Pituitary Gland ( hypophysis )
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Pituitary Gland (hypophysis)1 cm in diameter and 0.5 to 1 gm in weightLies in sella turcica (bony cavity at the base of brainConnected to hypothalamus by pituitary stalk
Physiologically it is divided into two partsAnterior pituitary (adenohypophysis)Posterior pituitary (neurohypophysis)Pars intermedia is small avascular zone between the two.
Embryologically the two parts develop from different sources
Anterior pituitary from Rathke’s pouch (invagination of pharyngeal epithelium)
Posterior pituitary from neural tissue outgrowth from hypothalamus
Anterior Pituitary Hormones
Cell Types in the Anterior PituitarySomatotropes (acidophils) (30-40%)Corticotropes (20%)ThyrotropesGonadotropesLactotropes
Posterior Pituitary HormonesAntidiuretic HormoneOxytocinThese hormones are secreted by magnocellular neurons located in the supraoptic and paraventricular nucleus of hypothalamus
Hypothalamus controls pituitary secretionThere are neural connections between the hypothalamus
and the posterior pituitary and vascular connections between the hypothalamus and anterior pituitary
Hypothalamic Releasing and Inhibitory HormonesThyrotropin-releasing hormoneCorticotropin-releasing hormoneGrowth hormone-releasing hormoneGrowth hormone-inhibitory hormoneGonadotropin-releasing hormoneProlactin inhibitory hormone
These hypothalamic releasing and inhibitory hormones are conducted through minute blood vessels called hypothalamic-hypophysial portal vessels to the anterior pituitary gland
The hypothalamic-hypophysial portal vessels form a direct vascular link between hypothalamus and the anterior pituitary
Hypothalamic- Hypophysial Portal Blood Vessels
Hypothalamic releasing and inhibitory hormones are secreted into median eminence
The neurons secreting these factors are present in different parts of hypothalamus
Their endings secrete these factors into the tissue fluid from where theses hormones are absorbed into hypothalamic-hypophysial portal system
Growth Hormone (somatotropic hormone or somatotropin)Protein hormone (191 amino acids), molecular weight
22,005Approximately 50% of the circulating pool of growth
hormone is in the bound form providing reservoir to prevent fluctuations
The half life of circulating growth hormone in humans is 6-20 min and daily growth hormone output is 0.2-1.0 mg/dl
The plasma growth hormone level is less than 3 ng/ml
Physiological Functions of Growth HormoneGrowth hormone promotes growth of almost all the
body tissuesIt promotes increase in size of cells, increased mitosis
and differentiation of certain type of cells such as bone growth cells, muscle cells
.
Effect of Growth hormone on Skeletal frame workIncreased deposition of protein by chondrocytic and
osteogenic cellsIncreased rate of reproduction of these cellsConversion of chondrocytes into osteogenic cells causing
bone deposition
Long bones grow in length at the epiphysial cartilages where epiphysis at the ends of long bones are separated from shaft.
Lengthening does not occur if epiphysis are united with the shaft
Growth hormone strongly stimulates osteoblasts so bones can become thicker under the influence of growth hormone through out life
Growth Hormone is Potent Protein Sparer
Growth hormone promotes protein deposition in tissues by increasing amino acid transport through cell membrane
Enhancement of mRNA TranslationIncreased nuclear transcription of DNA to mRNA (over
prolonged periods)Decreased catabolism of proteins and amino acids
Growth hormone enhances Fat utilization for energyFats are used for energy in preference to the use of
carbohydrates and proteinsRelease of fatty acids from adipose tissueConversion of fatty acids to acetyl-CoAMobilization of fat by growth hormone requires several hours whereas enhancement of protein synthesis can begin in minutes
Ketogenic Effect of Excess Growth HormoneThe growth hormone increases free fatty acid levels in the
blood which are utilized for providing energy. When excess quantities of growth hormone are present large quantities of acetoacetic acid formed by liver are released into body fluids causing Ketosis
Growth Hormone Decreases Carbohydrate UtilizationGrowth hormone increases blood glucose levelsDecreased glucose uptake in tissues such as muscle and
fatIncreased glucose production by the liverIncreased Insulin secretion (causes insulin resistance)
Growth Hormone Effects are Diabetogenic
Growth hormone causes insulin resistanceDecreased glucose utilization by the cellsRaising blood levels of fatty acids above normal decreases the sensitivity of liver and skeletal muscle to Insulin’s effects
Insulin and carbohydrates are necessary for growth hormone to exert its growth promoting action (insulin has also protein anabolic effect)
Growth hormone increases the ability of pancreas to respond to insulinogenic stimuli such as glucose
Growth Harmone effect on electrolytesGrowth hormone increases intestinal absorption of
calciumGrowth hormone reduces excretion of sodium and
potassium
Somatomedins(insulin like growth factors)The effects of growth hormone on growth, cartilage and
protein metabolism depend on interaction between growth hormone and somatomedins which are polypeptide growth factors secreted by liver and other tissues
IGF-I (somatomedin-C)Secretion of IGF-I before birth is independent of growth
hormone but is stimulated by growth hormone after birthIt has pronounced growth promoting activityIts concentration in plasma rises during childhood, peaks
at the time of puberty and declines in the old age. The IGF-I receptor is very similar to Insulin receptor
Duration of Action of Somatomedin CHalf life of somatomedin C is 20 hrs whereas that of
growth hormone is 20 minutesSomatomedin C is strongly attached to plasma protein and
released slowlyGrowth hormone is bound loosely
IGF-IIIGF-II plays important role in the growth of fetus and its
secretion is independent of growth hormone
Growth hormone and somatomedins can act both in cooperation and independently to stimulate pathways that lead to growth
Diurnal variations in Growth hormone SecretionThe levels of Growth hormone are low during the dayDuring sleep large pulsatile bursts of Growth hormone
secretion occur (specially in first two hours of deep sleep)Secretion of Growth hormone is under Hypothalamic
control
Regulation of Growth Hormone Secretion
GHRHGHIH (somatostatin)Ghrelin (it is mainly secreted in stomach but also
produced in hypothalamus and it has marked growth-hormone stimulating activity)
The balance between the effects of these hypothalamic factors on pituitary will determine the level of growth hormone release
PanhypopituitarismDecreased secretion of all the anterior pituitary
hormones. It may be Congenital
Acquired (tumor destroying the gland)
DwarfismCauses Panhypopituitarism during childhoodAll body parts develop in appropriate proportion but the
rate of development is slowThe child does not pass through pubertyIf there is only growth hormone deficiency the sexual
maturity occurs
African pygmy (Levi-Lorain dwarf)
Rate of growth hormone secretion is normal or high There is growth hormone insensitivity and there is
hereditary inability to form somatomedin C
TreatmentHuman Growth hormone preparation
Panhypopituitarsm in AdultsTumors (craniopharyngiomas, chromophobe tumors)Thrombosis of pituitary vessels, infarction of the gland
due to shock after delivery in women
EffectsHypothyroidismDecreased secretion of adrenal hormonesDecreased secretion of gonadotropic
hormones (sexual functions are lost)
GigantismCause Acidophilic tumors of anterior pituitary before pubertyAll the body tissues grow very rapidlyIf the condition develops before the union of
shaft and epiphysis the person becomes giantThere is hyperglycemia
TreatmentSurgical removal of tumor or irradiation
AcromegalyCause Acidophilic tumor after pubertyThe bones continue to become thicker Soft tissues also become thickerThere is enlargement of viscera specially
kidneys, liver, tongueThe bone enlargement is specially marked in
membranous bonesThere is kyphosis
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