Endocrine system

The endocrine system is a messenger system in an organism. It uses special chemicals called hormones. These hormones are released by glands into the circulatory system. They travel to distant organs to give instructions. In vertebrates, the hypothalamus is the main control center.

In humans, important glands include the thyroid, parathyroid, pituitary, pineal, and adrenal glands. The (male) testis and (female) ovaries are also part of this system. Other parts of the body, like the hypothalamus, pancreas, and thymus, help send messages too. The hypothalamus is in the brain and connects the endocrine system to the nervous system through the pituitary gland.

Hormones come in many types, such as thyroxine, growth hormone, melatonin, testosterone, estrogen, and progesterone. Some organs work together in a chain, like the hypothalamic–pituitary–adrenal axis. Even parts of the body not mainly for hormone work, such as bone, kidneys, liver, heart, and gonads, can release hormones. The study of how these hormones work is called endocrinology.

Structure

The endocrine system is a special messaging system in our bodies. It uses hormones, which are chemicals made by glands, to send signals to different parts of the body. These hormones travel through the blood to reach their targets.

The main glands in the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus, and adrenal glands. These glands release hormones directly into the blood, where they can travel to other parts of the body to do their jobs.

Some important jobs of these hormones include controlling how fast our bodies use energy, helping to manage water balance, and regulating things like growth and how our bodies process food. The hypothalamus and pituitary gland are especially important, as they help control many other glands and their hormones.

Development

Main article: Development of the endocrine system

The fetal endocrine system is one of the first systems to develop during prenatal development.

Adrenal glands

The fetal adrenal cortex forms within four weeks of gestation. The adrenal cortex comes from the thickening of the intermediate mesoderm. The mesonephros changes into a tissue called the genital ridge. This ridge makes steroidogenic cells for both the gonads and the adrenal cortex. The adrenal medulla comes from ectodermal cells. Cells that will become adrenal tissue move to the upper part of the mesonephros. At seven weeks of gestation, the adrenal cells join with sympathetic cells to form the adrenal medulla. By the end of the eighth week, the adrenal glands have formed above the developing kidneys. At birth, the adrenal glands weigh about eight to nine grams.

Thyroid gland

The thyroid gland forms from two clusters of embryonic cells. One part comes from the thickening of the pharyngeal floor, which makes thyroxine (T₄) producing follicular cells. The other part comes from the caudal extensions of the fourth pharyngobranchial pouches and makes parafollicular calcitonin-secreting cells. These two parts are visible by 16 to 17 days of gestation. Around the 24th day of gestation, the foramen cecum, a thin, flask-like diverticulum of the median anlage forms. By 50 days of gestation, the median and lateral anlage have fused together. At 12 weeks of gestation, the fetal thyroid can store iodine to make TRH, TSH, and free thyroid hormone. At 20 weeks, the fetus can control the making of thyroid hormones. During development, T₄ is the main thyroid hormone made.

Parathyroid glands

Once the embryo reaches four weeks of gestation, the parathyroid glands start to develop. The human embryo forms five sets of endoderm-lined pharyngeal pouches. The third and fourth pouch develop into the inferior and superior parathyroid glands, respectively. The third pharyngeal pouch meets the developing thyroid gland and moves to the lower poles of the thyroid lobes. The fourth pharyngeal pouch later meets the developing thyroid gland and moves to the upper poles of the thyroid lobes. At 14 weeks of gestation, the parathyroid glands grow from 0.1 mm in diameter to about 1 – 2 mm at birth. The developing parathyroid glands work starting in the second trimester.

Pancreas

The human fetal pancreas starts to develop by the fourth week of gestation. Five weeks later, the pancreatic alpha and beta cells begin to appear. By eight to ten weeks, the pancreas starts making insulin, glucagon, somatostatin, and pancreatic polypeptide. In early development, there are more pancreatic alpha cells than pancreatic beta cells. The alpha cells reach their highest number in the middle of gestation. From the middle stage until birth, the beta cells keep increasing. The endocrine cells spread out in the body by 10 weeks. At 31 weeks, the islets of Langerhans have formed.

Gonads

Main article: Development of the gonads

The reproductive system starts developing at four to five weeks of gestation with germ cell migration. The bipotential gonad forms from the medioventral region of the urogenital ridge. At the five-week point, the developing gonads separate from the adrenal primordium. Gonadal development begins 42 days after conception.

Male gonadal development

For males, the testes form at six fetal weeks and the sertoli cells start developing by the eighth week of gestation. SRY, the sex-determining locus, helps the Sertoli cells to develop. The Sertoli cells start the making of anti-Müllerian hormone. This hormone stops the growth of female internal features. At 10 weeks of gestation, the Leydig cells start making androgen hormones. The androgen hormone dihydrotestosterone helps make the male external genitalia.

