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Endocrine Physiology. The Pituitary Gland. The pituitary gland is a small reddish-grey body 12 x 8 mm. suspended from the base of the brain by a stalk or infundibulum . It lies in the pituitary fossa of the sphenoid bone above the back of the nasal cavity.
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The Pituitary Gland The pituitary gland is a small reddish-grey body 12 x 8 mm. suspended from the base of the brain by a stalk or infundibulum. It lies in the pituitary fossa of the sphenoid bone above the back of the nasal cavity.
From both a developmental and a physiological point of view, it is convenient to consider the pituitary gland in two parts: • The posterior pituitary (or neurohypophysis) which is derived from the brain. • The anterior pituitary (or adenohypophysis) which is derived from the mouth and nasal cavity. • The pituitary gland produces many hormones.
Posterior Pituitary Gland (Neurohypophysis) The neurohypophysis or posterior pituitary gland produces two hormones, the antidiuretic hormone (ADH, adiuretin) and the oxytocic hormone (Oxytocin). The antidiuretic hormone connected with the formation of urine by the kidneys.
ADH function is to regulate the reabsorption of water from the distal and collecting tubules and by so doing to regulate the volume of urine which passes to the bladder and is ultimately excreted. • The term antidiuretic hormone is a more appropriate name for this hormone than the older name vasopressin or vasopressor fraction.
Vasopressin was chosen when it was found that an injection of a relatively large amount of this hormone caused vasoconstriction and a rise of blood pressure. • However, it is very doubtful whether the amount of circulating ADH is sufficient to have any effect on the blood pressure, whereas it has a profound effect on the kidney tubules. • A deficiency of this hormone leads to diabetes insipidus.
Large volumes of urine are passed each day and the patient is excessively thirsty. The urine is very dilute. • In diabetes insipidus the urine was tasteless or insipid, hence the name of this condition; in diabetes mellitus where there is also an increase in urine production, the urine was sweet to the taste since it contained glucose.
Diabetes insipidus may be treated by administering ADH. • ADH has been synthesized and is a polypeptide made up of nine amino acids. • If two of the amino acids are changed, the second posterior pituitary hormone is obtained, the oxytocic hormone, or oxytocin.
Oxytocin (Greek Oxys=sharp. Tokos= birth) is so named because of its ability to contract the pregnant uterus. Its release is one of the many factors causing parturition (birth of a baby). • Injections of the hormone are used to induce uterine contractions and to cause the onset of labour and to enhance the uterine contractions during labour.
It also causes milk ejection from the breasts after child-birth and the synthetic hormone is used in the treatment of disorders of lactation. • Although these hormones are released from the posterior pituitary, they are probably manufactured in the nerves which run from the hypothalamus to the gland via the pituitary stalk.
These nerves are neuro-secretory, that is, nerve impulses bring about the release of a hormone instead of a chemical transmitter substance. • Such neuro-secretory nerves form a link between the two control mechanisms of the body, the nervous system and the hormonal system.
Anterior Pituitary Gland (Adenohypophysis) The anterior pituitary gland or adenohypophysis secretes a number of hormones. These are known as trophic hormones (Greek trophes= nourishment) since they regulate the growth and activity either of the body as a whole or of other endocrine glands.
A series of portal vessels link capillary networks in the median eminence of the hypothalamus with sinusoids in the anterior pituitary.
The release of the anterior pituitary hormones is probably controlled by the hypothalamus by a series of 'releasing factors' one for each of the anterior pituitary hormones, which pass along this portal system.
When histologically stained with both an acidic stain (eosin) and a basic stain (CarbolFuchsin, staining white blood cells), three types of anterior pituitary cell are found: • Eosinophil cells (α cells). • Basophil cells (β cells). • Chromophobe cells. The chromophobe cells are small cells which do not stain with the dyes. They may develop later into chromophil cells.
With special stainsattached to high-affinity antibodies that bind with the distinctive hormones, at least five cell types can be differentiated. These five cell types are: • 1. Somatotropes—human growth hormone (hGH). • 2. Corticotropes—adrenocorticotropin (ACTH). • 3. Thyrotropes—thyroid-stimulating hormone (TSH). • 4. Gonadotropes—gonadotropic hormones, which include both luteinizing hormone (LH) and folliclestimulating hormone (FSH). • 5. Lactotropes—prolactin (PRL).
Growth Hormone • (Somatotrophic Hormone, Sornato trophin, STH, Human Growth Hormone, HGH) • This hormone is concerned with somatic growth. It is produced by the eosinophil or α cells of the anterior pituitary.
One's height depends on inherited factors. The children of tall parents tend to be taller than the average but not so tall as their parents. They regress or tend to be nearer the mean than their parents. • Hence the term 'regression line' for this mean in statistics, a further factor, however, is the amount of circulating growth hormone play a major role in controlling the hight of person.
Under-production of this hormone during the period of growth leads to retarded growth and dwarfism. • Over-production leads to excessive growth of the bones of the body and also of the muscles and viscera leading to gigantism. • Most giants 7-8 ft. tall are examples of pituitary over activity.
Over activity later in lifedoes not increase the stature once the epiphyses of the long bones in the limbs have fused to the shafts (at an age of 18-24 years) and thenno further increase in length is possible.
The bones of the hands, feet and head, however, appear to retain their power of growth and the subject may first notice that his shoes no longer fit, and that his gloves and hats are becoming too small. • This condition is known as acromegaly (from Greek akron= extremity and megas=great).
Human growth hormone is a protein consisting of a chain of 187 amino acids. It has a molecular weight of 21,500. • Although the structural formula is known, it has not yet been synthesized, and its only source is from human pituitary glands. • It is species specific and growth hormone from other animals, except possibly the monkey, has no effect in man.
Trials with the limited amount of human growth hormone available have shown that it has its expected effect on growth, and dwarf children have increased their rate of growth markedly when under treatment with this hormone.
Typical variations in growth hormone secretion throughout the day, demonstrating the especially powerful effect of strenuous exercise and also the high rate of growth hormone secretion that occurs during the fist few hours of deep sleep.
Thyrotrophic Hormone • (Thyrotrophin, Thyroid Stimulating Hormone, TSH) • This hormone controls the activity of the thyroid gland. • Increased production of the hormone leads to over activity of the thyroid gland. • A decreased production of TSH leads to under activity of the thyroid.
Adrenocorticotrophic Hormone (Adrenocorticotrophin, Corticotrophin, ACTH) • This hormone is produced by the basophil or β cells. It is a peptidemade up of 39 amino acids (molecular weight 4,500), but a peptide containing only the first 24 has the same activity.
This hormone controls the activity of the adrenal cortex and the release of the corticoid steroids such as cortisol(hydrocortisone) and cortisone. • It does not control the release of aldosterone.
Gonadotrophic Hormones (Gonadotrophins) • The three gonadotrophic hormones of the anterior pituitary control the activity of the gonads. They are: • The follicle stimulating hormone (FSH). • The luteinizing hormone (LH). • The luteotrophic hormone (LTH, prolactin).
These hormones are produced in the female in a cyclical pattern which repeats itself about every 28 days from puberty to the menopause except during pregnancy. • In the male these hormones are produced at a steady rate without the cyclical variations. • The action of these hormones will be considered in Reproductive System.
