ENDOCRINE SYSTEM

The endocrine system consists of glands widely separated from each other with no physical
connection. Endocrine glands are groups of secretory cells surrounded by an extensive network of
capillaries that facilitates diffusion of hormones (chemical messengers) from the secretory cells
into the bloodstream. They are also referred to as ductless glands because hormones diffuse
directly into the bloodstream. Hormones are then carried in the bloodstream to target tissues and
organs that may be quite distant, where they influence cell growth and metabolism. Endocrine
glands are distinct from exocrine glands which release their secretions through ducts.

Diagram showing major endocrine glands

PITUITARY GLAND
The pituitary gland and the hypothalamus act as a unit, regulating the activity of most of the other
endocrine glands. The pituitary gland lies in the hypophyseal fossa of the sphenoid bone below the
hypothalamus. It is connected with the hypothalamus by the pituitary stalk or hypophyseal stalk.
It consists of two main parts which are; the anterior pituitary (adenohypophysis) and posterior
pituitary (neurohypophysis). Hormones from hypothalamus are transported to anterior pituitary
through portal blood vessels. But, the hormones from hypothalamus to posterior pituitary are
transported by bundle of nerve fibers. The hypothalamus controls release of hormones from both
the anterior and posterior pituitary but in different ways.

ANTERIOR PITUITARY
Anterior pituitary is also known as the master gland because it regulates many other endocrine
glands through its hormones. Hypothalamus controls anterior pituitary by secreting the releasing

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and inhibitory hormones (factors), which are called neurohormones. These hormones from
hypothalamus are transported to anterior pituitary through portal vessels.
Releasing and Inhibitory Hormones Secreted by Hypothalamus
1. Growth hormone-releasing hormone (GHRH): Stimulates the release of growth hormone.
2. Growth hormone-inhibitory hormone (GHIH) or somatostatin: Inhibits the growth hormone
release.
3. Thyrotropic-releasing hormone (TRH): Stimulates the release of thyroid stimulating hormone.
4. Corticotrophin-releasing hormone (CRH): Stimulates the release of adrenocorticotrophin.
5. Gonadotropin-releasing hormone (GnRH): Stimulates the release of gonadotropins, FSH and
LH.
Prolactin releasing hormone: stimulate the release of prolactin from the anterior pituitary gland.
7. Prolactin-inhibitory hormone (PIH): Inhibits prolactin secretion. It is believed that PIH is
dopamine.
Hormones Secreted by Anterior Pituitary
Six hormones are secreted by the anterior pituitary:
1. Growth hormone (GH) or somatotropic hormone (STH)
2. Thyroid-stimulating hormone (TSH) or thyrotropic hormone
3. Adrenocorticotropic hormone (ACTH)
4. Follicle-stimulating hormone (FSH)
5. Luteinizing hormone (LH) in females or interstitial cell-stimulating hormone (ICSH) in males
6. Prolactin.

Growth hormone (GH)

This is the most abundant hormone synthesized by the anterior pituitary. It stimulates growth and
division of most body cells but especially those in the bones and skeletal muscles. It also regulates
aspects of metabolism in many organs, e.g., liver, intestines and pancreas; stimulates protein
synthesis, especially tissue growth and repair; promotes breakdown of fats and increases blood
glucose levels. Its release is stimulated by growth hormone releasing hormone (GHRH) and
suppressed by growth hormone release inhibiting hormone (GHRIH), also known as somatostatin,
both of which are secreted by the hypothalamus.
Thyroid stimulating hormone (TSH)
The release of this hormone is stimulated by thyrotrophin releasing hormone (TRH) from the
hypothalamus. It stimulates growth and activity of the thyroid gland, which secretes the hormones
thyroxine (T4) and tri-iodothyronine (T3).
Adrenocorticotrophic hormone (ACTH)
ACTH is necessary for the structural integrity and the secretory activity of adrenal cortex.
Corticotrophin releasing hormone (CRH) from the hypothalamus promotes the synthesis and

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release of ACTH by the anterior pituitary. This increases the concentration of cholesterol and
steroids within the adrenal cortex and the output of steroid hormones, especially cortisol.
Follicle stimulating hormone (FSH)
Stimulates production of sperm in the testes, stimulates secretion of oestrogen by the ovaries,
maturation of ovarian follicles, ovulation.
Luteinising hormone (LH)
Stimulates secretion of testosterone by the testes, stimulates secretion of progesterone by the
corpus luteum.
Prolactin
This hormone is secreted during pregnancy to prepare the breasts for lactation (milk production)
after childbirth. The blood level of prolactin is stimulated by prolactin releasing hormone (PRH)
released from the hypothalamus and it is lowered by prolactin inhibiting hormone (PIH,
dopamine).

