the embryonic coelom.  The body cavities are the pleural cavities, pericardial cavity, peritoneal cavity and, in males, the cavities of the tunicae vaginalis testes.  The linings are, respectively, the pleurae (paired), peritoneum, pericardium and, in males, the tunicae vaginalis testes (paired, hopefully).  Organs protruding into the lining of these cavities become invested with a layer of mesothelium.   The mesentery is a fold in the peritoneum produced by an organ protruding into it; thus, it is covered with a layer of mesothelium  We will discuss the function of the mesentery when we examine the digestive system.   

a) Endothelium: Good examples of endothelia can be found in the blood vessels within s10 of the mesentery. Look on the lumenal side of the blood vessels. Blood vessels are usually paired with a vein next to an artery. The vein can be distinguished by its thinner wall and less robust elastin staining near the lumen. Identify the vein and study the lumen/wall junction. At 20X, you should observe nuclei that protrude into the lumen of the vessel. These are the nuclei of endothelial cells that line the lumen. Note that the layer is only one cell thick. The appearance of the endothelial layer of the blood vessel may be obscured in the larger vessels by the robust elastin staining. Capillaries are essentially a one cell-layer-thick tube made of endothelial cells and are readily found in this section. Look for a good example of a capillary (both longitudinally and transverse sectioned) where you can make out the endothelial cell nuclei. In transverse section, the endothelial cell appears as a crescent hugging a red blood cell. Longitudinal section of capillaries are not as easy to find but appear as string of red blood cells. Observation at higher magnification gives one an appreciation of the endothelial cell nuclei face on and in longitudinal section.

b) Mesothelium: Mesothelial cells can be found lining the exterior of the atrium on s19. This slide contains a section of both a ventricle and an atrium. The atrium is the smaller of the tissue sections on the slide and has a complete lumen. View the slide with the naked eye to locate the lumen and wall of the atria. View the atria with the microscope at low magnification to observe the wall of the organ. Observe the wall at higher magnification and you should observe a single layer of cells lining both the outside of the wall and the lumen. The mesothelial cells line the outer wall of the atria and at 40X, the nuclei are flat or squamous in appearance and one layer thick. On the lumenal side of the atria is the endothelial cell layer. Distinguishing the cell layer as either meso- or endothelium is not easy unless you know the topology of the tissue. There are some differences in the appearance of underlying tissue which can aide in the distinction between endo- and mesothelium and we will learn those later.


Another example of mesothelium can be found in s93 of the lung.  Look along the intact side of the tissue and your should observed a single layer of simple squamous epithelial cells lying atop a irregular connective tissue layer.  This is the mesothelium of the visceral pleura.


2) Simple cuboidal

Simple cuboidal cells are found both in endocrine and exocrine glands, in most of the tubular elements of the kidney, and in the duct portion of many glands. These cells are cubic in shape, as well as their nuclei. In some cases, the cell margins can be readily observed which is not the case with most simple squamous epithelia. The kidney section observed in s90 contains several profiles of tubular elements of the kidney and profiles of renal corpuscles. Pan across the section at low power; look for a field containing several good transverse sections of tubes, near or at the center of the section (medulla or middle). Change to high power and carefully examine these tubular elements. Some show more eosinophilic cytoplasmic staining than others. Note that the nuclei are round to cuboidal in shape. In the tubes with the lighter staining cytoplasm, you may be able to make out the cell boundaries by focusing up and down through the section. Also, in this cuboidal epithelia, the nuclei are closer to the base of the cell than the apex. Return to lower magnification and
staying in the medulla search until you find longitudinally sectioned tubes. You will not likely find perfect transverse sections but you can study the appearance of simple cuboidal epithelia of tubes in longitudinal section. If the histology gods are with you, you may find a good example of longitudinal section that shows obvious cell margins. If you are an extremely observant histologist, like Dr. Swartz, you may find a face on view of the lumen of the tube. You will observe an apparent square or hexagonal lattice within the lumen that is the apex of the simple cuboidal epithelia. The junctional complex of the epithelia (tight junction + zonula adherens + macula adherens) produces the lattice that marks the cell margins near the apex.

