Download CHAPTER 2: EPITHELIAL TISSUE TYPES

CHAPTER 2:
EPITHELIAL TISSUE
TYPES
Arrangements, Shapes, Special Features of Epithelial tissue and Observing
Epithelial Tissue .
“The deepest sin of the human mind is to believe things without evidence”._ Thomas Henry Huxley
(1825-1895).
CHAPTER 2: EPITHELIAL TISSUE TYPES
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Epithelium is membranous tissue covering the surfaces of the body like
external surfaces, internal organs and other internal surfaces, or lines a cavity or
the like, and that, in addition, performs any of various secretory, transporting, or
regulatory functions. As such, it serves many purposes, including diffusion,
protection, adsorption, excretion, secretion, filtration, cleaning, and Sensation.
Epithelium is one of the four basic types of animal tissue, along with connective
tissue, muscle tissue and nervous tissue. When considering the characteristics that
make a tissue epithelium, it is important to think about the following:
 Polarity: Epithelium is arranged so there is one free surface (apical surface)
and one attached surface (basal surface).
 Cellular nature: Cells in epithelium fit closely together side by side and
sometimes atop each other to form sheets of cells. These sheets are held
together by specialized junctions.
 Supported by connective tissue: Attachment to a layer of connective tissue
at the basal surface forms a layer called the basement membrane, an
adhesive layer formed by secretions from the epithelial cells and the
connective tissue cells.
 Avascular: Epithelium typically lacks its own blood supply.
 Regeneration: Epithelium cells can regenerate if proper nourished.
Classification of epithelium is based on the shape of the cells and the
arrangement of the cells within the tissue. Typically, the arrangement of the cells is
stated first, then the shape, and is followed by “epithelium” to complete the naming
(Ex. Simple Squamous Epithelium).
Arrangements:
1. Simple: Cells are found in a single layer attached to the basement
membrane.
2. Stratified: Cells are found in 2 or more layers stacked atop each other.
3. Pseudostratified: a single layer of cells that appears to be multiple layers
due to variance in height and location of the nuclei in the cells.
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4. Transitional: cells are rounded and can slide across one another to allow
stretching.
Shapes:
1. Squamous: (Latin, squama- scale) - flat, thin, scale-like cells.
2. Cuboidal: cells that have a basic cube shape. Typically the cell's height and
width are about equal.
3. Columnar: tall, rectangular or column-shaped cells. Typically taller than
they are wide.
Figure 2.1 Types of Epithelia.
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CHAPTER 2: EPITHELIAL TISSUE TYPES
TABLE 5.1 Types of Epithelium
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Special Features of Epithelium:
a. Cilia: (singular= cilium, Latin= eyelash) are common surface modifications
present on nearly every cell in the body. Hair-like appendages attached to
the apical surface of cells that act as sensory structures or to produce
movement.
G
C
Figure 2.2 Ciliated epithelium. Photomicrograph of an H&E–stained specimen of tracheal
pseudostratified ciliated epithelium. The cilia (C) appear as hair-like processes extending from the apical
surface of the cells. The goblet cells (G) can be readily identified by the presence of the apical mucous cup
(750×).
b. Goblet cells: specialized cells that produce mucus to lubricate and protect
the surface of an organ.
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Figure 2.3 Goblet cells. Pseudostratified ciliated columnar epithelium lining the human trachea
(400×).
c. Villi: (singular = villus, Latin = shaggy hair) - finger-like projections that
arise from the epithelial layer in some organs. They help to increase surface
area allowing for faster and more efficient adsorption.
d. Microvilli: smaller projections that arise from the cell's surface that also
increase surface area. Due to the bushy appearance that they sometimes
produce, they are sometimes referred to as the brush border of an organ.
Figure 2.4 Villi and Microvilli. (a) Diagram of small intestine showing some of the densely packed
microvilli that make up the striated border villi. Each microvillus is approximately 1um long by 0.1um in diameter
and contains a core of actin microfilaments. (b) Transmission electron microscope (TEM) image of a thin section cut
through a human jejunum (segment of small intestine) epithelial cell.
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A simple squamous epithelium is a single layer of flat cells in contact with
the basal lamina (basement membrane) of the epithelium. This type of epithelium
is often permeable and occurs where small molecules pass quickly through
membranes via filtration or diffusion. Simple squamous epithelia are found
in capillaries, alveoli, glomeruli, and other tissues where rapid diffusion is
required.
Simple cuboidal epithelia are epithelial cells in a single layer of cube-like
cells with large, spherical central nuclei. Simple cuboidal epithelia are found on the
surface of ovaries, the lining of nephrons, the walls of the renal tubules, and parts
of the eye and thyroid.
On these surfaces, the cells perform secretion and absorption.
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Function:






