ENDOCRINE SYSTEM, HUMAN

group of ductless glands that regulate body processes by their secretion of chemical substances called hormones, which are carried to specific target organs and tissues by the bloodstream. Diseases of the endocrine system result from too much or too little hormone secretion or from the inability of the body to utilize a hormone effectively. It is important to distinguish between an endocrine gland, which discharges hormones directly into the bloodstream or lymph system, and an exocrine gland, which secretes substances through a duct opening in the gland onto an external or internal body surface. Salivary and sweat glands, examples of exocrine glands, secrete saliva and sweat, respectively, which act locally at the site of duct openings. In contrast, hormones that are secreted in minuscule quantities by endocrine glands, are transported by the circulation to exert powerful effects on tissues remote from the site of secretion. As far back as 3000 BC, the ancient Chinese diagnosed some endocrinologic disorders and were able to provide effective treatments. For example, seaweed, which is rich in iodine, was prescribed for the treatment of goitre (enlargement of the thyroid gland). Perhaps the earliest demonstration in humans of direct endocrinologic intervention was the castration of men who could then be relied upon, more or less, to safeguard the chastity of women living in harems. During the Middle Ages and persisting well into the 19th century, it was a popular practice to castrate pubertal boys to preserve the purity of their treble voices. Castration established the testicle as the source of substances responsible for the development and maintenance of maleness. This knowledge led to an abiding interest in restoring or enhancing male sexual powers. John Hunter, an 18th-century Scottish surgeon, anatomist, and physiologist who practiced in London, transplanted successfully the testis (testicle) of a rooster into the abdomen of a hen. Charles-douard Brown-Squard, a 19th-century French neurologist and physiologist, asserted that testes contained an invigorating, rejuvenating substance. His conclusions were based, in part, on observations obtained after he had injected himself with an extract of the testicle of a dog or of a guinea pig to which water had been added. These experiments were advances in that they resulted in the widespread use of organ extracts (organotherapy). Modern endocrinology, however, is largely a creation of the 20th century. Its scientific origin is firmly rooted in the studies of Claude Bernard (181378), a brilliant French physiologist who made the key observation that complex organisms, such as humans, go to great lengths to preserve the constancy of what he called the milieu intrieur (internal environment). Later, an American physiologist, Walter Bradford Cannon (18711945), used the term homeostasis to describe this inner constancy. The endocrine system, in association with the nervous system and the immune system, regulates the body's internal activities and external interactions to preserve the static internal environment. This control system permits the prime functions of living organismsgrowth, development, and reproductionto proceed in an orderly, stable fashion; it is exquisitely self-regulating so that any disruption of the normal internal environment by internal or external events is resisted by powerful countermeasures. When this resistance is overcome, sickness ensues. Additional reading General works A comprehensive historical and biographical survey is provided by Victor Cornelius Medvei, A History of Endocrinology (1982). Comprehensive standard texts include Leslie J. DeGroot et al. (eds.), Endocrinology, 3 vol. (1979); and Francis S. Greenspan and Peter H. Forsham (eds.), Basic & Clinical Endocrinology, 2nd ed. (1986). For modern research in the field, see Recent Progress in Hormone Research: Proceedings of the Laurentian Hormone Conference (irregular); and Current Therapy in Endocrinology and Metabolism (biennial). Peter H. Wise, Endocrinology (1986), is a useful atlas.Briefer coverage is provided in Jay Tepperman and Helen M. Tepperman, Metabolic and Endocrine Physiology: An Introductory Text, 5th ed. (1987); Robert Volp (ed.), Autoimmunity and Endocrine Disease (1985); C. Donnell Turner and Joseph T. Bagnara, General Endocrinology, 6th ed. (1976); C.R. Kannan, Essential Endocrinology: A Primer for Nonspecialists (1986); E.D. Williams (ed.), Current Endocrine Concepts (1982); Brian K. Follett, Susumu Ishii, and Asha Chandola (eds.), The Endocrine System and the Environment (1985); and T.S. Danowski, Outline of Endocrine Gland Syndromes, 3rd ed. (1976). A survey of medical literature can be found in The Year Book of Endocrinology. Glands and hormones Seymour Reichlin, Ross J. Baldessarini, and Joseph B. Martin (eds.), The Hypothalamus (1978); Choh Hao Li (ed.), Hypothalamus Hormones (1979); Peter J. Morgane and Jaak Panksepp (eds.), Handbook of the Hypothalamus, 3 vol. in 4 (197981); Ajay