Istologia - Tessuto Epiteliale (INGLESE)

The apical surface can be specialised in different ways: we can have microvilli, with the function

of increasing cell surface, cilia, that move waste and dust or other molecules or organic objects,

and so on. Microvilli are specific of cells that absorb substances, since they increase the

available plasma membrane surface and therefore allow the cell to absorb more. They are

particular of the small intestine. Microvilli are covered by a glycocalyx, which acts as a filter

and, in the intestine, also contain enzymes. Microvilli can only be singularly seen with the TEM.

Cilia are longer than microvilli and can be therefore seen at the light microscope too. Cilia are

present, as an example, in the airways of the human body, where they block and send away

dust, small organisms and other things that could block the way.

A third possible apical specialisation are stereocilia, which can be very long and are structurally

similar to microvilli. Actually, since stereocilia are long microvilli, they should be considered

“stereovilli”. They have no axoneme but actin filaments (which means they’re not long cilia but

long microvilli). They are present in the male reproduction system and participate to the

secretion of factors needed for spermatozoa maturation.

On the basal surface, a possible specialisation is the basal labyrinth. It’s in cells involved in

reabsorption of substances, since there are enfoldments of the basal membrane. Close to them,

many mitochondria are present to provide energy for the mechanism of substances exchange.

On the lateral domain there might be 3 cell junction specialisations: occluding (tight) junctions,

with a very tight connection between the two cells; anchoring (adherens and desmosome)

junctions, since two cells are anchored thanks to proteins and better resist to stress;

communicating (gap) junctions, where two adjacent cells are able to communicate with the

passage of small molecules from one cell to the adjacent one. Junctions are made by different

components: a transmembrane protein in each cell membrane, connected to the nearby protein,

an intracellular link protein, that connects the transmembrane protein to the cytoskeleton.

Going from the free surface to the membrane of an epithelia tissue, we find different junctions

between cells. The one which is nearer to the free surface is the occluding (tight) junction. This

is a point—to-point fusion of the plasma membrane, which actually creates a very tight

connection with almost no extracellular matrix. The proteins that keep the tight junctions

together are some transmembrane proteins (occluding, claudin and JAM). In the cytoplasm they

are connected to cytoplasmic proteins such as ZO1, ZO2 and ZO3.

Going on from the free surface to the basal one, we find the adherens junctions. They are

connected to actin filaments and are made up by e-cadherin proteins. These proteins are

transmembrane proteins. Inside the cytoplasm, a cadherin protein connects with a beta-catenin,

which is connected to an alpha-catenin, then a vinculin protein and finally the actin filament.

This is a kind of anchoring junction.

Another kind of anchoring junction, which is just under the adherens junction, is the

desmosomes junction. They are similar to previous junctions, but in this case the desmosomes

are attached to a kind of plaque in the cytoplasm to which the intermediate filaments, and not

actin, are attached.

Another kind of junction is the gap junction, which is a communicating junction. It allows the

passage of material from a cell to the other, but it’s very selective. These junctions are

hydrophilic channels which allow the passage of small molecules and ions. They are made up by

connexin subunits which form a connexon (1 connexon = 6 connexin subunits). These channels

can also be regulated by a mechanism that use phosphorylation of the connexons. Gap junctions

also maintain the correct rate of cell growth/death by influencing apoptosis and other actions,

keeping a good homeostasis.

A final kind of junction, which is not between epithelial tissue cells but between the cell and the

basal membrane, is the hemidesmosome. It’s similar to demosomes only for morphology, since

different proteins are involved. These proteins are of the integrins family.

Histology - Lining epithelia

Epithelial tissues originate from all the 3 embryonic germs (ectoderm, endoderm and

mesoderm), depending on their position and function in the human body.

In lining epithelia, cells are closely apposed with tight junctions and there’s almost no extra-cel-

lular matrix between them. They are avascular, which means there’s no blood in this tissue and

they obtain nourishment from the connective tissue below them; they have different specialisa-

tions on the free (apical) edge and have many different intercellular junctions.

The functions of epithelial tissue is to cover and protect the body surface (skin epithelium is a

barrier that separates the inner and the outer environments), but also the internal parts of the

body from the internal cavities. This separation is not absolute, since many times metabolic ex-

changes (secretion, absorption) are required. Therefore, these barriers are specialised to allow

these exchanges. There is therefore a strict relationship between cells’ function and shape.

Some epithelia is specialized to receive sensitive stimulations. In the epithelium, although

there’s not a vascular network, there is a very developed nerve system to allow the sense of tact

in the skin.