The testicles move down during development in two stages that start at eight weeks of gestation and continue through the middle of the third trimester. During the first stage (8 to 15 weeks of gestation), the gubernacular ligament thickens. This stage is controlled by insulin-like 3 (INSL3), a relaxin-like factor made by the testicles. During the second stage (25 to 35 weeks of gestation), the testicles move into the scrotum. This stage is controlled by androgens. In the second and third trimester, testicular development finishes with the shrinking of the fetal Leydig cells and the growing of the seminiferous cords.

Female gonadal development

For females, the ovaries become visible by the 8th week of gestation. Without testosterone, the Wolffian structures shrink. The Müllerian structures stay and develop into the fallopian tubes, uterus, and upper part of the vagina. The urogenital sinus develops into the urethra and lower part of the vagina, the genital tubercle becomes the clitoris, the urogenital folds become the labia minora, and the urogenital swellings become the labia majora. At 16 weeks of gestation, the ovaries make FSH and LH/hCG receptors. At 20 weeks of gestation, theca cell precursors are present and oogonia mitosis is happening. At 25 weeks of gestation, the ovary is defined and folliculogenesis can start.

Pituitary gland

The pituitary gland forms from the rostral neural plate. The Rathke's pouch, a cavity of ectodermal cells of the oropharynx, forms between the fourth and fifth week of gestation and becomes the anterior pituitary gland. By seven weeks of gestation, the anterior pituitary blood system starts to develop. During the first 12 weeks of gestation, the anterior pituitary changes in cell structure. At 20 weeks of gestation, the hypophyseal portal system has formed. The Rathke's pouch grows towards the third ventricle and joins with the diverticulum. The posterior pituitary lobe forms from the diverticulum.

The working of the anterior pituitary depends on the timing of transcription factors in pituitary stem cells. Six weeks into gestation, the corticotroph cells can be seen. By seven weeks of gestation, the anterior pituitary can make ACTH. Within eight weeks of gestation, somatotroph cells start to develop with human growth hormone. By 12 weeks of development, thyrotrophs start making parts of TSH, while gonadotrophs start making parts of LH and FSH. At 24 weeks of gestation, prolactin-expressing lactotrophs start to appear.

Function

See also: List of human endocrine organs and actions

Hormones

Main article: Hormone

Hormones are special messages made by glands. These messages travel through the blood to different parts of the body. They help control important body processes like digestion, growing, sleeping, and feelings.

Hormones work by attaching to special spots on cells. This changes how those cells work. Some hormones act fast, while others cause slower changes.

Cell signalling

The endocrine system sends messages through the blood. But cells can also send messages in other ways, like within the same tissue or to nearby cells.

Autocrine

Main article: Autocrine signalling

Autocrine signaling is when a cell sends a message to itself.

Paracrine

Main article: Paracrine signalling

Paracrine signaling targets cells in the same tissue or organ. For example, cells in the pancreas can send messages to other cells in the pancreas.

Juxtacrine

Main article: Juxtacrine signalling

Juxtacrine signaling happens between cells that are close together. Messages pass through special channels between the cells.

Clinical significance

Disease

Main article: Endocrine diseases

Diseases of the endocrine system are very common. Some examples include diabetes, problems with the thyroid gland, and obesity. These diseases happen when hormones are not working right. This can be because a gland is missing, not making enough hormones, or making too many.

There are different types of endocrine problems. Some start in the glands themselves, while others start in the brain parts that control the glands. For example, Addison's disease and Cushing's disease affect the adrenal glands.

Graves' disease is another common problem that affects the thyroid gland. When the thyroid is too active, it can cause symptoms like feeling very hot, being tired, having high blood pressure, and sometimes swelling around the eyes.

DALY rates

A DALY, short for Disability-Adjusted Life Year, measures how much disease affects a country. It looks at early deaths and years living with health problems. Lower DALY rates mean fewer problems from endocrine disorders.

The map shows that many parts of Asia have lower DALY rates, so endocrine disorders are not a big problem there. But some places in South America and Africa, like Suriname and Somalia, have higher DALY rates, meaning more health challenges from these disorders.

Other animals

Animals with a nervous system have a special system called the neuroendocrine system. All animals with a backbone, called vertebrates, have a part of the brain called the hypothalamus connected to a gland called the pituitary.

Vertebrates also have a gland called the thyroid. In animals that change form during their life, like frogs, this gland helps them grow up. Vertebrates have tissue called adrenal glands, and mammals are special because theirs are organized in layers. They all have a system that helps control water in their bodies, and land animals have a special hormone called aldosterone that helps with this.

Additional images

Female endocrine system

Male endocrine system