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.