Simple Squamous Epithelium 

Intro to simple epithelia:

Simple epithelia consist of a single layer of cells covering surfaces. They commonly function in absorption or secretion or transport, and, since they provide little protection, they are found in regions of very low stress. Simple epithelia range from low (squamous) to high (columnar) and often have surface specializations like cilia and microvilli.

Simple squamous epithelium consists of flattened, irregularly-shaped cells that form a continuous sheet on a surface. The cells generally are attached to a basement membrane. It is found in those places where rapid and unimpeded movement of gases and transported molecules is required, as in blood vessels, lung alveoli, and the linings of the pleural, pericardial, and peritoneal cavities.

Surface View of Stratified Squamous Epithelium

Amphibian Skin 

Amphibian skin, surface view. This is actually a  stratified epithelium. You can see the edges of the cells in the layer below the top layer. Epithelial cells are  tightly apposed to each other. The dark cell borders in many sections is due to staining of structures involved in adhesion.

Higher magnification of the surface layer of stratified squamous amphibian skin

Interdigitated cell borders. Can see outlines of underlying cells. Higher mag than previous stratified squamous epithelium.

2 examples of simple squamous epithelium: blood capillary and a mesothelium that covers a connective tissue

There will be a basal lamina attached at the basal surface of these epithelial cells. In the case of the capillary, the basal surface is the one that faces surrounding connective tissue. The surface facing blood is the apical surface.

Simple squamous epithelium lining the inner surface of a blood vessel

Simple squamous epithleium in X-section. The blood vessel's innermost cells (endothelium) are simple squamous epithelium. Endothelium lines the lumens of all blood vessels. Endothelial cells are very thin so that nuclei can be seen as a bulge. This vessel is an artery, as seen by the X-sectioned smooth muscle cells of the vessel wall (Tunica media).

Simple squamous epithelial cells of the kidney: Bowman’s capsule, low magnification

Bowman's capsule of a kidney corpuscle. The parietal layer (the outer edge of the capsule) is a simple squamous epithellium. The capillaries inside the capsule (the glomerulus) are also lined by a squamous epithelium, the endothelium. It's hard to see. The tubules outside the capsule are composed of simple cuboidal epithelium.

The cells of simple cuboidal epithelia appear square, or nearly so, when sectioned perpendicular to the basement membrane (BM). When viewed from the surface, or sectioned parallel to the BM, the cells are polygonal, mostly hexagonal. Simple cuboidal epithelia are commonly found lining various ducts and tubules as in the kidney, salivary glands, the pancreas, and may other places, where they function in secretion and absorption.

Not all cuboidal cells are actually cuboidal. There are ‘tall’cuboidal and ‘low’ cuboidal, too. As there are ‘high’ squamous and ‘low’ squamous, 'tall' columnar and 'low' columnar epithelia. The progression from flat squamous to tall columnar is nearly continuous with many different examples of each of the 3 standard classifications showing much variation.

A simple cuboidal epithelium. The outer surface is 'basal' and has an associated basal lamina  along the entire length of the tube.

Simple cuboidal cells of a kidney tubule

H&E, kidney tubule. The dark dots of stuff in the tubule lumen is protein precipitated from the urine during fixation.

You cannot actually see the basement membrane in most H&E stained tissues.

Kidney tubule simple cuboidal epithelium

Longitudinal section of a proximal kidney tubule. These epithelial cells present nice regular square profiles. The basement membrane is not visible.  Note other tubules (yellow arrows). Capillaries are present, too, but aren't very obvious. Look for potentially simple squamous tubes (LS) or circles (XS).

The arrows point to different sorts of cuboidal cells – at the top are 2 proximal tubules cut in XS. The cells have a brush border of microvilli. At the bottom are two collecting tubules. Note the lack of a brush border of MVs on the apical domains of collecting tubule cells.

Simple columnar cells with brush border microvilli

Nuclei of columnar cells tend to be towards the basal end of the cell, sometimes in the middle, rarely apical.

There is a continuous range of epithelial cell types from low squamous to high columnar. Overlap occurs: for instance, a “high cuboidal” could be the same as a “low columnar” epithelium. It’s a matter of judgement.

Simple columnar epithelium of large intestine

The large intestine mucosal epithelium is composed mostly of goblet cells. The main function of the large intestine is to extract water from the feces. As more water is extracted the feces get more solid. The proteoglycan secreted from goblet cells serves to lubricate the passage of feces so as to prevent injury to the epithelial cells.

Stratified Epithelium

Stratified squamous, non-keratinized

   Stratified epithelia, especially squamous, are found where some resistance to stress is required. Stress comes in the form of abrasion, as in skin, movement of material through a GI tube, hydrostatic pressure as in the bladder, and the kinds of movements of all soft tissues as the body changes shape through muscle activity. 

   The multiple cell layers of stratified epithelia make it much more difficult for molecules to diffuse across the epithelium. Thus epithelia that carry out transport are almost never stratified.

   Stratified epithelia are identified as being squamous, cuboidal, or columnar according to the shape of the top layer of cells.

Non-keratinized stratified squamous epithelium- Esophagus

Esophagus, section, H&E. Human esophagi are covered by a non-keratinized stratified squamous epithelium. The top layer cells are squamous while the basal cells are cuboidal. Squamous cells are abraded off as food passes through esophagus. Species that eat very rough food (eg, rodents and many herbivores) often have keratinized esophageal epithelium. Don't use lack of keratinization as an absolute diagnostic criterion for esophagus! See capillaries in the loose connective tissue under the epithelium.

