Epithelium Handout Flashcards
1
Q
Basic Tissue Types
A
All of the organs of the body are composed of organized aggregations of cells that have the same structure, same origin, and perform the same function. They are called tissues. Generally histologists recognize 4 basic types of tissues: epithelial, connective, muscle, and nervous
2
Q
Location of epithelial tissue
A
they are limited in their distribution in the body. There are 3 main locations of epithelial tissues:
- cover exterior surfaces (barrier epithelium)
- Line internal cavities (barrier epithelium)
- Form secretory portion of glands and their ducts (glandular epithelium)
3
Q
Characteristics of Epithelial tissue
A
- The main structural difference of the epithelial tissue from other types is that it has an exposed free surface (external body surface like skin and the luminal surface like lining of the intestine).
- Epithelial tissue is a different in cellular composition from the other types. The cells are closely aggregated together and there is very little or no extracellular matrix. The adhesion between cells is very strong
- Epithelial tissue is avascular, there is no blood supply to epithelial cells, but epi cells themselves are found lining the walls of bv
4
Q
Morphologic polarity of epi tissue
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3 distinctive orientation in the organs. Thus, it is usually possible to identify the 3 main surfaces present in the cell: Apical, lateral, and basal.
5
Q
Apical domain
A
forms the external or luminal surface of the cell.
6
Q
The apical surface often exhibits what
A
various modifications, such as microvilli, stereocilia, or cilia
7
Q
the function of microvilli
A
to increase surface area, so they are found in the epithelia where absorption is important (small intestine or kidney).
8
Q
How are microvilli characterized
A
- Size: 1 x 0.01 um.
- The core of a microvillus is formed by actin filaments that are bound together by actin-binding proteins, such as fimbrin and fascin, and are anchored into the membrane by the lateral anchoring proteins, such as myosin I
- The amorphous apex of the microvillus is formed by villin into which the actin filaments are anchored.
9
Q
What are stereocilia
A
modified microvilli and are not related to cilia. These are long, sometimes branching projections with the core formed by actin filaments, similar to microvilli.. Stereocilia in the epididymis play an important role in the absorption of fluid. Stereocilia in the inner ear cochlea are sensory receptors.
10
Q
Why are cilia important
A
important to move substances (mucus) along the surface of the epithelium and are found in the areas where this transport is necessary like in the respiratory tract or the oviduct.
11
Q
Structure of the cilia
A
The core of the cilium is formed by 10 pairs of microtubules that form the axoneme. Cilia are thicker than microvilli and measure 2-10 x 0.25um.
- At its base, each cilium is attached to the centriole, known as a basal body. The skeleton of the basal body is formed by 9 triplets of microtubules without a central pair
12
Q
Genetic defects of ciliary proteins result in
A
the malformation of the skeleton of cilia. This causes uncoordinated or absent ciliary beating and results in the primary ciliary dyskinesia (immotile ciliary syndrome). This abnormality may cause several consequences like
a. Embryological pathologies, such as dextrocardia
b. Impaired development of skull air sinuses
c. No mucus removal from lungs causes recurrent and severe chest infections
d. Infertility is common and is due to the inability of the flagella of spermatozoa to beat in males and failure of the cilia to move the ovum in the oviduct in females
13
Q
where are secretory vesicles present
A
in the epithelial cells involved in the production and secretion of macromolecules, such as enzymes and mucins.
14
Q
Lateral domain is characterized by
A
the presence of cell junctions that allow the tissue to function as a whole. 3 major classes of cellular junctions are present in the epithelium.
15
Q
3 major classes of cellular junctions present in the epithelium
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- occluding junctions
- anchoring junctions
- communicating junctions
16
Q
how do junctions occur
A
together and form junctional complexes. in the light microscope these junctional complexes can be visualized as terminal bars.
17
Q
Tight or occluding junctions
A
they are mostly present towards the apical portion of the cell and are represented by a single type, zonula occludens. Occluding junctions are belt like junctions formed by sealing strands of transmembrane proteins that bind membranes of 2 adjacent cells
18
Q
where are occluding junctions found
A
in cells with secretory role (like stomach epithelia) or in cells with absorptive role (kidney).
19
Q
Zonula occludens is
A
virtually impermeable and its main function is to prevent diffusion by blocking paracellular pathways
20
Q
purpose of anchoring junctions
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to provide stability and mechanical strength to the tissue and allow it to function as one unit. Anchoring junctions connect cytoskeleton of 2 cell to cytoskeleton of an adjacent cell or to the extracellular matrix.