POSTERIOR PITUITARY
The posterior pituitary is formed from nervous tissue and consists of nerve cells surrounded by
supporting glial cells called pituicytes. These neurons have their cell bodies in the hypothalamus
and their axons are present in the posterior pituitary gland. Posterior pituitary hormones are
synthesized in the nerve cell bodies, transported along the axons and stored in vesicles within the
axon terminals in the posterior pituitary. Nerve impulses from the hypothalamus trigger exocytosis
of the vesicles, releasing their hormones into the bloodstream. Oxytocin and antidiuretic hormone
(ADH, vasopressin) are the hormones released from axon terminals within the posterior pituitary.

Diagram showing anterior and posterior pituitary lobes of the pituitary gland

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Oxytocin
Oxytocin stimulates two target tissues during and after childbirth (parturition): uterine smooth
muscle and the muscle cells of the lactating breast. During childbirth increasing amounts of
oxytocin are released from the posterior pituitary into the bloodstream to stimulate uterine
contraction. After delivery, oxytocin is also released to stimulate the contraction of the milk duct
resulting to ejection of milk from the mammary gland.
Oxytocin levels rise during sexual arousal in both males and females. This increases smooth
muscle contraction which is associated with glandular secretion and ejaculation in males. In
females, contraction of smooth muscle in the vagina and uterus promotes movement of sperm
towards the uterine tubes. It is believed that the smell of oxytocin may be involved in social
recognition and bonding (between mother and newborn baby).
Antidiuretic hormone (ADH, vasopressin)
The main effect of antidiuretic hormone is to reduce urine output. ADH acts on the distal
convoluted tubules and collecting ducts of the nephrons of the kidneys. It increases their
permeability to water and more of the glomerular filtrate is reabsorbed. After severe blood loss,
ADH can also causes smooth muscle contraction, especially vasoconstriction in small arteries.
This has a pressor effect, raising systemic blood pressure; the alternative name of this hormone,
vasopressin, reflects this effect.

THYROID GLAND
The thyroid gland is situated in the neck in front of the larynx and trachea. It resembles a butterfly
in shape, consisting of two lobes joined at the isthmus. The gland is composed of largely spherical
follicles formed from cuboidal epithelium. These secrete and store colloid, a thick sticky protein
material. Between the follicles are other cells called parafollicular cells or C-cells.
The arterial blood supply to the gland is through the superior and inferior thyroid arteries. The
venous return is by the thyroid veins, which drain into the internal jugular veins.

Thyroid Gland

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Thyroxine and Tri-iodothyronine
Iodine is essential for the formation of the thyroid hormones, thyroxine (T4) and tri-iodothyronine
(T3). The main dietary sources of iodine are seafood, vegetables grown in iodine-rich soil and
iodinated table salt.
Thyroid hormones are synthesized as large precursor molecules called thyroglobulin, the major
constituent of colloid. The release of T3 and T4 into the blood is stimulated by thyroid stimulating
hormone (TSH) from the anterior pituitary. Dietary iodine deficiency greatly increases TSH
secretion causing proliferation of thyroid gland cells and enlargement of the gland (goiter).
Between the two thyroid hormones, T4 is much more abundant. However, it is less potent than T3,
which is more physiologically important. Most T4 is converted into T3 inside target cells.
Thyroid hormones enhance the effects of other hormones, e.g., adrenaline (epinephrine) and
noradrenaline(norepinephrine). T3 and T4 affect most cells of the body by:
• Increasing the basal metabolic rate and heat production
• Regulating metabolism of carbohydrates, proteins and fats.
T3 and T4 are essential for normal growth and development, especially of the skeleton and nervous
system.
Calcitonin
This hormone is secreted by the parafollicular or C-cells in the thyroid gland. Calcitonin lowers
raised blood calcium (Ca2+) levels. It does this by acting on:
• Bone cells promoting their storage of calcium
• Kidney tubules inhibiting the reabsorption of calcium.

