The endocrine system in human beings is a complex network of glands that secrete hormones into the bloodstream and then carry them to different organs and tissues. The hormones affect growth and development, metabolism, and sexual function.
Many endocrine glands regulate hormone levels through complex feedback mechanisms that keep the body’s internal environment consistent and stable. This constancy is called homeostasis.
The Pituitary Gland
The Pituitary Gland is a small pea-sized gland, no larger than a cherry, that sits at the base of the brain beneath the hypothalamus and is referred to as the “master gland”. It secretes hormones that influence many different body functions.
Its specialized cells produce and release several hormones that control various processes in the body, including metabolism, growth, reproduction, blood pressure and many others. The hormones are released in bursts throughout the day, and levels vary by time of day and other factors.
Most of the pituitary’s hormones are synthesized by specialized cells called thyrotrophs, which secrete a hormone known as thyrotropin (TSH). The gonadotrophs secrete luteinizing hormone and follicle-stimulating hormone; the corticotrophs release adrenocorticotropic hormone (ACTH); and the somatotrophs produce growth hormone (GH) and prolactin.
These cells are located in the anterior lobe of the pituitary, a portion that has an embryological origin as an epithelial upgrowth from the foetal oral cavity called Rathke’s pouch. The lobe is divided into two parts: pars tuberalis, which surrounds the infundibular stalk; and pars distalis, which consists of strings of specialized cells that secrete hormones associated with growth and development.
This part of the pituitary lobe is separated from the posterior lobe by the pars intermedia, a thin band of cells that separates them. The pars tuberalis and the pars distalis make up about 80% of the pituitary’s front-facing surface.
The pituitary’s cells use a type of signaling known as endocrine signaling, which uses the circulatory system to reach distant target organs. Most of the endocrine signals are paracrine, meaning that the messages are transmitted through blood vessels to cells that then bind to receptors on target tissues. Some are autocrine, which are sent directly from one cell to another.
The Hypothalamus
The hypothalamus is the brain region that links the nervous system to the endocrine system. The neuron cells of the hypothalamus make chemicals that control the hormones made by the pituitary gland and other endocrine glands.
The main function of the hypothalamus is to maintain homeostasis, or internal balance. It does this by gathering information from the senses, such as temperature, light exposure and feelings, and sending it to the pituitary gland. The pituitary gland then makes hormones that control many functions in the body.
To reach the pituitary, hypothalamic hormones pass through a vascular system called the hypothalamo-hypophyseal portal system. This vascular system is formed by fenestrated capillaries.
In addition to delivering hormones to the pituitary, the hypothalamus also has an important role in regulating behavior and emotions. This is because it stimulates and inhibits a number of fundamental behaviors, such as eating, drinking, and sexual behavior.
Another function of the hypothalamus is to regulate temperature and water balance in the body. This is done by releasing a hormone that helps the body cool down during exercise or hot weather, and by producing a hormone that helps keep the body warm during cold weather.
It is also responsible for regulating sleep. The hormone melatonin helps the body know when it is time to sleep.
The hypothalamus is divided into several regions and zones. The lateral (prechiasmatic) region is above the optic chiasm, while the medial and periventricular regions are located below the optic chiasm. It is also divided by a coronal plane passing through the infundibulum.
The Adrenal Gland
The adrenal gland is one of the most important endocrine organs in the human body. It makes hormones that control your metabolism, blood pressure, immune system and stress response. Adrenal problems can be caused by a number of conditions, including infections, tumors and autoimmune diseases.
The outer part of each adrenal gland is called the cortex. This area contains cells that synthesize and secrete chemical derivatives (steroids) of cholesterol, which are then converted into glucocorticoids, aldosterone and other hormones.
These hormones have a variety of effects, from increasing your heart rate and elevating your blood pressure to boosting your energy supply and sharpening your concentration. When you’re under stress, the adrenal glands release these hormones to get your body ready for a fight or flight.
Some of these hormones also boost your mood and make you feel more alert. Normally, the adrenal glands produce just enough of these hormones to keep your metabolism and immune system in balance.
But some conditions can cause the adrenal glands to make too much or not enough of these hormones, resulting in symptoms like fatigue, weight gain and high blood sugar levels. This condition is known as adrenal fatigue.
The adrenal glands are divided into two parts: the cortex and the medulla. The cortex contains nerve fibers that connect to the pituitary gland, which releases a hormone called ACTH that stimulates the adrenal cortex.
The Parathyroid Gland
The parathyroid gland is one of four endocrine glands found in the neck of humans and other tetrapods. It makes and releases the hormone parathyroid hormone (PTH) to control blood calcium levels.
The blood calcium level should be in a narrow range, so that the nervous and muscular systems can function properly. Too little calcium can make your bones weak and brittle (osteoporosis).
In normal people, the parathyroid glands measure blood calcium levels each minute. If the levels get down a bit, they know it and make PTH which goes to the bones and takes some calcium out of the calcium vault and puts it into the blood.
But sometimes the parathyroid glands do not make enough PTH or keep the calcium level in the standard range. This is called hyperparathyroidism and occurs around one in 2,000 people.
It is usually caused by a noncancerous growth (adenoma) on one of the parathyroid glands. But in rare cases, a tumour can cause the problem.
There are several different ways that your doctor can diagnose a condition like hyperparathyroidism or a parathyroid adenoma. For example, your doctor might test your calcium and PTH levels in your blood or take a sample of your parathyroid tissue via biopsy or surgery.
Your doctor might also give you a bone densitometry test that uses a small dose of ionizing radiation to show how much calcium is in your bones. This is usually more accurate than using a simple test of your blood calcium levels.
Your doctor might also use a procedure called minimally invasive parathyroidectomy to remove the overactive gland. This is usually done through a small incision in your neck.
The Adrenal Cortex
The adrenal cortex is the endocrine gland that produces steroid hormones, and is located in a triangular region above each kidney. It is composed of an inner medulla and an outer cortex. The inner medulla is an extension of the sympathetic nervous system, and produces catecholamines (epinephrine and norepinephrine) that are released into the bloodstream to stimulate the fight-or-flight response.
The cortex is composed of three zones, the outermost zone is called zona glomerulosa, and it produces aldosterone under angiotensin II stimulation; the middle zone is called zona fasciculata, and it produces cortisol; and the inner zone is called zona reticularis, and it produces dehydroepiandrosterone sulfate. These hormones are responsible for a wide range of functions, ranging from regulation of metabolism to the production of sex steroids and their secondary effects on sexual development.
In mammalian embryos, the adrenogonadal primordium first expresses GATA4 and WT1 along with NR5A1, and is established by thickening of coelomic epithelium (CE) cells. After this step, the mediodorsal portion of the primordium migrates dorsally to become the adrenal cortex, whereas the ventral portion becomes the gonads.
These findings reveal an early developmental mode for specifying the adrenogenic lineage and highlight the role of WNT4 signaling as a control mechanism for dynamic gene expression in the adrenocortical cortex. In addition, these results suggest that the adrenogonadal lineage is derived from a common progenitor that is also specified in the gonads.
In addition, we discovered that the adrenocortical lineage is specified within a region of CE encompassing a cluster of cells expressing adrenocortical markers NR5A1 and KRT19, but lacking the gonadal markers GATA4 and LHX9. This cluster is defined as adrenogenic CE and represents a recurrent pattern of HOX gene expression in humans.
