Lecture #4

Lecture #4

In the name of Allah the most gracious the most merciful

The Epithelium

((I’m sorry for the inconvenience but the doctor didn’t give us the slides. I couldn’t find the exact images in the atlas nor the text book’s CD so use the atlas for now which is useful until we get the slides from the doctor))

The most important thing is to understand things!! (meaning understand the figures so you can notice the types of the epithelium structure when seen for the first time).

There are four basic types of tissues:

1. Epithelial2. Connective3. Muscular4. Nervous

You will learn about each of these in different lectures

Epithelial tissue is composed of aggregated polyhedral cells with small amount of extra cellular matrix, lining the body cavities and involved in glandular secretion.

You know that tissues form organs!

Organs can be divided into two components:

1. Parenchyma2. Stroma

Parenchyma is composed of cells that are responsible for the main function of the organ. Whereas stroma is the supporting tissue , usually the connective tissue.

The epithelium is the tissues composed of tightly adherent polyhedral cells with little inter cellular space that covers surfaces and line cavities of the body. As said previously it is involved in glandular secretion. Now an exception to this definition

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regarding to covering (lining) surfaces of body cavities is the epithelium forming endocrine glands where the endocrine glands (cluster of cells) having no reference to the surface. You will learn more about that in the endocrine system.

As we said epithelium is composed of cells and as we all know cells have nuclei. The nuclei of the epithelium have distinctive shape which means different types of epithelium=different shapes of cells =different shape of nuclei. so remember the nucleus refer to the cell’s shape; if you say the cell is flattened therefore the nucleus is flattened and so on.

The long axis of the nucleus is parallel to the main axis of the cell. The nuclei can serve as a clue to the no. and shape of the cells. We do that when the cell membranes can’t be distinguished with a light microscope which is in most cases. The nuclei can be useful for cell classification when you can’t distinguish or see clearly the plasma membrane of the cells.

Epithelium divided into to two main groups based on its structure and function:

1. Covering/lining epithelium2. Glandular epithelium

Functions of epithelium:

Covering/lining of epithelium we can expect that it functions to protect the underlying structure and tissues. [Epithelia (plural) epithelium (singular)]. This group of epithelium can be classified according to cell layers and the shape of the cells in the outer most layer. Remember the epithelium may seem sometime composed of many layers. Whenever you want to classify the epithelium ignore all the layers and look at the outer most surface/superficial layer.

Simple epithelium has one layer only.

Stratified epithelium has more than one layer.

Cells of epithelium if single can be flattened, cuboid or tall to form squamous, cuboidal or columnar respectively.

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Two specialized types of epithelia:

1. Pseudo stratified epithelium; (pseudo means false, stratified means many layers) so it look it has many layers but it does not show the typical criteria for many layers. Which means, of course the epithelium rests on basement membrane, which appears as a line in the light microscope occasionally (may not always appear). All the cells rest at the basement membrane. However, not all of them reach the surface .Also you can see different levels of nuclei.

2. Transition epithelium; present in the urinary system ONLY. That’s why it is sometimes called urothelium.

Glandular epithelium (meaning presence of secretion) also as other functions such as contractility and a fourth one (the professor couldn’t remember).

Description of each type of epithelium (imp for lab!)

[Image A] all are for simple squamous epithelium. We have a cavity (lymphatic channel) called lumen. The layer lining the lumen is made up of simple squamous epithelium, a single sheet of flattened cells which you can see below. Do you see them cubed? Tall? No then must be squamous.

[Image B] in the kidney. Cells are flattened so also simple squamous

[Image C] all flattened cells forming one sheet, simple squamous. Can see nucleus and cytoplasm flattened.

The point is not to memorize the images but know the basics. So you will be able to recognize it regarding from where the image is extracted for the first time

Images of simple cuboidal cells are tall and wide.

[Image A] can see one layer and cells are cubed, simple cuboidal.

For simple columnar epithelium the cells are taller than they are wide. One layer.

