Classification of Epithelial Tissues: Glandular Epithelium

The glandular epithelium comprises secretory cells that obtain molecules from the blood to produce secretions. Secretions can be water-based, such as  sweat, or lipid-rich, such as sebum.

Glandular epithelia can be unicellular or multicellular. Unicellular glands contain single secretory cells like goblet cells dispersed throughout the epithelial layer.

Multicellular glands are formed when a group of epithelial cells termed  connecting cells invaginate into the connective tissue layer.

The connecting cells develop into hollow tubular structures known as ducts and help release glandular secretions onto the epithelial surface. Such multicellular glands are termed  exocrine glands. Exocrine glands include sweat glands on the skin, and  lacrimal glands that produce tears in the eyes.

Some other multicellular glands lose the connecting cells during embryonic development and form ductless endocrine glands. They synthesize hormones and secrete them into the bloodstream as chemical messengers.

For example, the pituitary gland produces thyroid-stimulating hormone, which stimulates the thyroid gland to synthesize thyroid hormones.