Epithelia can be divided in three groups, according to cells’ structure: it can be squamous (flat

cells), cuboidal (cuboidal cells) or columnar (tall and thin cells). Another way to classify epithe-

lia is based on whether there is only one layer of cells (simple or single layer epithelium) or

more than one (composed or stratified epithelium). In stratified epithelia, the shape classifica-

tion depends on the superficial layer. In the case of stratified squamous epithelia, it can be ker-

atinised or non-keratinised. A particular stratified epithelium is the transitional epithelium,

which has no specific shape because it depends on the situation of the organ where it is. An ex-

ample is the urinary bladder, where cells are cuboidal when it’s empty but squamous when it’s

full.

Simple squamous epithelia can be found in: lung alveoli; the lumen of blood and lymph vessels

(called “endothelium”); the serosa (called “mesothelium”, is a tissue that lines some internal

cavities); the Bowman capsule. Simple cuboidal epithelia can be found in: ovary surface; kidney

tubules; gland ducts; terminal bronchioles; retina. Simple columnar epithelia are typically char-

acterised by apical specialisations such as microvilli, cilia or stereocilia. The position of the nu-

cleus is important during microscope observation. This epithelium can be generally found in or-

gans and glands developed to absorb or secrete substances (uterine tube, small bronchioles,

small intestine, stomach, gall bladder, excretory ducts).

Pseudostratified epithelium is a particular simple epithelium, always columnar. Cells have a dif-

ferent morphology, since some are small and don’t reach the apical surface, others do, but the

main fact is that all cells reach the basal lamina. It is therefore considered a simple epithelium.

A stratified epithelium contains generally three types of layers: the basal layer, with cuboidal

cells (sometimes stem cells); the spinal layer, which can be made of several layers, more flat-

tened; the superficial layer, with flat cells. Cells on the basal layer replicate and differentiate,

then move to the upper layers until they get to the surface, when they are eventually removed.

The difference between keratinised and non-keratinised epithelia is that in keratinised epithelia

the superficial layer is formed by dead (keratinised) cells. The main keratinised lining epithelium

in humans is the epidermis, covering the whole body surface. Animals have also internal kera-

tinised epithelia, such as the rat oesophagus.

The epidermis has different layers. From the bottom to the top, we have: the basal layer or

stratum basale; the spinous layer (8-10 layers of cells); the stratum granulosum (3-5 layers); the

stratum lucidum (which is typical of thick epithelia); the stratum corneum (15-30 layers of dead

cells). In the basal layer, made up by stem cells, new cells are produced by mitosis with a

turnover rate of 25-50 days. These cells move to the spinous layer, where they specialise. Then

they gradually move up while new cells are produced; in the stratum granulosum they begin to

keratinise; in the stratum lucidum they die, lose their nucleus and then form the stratum

corneum. Epidermis cells are: keratinocytes (most of the epithelium); melanocytes (melanin syn-

thesis); Langerhans cells (defence processes); Merkel cells (tactile receptors). Both melanocytes

and Merkel cells are located in the basal layer. Merkel cells are connected with the axon of a

nearby neuron; melanocytes have different protrusions, through which they distribute the

melanin in the skin, where it is phagocytosed by keratinocytes. Langerhans cells are defensive

cells in the spinous layer. When an external microorganism that passes through the skin reaches

them, they recognise the antigen, move under the basal lamina, enter a lymphatic vessel and

reach the lymph node, activating T cells and therefore the immune reaction. Stratified squa-

mous epithelia can be find in: epidermis; cornea; oral cavity, larynx, oesophagus and rectum

mucosa; vagina.

Stratified cuboidal epithelium is particularly present in big gland ducts, such as sweat glands,

sebaceous glands, exocrine pancreas. The stratified columnar epithelium is not much present in

our body. It can be found in: the larynx mucosa; male urethra; conjunctiva (a layer of the eye);

big glands ducts.

Transitional epithelium is not well defined and changes depending on the situation, since it can

be stretched and released. It is formed by different layers of cells, with polygonal basal layer

cells and big, sometime bi-nucleated superficial cells. If we stretch this epithelium, superficial

cuboidal cells become squamous. These superficial layer cells are described as “dome-shaped

cells”. On the apical surface of these cells, there are some small vesicles that fuse on the sur-

face when it is stretched, increasing the surface extension.

In some pathological conditions, epithelial cells are transformed and become similar to mes-

enchymal cells (typical embryonic mesoderm stem cells). They are fusiform, not connected by

cell junctions, are non-polar and are able to move on a substrate. This transition is typical of

fibrogenesis, tumor invasions but also in physiological conditions, such as embryogenesis, when

some organs are generated by this transition. This transition is called EMT (epithelial to mes-

enchymal transition). During this transition, epithelial cells lose come typical markers and gain

other markers.

Histology - Exocrine glands

Cells in the glandular epithelia are specialised in producing m