Other examples of simple cuboidal epithelia can be found in the duct-portion of salivary glands. Observe the submandibular gland (s45) at low power and look for transverse sections of tubular elements. The lumens of the ducts are devoid of material and the cytoplasm of the cells usually stains light pink. Observe the ducts at 40X and note that the cell margins are not readily visible and that the nuclei are positioned more in the center of the cell than observed in the cuboidal epithelia of the kidney.


The follicular cells of the thyroid are an example of simple cuboidal epithelial cells of an endocrine gland. Study s86 at low power and you will observe eosinophilic sacs (follicles) and at higher power you will observe that these follicles are surrounded by a layer of cells. The eosinophilic material is colloid (protein). The cells lining the interior to the follicles are called thyroid follicular cells and are simple cuboidal epithelia. The slide that we have for study is an example of an inactive thyroid gland in which the follicular cells are somewhat flat to low cuboidal. However, the nuclei are still round/cuboidal in appearance.


3) Simple columnar

Simple columnar epithelia are most readily found lining the lumen of the small intestine and gall bladder. These cells are usually involved in absorption of nutrients and/or water. The cells are elongate with their long axis perpendicular to the lumen. The nuclei are also elongate. We will first observe the lining of the small intestine. Scan s95 (jejunum) at low power and you will observe one surface which is thrown into folds (villi). Examine this region of the section at higher power and you will observe the simple columnar cells of this epithelia. At 40X, you can scan along the lining and look for a good example of the simple columnar epithelial cells in longitudinal section. The nuclei are near the cell base and in some cases the cell margins can be observed.  The lumenal border of the cell appears indistinct and is called the striated border and results from super-positioning of the microvilli capping the cell.  Occasionally, you might observe vertical bars along the apical junctions of adjacent cells; these are called terminal bars and they are the light microscopic manifestation of the junctional complex. In some of the lining cells, you will note a lack of cytoplasmic staining and terminal bars at the apex. These round cells are goblet cells (unicellular gland cells). We will observe better examples of this cell type in other sections.

Another example of simple columnar epithelia can be found along the lining of the gall bladder. Examine s63 with the naked eye. This slide contains the gall bladder and some liver. The large lumen that you observe with the naked eye is the lumen of the gall bladder. Examine the lining of this lumen at 10 - 40X and observe that the epithelia does not always appear as a single layer of cells; this is due to the super-positioning (overlaying) of cell layers in this relatively thick section. Keep scanning along the lining until you find an area that is "one nucleus" thick (i.e., one cell layer thick). At this location, the cells will show a simple columnar appearance and a striated border indicating the presence of apical microvilli.  This epithelia does not contain goblet cells.


4) Stratified squamous

Stratified squamous epithelia line surfaces that are subject to abrasion. Those that are exposed directly to the environment (exterior skin) are keratinized (a layer of dead cells), while those that are not directly exposed (e.g. surfaces of the oral cavity, internal eyelid, vagina, terminal portion of the penile urethra) are  only partially keratinized or not keratinized at all.. The cells within the epithelial layer are of various shapes but the type of epithelia is based upon the shape of the outermost layer of cells which are squamous.

a) Keratinized

Keratinized stratified squamous epithelia is observed in s30 (abdominal skin). Examine the slide at low power and you will observe one surface which is more intensely stained than the other. This layer is the epidermis and contains keratinized stratified squamous epithelia. Study the epidermis at higher magnification, you will observe the various cell and nuclei shapes within the layer. The cells near the base of the epithelia are cuboidal to columnar in appearance, but that the outermost layer of cells is squamous in shape. Within the outer layers, the nuclei are not readily apparent because either they are visually occluded by granules within the cytoplasm or have been shed by the cell. Fully keratinized cells are observed as multiple layers of pink to purple fibrous material sloughing off of the surface of the epithelium.

b) Non-keratinized

The eyelid contains both keratinized and non-keratinized stratified squamous epithelia. Study s98 at low power to get an orientation of the section. The bulbous portion of the section is the junctional part of the eyelid which meets with the other eyelid and contains the hair follicles of the eyelid. There is a transition from keratinized (outer) to non-keratinized (eye side) of the stratified squamous epithelia. Observe this region of transition at 10- 40X. As you move towards the eye side, you loose the obvious keratin layer and can, maybe, make out the squamous (flattened) nuclei of the outermost layer of cells. There is pigmentation that caps the nuclei near the base of the epithelia and we will learn more about this later.

