Histology

%%Introduction%%

• Histology is the area of Biology responsible for the study of tissues. In animals, There are four main types of tissue: Epithelial, connective, muscular, and nervous. In this summary, the tissue analyzed will be epithelial.

%%Epithelial tissue%%

• Epithelial tissues, or epithelium, have juxtaposed cells -that is, very close to each other.
• Thus, is observed little extracellular substance.
• %%Epithelium avascular tissues%%, i.e., do not present blood vessels. Epithelial cells receive nutrients and oxygen from blood vessels- in the underlying connective tissue layer.
• Between the epithelial cells and the connective tissue is the %%basal membrane%%, a protein structure that allows the fixation of epithelial cells.
• The basal membrane is permeable to the passage of gases and nutrients, allowing the maintenance of the epithelium.

Epithelial tissues have two basic types: %%coating epithelia%% and %%secretor epithelial%% (glandular).

%%Coating epithelia%%

• Coating epithelia, as the name suggests, %%coat organs, surfaces, and cavities acting on the protection%% and, in some situations, on the absorption of substances.
• %%Coating epithelial%% cells may present specializations in their plasma membranes, such as domes and intertype, structures that increase the binding between cells.
• There are situations in which %%hemidesmosomes%% are observed, which attach the epithelial cells to the protein lamina existing between the epithelium and the connective tissue that supports it.
• Intertyping also increases the area of contact between neighboring cells, potentiating the exchange of nutrients, signaling molecules, and vesicles between them.
• The %%coating epithelia%% that act on the absorption of substances present microvilli, that is, projections of the plasma membrane that increase the area of contact with the medium, resulting in a larger surface of absorption of substances.

%%Zonulas%%

• There are the zonules of %%occlusion and adhering%%.
• The %%occlusion zonula%% is a band that surrounds the cell in its apical region. Resulting in the seal of intercellular space.
• Below this is the %%adhesion zoo%%, it surrounds the entire cell, contributing to the adhesion between neighboring cells.

%%Classification epithelial tissues%%

According to the number of cell layers, the lining epithelial tissues can be classified into different types.

• The %%simple epithelium%% is that formed by only one layer of cells, while the stratified epithelium has two or more layers.
• There is also the %%pseudostratified epithelium%%, tissue with only one layer of cells, which appears to be formed by more than one cell layer- due to the variation in the size and position of the nucleus.

It is also possible to differentiate epithelium according to the shape of their cells.

• The %%cubic epithelium%% is equipped with cube-shaped cells.
• The %%columnar epithelium%% (prismatic) has elongated cells.
• And the %%pavement epithelium%% (squamous) consists of flattened cells.
• Finally, there is the %%transition epithelium%%, whose shape and size of cells vary according to the degree of distension of the organ in which it is present.
• For example, when there is only one cell layer, and these cells are cubic, the epithelium in question is called %%cubic simple%%.

%%Secretion epithelium%%

• Secretion epitheliums, or glandular epithelia, are derived from differentiation that occurs in the lining epithelium, originating from the glands.

• The %%glands have cells specialized in producing and releasing substances vital to the functioning of the body%%- such as hormones.

• As for the number of cells, the %%glands can be unicellular%%, that is, formed by only one cell, such as mucus-producing goblet cells, present in the trachea and intestine.

• There are also %%multicellular glands%%, such as salivary, sudoriferous, and adrenal glands.

It is also possible to classify the glands according to the place where the secretion is released.

• The %%exocrine glands%% have a connection with the epithelium from which they originated. This connection corresponds to ducts through which secretions follow to the surface of the body or a cavity.
• %%Endocrine glands%%, such as pituitary glands, thyroid, and parathyroids, are devoid of ducts, and their secretions are released into the blood.
• There are also %%amphitrite or mixed glands%%, in which an endocrine and exocrine part is observed.
• The endocrine part of the pancreas secretes in the blood the hormones insulin and glucagon, while the exocrine portion secretes pancreatic juice in the pancreatic duct.

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