S. Bhatnagar (ed.), The Anterior Pituitary Gland (1983); Peter H. Baylis and Paul L. Padfield (eds.), The Posterior Pituitary: Hormone Secretion in Health and Disease (1985); George T. Tindall, Daniel L. Barrow, and Joseph B. Martin, Disorders of the Pituitary (1986); Sidney H. Ingbar and Lewis E. Braverman (eds.), Werner's The Thyroid: A Fundamental and Clinical Text, 5th ed. (1986); Patrick J. Mulrow (ed.), The Adrenal Gland (1986); Russel J. Reiter (ed.), The Pineal Gland (1984); G.M. Brown and S.D. Wainwright (eds.), The Pineal Gland: Endocrine Aspects (1985); and R.J. Wurtman and F. Waldhauser (eds.), Melatonin in Humans (1986). Gynecological and reproductive endocrinology Samuel S.C. Yen and Robert B. Jaffe, Reproductive Endocrinology: Physiology, Pathophysiology, and Clinical Management, 2nd ed. (1986); Philip Rhodes, Reproductive Physiology (1969); Daniel R. Mishell, Jr., and Val Davajan (eds.), Infertility, Contraception, & Reproductive Endocrinology, 2nd ed. (1986); Kyoichiro Ochiai et al. (eds.), Endocrine Correlates of Reproduction (1984); John E. Tyson (ed.), Neuroendocrinology of Reproduction (1978); and Eugene D. Albrecht and Gerald J. Pepe (eds.), Perinatal Endocrinology (1985). Diabetes mellitus and hypoglycemia Sydney S. Lazarus and Bruno W. Volk, The Pancreas in Human and Experimental Diabetes (1962); Bruno W. Volk and Edward R. Arguilla (eds.), The Diabetic Pancreas, 2nd ed. (1985); Elliott P. Joslin, Joslin's Diabetes Mellitus, 12th ed., edited by Alexander Marble et al. (1985); Mayer B. Davidson, Diabetes Mellitus: Diagnosis and Treatment, 2nd ed. (1986); Bernard N. Brodoff and Sheldon J. Bleicher (eds.), Diabetes Mellitus and Obesity (1982); and Dorothy Reycroft Hollingsworth, Pregnancy, Diabetes, and Birth (1984). For current research, see Diabetes (monthly). Neuroendocrinology Bernard T. Donovan, Hormones and Human Behaviour (1985); Kenneth W. McKerns and Vladimir Pantic (eds.), Neuroendocrine Correlates of Stress (1985); Nandkumar S. Shah and Alexander G. Donald (eds.), Psychoendocrine Dysfunction (1984); Derek Gupta, Patrizia Borrelli, and Andrea Attanasio (eds.), Paediatric Neuroendocrinology (1985); Joseph B. Martin and Seymour Reichlin, Clinical Neuroendocrinology, 2nd ed. (1987); and Joseph Meites (ed.), Neuroendocrinology of Aging (1983). For current research, see Frontiers in Endocrinology (irregular). Theodore B. Schwartz General features The ovary Anatomy The ovaries, in addition to producing egg cells (ova), secrete and are acted upon by various The ovaries are multipurpose organs. They harbour, nurture, and guide the development of the egg so that when it is extruded from the ovary (ovulation) it has been prepared for its migration down the fallopian tube, its penetration by sperm, and its eventual implantation in the wall of the uterus. Additionally, the ovary is a sophisticated endocrine structure. It secretes hormones essential for the onset of menstruation (menarche) and its cyclical perpetuation. At the same time, the ovary produces profound alterations in body physique that transform a prepubertal girl into a mature woman. The mature ovary is a roughly bean-shaped structure weighing about 14 grams. It, like the adrenal gland, consists of an outer cortex and a central medulla with the addition of an inner hilus (depression or pit) that serves as the point of entry and exit of blood vessels and nerves. The ovaries are located in the pelvis, attached to a structure called the broad ligament (see reproductive system, human). Immature follicles (primordial follicles) embedded in fibrous tissue (stroma) enlarge as the follicle matures and moves through the cortex toward the outer surface of the ovary. The cells lining the follicle multiply and become layered into a zona granulosa. Along with this change the stromal cells immediately surrounding the follicle arrange themselves concentrically to form a theca (an enclosing sheath). This egg-containing mature structure is known as a Graafian follicle. Both granuloma cells and thecal cells secrete steroid hormones known as estrogens. The follicular fluid bathing the ovum is an extraordinarily complex liquid containing not only high concentrations of estrogens but also other steroids (progestogens and androgens), pituitary hormones (FSH, LH, prolactin, oxytocin, and vasopressins), and numerous enzymes and bioactive proteins. During the maturation (follicular) phase of the menstrual cycle, follicles continue to enlarge until one (or, rarely, two) follicles rupture at the ovarian surface. The egg is extruded and promptly enters the fallopian tube to begin its journey to the uterus. The supportive role of the follicle does not end with the discharge of the egg. Thecal cells penetrate the emptied follicle and, together with persisting but modified granulosa cells, fill the follicle, now called a corpus luteum, which is the source of serum progesterone during the postovulatory (progestational or luteal) phase of the menstrual cycle. With menstruation, the corpus luteum