GLANDULAR EPITHELIUM: CHARACTERISTICS, CLASSIFICATION AND FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The glandular epithelium is a type of tissue responsible for lining and covering the organs associated with the secretion of substances. The cells that make up these glandular tissues are capable of secreting products of a diverse nature, such as hormones or sweat.

The glands can secrete their products to a free surface through a conduit (exocrine glands), or they can direct the secretion into the bloodstream (endocrine glands). The glands are widely varied in their histology, function, and secretion product.

characteristics

The glandular epithelium is responsible for the secretion of a wide range of substances with multiple functions: from hormones and lipids to mucus. This epithelium is included in the connective tissue, forming organs called glands.

The cells that form it are closely grouped, leaving little or no space between the cells.

The cells are characterized by having a single nucleus and are generally of the cuboidal type. The epithelium is formed by multiple cell layers with a basement membrane that separates it from the other tissues.

The cytoplasm is abundant and of a clear or transparent appearance. The division of this tissue occurs by a normal process of mitosis.

Classification

Glands can be classified in three different ways: according to the place where the secretions are released, according to the number of cells that form them or according to the secretion mechanism.

Exocrine glands

Glands that have a duct and deliver their secretion through it to an internal surface (the intestinal surface, for example) or external are known as exocrine. This group is subdivided as follows:

Unicellular exocrine glands

Within the group of unicellular glands, the goblet glands stand out. They are generally found in the epithelium linings in the airways, on the nasal mucosa, and in the large and small intestine.

Goblet cells are calyx-shaped and their main function is the production of mucus. The dark-appearing nucleus is located at the base of the cell along with other organelles, such as the smooth endoplasmic reticulum and the Golgi apparatus.

The upper cell regions are filled with membrane-bound granules of mucus. Secretion occurs through exocytosis and continuously.

Multicellular exocrine glands

Multicellular glands are more complex than the previous group and are composed of a duct and a secretory unit, surrounded by connective tissue.

Generally, the glands are composed of a group of numerous cells under the lining epithelium and are called extraepithelial glands.

In contrast, the type of gland that has small cell groups and is located in the lining epithelium is known as intraepithelial glands and they are not very common.

The extraepithelial glands consist of regions with secretory functions called adenomers and they continue with the secretory ducts of other structures. The former are responsible for producing the secretion and the ducts transport them.

These adenomers can be grape-shaped (acinous), sac-shaped (alveolar) or tubular, although intermediate forms may appear.

The secretion of these glands can be serous (secretion high in albumin) or mucosa (production of mucin, a viscous substance). There may be mixed glands that contain secretory cells of both types.

Types of discharge

The secretion of the gland can be merocrine (also called eccrine), in which the product is found in the Golgi apparatus within membranous structures and is exported abroad by exocytosis.

The secretory granule fuses with the cell membrane and the granule opens. In this type of secretion there is no loss of cell membrane or cytoplasm.

The apocrine glands accumulate proteins and lipids for their secretion. The cellular region where the accumulation occurred is compressed, and subsequently detaches, forming an aposome (extruded portion). Note that the secretory proteins do not possess any signal peptides and are not packaged in vesicles.

The glands located in the armpits, external auditory canal, eyelids, nipples, labia majora, mons pubis and perianal region are specific examples of apocrine glands. This type of glands are associated with a hair.

The holocrine glands are only the sebaceous glands (in humans) and involve the total shedding of the cell with the secretion product. The secretion, lipid in nature, accumulates in the form of drops inside the cell. They may or may not appear accompanied by hairs.

Endocrine glands

Glands that are secreted by blood and do not have a secretory duct are called endocrine. They are characterized by having an effective irrigation system.

The endocrine glands are formed from the three embryonic leaves and are distributed throughout the body.

Its function is the production of hormones, essential molecules for hundreds of reactions in metabolism. Most hormones are of the steroid or protein type and can be secreted by individual cells or by glands.

The typical glands are the adenohypophysis, thyroid, parathyroid, and adrenal glands, as well as the testes and ovaries. The glands are organized in a hierarchical way, in a "master" gland -for example, the pituitary- that controls the others with its secretions.

The cells that make up the endocrine tissues that secrete steroids have an abundant smooth endoplasmic reticulum and mitochondria in contrast to the protein-producing cells, which have a large number of secreting granules.

Type of signage

The type of signaling can be endocrine, where hormones are released into the bloodstream. It is also known as hemocrine.

The paracrine mechanism consists of hormonal secretion that reaches its cellular target by diffusion mechanisms in the connective tissue. Lastly, autocrine signaling occurs when the molecule acts on the same producer cell.

Features

The main function of the glandular epithelium is the secretion of various substances. Different types of glandular epithelium can secrete different compounds, including: hormones (chemical messengers), milk (in the mammary glands, feeding functions), mucus and saliva (protection), sweat (thermoregulation).

It also has functions related to the sexual act, since the glandular epithelium produces secretions that lubricate the sexual organs.

References

1. Rehfeld, A., Nylander, M., & Karnov, K. (2017). Glandular Epithelium and Glands. In Compendium of Histology (pp. 101-120). Springer, Cham.
2. Ross, MH, & Pawlina, W. (2007). Histology. Text and Color Atlas with Cellular and Molecular Biology. 5aed. Panamerican Medical Ed.
3. Thews, G., & Mutschler, E. (1983). Anatomy, physiology and pathophysiology of man. I reversed.
4. Welsch, U., & Sobotta, J. (2008). Histology. Panamerican Medical Ed.
5. Zhang, SX (2013). An atlas of histology. Springer Science & Business Media.