EPITHELIA: GLANDULAR

Right now, I want you to be able to identify endocrine, unicellular exocrine, multicellular exocrine glands when you see them. I also want you to tell me where they are located in the body and what are the basic differences between the three. I am not asking you to recognize specific glands (eg. salivary gland, pancreatic gland...). You will learn to do that later on.

A GLAND may consist of one cell or a group of specialised cells that make and secrete substances. Glands are classified as ENDOCRINE or EXOCRINE depending on their route of secretion.


ENDOCRINE GLANDS: their products, called hormones, are secreted directly into the blood and circulate throughout the body to their target areas. Hormones act as chemical messengers to regulate specific body functions. MOST OF THE ENDOCRINE GLANDS ARE EPITHELIAL DERIVATIVES. Endocrine glands are formed by invagination from an epithelial sheet and initially have ducts connecting them to the free surface of the epithelial sheet. During embryonic development, they will lose their ducts and thus are called ductless glands (No ducts).
Under the microscope, they look like any stratified epithelial tissues with one big difference: THEY DO NOT HAVE A FREE SURFACE.... and are surrounded directly by other tissues.


We will learn more about them later in the year.


EXOCRINE GLANDS: release their products onto the free surface of the skin or of the open cavities of the body such as the digestive, respiratory or reproductive tracts. Their products are NOT released into the blood.

Exocrine glands are also classified on the basis of their structure:
Unicellular glands
consist of single cells specialized for secretion scattered amongst other non-secretory epithelial cells of a surface membrane. They have no ducts, of course, but they secrete their products directly on the free surface of open body cavities and thus, are considered exocrine. The most common unicellular exocrine glands are the goblet cells (mucus secreting cells) found in the epithelium of the trachea and the digestive tube.
Multicellular glands
are multicelled glands. They are formed by invagination from an epithelial sheet (like the endocrine glands) but will never lose the duct connecting them to the free surface of the epithelial sheet (surface of the skin or lumen of the open body cavities). Exocrine multicellular glands occur in several forms.

They develop from covering epithelia (in blue) by invagination. The epithelium dips or grows down from the surface into the underlying tissues to form a simple or branched tube (in green). The blind ends of these tubes constitute the secretory parts of the gland (in red)) and may stay tubular or expand to form cup-like acini (s. acinus). A few examples are the sweat glands, mammary glands, salivary glands....
We will learn more about them later in the year.