Glands are divided based on their function into two groups: endocrine and exocrine.
Endocrine glands Endocrine glands secrete substances that circulate in the blood. The glands secrete their products through
basal lamina into the bloodstream. Basal lamina typically can be seen as a layer around the glands to which more than a million tiny blood vessels are attached. These glands often secrete
hormones which play an important role in maintaining
homeostasis. The
pineal gland,
thymus gland,
pituitary gland,
thyroid gland, and the two
adrenal glands are all endocrine glands.
Exocrine glands Exocrine glands secrete their products through a
duct onto an outer or inner surface of the body, such as the skin or the
gastrointestinal tract. Secretion is directly onto the
apical surface. The glands in this group can be divided into three groups: •
Merocrine glands – cells secrete their substances by
exocytosis. (e.g. mucous and serous glands; also called "
eccrine", e.g. major sweat glands of humans, goblet cells, salivary gland, tear gland and intestinal glands) •
Apocrine glands – a portion of the secreting
cell's body is lost during secretion. The term
Apocrine gland is often used to refer to the
apocrine sweat glands, however it is thought that apocrine
sweat glands may not be true apocrine glands as they may not use the apocrine method of secretion. (e.g. mammary gland, sweat gland of arm pit, pubic region, skin around anus, lips and nipples) •
Holocrine glands – the entire cell disintegrates to secrete its substances. (e.g.
sebaceous glands:
meibomian and
zeis glands) Exocrine glands can further be categorized by their product: •
Serous glands secrete a watery, often protein-rich, fluid-like product, e.g. sweat glands. •
Mucous glands secrete a viscous product, rich in
carbohydrates (such as glycoproteins), e.g.
goblet cells. •
Sebaceous glands secrete a
lipid product. These glands are also known as oil glands, e.g.
Fordyce spots and
meibomian glands. ==Clinical significance==