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Samenvatting Cell Biology - Histology
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[Meer zien]Voorbeeld 4 van de 31 pagina's
Voorbeeld 4 van de 31 pagina's
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Voorbeeld van de inhoud
EPITHELIA AND GLAND TISSUEThere are ± 200 different cell types, which are organised in tissues. These tissues can be classified in 4
basic types of tissues:
1. Epithelial tissue
2. Connective tissue
3. Muscle tissue
4. Nervous tissue
These basic tissue types are functionally organized in organs (e.g. liver, hart, gut). Cell types that are part
of the same basic tissue type can look much different. What these cells have in common is that they
share the same embryonic origin.
Epithelial tissue
An epithelium is a tissue existing of cells that are tightly connected to each other. It is derived from one of
the three primary germ layers, which are ectoderm (nervous tissue), endoderm (digestive tissue) and
mesoderm (muscle/connective tissue). There are 2 types of epithelial tissues:
1. Covering epithelia (e.g. skin)
2. Glands (invagination of epithelial layers)
An epithelium is classified based on its embryonic origin, morphology and cell organization. The first one
is if it is ecto-, endo- or mesodermal. Secondly, the shape of the cell and lastly, the number of cell layers
(strata). The different existing cell shapes are:
Squamous epithelium Cuboidal epithelium Columnar epithelium
Transition epithelium is when the cells are changing between squamous and cuboidal epithelium. The
different types of cell layers are:
Simple epithelium Stratified epithelium Pseudo-stratified epithelium
The characteristics of epithelia are that there is no blood circulation, the cells are polarised (apical and
baso-lateral domains), the surface has specializations and there is presence of a basal membrane. The
apical domain contains microvilli, which increase the surface area. It helps the cell to have more contact
with the extracellular environment to take up components from the liquid top of the cell. Another
structure on the apical domain are the cilia. These are the longer structures (5x larger), which can be
used for movement or sensory function. The microvilli have a cytoskeleton of actin filaments (extension
from inside the cell) and the plasma membrane is fold around this. This results in more surface to act
with extracellular environment. For cilia, the cytoskeleton is made of microtubuli. These can be changed
in length, which results in movement of the cilia if one side contracts and the other side doesn’t.
,Intercellular connections
Tight junction
On top of the cell, there are the tight junction and the adhesion belt. The
tight junction is also called the zonula occludens and it prevents small
Adhesion
molecules to go in between the epithelial cells. This is possible because the
belt
Terminal
tight junction proteins function like a zipper, which causes fusion of the
web
Button
plasma membranes. This happens around the whole cell.
desmosome
Another function of the tight junction is to make sure that
certain proteins can’t go from the top of the cell to the
basal side of the cell. Therefore, the membrane proteins of
both cells are compartmentalized.
Hemidesmosome Below the tight junction, there is the adhesion belt (also
(a)
Gap junctions called zonula adhaerens). This is also around the whole
cell and this adhesion between cells is made by transmembrane-linker proteins called
cadherins. Adhesion molecules of neighbouring cells bind to each other. Due to these
molecules, the plasma membrane is a bit widened, because there needs to be space for the adhesion
molecules. These adhesion molecules on the inside (intercellularly) attach to the cytoskeleton of actin.
This connects the cytoskeletons of cell together.
Gap junctions are located lower and those proteins are located in the membrane,
where one cell connects to another cell. This is also called a nexus. This is for transport
between cells of very small molecules, such as ions, amino acids and certain hormones.
The proteins that make up the gap junctions are called connexins. 6 of those form a
pore and when the pore of one cell connects to the pore of another
cell, there is a tubule.
A button desmosome can be used to connect parts. This looks a bit
like the adhesion belt, because there are, again, adhesion molecules.
But now, on the inside, it is not connected to the actin cytoskeleton, but to the
intermediate filaments. These are thicker than actin and, therefore also stronger. It
makes the strongest connection between cells. The intermediate filaments are, for
instance, keratin, vimentin, desmin, etc.
The epithelial cell also uses a structure similar to the button desmosome to
bind to the connective tissue underneath. This is called the hemidesmosome.
Hemi means half, because it is just like a button desmosome, but only half of it.
This half is binding to fibres outside the cell (via integrins). These fibres are
present in the basal lamina on the basal side of the cell. The 2 layers of the
basal lamina are the lamina lucida (light) and the lamina densa (dense, because
more fibres). The hemidesmosome binds to the fibres in the basal lamina and the connective tissue
underneath is also bound to this basal lamina.
At the basal domain, there are invaginations (basal labyrinth) of the plasma membrane, which is again to
increase the surface area. Here, a lot of mitochondria are located, because this is where the glucose and
oxygen comes in, which are used to make energy out of.
