Endocrine system

The endocrine system is a complex messenger network within an organism. It uses chemical messengers called hormones to regulate distant organs. These glands release hormones directly into the circulatory system rather than through a duct. This is different from exocrine glands, like salivary glands, which use ducts to send secretions to the body's surface. Because endocrine glands are vascular, they can release substances into the spaces between cells. These substances are then absorbed into the bloodstream to travel throughout the body. The study of this system and its many disorders is called endocrinology.

At the center of this system is the hypothalamus. Located in the brain, the hypothalamus acts as a neural control center. It serves as a vital link between the endocrine system and the nervous system. It works closely with the pituitary gland, which is often called a neuroendocrine organ. Together, they manage several important feedback loops. A feedback loop is a process where the system responds to changes to maintain balance. Some of these important pathways include the hypothalamic–pituitary–adrenal axis and the regulation of thyroid hormones. When glands signal each other in a specific sequence, scientists refer to this chain as an axis.

Different glands produce specific hormones to perform unique tasks. The thyroid gland secretes thyroxine, which regulates metabolic activity and cell growth. The pineal gland produces melatonin, which is involved in sleep cycles. The pituitary gland secretes growth hormone and various tropic hormones. These tropic hormones, such as TSH and ACTH, act as signals to other glands. The pancreas contains specialized clusters of cells called the islets of Langerhans. Within these islets, alpha cells produce glucagon to raise blood sugar. Meanwhile, beta cells produce insulin to decrease blood glucose levels. The ovaries secrete estrogen and progesterone, while the testes produce testosterone.

Many organs outside the primary endocrine system also have secondary functions. For example, the kidneys secrete the hormone erythropoietin. The liver, heart, and bones also play roles in hormone secretion. The parathyroid glands are another critical component. These glands secrete parathyroid hormone, or PTH. This hormone acts on the bones, kidneys, and the gastrointestinal tract. It works to increase calcium reabsorption and phosphate excretion. It also helps convert Vitamin D into its most active form to assist with calcium absorption. This shows how many different body systems must work together to maintain homeostasis.

This system begins to develop very early during prenatal development. The fetal endocrine system is one of the first systems to form. For instance, the fetal adrenal cortex can be identified by four weeks of gestation. The adrenal medulla is derived from ectodermal cells. By the end of the eighth week, the adrenal glands become distinct organs. At birth, these adrenal glands weigh approximately eight to nine grams. This is actually twice the weight of an adult's adrenal glands. They also make up about 0.5% of a newborn's total body weight.

The thyroid gland also undergoes a complex development process. It begins forming from two different clusters of embryonic cells. These structures become apparent by 16 to 17 days of gestation. By 50 days, the different parts of the thyroid have fused together. At 12 weeks of gestation, the fetal thyroid can store iodine. By 20 weeks, the fetus can use feedback mechanisms to produce hormones. The parathyroid glands also start developing at four weeks of gestation. They grow from pharyngeal pouches and migrate to the thyroid gland. By birth, these glands have grown from 0.1 mm to about 1 to 2 mm in diameter.

The pancreas follows its own unique timeline of development. It begins to develop during the fourth week of gestation. By the eighth to tenth week, it starts producing insulin and glucagon. During early development, alpha cells are more numerous than beta cells. As gestation continues, the number of beta cells increases. By the end of development, the ratio of alpha to beta cells reaches approximately 1:1. The concentration of insulin in the fetal pancreas also rises significantly. It starts at 3.6 pmol/g at seven to ten weeks. By the time the fetus is near term, it reaches 93 pmol/g. This careful growth ensures the body can manage energy as soon as it is born.