The pancreas, testis and ovaries have both endocrine and exocrine glands called heterocrine or composite glands. The endocrine part of the pancreas is formed of Islets of Langerhans, which comprise of alpha cells, beta cells, delta cells and F cells. Alpha cells secrete glucagon, while the beta cells secrete insulin. Delta cells secrete somatostatin whereas F cells secrete the pancreatic polypeptide.
Glucagon secreted by alpha cells acts on the target organs like the liver and the adipose tissues. The secretion of glucagon is stimulated by low blood glucose levels, which increases the blood glucose level by glycogenolysis and gluconeogenesis and by inhibiting the conversion of glucose into lactic acid. Insulin, secreted by the beta cells, act on the target organs like the liver, adipose tissue and the muscles. The secretion of insulin is stimulated by high glucose levels in blood and stimulates glycogenesis in muscle and liver cells. Furthermore, insulin prevents gluconeogenesis and promotes glycolysis. The degeneration of beta cells leads to the deficiency of insulin, which causes insulin-dependent diabetes mellitus. Somatostatin, secreted by the delta cells, act on the target organs like the pancreas and intestine. It inhibits the secretion of both glucagon and insulin, along with the intestinal absorption of glucose. Pancreatic polypeptide secreted by F cells increases glycogenolysis and regulates gastrointestinal activity.
The exocrine part of the testis and the ovaries produce gametes while the endocrine part secretes hormones. The interstitial cell-stimulating hormone or ICSH of the anterior pituitary stimulates the testis to secrete androgens, namely testosterone, androsterone, epiandrosterone and dehydro-epiandrosterone. Testosterone controls the growth and development of male secondary sex organs, secondary sexual characteristics and stimulates spermatogenesis.
Each ovary secretes estrogen and progesterone. Estrogen is secreted by the growing ovarian follicles and stimulates the growth and functioning of the female secondary sex organs and release of egg from the ovum. After ovulation, the ruptured follicle converts into a corpus luteum. The luteinizing hormone of the anterior pituitary gland stimulates the secretion of progesterone from the corpus luteum, which in turn stimulates the proliferation of the endometrium of the uterus and prepares it for implantation. Moreover, it helps in placenta formation and the development of the foetus in the uterus. Progesterone also acts on the mammary glands and stimulates the formation of alveoli, which store milk.