Non-keratinized stratified squamous -Vagina

Vagina, section, H&E.  Basal layer cells tend to be more darkly stained and more basophilic than the upper layers because the basal cells are highly active in protein synthesis and thus have lots of ribosomes and mRNA (why are they making more protein than the upper-level cells?). The upper layer cells are unusually swollen and poorly stained. Not good prep technique. 

Keratinized stratified squamous epithelium ---Epidermis (thin skin), H&E --

Keratinization involves loss of cell water and replacement of a normal cytoskeleton with a very dense, cross-linked matrix of hydrophobic proteins like keratin. All cell organelles are destroyed by autophagy before dehydration and keratinization. The top layer of squamous cells are not tightly adherent and flake off due to abrasion.

Keratinized stratified epitheliumThick skin

Thick skin section, H&E stain. Much thicker keratinized layer than previous slide. The cells are more tightly adherent. The dark granular layer is the last layer to contain cellular organelles; all cells above the granular layer are keratinized.

Stratified cuboidal epithelium The TOP(apical) layer is cuboidal

The inner (apical) layer of cells are “low cuboidal”. Most stratified cuboidal or columnar epithelia have only 2-3 cell layers, unlike squamous which commonly has many.

To remember which layer is “top” or ”apical” remember that the basal layer is the one that attaches to the basal lamina and thus to connective tissue.

Stratified columnar epithelium

Male urethra

Diagram. Stratified columnar from male urethra. Cuboidal cells are seen at the basal layer and columnar bordering the lumen. 

Pseudostratified columnar epithelium, ciliated

In pseudo-stratified epithelia there is not a single layer of nuclei as in simple nor clearly multiple layers either, it is in between the two. Note that some cells, the columnar ones, touch both the basal lamina and the lumen of the trachea. But the cuboidal basal cells touch only the basal lamina

Trachea –the cell ‘layers’ are irregular

Trachea, sec, H&E. The layers if epithelial cell nuclei are irregular, not arrayed in clear layers. Note the relatively small, dense nuclei basally and the elongated, more pale nuclei centrally. Since there are no clear layers of nuclei it is not stratified. When trying to decide if an epithelium is simple, pseudostratified, or stratified, examine as much of the tissue as you can. That is, look along the entire length of it to try to find regions of a clean X-sec. Oblique sections tend to make all epithelia look stratified 

Trachea

Transitional epithelium is found only in the ureter and bladder of the urinary system

Found in the urinary tract, transitional epithelium undergoes changes in shape  when the bladder or ureters change from contracted to distended or vice versa. When the bladder is distended (full), the top layer is essentially squamous; when contracted (empty), the top layer cells are domed and extend out into the lumen. 

A contracted (empty) bladder transitional epithelium

Contracted bladder section. Domed apical cells, cuboidal basal cells and a clear basal layer of nuclei as in all stratified epithelia. Be sure to recognize the transition from epithelium to connective tissue. A well-organized row of dark nuclei represent the bottom of the epithelium.

Contracted bladder

Distended bladder looks almost like stratified squamous

Specializations of epithelial cells Found only in columnar and cuboidal cells

Cilia are longer than microvilli and have a core of microtubules; MVs have a microfilament core.

The terminal bar is shaped like an O-ring. It extends entirely around the apical end of the cell and is made of 3 components – the Zonula Occludens, Zonula Adherens, and Macula Adherens (desmosome). The desmosome is not really part of the terminal bar but most include it when describing the bar. In light microscopy the part that stains (poorly) is the Zonula Adherens, a complex of microfilaments and X-linker proteins.

The terminal bar of some col/cub epithelial cells is the zonula adherens, a ring of cytoskeletal elements and X-linkers that stains with some dyes

The hexagonal shape of the cross-sectioned cells is a consequence of the cells attaching to each other by way of a continuous ring. The hexagonal shape is the best (most common anyway) one to allow dense cell packing and continuous cell-cell contact.

Sections at 900to the plane of the terminal bar show the bar as spots (when stained with, eg, iron-hematoxylin)

The dark spots in cross-sections indicate the points at which adjacent cells are attached to each other at the terminal bar.

Simple cuboidal epithelial cells with brush border (kidney proximal tubule)

H&E stain of kidney. Note that some XS tubules have microvilli and some don't. the presence of MV suggest the cell is transporting molecules from one side of the simple epithelium to the other. Kidney proximal tubules transport valuable stuff from the pre-urine into neighboring blood capillaries. The tubule with no MVs is a collecting duct. By the time urine reaches the collecting duct all the transportable molecules have been removed thus there's no need for a huge apical PM domain surface area to put transporters in and thus no need for MVs.

Small intestine simple columnar cells with microvillar brush border, H&E

Small intestine, section. The brush border stands out well. Note that each cell extends to the BM and that there is a single row of nuclei. Therefore, it is a simple epithelium. The microvilli of the brush border serve to greatly increase the surface area of apical PM and to greatly increase the rate of absorption across the PM as a result. Absorptive cells tend to have brush borders, secretory cells tend not to have BBs. Note the dark nuclei of patrolling lymphocytes (probably T-cells)migrating into this epithelium.

Brush border, H&E

Simple columnar epithelial cells with stereocilia Stereocilia are not cilia, they are super-long microvilli

Epididymis of testis, section. Simple columnar epithelium with stereocilia. Sterocilia are very long microvilli, not cilia. They are often matted down on to the tops of the cells and are hard to see except when extending out from the cell.

Microvilli are shorter than cilia and cilia are shorter than stereo cilia.