21
Q
2 main types of anchoring junctions are found where
A
along the lateral surface of the epithelial cells: zonular adherens and macula adherens (= desmosome)
22
Q
Zonula adherens is what
A
a belt like junction that connects actin filaments of 1 cell to actin filaments of another cell. It is not as tight as the occluding junction and the cleft between 2 cells is usually about 20 nm
23
Q
Actin binding proteins (zonula adherens)
A
vinculin, alpha actinin
24
Q
Peripheral protein (zonula adherens)
A
Catenin
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Transmembrane link protein (zonula adherens)
cadherin family (Calcium dependent)
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Cadherin clinically
plays an important role in the control of cell behavior and its loss is often associated with an acquisition of invasive behavior by tumor cells (metastasis) for example, gastric cancer
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Adherens junction is composed of
1. Actin binding proteins
2. Peripheral protein
3. Transmembrane link protein
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Macula adherens
or a desmosome is a spot like junction that connects intermediate filaments of 2 cells.
29
where are desmosomes common
in the epidermis of the skin and provide mechanical strength for that tissue, but they also occur in other types of tissue, such as muscle.
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What are desmosomes composed of
1. Intracellular plaque
| 2. transmembrane proteins
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Intracellular plaque is made of what
desmoplakin. Intermediate filaments go through the plaque
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Transmembrane proteins belong to what family
the cadherin family
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Pemphigus
disease of cell junctions - the body produced abnormal antibodies to the proteins forming desmosome junctions in the skin. This prevents normal adhesion between cells and causes widespread skin and mucous membrane blistering as the intraepidermal desmosomes fall apart
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What does communicating or gap junctions allow for
diffusion of small molecules and ions between the cytoplasm of adjacent cells.
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how does diffusion occur through gap junctions
through narrow intercellular channels that are 2.8nm in diameter
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Formation of gap junction channels
each channel is formed by a pair of connexons aligned with each other
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Structure of connexon
consists of 6 protein subunits, connexins
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How do gap junctions usually occur
in patches.
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where are gap junctions most common
in such tissues as epithelia, cardiac, and smooth muscle, and embryonic tissues and allow communication between cells and have a role in their control of cell proliferation and differentiation
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Basal domain
faces the basement membrane and is important for its interactions with the underlying tissue (usually connective tissues).
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Several important components of the basal domain
1. Basement membrane
| 2. Anchoring junctions
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what does the basement membrane do
separates epithelium from the underlying connective tissue and is formed by 2 layers: basal lamina and reticular lamina
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basal lamina
the most external layer of the basement membrane and belongs to the epithelium. it is composed of sheets of collagen type IV and laminin
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Reticular lamina
the deeper layer of the basement membrane and belongs to the connective tissue. it is composed mainly of thin collagen fibers.
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how are epithelial cells attached to the basement membrane
2 types of anchoring junctions: focal adhesions and hemidesmosomes
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Focal adhesions
attach actin cytoskeleton of a cell to the extracellular matrix. These junctions are similar to the adherens junctions, but instead of attaching 2 cells to each other, they attach cells to extracellular matrix.
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proteins in the focal adhesion junction
they are different than the proteins in the adherens junction.
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Main components of the focal adhesion
1. Actin binding proteins
2. Peripheral proteins
3. Transmembrane protein
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Actin binding proteins (focal adhesions)
(vinculin, alpha actinin) attach to microfilaments
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Peripheral protein (focal adhesions)
talin, connects actin binding protein to the transmembrane protein
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Transmembrane protein (focal adhesions)
integrin belongs to the calcium independent integrin family, instead of cadherins found in the zonula adherens.
52
Hemidesmosome
resembles the desmosome as it connects the intermediate filaments, but these junctions attach cell cytoskeleton to the extracellular matrix.
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Main components of the hemidesmosom
Intracellular plaque and transmembrane protein
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Intracellular plaque of hemidesmosome
made of desmoplakin. Notice that the intermediate filaments end on the plaque
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Transmembrane proteins of the hemidesmosome
also integrin family - attach to the extracellular proteins
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How can epithelial tissues be classified
1. By number of layers
2. By shape of cells
3. By apical cell modifications
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Simple cell
single layer in the epithelium
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pseudostratified
if all cells of the tissue rest on the basement membrane, but not all of them reach the surface
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squamous
cells are flat
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cuboidal
if the transverse diameter of the cell is roughly equal to the cell height
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columnar
if the cell height is considerable greater than the transverse diameter
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ciliated
if cilia are present
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non ciliated
cilia are not present
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Simple squamous epithelium
formed by a single layer of squamous cells with flattened nuclei.
