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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 septal cartilage
Arthur Reply
I don't understand please
Esther Reply
hey
Austine
good evening boss
Austine
Evening
Esther
good evening
Melissa
evening
Vitus
hi
Rodgers
good morning
Aurelia
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Helsa
you guys are from ?
Aurelia
Philippines
Helsa
im from india
Aurelia
Hello
Amos
why
Sami
wat
Rodgers
what's that
Gift
what's the medulla?
alhussiney Reply
The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum
Ludmila
define bone in simple terms
Mhoses Reply
bones are made up by collagen and calcium phosphate
Aurelia
yes
Confidence
Bones are the hard large calcareous connective tissue of which the adult skeleton of most vertebrates is chiefly composed
Olalekan
Bone is the substance that forms skeleton
Frances
good morning
Mrinal
Skelton muscles is any disorder of calcium
Mrinal
Bones protect the various organs of the body
Celina
Bone is a hard strong and durable type of connective tissue
Bkv
bone is a hard, calcareous connective tissue which gives structural support to the body and helpless in its locomotion!
Divya
It also is the main site for synthesis of the RBCs
Divya
thanks for your answers
Mhoses
A diagram of epithelial tissue
Emmanuel Reply
What does the urinary regulate blood pressure
Jennifer Reply
functions of melatonin
marion Reply
d. inversely proportional
John Reply
Airway resistance in the human lung is? a. Greatest in the generation of distal bronchioles in healthy people because these airways have the smallest radii b. Not increased during a forced expiration in a healthy subject c. Decreased when breathing through the mouth d. Inversely proportional
Maryam Reply
1. Chemical level 2. Cellular level 3.Tissue level 4. Organ level 5.Organ system level 5.Organismal level
Jamela Reply
d
HMD
High
HMD
why there is bleeding in menstruation?
KUHELI Reply
menopause
Vasu
no
KUHELI
no one ready to help me
Maryam
skin
The following hormones increase the blood glucose level except_____________? a. Parathormone b. Growth Hormone c. Epinephrine d. Thyroxine
Maryam
c.epinephrine
Divya
Which of the following is NOT a characteristic feature of alveoli in the lung? a. Poor blood supply b. A large surface area c. Thin walls d. Chemical layer called surfactant
Maryam
a.poor blood supply
Divya
poor blood supply
Tanvi
Excess of cortisol causes______________? a. Acromegaly b. Conn’s syndrome c. Diabetes insipidus d. Cushing syndrome
Maryam
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Maryam
cushing syndrome
Tanvi
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Maryam
Airway resistance in the human lung is? a. Greatest in the generation of distal bronchioles in healthy people because these airways have the smallest radii b. Not increased during a forced expiration in a healthy subject c. Decreased when breathing through the mouth d. Inversely proportional
Maryam
what happens to the brain when one cannot sleep
aghedo
it means melatonin is not released enough to make u fall asleep or there is a disturbance or destruction for melatonin to be released on time
Mhoses
name temporary endocrine gland in female.
KUHELI Reply
when does 1st meiotic division occur in male ?
KUHELI Reply
spermatogenesis
Vasu
) Which of the following best describes the human body's defense mechanism against environmental bacteria?
Homeostasis
Zeinab
What the answer plz
Zeinab
skin
KUHELI
Kuheli but I think skin is an organ not defence mechanism
Zeinab
Homeostasis is the system responsible for body response to external changes
Eliaz
skin
skin
Kharim
what is an inflammation
Lodrick Reply
it's a reaction from a body tissue
Perpenjeng
is the body own mechanism of fighting against disease especially infections or injuries.
Azapa
Inflammation is a tissue response to damage tissues
Zeinab
The following hormones increase the blood glucose level except_____________? a. Parathormone b. Growth Hormone c. Epinephrine d. Thyroxine
Maryam

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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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