becomes scarred and contracted (atretic), remaining as a corpus albicans. In the event that the extruded egg is fertilized and pregnancy ensues, the corpus luteum persists and continues to secrete increasing amounts of progesterone during the first trimester. As might be expected, these changes are controlled by secretions from the hypothalamus and the anterior pituitary gland. Regulation of hormone secretion Before the onset of puberty the ovaries are quiescent, and the stroma of the cortex and medulla are studded with multiple primordial follicles. Puberty is heralded by subtle but far-reaching changes. Some undefined event stimulates the secretion of luteinizing hormone-releasing hormone (GnRH) from the hypothalamus, and GnRH secretion becomes pulsatile. Animal studies support the notion that puberty is precipitated by a reduction in the secretion of melatonin, a hormone of the pineal gland. There is, however, no evidence that melatonin has a role in the onset of puberty in humans. Secretion of GnRH activates gonadotrophs from the anterior pituitary, resulting in enhanced secretion of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The secretion of these hormones, particularly LH, is much enhanced shortly after the onset of sleep; increased nocturnal secretion of LH is the earliest change detectable in the pubertal child. It appears that GnRH secretion is inhibited by neurons that secrete dopamine and is stimulated by noradrenergic neurons (involved with norepinephrine). Endogenous opiates, especially beta-endorphin and dynorphin, also play important roles in regulating the frequency and strength of GnRH secretion. The increased secretion of estrogens from the ovaries, stimulated by LH secretion coupled with maturing Graafian follicles (resulting from the increased FSH secretion) leads to menarche. Before long, the cyclic activity characteristic of the normal female hypothalamus appears. Immediately following the cessation of menstruation, the sequence begins with a gradual rise in the blood level of estradiol (the most potent of the estrogens), paralleled by a slow rise in serum LH. An inconspicuous rise in androgens also occurs while progesterone and its precursor, 17-hydroxyprogesterone, remain suppressed. Finally, the rising estradiol level trips off a mid-cycle surge of LH and FSH (an example of a positive feedback mechanism). The abrupt rise in gonadotropins precipitate ovulation, ending the follicular phase. With the formation of the corpus luteum, estrogen levels fall but not back to baseline, while the levels of 17-hydroxyprogesterone and progesterone are much elevated. At the end of the luteal phase all hormonal levels return to baseline, and the withdrawal of the estrogens precipitates the next menstrual period. The normal menstrual cycle is 28 days long although it varies considerably from one woman to another and occasionally in the same woman, with irregularities occurring most frequently shortly after puberty or before the menopause. The premenstrual fall in levels of estrogen and progestins occurs because of a degeneration and loss of function of the corpus luteum (luteolysis) that results from a faltering of LH pulses from the pituitary. The endometrium, which had become increasingly thickened and vascular, undergoes a constriction of small arteries. Cutting off oxygen and nutrient supplies to the endometrial lining leads to cell death and the subsequent sloughing and bleeding characteristic of menstruation. It should be noted that the basal body temperature, which fluctuates only mildly during the follicular phase, shows a rather abrupt progressive rise after ovulation, paralleling the increase in progesterone. This thermogenic action results from the effect of the elevated progesterone levels on temperature-regulating centres in the base of the brain. (The structural and functional changes that occur in the fallopian tubes [oviducts], lining of the uterus [endometrium], distal opening to the uterus , and vagina and that accompany the endocrinologic fluctuations are discussed in the article reproductive system, human. The extension and accentuation of these changes, which occur in the event of pregnancy, are also discussed there.) General features Growth and development The processes of growth and development are usually accepted as facts of everyday life; however, when one considers the powerful forces at work and the many harmoniously intermingled regulators that harness them, the emergence of a mature adult human being is a source of wonder. The carefully monitored conversion of a crude mixture of nutrients, often ill-balanced, into growing body tissues is integral to the purview of the endocrine system, although the nervous and immune systems play important roles as well. From the 10th to the 20th week of pregnancy, the fetus grows at a rate of 52 inches (132 centimetres) per year. This phenomenal growth rate tapers rapidly as birth approaches. Weight at birth is an important marker. Low birth weight is not surprising in infants