Gland tissue
The gland tissue is a specific type of epithelia. It is an invagination of epithelial
tissue. The proteins in this fold make proteins that they secrete. Now is talked
about gland tissue. The outer layer of the gland tissue is the parenchyma
(gland cells), which makes the function of this tissue. Around the gland tissue,
there is connective tissue, which is called stroma. These are supporting the
morphology of the whole gland. The secretion of product often goes via secretion vesicles. This secretion
by gland tissue happens via 2 ways: exocrine glands or endocrine glands. Exocrine glands release its
product exogenous via a duct to the epithelial surface, which means to the outside world (like pancreas).
Endocrine glands release its product (hormones) endogenous directly to the lumen (blood or lymph).
There are 3 methods of secretion:
1. Merocrine → Product is secreted via vesicles to the outside of the cell (salivary gland)
, 2. Apocrine → Top of the cell contains product, top of the cell gets detached and is released
(mammary gland)
3. Holocrine → Cells that contain product burst (apoptosis) and release product (sebaceous gland)
Merocrine secretion Apocrine secretion Holocrine secretion
The primary components for the secretion process of the protein-producing glands are ribosomes, rough
endoplasmic reticulum (RER) and the golgi apparatus. A chain of mRNA binds to a ribosome (in cytosol),
tRNA’s are bound by the ribosome to the mRNA (depending on the code they are carrying) and a
polypeptide will be formed from the amino acids that the tRNA’s are carrying. The ribosomes are bound
to the outside of RER and the formed polypeptide is transported inside. The part of the endoplasmic
reticulum that doesn’t contain ribosomes is called smooth endoplasmic reticulum (SER). When a protein
is made in the ER, it is secreted via transport vesicles to the golgi for maturation and to put them on
transport to the right location. The functions of the golgi apparatus are:
• Glycosylation of proteins → Putting a sugar group to the proteins to get them to the outside
world.
• Sorting of proteins → To get the proteins to the right location
o Secretion vesicles → For secretion, also called exocytosis (release of product from the
cellular surface)
o Cell membrane (membrane proteins) → Put on membrane
o Lysosomes → To be degraded, when there is too much
There are also many glands producing lipids, such as steroids. Steroids are made by enzymes in the SER.
Part of those enzymes that make lipids can be found in mitochondria, specifically the synthesis of
cholesterol. Steroids and lipids can’t be stored in the cell, this is because the building blocks of lipids are
the same as the plasma membrane and, therefore, the lipids can go through the plasma membrane.
, CONNECTIVE TISSUE
In connective tissue, the cells are separated by extracellular matrix. All types of connective tissue are
derived from mesenchyme, which is the embryonic connective tissue. There are 4 large classes of
connective tissue:
1. Loose and dense connective tissue
2. Cartilage and bone
3. Blood and lymph
4. Endothelium and mesothelium
The types of connective tissue are different in
their strength, which is determined by their
extracellular matrix. Another way to classify
connective tissue is whether it is supportive or
metabolic. Among the metabolic connective
tissue are blood, fat cells and the cells of the
immune system. The other ones are supportive
connective tissue. There is also a distinction made between resident cells (figure) and wandering cells
(blood). The resident cells have a particular name, depending on the tissue they are in. Not included in
the figure are mast cells and macrophages, which are also resident cells. These cells normally wander in
the blood, but they stick to the tissue at a certain moment and are becoming resident cells in that tissue.
In the tissue, mast cells release vasodilating factors (histamines), which dilate blood vessels.
Macrophages start in the blood as monocytes and as soon as they migrate into the tissue, they are called
macrophages. Depending on which tissue, the macrophages get a different name: skin (cells of
Langerhans), liver (Kupffer cells), lung (dust cells), brain (microglia). In all these tissues, the macrophages
are part of the immune system.
Dense connective tissue
Fibroblasts are an example of dense connective tissue. The cells with
the nuclei are the fibroblasts and the tissue in between is the matrix.
This matrix consists of 2 components: fibres (like collagen) and a
ground substance (glue between fibres). Since the collagen fibres can
become very long, the building blocks are secreted by the cell and
the fibres are formed in the matrix. The long stripes are fibres and
the dots are cross-sections of these fibres. The empty spaces
between the fibres contain the ground substance. This ground
substance mainly contains glycoproteins. These macromolecules
interact with each other, with fibres and with cells (of connective tissue, epithelia, etc.). There are 3 types
of fibres possible in dense connective tissue:
1. Collagen fibres
2. Reticular fibres
3. Elastic fibres
If there is not enough vitamin C, there can’t be collagen fibres made, which deteriorates the connective
tissue causing bleedings. There are 15 types of these collagen fibres:
• Type I → In tendons (connection muscle to bone) and in bone
• Type II → Thin fibres in cartilage
• Type III → Thin fibres in reticular connective tissue
• Type IV → Basal lamina
The synthesis of this collagen happens (like every protein) in the RER and golgi apparatus. There is a
continuous secretion of collagen by the fibroblasts. Because the collagen fibres are too long to be
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