Hypothalamus-Pituitary-Gonad Axis (HPG)
Hypothalamus-Pituitary-Gonad Axis (HPG) refers to the hypothalamus, pituitary gland, and gonads, just like these individual endocrine glands are single entities. Because these glands usually work together, physiologists and endocrinologists describe them as a single system.
The HPG axis plays a vital role in the development and regulation of many human systems (such as the reproduction and immune system). The fluctuation of this axis will cause changes in the hormones produced by each gland, and produce various local and systemic effects on the human body. This axis controls the development, reproduction and aging of animals. Gonadotropin-releasing hormone (GnRH) is secreted from the hypothalamus by neurons expressing GnRH. The front part of the pituitary gland produces luteinizing hormone (LH) and follicle-stimulating hormone (FSH), while the gonads produce estrogen and testosterone. HPA, HPG and HPT axes are the three ways that the hypothalamus and pituitary gland directly regulate neuroendocrine function.
Highlighted Functions
Fig.1 HPG axis
Reproduction - One of the most important functions of the HPG system. HPG axis is to regulate reproduction by controlling the uterine and ovarian cycles. In males, the production of GnRH, LH and FSH is similar, but the effects of these hormones are different.
Life cycle - Activation and deactivation of the HPG axis also help to regulate the life cycle.
Sexual dimorphism and behavior - Sex steroids also affect behavior, because sex steroids affect the structure and function of the brain. During development, hormones help determine how neurons synapsed and migrate to cause sexual dimorphism. It is believed that FSH may play an important role in brain development and differentiation.
Mechanisms
Fig.2 HPG regulation in males.
The hypothalamus is located in the brain and secretes GnRH. GnRH moves down the front part of the pituitary gland through the pituitary system, and binds to receptors on secretory cells of the pituitary gland. In response to GnRH stimulation, these cells produce LH and FSH and enter the bloodstream. These two hormones play an important role in communication with the gonads. In women, FSH and LH mainly act to activate the ovaries to produce estrogen and inhibin, and regulate the menstrual cycle and ovarian cycle. In men, LH stimulates the interstitial cells located in the testis to produce testosterone, while FSH plays a role in spermatogenesis. Recent studies have shown that there is a neurosteroid axis, which helps the cortex to regulate GnRH production in the hypothalamus.
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