coming from families whose histories include low birth weight, but it may also be an indication of premature birth or of poor intrauterine nourishment from a mother living in poverty or with poor hygiene. Growth during infancy remains rapid and then progresses at a slower but steady rate until the onset of puberty, when there is a striking acceleration. The pubertal growth spurt lasts about two years, and it is accompanied by the appearance of secondary sexual characteristics. With puberty, there ensues an increase in nocturnal secretion of growth hormone. Endocrine influences Accurate estimates of bone age are made by examining radiographs (a film record of a structure using X rays) of the hands and wrists of large numbers of normal children. In children with endocrine disorders, bone age may not correlate closely with chronological age; bone age is retarded in growth hormone-deficient children and increases in children with growth hormone-producing tumours. Hyperthyroidism, even when it occurs in the developing embryo, is associated with an advanced bone age, while the opposite is true with thyroid deficiency. Children with Cushing's syndrome not only have osteoporosis but retarded bone age as well. An excess both of androgens and of estrogens is associated with a relatively advanced bone age, while a partial androgen deficiency leads to an increase in prepubertal growth of the extremities, resulting in adults with long arms and long legs attached to a short trunk (eunuchoid habitus). Insulin is a potent growth hormone, and childhood diabetics are notoriously small for their ages. Indeed, like hypothyroid children, some never advance to the pubertal state unless proper insulin replacement therapy is provided. General features General aspects Integrative functions The endocrine systems of humans and other animals serve an essential integrative function. Inevitably, humans are beset by a variety of insults, such as trauma, infection, tumour formation, genetic defects, and emotional damage. The endocrine glands play a key role in responding to these stressful stimuli. Less obvious are the effects of subtle changes in the concentrations of key elements of the body's fluids on the storage and expenditure of energy and the steady and timely growth and development of a normal human being. These more subtle changes largely result from the monitoring by and the response, sometimes minute by minute, of the endocrine system. The menstrual cycle in the normal, mature female and the reproductive process in males and females are under endocrine control. Beyond this, lactation and probably some forms of parental behaviour are strongly influenced by endocrine secretions. The endocrine system works in concert with the nervous and the immune systems to permit the orderly progression of human life, and these systems provide the body's bulwark against threats to health and life. Anatomical considerations There are some characteristics shared by all endocrine glands. Some glands, for example the thyroid gland, are discrete, readily recognized organs with defined borders that are easily separable from adjacent structures. Others are embedded in other structures (for example, the islets of Langerhans are found in the pancreas) and may be clearly seen only under the microscope. The boundaries of endocrinology, however, have yet to be sharply defined, and endocrine tissue has been identified in surprising locations, such as the heart. Under the microscope, endocrine cells appear to be rather homogeneous, usually cuboidal in shape, with a rich supply of small blood vessels. Sometimes, as is the case in the thyroid gland, endocrine cells are intermixed with other, distinctly different endocrine cells with a different embryological origin and an entirely different set of hormonal secretions. Finally, all nerve cells are capable of secreting neurotransmitters into the synapses between adjacent nerves, although some nerve cells, for example those of the neurohypophysis (posterior pituitary gland), also secrete neurohormones directly into the bloodstream. Endocrine glands with mixed cell populations have not evolved by chance. The hormonal secretions of one set may modulate directly the activity of adjacent cells with different characteristics. This direct action on contiguous cells of different types, which diffuses the hormone to target cells without moving it through the general circulation, is known as paracrine function. Even in homogeneous glandular tissues (i.e., tissues comprising one cell type), the direct proximity of the cells in some way enhances the amount of hormonal secretions since isolated cells are less vigorous in their activity, under laboratory conditions, than are sheets of attached cells, a phenomenon known as autocrine function. On the other hand, hormonal secretions themselves inhibit further hormonal secretion if they remain in the vicinity of the parent cell. Figure 1: Intracellular structure of a typical endocrine cell. When viewed under the