The brain is the controller of the nervous system, but it is also
one of the most important endocrine glands. Specialized nerve cells, notably in
the hypothalamus, synthesize hormones which are transported along the axon to
the nerve terminal. Here they are released into the portal blood system, which
carries them to the pituitary gland. In some cases, the axon of the neuroendocrine
cell projects down to the pituitary cell itself. The principal hypothalamic
neurohormones are:
1 corticotrophin-releasing
hormone (CRH), controls the release
of ACTH;
2 dopamine
inhibits prolactin release;
3 growth-hormone-releasing
hormone (GHRH) causes growth hormone release;
4 somatostatin
inhibits growth hormone release;
5 gonadotrophin-releasing
hormone (GnRH) causes luteinizing hormone (LH) and follicle-stimulating
hormone (FSH) release;
6 thyrotrophin-releasing
hormone (TRH) causes thyroid- stimulating hormone (TSH) release;
7 oxytocin
causes milk ejection and contraction of the uterus in labour – it is
synthesized in the hypothalamus and is stored in and released from the
posterior pituitary gland;
8 vasopressin
(antidiuretic hormone, ADH) promotes water reabsorption from the kidney
tubules – it is synthesized in the hypothalamus, and stored in and released
from the posterior pituitary gland.
The pituitary gland is composed of two lobes, anterior and
posterior, which arise from different embryological origins – the anterior
originates from the embryonic oral cavity and the posterior from the base of
the brain (i.e. a neural origin). The two lobes become closely apposed to each
other to form the pituitary gland. Humans have a non-functional intermediate
lobe, which is much larger in some other animals. The principal hormones of the pituitary are:
1. anterior:
1. corticotrophin
(adrenocorticotrophic hormone; ACTH) releases glucocorticoids and other
steroids from the adrenal cortex;
2. follicle-stimulating
hormone (FSH) promotes spermatogenesis in males andovarian follicular maturation in females;
3. luteinizing
hormone (LH) promotes testosterone synthesis in males and causes ovarian
follicular rupture and ovulation in females;
4. prolactin
(PRL) promotes lactation and may have an immunomodulatory role in
non-lactating females and males;
5. thyrotrophin
(thyroid-stimulating hormone; TSH) pro- motes thyroid hormone production
and release from the thyroid gland;
6. growth
hormone (also called somatotrophin; GH) pro- motes muscle and skeletal
growth.
2. posterior:
(a) oxytocin causes milk ejection and
contraction of the uterus in labour;
(b) vasopressin (antidiuretic hormone, ADH)
promotes water reabsorption from the renal tubules.
The thyroid gland is situated just in front of the trachea in
humans. The thyroid-hormone-producing cells are arranged in follicles, and
concentrate iodine which is used for the synthesis of the thyroid hormone. The
circulating hormones are thyroxine (T4) and tri-iodothyronine (T3).
The parathyroid glands are embedded in the thyroid, and produce parathyroid
hormone (parathormone; PTH). PTH is important in the control of calcium and
phosphate metabolism. The parafollicular cells are in the thyroid,
scattered between the follicles. They produce calcitonin, which inhibits
bone calcium resorption.
The adrenal glands are situated just above the kidneys, and are
composed of an outer layer, or cortex, and an inner layer, or medulla (a
modified ganglion). The hormones produced are:
1.
cortex:
(a) glucocorticoids, principally cortisol in
humans, are involved in carbohydrate metabolism and the response to stress;
(b)
mineralocorticoids, principally
aldosterone, control electrolyte balance;
(c) androgens, principally testosterone,
dehydroepiandrostenedione sulphate (DHEAS) and 17-hydroxyprogesterone, modulate
secondary sexual characteristics and have anabolic effects.
2.
medulla:
(a)
epinephrine modulates cardiovascular and
metabolic response to stress;
(b) norepinephrine, principally a
neurotransmitter in the peripheral sympathetic nervous system;
(c) dopamine, a neurotransmitter in the
autonomic nervous system.
The endocrine pancreas consists of islet cells scattered in the
larger exocrine pancreas, which lies posteriorly in the upper abdomen.
(‘Exocrine’ refers to glands which have ducts, and which are not covered in
this book.) The endocrine pancreas secretes:
1
insulin, which regulates glucose and
lipid metabolism;
2
glucagon, a counter-regulatory
hormone to insulin that elevates blood glucose;
3
somatostatin, which regulates
gastrointestinal motility;
4
pancreatic polypeptide, which
regulates gastrointestinal secretion.
The ovary is the major female reproductive gland, and produces:
1 estrogens, which regulate reproductive
function and secondary sexual characteristics;
2 progesterone, which stimulates
endometrial vascularization and maintains pregnancy;
3 relaxin, a polypeptide also found in
the placenta and uterus, which may be important in parturition by softening the
cervix and relaxing the pelvic ligaments;
4 inhibin, which inhibits FSH
production.
The placenta is the organ of pregnancy serving the developing
fetus. Hormones produced by the placenta include:
(a) chorionic
gonadotrophin (CG; hCG; h = human) which maintains placental progesterone synthesis;
(b) placental
lactogen (PL);
(c) estriol,
the major form of estrogen secreted by the placenta;
(d) progesterone
which maintains the reproductive organs in pregnancy;
(e) relaxin.
The testis is the major male reproductive gland, producing:
1 testosterone which controls
reproductive function and secondary sexual characteristics;
2 inhibin, which inhibits FSH
secretion;
3 Müllerian inhibiting hormone (MIH),
a fetal hormone which dedifferentiates the Müllerian duct.
The gastrointestinal tract (GIT) is the largest endocrine organ
and produces several autocrine, paracrine and endocrine hormones including:
1
cholecystokinin (CCK);
2
gastric inhibitory peptide (GIP);
3
gastrin;
4
neurotensin;
5
secretin;
6
substance P;
7
vasoactive intestinal peptide (VIP).
Adipocytes produce the peptide hormone leptin which is important
in the control of feeding and energy expenditure.
The kidney produces hormones involved in the control of blood
pressure and in erythropoiesis. Renin cleaves angiotensinogen to
angiotensin I in the kidney and plasma. Erythropoietin stimulates
production of red blood cells in the marrow.
The skin, liver and kidney produce vitamin D which has certain
endocrine functions.
The heart produces atrial natriuretic peptide. Circulating
blood elements, including macrophages, produce peptides such as the
cytokines, which are involved in immune function.
The pineal gland is situated in the brain and is involved with
rhythms, for example the reproductive rhythms of animals which breed
seasonally. Its role in humans is not known for certain. The pineal gland
produces melatonin.
Readers should be aware that putative endocrine hormones continue to be reported.
