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 secrete their products, called hormones, directly into the blood. Hormones circulate throughout the body to their target areas and act as chemical messengers to regulate specific body functions. MOST OF THE ENDOCRINE GLANDS ARE EPITHELIAL DERIVATIVES: they 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: they do NOT have ducts connecting them to blood vessels ). Their secretory products are released in the interstitial space between cells and diffuse into the blood of the nearest capillaries. Under the microscope, endocrine glands 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 onto the free surface 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 classified on the basis of the number of cells making up the glands. Exocrine glands that are made of one secretory cell are called unicellular and glands that are made of several secretory cells adjacent to each other and forming distinct structures are called multicellular.

Multicellular exocrine glands can be further classified on the basis of:
1. their duct structures (simple versus ramified);
2. the shape of the secretory parts (at the end of the ducts the secretory cells congglomerate together to form tubes or round little bags);
3. the way they secrete their products.

Objective: Right now, I just want you to be able to classify exocrine glands when you see them. I am not asking you to recognize specific glands (eg. salivary gland, sebaceous gland, pancreatic gland...). You will learn to do that later on as we study the organ systems.