PARATHYROID GLAND
Normally there are four parathyroid glands in humans; they are located immediately behind the
thyroid gland. Each parathyroid gland is about 6 millimeters long, 3 millimeters wide, and 2
millimeters thick and has a macroscopic appearance of dark brown fat. They contain mainly chief
cells and a small to moderate number of oxyphil cells, but oxyphil cells are absent in many animals
and in young humans.
The chief cells are believed to secrete most, if not all, of the Parathyroid hormone or parathormone.
The function of the oxyphil cells is not certain, but the cells are believed to be modified or depleted
chief cells that no longer secrete hormone. The number of oxyphil cells increases after puberty.
Function: Parathormone (PTH) plays an important role in maintaining blood calcium level.
Primary action of PTH is to maintain the blood calcium level within the critical range of 9 to 11
mg/dL. It maintains the blood calcium level by acting on bones, kidneys and gastrointestinal tract.
It also controls blood phosphate level.

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 ADRENAL GLAND
There are two adrenal glands. Each gland is situated on the superior pole of each kidney. Based
on the position of the gland they can also be called suprarenal glands. Each gland weighs about
4 g. The adrenal glands have two distinct parts namely; adrenal cortex and adrenal medulla.

ADRENAL CORTEX
The adrenal cortex is the outer portion of the gland, constituting about 80% of the gland. It
develops from the mesonephros, which give rise to the renal tissue. It secretes hormones known
as corticosteroids. Adrenal cortex is formed by three distinct layers of structure;
1. Zona glomerulosa: This is the outer zone and it constitutes about 15% of the mass of the gland.
The cells of this zone are able to form new cells that regenerate the other zones. Cells of this layer
secrete mineralocorticoids such as aldosterone and Deoxycorticosterone. Mineralocorticoids are
the corticosteroids that act on the minerals (electrolytes), particularly sodium and potassium. 90%
of mineralocorticoid activity is provided by aldosterone. Aldosterone is very essential for life and
it maintains the osmolarity and volume of ECF. Aldosterone has three important functions, It
increases:
• Reabsorption of sodium from renal tubules.
• Excretion of potassium through renal tubules.
• Secretion of hydrogen into renal tubules.

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2. Zona fasciculata: This is the middle zone. It is the largest zone since it constitutes about 50 %
or the mass of the gland. The cells in this layer secrete glucocorticoids such as cortisol, cortisone
and corticosterone. Glucocorticoids act mainly on glucose metabolism. Cortisol or hydrocortisone
is more potent and it has 95% of glucocorticoid activity. Aldosterone is a life-saving hormone,
whereas cortisol is a life-protecting hormone because, it helps to withstand the stress and trauma
in life. Glucocorticoids have metabolic effects on carbohydrates, proteins, fats and water.
3. Zona reticularis: This is the inner zone. It is the smallest zone since it constitutes only about
7% of the mass or the gland. The cells of this layer secrete sex hormones such as androgens
(testosterone), which the main sex hormone secreted by adrenal cortex. Oestrogen and
progesterone are produced in small quantity. Androgens, in general, are responsible for masculine
features of the body. But in normal conditions, the adrenal androgens have insignificant
physiological effects, because of the low amount of secretion both in males and females.

ADRENAL MEDULLA
Adrenal medulla is the inner part of adrenal gland and it forms 20% of the mass of adrenal gland.
It develops from the neural crest, which give rise to the nervous system. It is made up of interlacing
cords of cells known as chromaffin cells. The cells of the adrenal medulla secrete hormones
such as epinephrine/adrenaline, norepinephrine/noradrenaline and dopamine.
Adrenaline and noradrenaline stimulate the nervous system. Adrenaline has significant effects on
metabolic functions and both adrenaline, noradrenaline and dopamine have significant effects on
cardiovascular system.

THE ENDOCRINE PANCREAS

The pancreas is a gland that consists of 2 different types of tissues namely; pancreatic acini and
the islet of Langerhans. The pancreatic acini are responsible the exocrine function of the pancreas,
which is secretion of pancreatic juice and the endocrine function of the pancreas is performed by
the islets of Langerhans.