5) Stratified cuboidal and stratified columnar

Stratified cuboidal and stratified columnar epithelia are most commonly observed in the terminal portions of exocrine gland ducts. Return to slide 45 (submandibular gland) and look for large non-vascular lumen, typically found in the connective tissue septa outside of the secretory portions of the gland.  Examination at higher magnification will reveal the walls of these ducts are comprised of 2 or more layers and the lumenal layer is cuboidal to columnar.  In general, as exocrine ducts approach their secretory surface, their morphology becomes more like the epithelium lining that surface.  In this case, the morphology of the submandibular gland ducts eventually become a stratified squamous epithelium like that lining the oral cavity.

6) Pseudostratified columnar

Pseudostratified columnar epithelia are so called because of their apparent stratification. However, close examination of the cells shows that while some of the cells span the thickness of the epithelium to reach the lumenal surface, all of the cells are attached to the basal membrane.   This arrangement results in the nuclei of this epithelium lying at different levels thus giving the illusion/impression of stratification. This type of epithelia is always ciliated and is commonly found in the respiratory tract of both sexes.  [It is also found in the male reproductive tract but this will be examined at a later time.] Slide 48 is a transverse section of both the esophagus and the trachea. Given that the lining of the esophagus is non-keratinized stratified squamous epithelia, you should be able to distinguish which lumen is the esophagus and which is the trachea. Carefully examine the lining of the trachea and focus upon the lumenal most aspect of the lining cells. You should observe (at 40X) fuzzy or hair-like projections from the apex of the cells. These are cilia. Observe the stratification of the nuclei and notice how most cells can be observed to traverse the thickness of the epithelia. Goblet cells are present in this epithelia. These cells are recognized by their pale-staining cytoplasm in the shape of a goblet with the nuclei located in the stem portion. Also recognized in this section is the basal membrane of the epithelia. This is the pale, eosinophilic layer just beneath the epithelial cells. It would be shown to more advantage if the specimen was stained with PAS to emphasize the carbohydrate content of this layer.


7) Transitional

Transitional epithelia is a special type of epithelia restricted to the excretory passages of the urinary system (renal pelvis, most of the urethra and urinary bladder). We will focus our study on the urinary bladder as it presents the best example of transitional epithelia. It appears as stratified cuboidal to squamous epithelia. However, a key features of this epithelia is its lability in terms of thickness and the shape of the outermost cells. When the bladder is empty, the outermost cells are cuboidal or dome-shaped, but when the bladder is full (distened), the thickness of the epithelial layer is less and the outermost cells appear more squamous. Thus the name transitional epithelia. Slide 69 contains a transverse-sections of an empty urinary bladder. In the center of the section, you will observe a convoluted empty space which is the lumen of the bladder. Study the lining of this lumen which is transitional epithelia. A demonstration slide has sections of an empty and full bladder. For these, focus on the difference in the appearance of the epithelia between the two states of the bladder. The thinner of the two is the full bladder while the thicker is the empty bladder.

II. Glands

Glands are single cells or groups of cells which secrete protein, mucous, or lipid. These products can be secreted internally (endocrine) or into the external environment (exocrine). When exocrine glands are organized as groups of cells (i.e., multicellular), they often contain a secretory portion which synthesizes and secretes the products and a ductile portion which channels the product to the exterior and may also further modify the secretory product.

1) Endocrine

We have already viewed an endocrine gland when we observed the simple cuboidal epithelia lining the follicles of the thyroid gland (s87). The secretory product from these cells is released into capillary beds that surround each follicle. Re-examine the slide to identify the capillaries between the follicles. Another example of an endocrine gland is the suprarenal [TA; adrenal] gland (s89). View this slide at low power and notice that the parenchyma can be roughly divided into a cortex region (outer) and a medulla region (inner). Direct your study to the cortical (outer) region. The key features to note in this region are the rich capillary beds and the foamy appearance of the cytoplasm of the cells. This latter feature is the result of these being steroid-secreting cells and thus originally contained a lot lipid which was lost during tissue processing.

