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Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells.

The epithelial cell

Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.

Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach.

Cell to cell junctions

Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions ( [link] ).

Types of cell junctions

These three illustrations each show the edges of two vertical cell membranes. The cell membranes are viewed partially from the side so that the inside edge of the right cell membrane is visible. The upper left image shows a tight junction. The two cell membranes are bound by transmembrane protein strands. The proteins travel the inside edge of the right cell membrane and cross over to the left cell membrane, cinching the two membranes together. The cell membranes are still somewhat separated in between neighboring strands, creating intercellular spaces. The upper right diagram shows a gap junction. The gap junctions are composed of two interlocking connexins, which are round, hollow tubes that extend through the cell membranes. Two connexins, one from the left cell membrane and the other from the right cell membrane, meet between the two cells, forming a connexon. Even at the site of the connexon, there is a small gap between the cell membranes. On the inside edge of the right cell membrane, the gap junction appears as a depression. Three connexins are embedded into the membranes like buttons on a shirt. The bottom images show the three types of anchoring junctions. The left image shows a desmosome. Here, the inside edge of both the right and left cell membranes have brown, round plaques. Each plaque has tentacle-like intermediate filaments (keratin) that extend into each cell’s cytoplasm. The two plaques are connected across the intercellular space by several interlocking transmembrane glycoproteins (cadherin). The connected glycoproteins look similar to a zipped-up zipper between the right and left cell membranes. The right image shows an adheren. These are similar to desmosomes, with two plaques on the inside edge of each cell membrane connected across the intercellular space by glycoproteins. However, the plaques do not contain the tentacle-like intermediate filaments branching into the cytoplasm. Instead, the plaques are ribbed with green actin filaments. The filaments are neatly arranged in parallel, horizontal strands on the surface of the plaque facing the cytoplasm. The bottom image shows a hemidesmosome. Rather than located between two neighboring cells, the hemidesmosome is located between the bottom of a cell and the basement membrane. A hemidesmosome contains a single plaque on the inside edge of the cell membrane. Like the desmosome, intermediate filaments project from the plaque into the cytoplasm. The opposite side of the plaque has purple, knob-shaped integrins extending out to the basal lamina of the basement membrane.
The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.

At one end of the spectrum is the tight junction    , which separates the cells into apical and basal compartments. An anchoring junction    includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections. There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens. Desmosomes occur in patches on the membranes of cells. The patches are structural proteins on the inner surface of the cell’s membrane. The adhesion molecule, cadherin, is embedded in these patches and projects through the cell membrane to link with the cadherin molecules of adjacent cells. These connections are especially important in holding cells together. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix, for example, the basal lamina. While similar in appearance to desmosomes, they include the adhesion proteins called integrins rather than cadherins. Adherens junctions use either cadherins or integrins depending on whether they are linking to other cells or matrix. The junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane. The actin can connect isolated patches or form a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue.

Questions & Answers

what is a local potential
Nandi Reply
potential of neurons
vipin
response of neurons against sodium ion Chanel
vipin
The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.
Adnan
what is tissues
Addai Reply
what is meant by control center?
Freeman Reply
what is anatomy
Ajibola Reply
is the study of the structure of the body and their relationship to each refers to the shapes iyo sizes
Khadar
If water touches hydrophobic tail, what will happen as negative effect?
Sayo
is the study of structural of the human body and their function
Masiame
What is the function of the Trachea
Samuel
what is a blastomere?
Toluba Reply
In biology, a blastomere is a type of cell produced by cleavage (cell division) of the zygote after fertilization and is an essential part of blastula formation.
Adnan
what is cell
Shivaani
Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. ... Cells have many parts, each with a different
Adnan
function
Adnan
cell is a membrane bound unit that contains the fundamentals molecules of life
Sushma
A1
Adnan
excellent
Adnan
movement,reproduction, excretion, respiration ,growth ,nutrition, response to external stimuli.
Sushma
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Adnan
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Adnan
questions pushy ja sukty hy kya
Maryam
jwab forun milta hy ya ak do din k bad
Maryam
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Maryam
inshallah
Adnan
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Adnan
question kari
Adnan
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Ziya
Which muscle of the gluteal region originates from lumber spine? a. Gluteus medius b. Psoas major c. Iliacus d. Gluteus maximus
Maryam
sweet sister's
Adnan
question tu kari Inshallah Allah behtar kariga
Adnan
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Maryam
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Maryam
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Adnan
what is the functional unit of kidneys
Khadar
nephron is the functional unit of kidney
Maryam
Maryam Riaz sister option. d. gluteus maximus
Adnan
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Maryam
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Adnan
option b. psoas major
Adnan
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Adnan
nephron is the functional unit of kidney
Ihsan
I discussed with to my teachers
Adnan
d
Ihsan
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Adnan
yes
Ihsan
what is cell
Shilpa
cell is the smallest unit of life
Maryam
ok
Shilpa
Cells are the basic building blocks of all living things. The human body is composed of trillions of cells. They provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialized functions. ... Cells have many parts, each with a different
Adnan
function
Adnan
Maryam Riaz sister I m clear option d. also right thank you ihan bro
Adnan
type structure function origin insertion action of muscles
DAMINI Reply
oxygn amount in our body
Tg Reply
A normal level of oxygen is usually 95% or higher. Some people with chronic lung disease or sleep apnea can have normal levels around 90%. The “SpO2” reading on a pulse oximeter shows the percentage of oxygen in someone's blood
Adnan
What is the difference between cytoplasmic and endoplasmic
Ajibola Reply
The cytoplasm is 90℅ water and 10℅ of organic and inorganic compounds. The cytoplasm contains mitochondria. ... Lysosomes of Cytoplasm contains 50 different enzymes that digest proteins, lipids, and nucleic acids. The endoplasmic reticulum produces process and transport proteins, lipids for all the
Adnan
tarsal are cube shaped broad and long whiles metatarsal they are longer than they are wide
Priscilla Reply
articular cartilage serve as shock absorber and also reduces friction
Priscilla
articular cartilage serve as shock absorber and also reduces friction
Priscilla
when a bone joint with another bone
Priscilla
to know how you body function to know the relationship between the body parts to know specific functions of a certain body parts
romeo Reply
To know our body function we shd know chemical secretion of our glands, we can map all the function of our body and also relation ship between the body parts.
KUSUMA
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kanaku
can you get fungi infection by seating on a public toilet?
Calls Reply
explain how blood acts as a connective tissue in the body
Rymz Reply
how does blood act as a connective tissue in the body
Rymz
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Victoriah Reply
fat deficiency in the body
Agbor Reply
Explain how body exercise regulates blood pressure
dranimva Reply
thank you
Thelma

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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