8.8:
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.
8.8:
Classification of Epithelial Tissues: Glandular Epithelium
The glandular epithelium is made of one or more epithelial cells modified to synthesize and secrete chemical substances. Glandular epithelia can be classified based on cell number. Unicellular glands have individual secretory cells scattered across the epithelial monolayer. In contrast, multicellular glands consist of a hollow tubular duct attached to the cluster of secretory cells located in the deep pockets.
Multicellular glands are formed during early development when epithelial budding occurs. The ingrowth of the bud into the underlying connective tissue is initiated by the interaction of various growth factors and cell adhesion molecules. The process of budding is followed by ductal elongation and cellular proliferation into a cluster of secretory cells. These glands with ducts release their products directly onto the epithelial surface and are known as exocrine glands. Some common examples are sweat glands, salivary glands, lacrimal glands, mammary glands, and sebaceous glands.
In some other cases, the ducts are lost during embryonic development, and the cluster of secretory cells is surrounded by various blood vessels. Such glands which are ductless and release their secretions directly into surrounding vessels are referred to as endocrine glands. The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, from where they diffuse into the bloodstream to be delivered to their target cells or organs with appropriate receptors. The endocrine system coordinates the regulation and integration of body responses. A few examples of endocrine glands include the pituitary, thyroid, adrenal cortex, and gonads.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: epithelial tissues