The adrenal glands, also known as the suprarenal glands, are part of the endocrine system. The endocrine system produces hormones, substances that regulate the activity of body organs and tissues. Beginning in the womb, the adrenal glands produce several hormones, including epinephrine, norepinephrine, aldosterone, cortisol and androgenic steroids. These hormones affect a variety of body functions including water and salt balance, glucose metabolism, sexual development and immune function.
Adrenal Gland Formation
The adrenal glands begin forming by the fourth week after conception. They are producing the hormone cortisol by week eight, notes Dr. Mark Hill in the online text, "UNSW Embryology." The fetal adrenal glands work in concert with the maternal placenta to produce hormones required to sustain the pregnancy in the second and third trimesters.
The adrenal glands have two separate regions, which are derived from different cell types in the developing fetus. The outer portion of the adrenal gland is the adrenal cortex. The adrenal medulla is the inner portion of the gland. Interestingly, the adrenal glands are proportionately much larger at birth than they are in adulthood, reflecting the important role of fetal adrenal hormone production during pregnancy.
Adrenal Glands After Birth
The fetal adrenal cortex comprises much of the gland at birth. During the first three years of life, the fetal adrenal cortex recedes and is replaced by the mature, or adult, adrenal cortex. The adult adrenal cortex has three zones: the zona glomerulosa, zona fasciculata and zona reticularis (ZR). The ZR is not present at birth; it develops at approximately age 4.
In a 2009 article in the "Journal of Endocrinology," Dr. X. Hui and colleagues noted evidence of increased androgen hormone production in the ZR near age 9. Androgens are sex hormones responsible for the growth of pubic and other body hair beginning at puberty. The onset of androgen production by the adrenals heralds the onset of puberty.
Adult Adrenal Cortex
As explained in the medical text "Endocrinology: An Integrated Approach," the adult adrenal cortex produces the hormones cortisol, aldosterone, androstenedione and dehydroepiandrostenedione (DHEA). Cortisol has wide-ranging effects on many tissues throughout the body. They include increased blood sugar concentration, maintenance of blood pressure, and suppression of inflammation.
Aldosterone acts primarily on the kidneys to promote retention of salt and water. These actions are important in maintaining normal blood pressure, and salt and water balance in the body. Androstenedione and DHEA are male sex hormones. In women, adrenal cortical production of these hormones is essential for normal reproductive function.
Adult Adrenal Medulla
In "Endocrinology: An Integrated Approach," the authors note that the adult adrenal medulla comprises less than 20 percent of the gland. The adrenal medulla produces the hormones epinephrine (adrenalin) and norepinephrine. These are the "fight or flight" hormones released in response to startling or frightening situations. Adrenalin and norepinephrine cause increased heart rate and blood pressure, relaxation of the airways with increased airflow, a surge of blood sugar, and increased blood supply to the voluntary muscles. The adrenal medullary hormones are quickly metabolized, limiting their effects on the body to a short time frame.
References
- UNSW Embryology: Endocrine Development, Adrenal Glands
- Endocrinology: An Integrated Approach: Embryology of the Adrenal Gland
- Endocrinology: An Integrated Approach: The Adrenal Glands
- Endocrinology: An Integrated Approach: Anatomical and Functional Zonation in the Adrenal Cortex
- "Journal of Endocrinology"; Development of the Human Adrenal Zona Reticularis: Morphometric and Immunohistochemical Studies from Birth to Adolescence; X.G. Hui, et al; November 2009