ultramicroscope (a microscope of extraordinary magnifying power), the endocrine cell has the fine structure that is illustrated in Figure 1. Many of the various intracellular structures, called organelles, are involved in the sequence of events that occurs during the synthesis and secretion of hormones. General features The anterior pituitary Anatomy The pituitary gland lies at the base of the skull, nestled in a bony structure called the sella turcica. The gland is attached to the hypothalamus by the pituitary stalk, around and through which course the veins of the hypophyseal-portal plexus. In most species the gland is divided into three lobes: anterior, intermediate, and posterior. In humans the intermediate lobe does not exist as a distinct anatomic structure but rather remains only as dispersed cells. Despite its proximity to the anterior pituitary, the posterior lobe of the pituitary is functionally distinct and is an integral part of a separate neural structure called the neurohypophysis (see below The posterior pituitary [neurohypophysis]: Neurohypophyseal unit). The cells comprising the anterior lobe are derived embryologically from an extension of the roof of the pharynx, known as Rathke's pouch. While the cells appear to be relatively homogeneous (of the same type) under a light microscope, there are in fact five different types, each of which, except in pathological states, secretes the same hormone or hormones throughout its existence. The thyrotroph synthesizes and secretes thyrotropin (thyroid-stimulating hormone, or TSH); the gonadotroph, both LH and FSH; the corticotroph, corticotropin (also called adrenocorticotropic hormone, or ACTH); the somatotroph, somatotropin (also called growth hormone, GH); and the lactotroph, prolactin (PRL). These hormones are proteins that consist of large polypeptide chains. Furthermore, the gonadotropins and TSH are glycoproteins in which there is linkage to carbohydrates known as polysaccharides. Each of these three hormones is composed of two glycopeptide chains; one of which, the alpha chain, is identical in all three hormones, while the other, the beta chain, differs in structure for each hormone, lending specificity for individual hormone action. As is the case in all protein hormones, hormones of the anterior pituitary are synthesized initially in the cytoplasm of the cell as larger, inactive molecules called prohormones, which are split into the active hormone molecules at the time of secretion into the circulation. Hormones Thyrotropin Thyrotropin is also called thyroid-stimulating hormone (TSH). Thyrotropin-producing cells (thyrotrophs) make up about 10 percent of the anterior pituitary and are located mainly in the centre of the gland. Thyrotropin becomes attached firmly to receptors on the surface of the thyroid cells, forming thyroid follicles in the thyroid gland. Following binding, a complex train of events occurs so that preformed thyroid hormones are secreted and steps are set in motion for the synthesis of additional thyroid hormones. Thyrotropin exerts other pervasive effects. It stimulates the growth of thyroid cells and leads to increased blood flow through the gland. It also enhances the breakdown of thyroglobulin, a large thyroid-hormone-containing glycoprotein that is stored within the follicles of the thyroid gland. The levels of thyrotropin in circulating fluids become elevated during thyroid hormone deficiency because there is no negative feedback inhibition of pituitary thyrotropin release by thyroid hormone. Elevated thyrotropin levels are found in other pathological states, including the presence of a thyrotropin-producing pituitary tumour. Low serum thyrotropin levels occur following damage to cells in the hypothalamus that produce thyrotropin-releasing hormone (TRH), following damage to the pituitary stalk, or, finally, following damage to the thyrotrophs themselves. Tests of increased sensitivity have made the measurement of thyrotropin in blood valuable in detecting subtle changes of both thyroid hyperfunction and hypofunction. General features The thyroid gland All animal life requires oxygen for sustenance, and the human species is no exception. Oxygen drives the basic metabolic processes that permit growth, development, reproduction, physical movement, and constant body temperature. The complex of chemical interactions necessary to sustain these processes is called metabolism, and the prime, overall regulators of metabolism are the thyroid hormones. Anatomy The thyroid gland is located in the anterior part of the neck in the midline. It consists of two lateral lobes lying on each side of the thyroid cartilage (Adam's apple) and connected by a band of tissue called the isthmus. It is one of the larger endocrine glands, and its capacity to grow is phenomenal. Any enlargement of the thyroid, regardless of cause, is called a goitre. The thyroid arises in the embryo from a downward outpouching of the floor of the fetal pharynx, and a persisting remnant of this migration is known