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