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Simple squamous epithelium can be found where
in the lining of the vascular system, body cavities, parietal layer of the Bowmans capsule of the kidney, and in the alveoli of the lung
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Major function of the simple squamous epithelium
barrier and diffusion of oxygen and carbon dioxide
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endothelium
simple squamous epithelium that lines the inner walls of blood vessels and heart
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mesothelium
the simple squamous epithelium that covers internal organs within the pleural, pericardial, and peritoneal cavities. The cancer derived from this type of epithelium is called mesothelium. Plural mesothelioma is an aggressive malignant tumor often caused by an exposure to asbestos
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Simple cuboidal epithelium
is formed by a single layer of cuboidal cells, which have a width that is roughly equal to cell height. Cells of this kind have nearly spherical nuclei.
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where is simple cuboidal epithelium most commonly found
in the walls of ducts of various glands, in the kidney tubules, in the germinal epithelium of the ovary. and the thyroid follicles.
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Main functions of simple cuboidal epithelium
Barriers (ducts), Secretion (thyroid), and Absorption (kidney)
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Simple columnar epithelium
consists of a single layer of tall columnar cells with oval shaped nuclei.
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2 types of surface modifications associated with simple columnar epithelium
microvilli (found in the nonciliated simple columnar epithelium) and cilia (found in the ciliated simple columnar epithelium
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Can individual microvilli be seen on a microscope
no, but densely packed microvilli form a thin layer called the striated, or microvillar border
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Nonciliated simple columnar epithelium is found
in the lining of the GI tract and in the walls of the gallbladder
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Ciliated simple columnar epithelium is found
in the lining of the oviduct, has well developed cilia, which are used for the transport of the ova.
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Main functions of the simple columnar epithelium
absorption, secretion, lubrication, and transport
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Pseudostratified columnar epithelium
similar to simple columnar epithelium in the columnar shape of most cells, but there appears to be an additional later of cells of more rounded shape, that do not reach the surface. These are called basal cells. All the cells in this tissue rest on the basement membrane, and because of that the epithelium is called pseudostratified. The basal cells are the replacement, or progenitor, cells in this epithelium.
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subdivisions of Pseudostratified columnar epithelium
Ciliated and non ciliated
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where is Pseudostratified columnar epithelium most commonly found
in the respiratory tract.
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What do epithelial cells in the respiratory tract have
numerous cilia, and the epithelium is referred to as ciliated pseudostratified columnar, often simply called respiratory epithelium
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Function of respiratory epithelium
move the mucus that collects dust and other particles up the respiratory tract.
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light microscope and Pseudostratified columnar epithelium
cilia make an irregular brushy surface
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what happens to Pseudostratified columnar epithelium under the influence of certain factors
the respiratory epithelium can change into another type of epithelium, for example unto stratified squamous. If the factors are removed, the epithelium can change back. This is called metaplasia and is common in heavy smokers. The loss of cilia results in the accumulation of mucus in lungs and inflammation.
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where does metaplasia occur
mainly in respiratory epithelium, but can occur in other types of epithelia as well, in the simple columnar epithelium
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cilia in the Pseudostratified columnar epithelium in the epididymis
has immotile stereocilia instead of cilia. This is the nonciliated pseudostratigied columnar epithelium.
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what are stereocilia in the epididymis used for
absorption.
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inidividual stereocilia in Pseudostratified columnar epithelium on a light microscope
they are beyond the resolution of the light microscope, so both on the picture and in the lab we will be looking at clumps of stereocilia.
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Stratified squamous epithelium consists of
several layers of cells of various shapes. The cells of the most superficial layer are flat and for that reason the epithelium is called squamous.
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what does the stratified squamous epithelium do
form a barrier between the environment and the underlying tissue and protects it
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2 principle types of stratified squamous epithelia
1. keratinized
| 2. Non keratinized
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keratinized type is characterized by
the presence of the dead cornified layer on the surface. This type is found in the epidermis of the skin.
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what is keratin
an intermediate filament, principally found in epithelial cells.
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what is presence of keratin in a malignant tumor indicative of
its epithelial origin
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Non keratinized type is what
similar to the previous type, but does not have the dead layer of keratin on the surface.
96
where is the non keratinized type found
at the beginning of the GI tract (oral cavity, esophagus) and in the female reproductive system (vagina)
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Stratified cuboidal epithelium
rather rare type that is found in the ducts of some glands.
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where is the stratified cuboidal epithelium most common in
the ducts of the sweat glands of the skin
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stratified cuboidal epi consists of
2 layers of cells that are cuboidal in shape.
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the major functions of the stratified cuboidal epi
barrier and conduit
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stratified columnar epithelium can be found
in the larger ducts of several glands, including the parotid and submandibular salivary glands. (this is the more rare type)
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basal layer of stratified columnar epithelium is composed of
cuboidal cells, while the surface cells are columnar.