as a thyroglossal duct. If viewed under a three-dimensional microscope, the resting thyroid is seen as a collection of small, generally globular sacs, called follicles, filled with the prohormone thyroglobulin. The cells lining these globules are called follicular cells, and it is their function to synthesize thyroid hormones as part of the prohormone thyroglobulin and either to secrete them directly into the circulation or store them within the follicles. When the individual's requirement for thyroid hormone increases, thyroglobulin is split into its component parts, and the thyroid hormone thus released passes through the follicular cells to enter the circulation. Nestled in the spaces between the follicles are parafollicular cells. These, in essence, form a separate endocrine organ. They have an entirely distinct embryological origin, and they are not embedded in the substance of the thyroid gland, in many species other than man (see below The parathyroid glands: Calcitonin). General features The pancreas The discovery of insulin in 1921 by a Canadian surgeon, Frederick Banting, with the assistance of a medical student, Charles Best, was one of the most dramatic events in modern medicine. It not only saved the lives of innumerable patients affected with childhood diabetes but it also ushered in present-day understanding of the complexities of the endocrine pancreas. The importance of the endocrine pancreas lies in the fact that its principal hormone, insulin, plays a central role in the regulation of energy metabolism and that a relative or absolute deficiency of insulin leads to diabetes mellitus, still a leading cause of disease and death throughout the world. Anatomy In humans the pancreas weighs approximately 80 grams, has roughly the configuration of an inverted smoker's pipe, and is situated in the upper abdomen. The head of the pancreas (equivalent to the bowl of the pipe) is immediately adjacent to the duodenum, while its body and tail extend across the midline nearly to the spleen. The bulk of pancreatic tissue is devoted to its exocrine function, the elaboration of digestive enzymes that are secreted via the pancreatic ducts into the duodenum. The endocrine pancreas consists of the islets of Langerhans. Approximately 1,500,000 islets, weighing about one gram in total, are scattered throughout the gland. The embryonic origin of the cells that make up the islets is not clear; both endodermal and neuroectodermal precursors have been proposed. Approximately 75 percent of the cells in each islet are the insulin-secreting beta (B) cells, which tend to cluster centrally. Around the periphery lie the alpha (A), delta (D), and F (or PP) cells, which secrete glucagon, somatostatin, and pancreatic polypeptide, respectively. Each islet is supplied by one or two minute arteries that branch into numerous capillaries; from this network, capillaries emerge to coalesce into small veins outside the islet. The islets also are richly supplied with autonomic nerves. Thus, islet function may be modulated by neural control, by circulating metabolites and hormones, and by secretion of hormones locally (paracrine effects). The principal function of the endocrine pancreas is the secretion of insulin and other polypeptide hormones necessary for the orderly cellular storage and retrieval of such dietary nutrients as glucose, amino acids, and triglycerides. General features The adrenal cortex Anatomy The adrenal glands lie on the upper inner surface of each kidney. Each gland consists of two parts that are quite distinct anatomically, embryologically, and functionally. The inner core (adrenal medulla) is discussed separately below. The outer covering (adrenal cortex) is derived from the fetal mesodermal ridge, a structure that also gives rise to the kidneys so that the juxtaposition of the two organs is not surprising. Within the adrenal cortex are three zones known as the outer (zona glomerulosa), the middle (zona fasciculata), and the inner (zona reticularis). Under the microscope the cells are rather typical endocrine cells; the distinction between zones is made by differing staining characteristics. Hormones Adrenocortical cells synthesize and secrete chemical derivatives (steroids) from cholesterol, the major animal sterol. While cholesterol can be synthesized in many body tissues, further differentiation into steroid hormones takes place only in the adrenal cortex and in its embryological cousins, the ovaries and the testes. The adrenal cortex is capable of synthesizing all of the steroid hormones produced by the body, including the progestogens and estrogens (see below The ovary), androgens (see below The testis), mineralocorticoids (which are secreted from the zona glomerulosa), and glucocorticoids (which are synthesized and released from the zona fasciculata and zona reticularis of the adrenal cortex). Although upwards of 60 steroids are manufactured in the adrenal cortex, only a few members of these three major categories are important in body functioning.