pituitary gland ( hypophysis )

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