A student asked what is the difference between the shape of nucleus between cuboidal and columnar?

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Cuboidal may be more spherical, wider than but not as elongated as the squamous. But for columnar is somewhat longer why? Because the long axis of the nucleus is parallel to the long axis of the cell so expect to be somewhat elongated (oval shape).

For pseudo stratified columnar epithelium image have many nuclei present at different levels. You can see a layer of the basal nuclei, a layer of nuclei in the intermediate layer then a layer of cytoplasm close to the surface. Not all the cells reach the surface! There are specific cells as you can see they don’t reach the surface and there is no cell overlying it. You can’t say there is a layer over it.

[Image] For stratified squamous epithelium, there is more than one layer. How to decide if the stratified layer is squamous cuboidal or columnar? You go to the surface layer.

For stratified cuboidal epithelium, you can see the tubules and the surface layer is toward the lumen. When you look at these tubules you can see two layers at the base of tubule and at surface so must look at the surface layer and the type of epithelium is stratified cuboidal epithelium.( You may think these are not cuboidal but ask your self do they seem flattened or tall? No then definitely cuboidal.

Stratified cuboidal/columnar epitheliums are rarely found in the body!!

For stratified columnar epithelium, which is lined in the tubule, you can see two layers, ignoring the connective tissue. You have the basal layer, seeing the nuclei, and other layer which is columnar. So It can be found in certain tubules. You can see all the cells reach the surface so its definitely not pseudo stratifies. Only two layers of nuclei; the basal one and the other one. Unlike the pseudo stratified can see more than two.

[Image] Back to transitional epithelium we said that it’s a specialized epithelium in the urinary system only. More than one layer. If you look at the epithelium the surface layer has cells that change in shape according to the status of the organ whether it’s stretched or not. If stretched they would be more flattened. Otherwise, more doomed shape going into the lumen. Occasionally cells can be binucleated; same cells at higher magnification having two nuclei.

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Glandular epithelium is the one that has specialized cells for secretion and can be classified into two groups:

1. Unicellular glands; composed of one cell, they are large, isolated cells responsible for secretion. Ex: goblet cells

2. Multicellular glands; composed of clusters of cells

[Image] the cells that are not stained are goblet cells. Remember they can only be stained by PAS (periodic acidic Schiff) due to composition of glycoprotein which will look like the one on the right.

[Image] During prepare development, the cells of the covering epithelium proliferate, invading the adjacent connective tissue forming a cluster. This cluster either maintains its connection with the surface of epithelium forming exocrine glands or not having a connection forming endocrine gland. This connection in the exocrine glands is used as a duct for secretion. However the secretion for the endocrine gland is picked up by the blood vessels and carried into the circulatory system. Endocrine glands are ductless glands.

The secreting clusters are either

1. Course2. Follicles3. acinine

For exocrine glands consists of two portions

1. Secratory; consists of cells that produce the secretion can be round called acinar (alveolar) or tubular glands

2. Duct; can be branched or unbranched

Exocrine glands can be classified functionally into three types

1. Merocrine glands

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2. Holocrine glands3. Apocrine

Merocrine can be further subdivided into two groups based on the nature of their secretions:

1. If proteins to be secreted they are called serous.2. If glycoproteins to be secreted they are called mucous.

[Image] merocrine gland secretion occurs by exocytosis of proteins or glycoproteins. There are no physical changes of the cells after the elaboration of secretion with the light microscope

[Image] holocrine glands, you can see that the secretary cell is filled with secretary product and afterwards the whole cell is disrupted and shed. Therefore this process takes a constant cellular mitosis for proliferation to replace the shredded cells, a result of being secreted. Ex: sebaceous glands

[Image] apocrine glands as we said only the secretary product is released. It is an intermediate between the merocrine and the holocrine so the secretary product and the apical part of the cytoplasm and plasma are librated in secretion. However, in holocrine gland the whole cell is released.

Good luck!!

Amineh Al-Farraj