There are basically two types of endocrine glands:
Cordonal endocrine glands – formed by cells organized in plates or sheets. These plates have different shapes and are surrounded by blood spaces or blood capillaries that receive the secretion products and distribute them throughout the body by the blood circulation.
In histological sections the cell plates are seen as ribbons or cords and for this reason this type of endocrine gland is called “cordonal”.
Follicular endocrine glands are formed by thousands of small spheres whose walls are usually a simple cubic epithelium. These spheres, called follicles store the secretory product of the gland. An example of a large follicular endocrine gland is the thyroid. Other examples: adrenal (suprarenal) gland, parathyroid, adenohypophysis, corpus luteum.
In the image: an islet of Langerhans, small cordonal endocrine glands surrounded by pacreatic acini. The plates are seen in sections as rows of cells (highlighted in light blue after moving the cursor or clicking). They are surrounded by blood spaces highlighted in pink (the blood spaces of this specific section do not contain blood because the animal was fixed by intravascular perfusion of the fixative).
Depending on the animal size the pancreas may contain hundreds ou thousands islets.