2) Exocrine

Exocrine glands secrete their products into a lumen or onto the surface of the skin. The former are associated with the digestive and respiratory tracts and the latter includes sweat and sebaceous glands of the integument. These glands can be unicellular or multicellular. The multicellular glands can have different types of organization (tubular, compound tubuloacinar, etc) which are sometimes difficult to interpret from a one dimensional image of the gland. The cells of these glands secrete primarily proteins or mucous. An exception to this is the mammary gland whose secretion is rich in both proteins and lipids. Cells that secrete mucous have pale-staining cytoplasm when stained with H&E but red-staining when stained with PAS. Cells that secrete protein stain highly eosinophilic with H&E because of the high content of protein in the cytoplasm.

 a) unicellular (goblet cells)

You have already observed the goblet cells within the cilated pseudostratified columnar epithelia found in the trachea (s48). Examine s95 (jejunum) and identify the goblet cells in the epithelial layer of this section. The goblet cells are somewhat swollen in this specimen relative to the goblet cells observed in the trachea.

b) multicellular

i) simple tubular

An example of simple tubular glands can be observed on s99 (colon) which secretes mucous. Scan along the epithelial surface at low power and you should observe tubular glands both in longitudinal and transverse section. Study each view of the gland at 10 - 40X. The lumen of the gland should be readily observed in the transverse sections. In longitudinal section, the lumen is not as obvious and the mucous-secreting goblet cells are not as prevalent near the opening of the gland compared to the base. The cells that make this up simple tubular gland both secrete and channel the secretory product into the lumen (i.e., it is ductless).

ii) simple coiled tubular

The sweat glands of the skin are simple coiled tubes  and are located in the dermis of the skin. Study s30 (abdominal skin) at low power and the dermal layer is evident as the more eosinophilic layer. Look for a cluster of cells surrounded by the rather acellular eosinophilic connective tissue. At higher power, the gland will show several different aspects of the coiled tubular gland including longitudinal and transverse-sections of tubes. The cells of the glands are simple to stratified cuboidal epithelia with eosinophilic cytoplasm. These glands have a distinctive secretory portion and duct portion. The secretory portion is made of simple cuboidal epithelia with a pailer staining cytoplasm than the duct portion. The duct portion continues out to the epidermis where it fuses with the epidermis and releases the glandular products onto the epithelia.

iii) compound tubuloacinar

The submandibular gland (s45) is a gland that secretes saliva into the oral cavity. It is classified as a compound tubuloacinar gland. The acinar aspect of the gland results from the sac-like appearance of the terminal secretory portions of the gland (L. acinus, berry, grape). The acini also have a grape-cluster-like arrangement. View the slide at low power and you will observe lobules of parenchyma. Examine the lobules at higher power and you will observe the simple cuboidal cells of the ducts of the gland. You will observe groups of pale-staining cells and highly eosinophilic cells. The pale staining cells are typically arranged in tubes and are called mucous tubules. These contain mucous secreting cells and their nuclei are located at the base of the cells. The highly eosinophilic cells are arranged in serous acini and these contain serous or protein secreting cells. The eosinophilic material is in small granules (secretory granules or zymogen) within the cytoplasm. In some cases you will observe mixed secretory units (mixed tubulo-acini) consisting of a  mucous secreting tube capped by a group of serous cells. This cap is termed a serous demilune for obvious reasons.

III. Surface (apical/basal) specializations of epithelia

1) microvilli

We previously looked at the jejunum (s95) and gall bladder (S63) as examples of simple columnar epithelia. These cells are capped with actin-dense microvilli. Briefly review these slide to locate the striated border discussed earlier.


2) cilia

We observed cilia in the trachea (s48). Review this slide and note the fuzzy surface of the epithelia which represents the cilia which reside on the apex of the cells.

3) junctional complex

The junctional complex or terminal bar (zonlua occludens, zonula adherens and maybe macula adherins) is difficult to observe with the light microscope/H&E sections. With some knowledge of how to use a microscope and the three dimensional nature of the tissue, you may be able to find regions of the kidney (s90) in which a tube has been cut en-face very near the lumenal surface of the epithelial cells.  Terminal bars can occasionally be observed among the columnar epithelium of the jejunum (s95) . 

4) basement membrane

The basement membrane of the epithelia can be observed also on s48 (trachea) just beneath the epithelial cells. If you did not notice this previously, carefully examine the epithelia and note the appearance and location of the basement membrane.  Remember that all epithelia has a basement membrane; it is just particularly well developed and observable in the trachea.


Lab:  Epithelial Tissue