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function of the stratified columnar epi
barrier and conduit
104
Transitional epithelium is sometimes called
urothelium, because it is restricted to the urinary system
105
Transitional epithelium classification
can not be classified as squamous, cuboidal, or columnar. It is composed of multiple layers of cells of different shape. The surface layer often contains large rounded dome cells that are often binucleated.
106
the most important feature of transitional epi
the ability to stretch enormously, when the organ becomes dilated - when this happens, the shape of the epi cells changes dramatically. From rounded or cuboidal shape the cells change into flat, squamous cells and back, when the organ contracts again.
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where is the transitional epi found
in the urinary bladder, ureter, and in the pelvis and calyces of the kidney
108
Glandular epi is formed by
the result of invagination of the epi tissues. Epi cells then start secreting various materials, such as mucins, steroids, etc
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How can glands be classified
based on the distribution of products.
Endocrine and Exocrine
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Endocrine glands
If the glands secrete their product into the intercellular space or underlying connective tissue and the product diffuses into blood. They are called endocrine glands. Cells within endocrine glands are usually arranged in cords that follow the capillaries that supply the gland. Endocrine glands do not have ducts.
111
Exocrine Glands
Exocrine glands maintain their connection to the originating epi and secrete into ducts or spaces lined within the originating epi. Cells within the exocrine glands are usually arranged into acini or tubules.
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how can exocrine glands be classified
based on the mechanisms of secretion
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3 major mechanisms of secretion
merocrine, apocrine, and holocrine
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Merocine glands
the secretory product is delivered in membrane bound vesicles to the apical surface, where the vesicles fuse with the PM to release their content via exocytosis. Based on the type of product that is produced merocrine glands can be further subdivided into serous, mucous, and mixed
115
serous glands
(merocrine gland) secrete a thin watery, protein rich secretion. The secretory vesicles are small and densely staining. These cells have better developed rough ER than mucous cells, and consequently their cytoplasm stains more basophilic. The rough ER occupies the basal part of the cell, and as a result the nuclei are shifted towards the middle of the cell.
116
usual shape of serous glands
usually pyramidal in shape and as a result serous acini are rounded in shape (ex = pancreas and parotid salivary gland)
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Mucous glands
(merocrine gland) produce mucins that consist of long chains of negatively charged carbs attached to a small protein core. Mucins are viscous and are important for lubrication and protection. The secretory granules are large and stain very lightly with hematoxylin and eosin. The rough ER is not well developed. The nuclei are heterochromatic and occupy basal position in the secretory cells of mucous glands.
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shape of mucous glands
usually columnar and mucous acini typically have tubular shape (ex: goblet cells, glands of the soft palate, Brunners glands)
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Mixed or seromucous glands
(merocine gland) have both serous and mucous cells. Mucous cells form tubules, capped by serous cells that form serous demilunes.
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Apocrine type of secretion involves
pinching of the apical cytoplasm with the formation of secretory vesicles. This mechanism is found in the mammary gland, which is responsible for releasing large lipid droplets into milk
121
Holocrine type of secretion involves
the who cell, rather than its parts. As the cell accumulates the secretory product, it goes through apoptosis, and both the cellular content and cell debris are released into the lumen of the gland. This type of secretion is typical for sebaceous glands of the skin.
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Unicellular glands
individual secretory cells scattered within the non secretory epithelium
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Goblet cells
represent the most common type of unicellular glands. Goblet cells, which are specialized for the secretion of mucus, are scattered within the columnar epithelial cells of the GI and respiratory tracts
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Multicellular exocrine glands
composed of more than 1 cell. Most multicellular glands form tubular invaginations from the surface.
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what does invaginating the epi allow
increase surface size
126
Terminal part of the invagination of epi
secretory portion; the portion that connects the secretory cells to the surface serves as a duct. Secretory parts can be shaped differently.
127
If the secretory part is shaped like a flask
the gland is alveolar, or acinar. These glands are most often serous
128
If the secretory part is shaped like a tube
it is called tubular. These are glands that are most often mucous
129
If the secretory part is shaped like a flask shaped dilation
the tube is tubuloalveolar. These glands are always mixed, or seromucous
130
Myoepithelial cells
contractile cells, present in some glands between the basal lamina and in the secretory portion of glands and ducts. These cells contain contractile proteins, such as myosin; contraction causes expulsion of the secretory product out of the gland. These cells are present in sweat glands, salivary glands, and in the mammary gland.
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Cancer of epithelium
Because epi tissues are on the surface theya re exposed to a variety of carcinogens and are often sources of various types of malignant tumors. The 2 principle types of cancers derived from the epi tissue are carcinoma and adenocarcinoma
132
carcinoma
derived from barrier epi
133
adenocarcinoma
derived from glandular epi