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 Structure of the pancreas and it secretory cells
Human pancreas contains about1 to 2 million islets. Islets of Langerhans consist of four types of
cells:
1. A cells or α-cells: these cells secrete glucagon. Glucagon increases blood glucose levels by
stimulating:
• Conversion of glycogen to glucose in the liver and skeletal muscles (glycogenolysis).
• Gluconeogenesis (synthesis of glucose from non-carbohydrate sources e.g. amino acid,
fat, etc).
2. B cells or β-cells: these cells secrete insulin. Its main function is to lower raised blood nutrient
levels, not only glucose but also amino acids and fatty acids. When nutrients, especially glucose,
are in excess of immediate needs insulin promotes their storage by:
• Stimulating uptake and use of glucose by muscle and connective tissue cells
• Increasing conversion of glucose to glycogen (glycogenesis), especially in the liver and
skeletal muscles
3. D cells or δ-cells: these cells secrete somatostatin. This hormone, also produced by the
hypothalamus, inhibits the secretion of both insulin and glucagon in addition to inhibiting the
secretion of growth hormone from the anterior pituitary.
4. F cells or PP cells: these cells secrete pancreatic polypeptide. Exact physiological action of
pancreatic polypeptide is not known. It is believed to increase the secretion of glucagon from α-
cells in islets of Langerhans.

PINEAL GLAND
Pineal gland or epiphysis is situated above the hypothalamus. It is a small cone shaped structure
with a length of about 10 mm. The pineal gland is a small body attached to the roof of the third
ventricle and is connected to it by a short stalk containing nerves, many of which terminate in the

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hypothalamus. Pineal gland has two types of cells: Large epithelial cells called parenchymal cells
and neuroglial cells. In adults, the pineal gland is calcified. But, the epithelial cells exist and secrete
the hormonal substance known as melatonin.
Function: melatonin is believed to be associated with:
• Coordination of the circadian and diurnal rhythms of many tissues, possibly by influencing the
hypothalamus.
• Inhibition of growth and development of the sex organs before puberty, possibly by preventing
synthesis or release of gonadotrophins.

THYMUS GLAND
Thymus is situated in front of trachea, below the thyroid gland. Thymus is small in newborn infants
and gradually enlarges till puberty and then decreases in size. The endocrine function of the thymus
gland is secretion of two hormones namely; thymosin and thymin. This thymosin stimulate growth
and development of T lymphocytes, while thymin suppresses the neuromuscular activity by
inhibiting acetylcholine release.
GONADS
Gonads refer to the primary sex organs in male and female. The male gonad are the testes, while
female gonad are the ovaries.
Testes
These are ovoid shaped structures suspended by the spermatid cord. They are surrounded by three
layers of tissue: tunica vasculosa(inner), tunica albuginea(middle) and tunica vaginalis(outer). The
hormone secreting cells present in the testes are: interstial cell of Leydig – which secrete
testosterone; and Sertoli cell – which secrete inhibin.
The inhibin inhibits FSH release from anterior pituitary gland, while testosterone perform different
functions in both fetal and adult life.
Function of testosterone in fetal life;
• Sex differentiation in fetus
• Development of accessory sex organ
• Descent of the testes
Funtions of testosterone in adult life;
• Increase in size of the penis and scrotum.
• Effect on secondary sexual characters: increased muscular and bone growth, broadening of
shoulders, increased thickness of the skin and hair distribution.

Ovaries
Ovaries are almond shaped structures attached to the superior poles of the uterus by the ovarian
ligament. The have an outer cortex and inner medulla. The cortex contains the ovarian follicles,
which in turn houses the ova. Furthermore, the theca interna cell of the ovarian follicles secrete
hormones such as oestrogen and progesterone.

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Functions of oestrogen;
• Growth and development of uterus
• Promote growth of the ovarian follicles
• Responsible for development of secondary sexual characteristics
• It increases osteoblastic activity
Functions of Progesterone;
• Promotes the secretory activities of fallopian tubes and uterus during the secretory phase
of the menstrual cycle
• It prepares the uterus for implantation of fertilized ovum by stimulating its growth and
development.
• It promotes the development of mammary gland.

BY: SIMON OCHE

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