These cells provide protection and may be active (pumping material in or
out of the lumen) or passive, depending on the location and cellular
specialization.
They are also found in kidney tubules, glandular ducts, ovaries, and the
thyroid gland.
Simple cuboidal cells are found in single rows with their spherical nuclei in
the center of the cells and are directly attached to the basal surface.
Simple cuboidal epithelium commonly differentiates to form the secretory
and duct portions of glands.
They also constitute the germinal epithelium which covers the ovary (but
does not contribute to ovum production) and the internal walls of the
seminiferous tubules in the male testes.
These cells offer some protection and function in absorption and secretion.
A simple columnar epithelium is a columnar epithelium that is unilayered. In
humans, a simple columnar epithelium lines most organs of the digestive
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tract including the stomach, small intestine, and large intestine. Simple ciliated
columnar epithelia line the uterus. Simple columnar epithelia are further divided
into two categories: non-ciliated and ciliated.
Ciliated columnar epithelia move mucus and other substances via cilia, and
are found in the upper respiratory tract, the Fallopian tubes, the uterus, and the
central part of the spinal cord.
A ciliated columnar epithelium lines the lumen of the uterine tube, where
currents generated by the cilia propel the egg cell toward the uterus.
A pseudostratified epithelium is a type of epithelium that, though
comprising only a single layer of cells, has its cell nuclei positioned in a manner
suggestive of stratified epithelia. As it rarely occurs as squamous or cuboidal
epithelia, it is usually considered synonymous with the term pseudostratified
columnar epithelium.
The term pseudostratified is derived from the appearance of this epithelium
in section which conveys the erroneous (pseudo means false) impression that there
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is more than one layer of cells, when in fact this is a true simple epithelium since
all the cells rest on the basal lamina. The nuclei of these cells, however, are
disposed at different levels, thus creating the illusion of cellular stratification. Not
all ciliated cells extend to the luminal surface; such cells are capable of cell
division providing replacements for cells lost or damaged.
Pseudostratified epithelia function in secretion or absorption. If a specimen
looks stratified but has cilia, then it is a pseudostratified ciliated epithelium, since
stratified epithelia do not have cilia.
Examples:

Ciliated pseudostratified columnar epithelia are found in the lines of
the trachea as well as the upper respiratory tract.

Non-ciliated pseudostratified columnar epithelia are located in the
membranous part of male vas deferens.

Pseudostratified columnar epithelia with stereocilia are located in
the epididymis. Stereocilia of the epididymis are not cilia because their
cytoskeleton is composed of actin filaments, not microtubules. They are
structurally and molecularly more similar to microvilli than to true cilia.
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A stratified squamous epithelium consists of squamous (flattened)
epithelial cells arranged in layers upon a basal membrane. Only one layer is in
contact with the basement membrane; the other layers adhere to one another to
maintain structural integrity. Although this epithelium is referred to as squamous,
many cells within the layers may not be flattened; this is due to the convention of
naming epithelia according to the cell type at the surface. In the deeper layers, the
cells may be columnar or cuboidal. This type of epithelium is well suited to areas
in the body subject to constant abrasion, as it is the thickest and layers can be
sequentially sloughed off and replaced before the basement membrane is exposed.
It forms the outermost layer of the skin and the inner lining of the mouth,
esophagus, and vagina.
Non-keratinized: Non-keratinized surfaces must be kept moist by bodily secretions
to prevent them from drying out. Types of non-keratinized stratified squamous
epithelium include cornea, oral cavity, esophagus, anal canal, vagina, and the
internal portion of the lips.
Keratinized: Keratinized surfaces are protected from abrasion by keratin and kept
hydrated and protected from dehydration by glycolipids produced in the stratum
granulosum. Examples of keratinized stratified squamous epithelium include
epidermis of the palm of the hand and sole of the foot, and the masticatory mucosa.
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It is a rare type of epithelial tissue composed of cuboidal shaped cells
arranged in multiple layers. Only the most superficial layer is made up of cuboidal
cells, and the other layers can be cells of other types. This is because,
conventionally, naming of stratified epithelium is based on the type of cell in the
most superficial layer.
They protect areas such as the ducts of sweat glands, mammary glands,
and salivary glands.
Transitional epithelium (also known as urothelium) is a type of tissue
consisting of multiple layers of epithelial cells which can contract and expand.
These cells, part of the epithelium, are found in the urinary bladder, in the ureters,
and in the superior urethra and gland ducts of the prostate.
Appearance:
These cells appear to be cuboidal with a domed apex when the organ or the
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tube in which they reside is not stretched. When the bladder fills, the tissue
actually compresses and the cells become stretched, and they appear to be flat,
irregular, and squamous.
Functions:
The transitional epithelium cells accommodate fluctuation of volume of the
liquid in an organ or tube, such as the urinary bladder, and protect against the
caustic effects of urine. Location: urinary bladder, kidneys, ureters, and urethra.
Also it provides a large amount of sperm.
Surfaces:
The free surface of the tissue (the part that faces outward into the organ or
tube) is called the apical surface or free surface. The part that lines the bottom of
the tissue is called the basement membrane.
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1. Epithelial tissue covers the whole surface of the body. It is made up of cells closely
packed and ranged in one or more layers. This tissue is specialized to form the covering
or lining of all internal and external body surfaces.
2. Epithelial tissue can be divided into two groups depending on the number of layers of
which it is composes. Epithelial tissue which is only one cell thick is known as simple
epithelium. If it is two or more cells thick such as the skin, it is known as stratified
epithelium.
3. Both simple and stratified epithelium can be subdivided according to the shape and
function of its cells.
4. Squamous cells have the appearance of thin, flat plates. The shape of the nucleus usually
corresponds to the cell form and help to identify the type of epithelium.
5. Cuboidal cells are roughly square or cuboidal in shape. Each cell has a spherical nucleus
in the center.
6. Columnar epithelial cells occur in one or more layers. The cells are elongated and
column-shaped. The nuclei are elongated and are usually located near the base of the
cells.
7. When the epithelia are composed of several layers of cells and are then called compound
or stratified epithelium. The top cells are flat and scaly and it may or may not be
keratinized.
1)
2)
3)
4)
What is the epithelial tissue? And what are the purposes of it?
What are the basic characteristics of epithelial tissue?
What are the differences between the cilia and the microvilli?
Classify the Epithelial tissues based on the both the shape and the arrangement of the
cells and write suitable location for each one.
5) Classification of epithelium is based on the shape of the cells and the arrangement of the
cells within the tissue. Describe both in brief.
6) What are the Special structural Features that present within Epithelium? Describe with
mention of the functions for each one.
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