The pituitary Gland College of Dentistry Dr Mouaadh Abdelkarim

Pituitary gland is located in the diencephalon. Pituitary gland – two-lobed organ that secretes important hormones Adenohypophysis – anterior lobe, made up of glandular tissue Synthesizes and secretes a number of hormones Neurohypophysis – posterior lobe (neural tissue) Receives, stores, and releases hormones from the hypothalamus Pituitary (Hypophysis) Gland

The six hormones of the anterior pituitary: 1- Growth hormone (GH) 2- Prolactin (PRL) 3- Thyroid-stimulating hormone (TSH) 4- Adrenocorticotropic hormone (ACTH) 5- Follicle-stimulating hormone (FSH) 6- Luteinizing hormone (LH) Anterior Pituitary Hormones

Pituitary Hormones Anterior Pituitary: Trophic effects: High blood [hormone] causes target organ to hypertrophy. Low blood [hormone] causes target organ to atrophy.

Hypothalamic Control of the Anterior Pituitary Hormonal control rather than neural. Hypothalamus neurons synthesize releasing and inhibiting hormones. Hormones are transported to axon endings of median eminence. Hormones secreted into the hypothalamo-hypophyseal portal system regulate the secretions of the anterior pituitary There is no direct neural contact with the hypothalamus There is a vascular connection, the hypothalmic- hypophyseal portal system

Hypothalamic Control of Posterior Pituitary Hypothalamus neuron cell bodies produce: ADH: supraoptic nuclei. Oxytocin: paraventricular nuclei. Transported along the hypothalamo-hypophyseal tract. Stored in posterior pituitary. Release controlled by neuroendocrine reflexes.

Pituitary (Hypophysis)

Feedback Control of the Anterior Pituitary Long Feed back loops: Anterior pituitary and hypothalamic secretions are controlled by the target organs they regulate. Negative feedback at 2 levels: The target gland hormone can act on the hypothalamus and inhibit secretion of releasing hormones. The target gland hormone can act on the anterior pituitary and inhibit response to the releasing hormone.

Feedback Control of the Anterior Pituitary Short feedback loop: Hormone released by anterior pituitary inhibits secretion of releasing hormone. Long Feedback loop Hormone released by target organs or tissue inhibits anterior pituitary and hypothalamus Positive feedback effect: During the menstrual cycle, estrogen stimulates “LH surge.”

Physiology of posterior Pituitary gland Posterior pituitary hormones ADH also known as Vasopressin Oxytocin These hormones are synthesized in the neuron cell bodies in the hypothalamus and packed in secretory vesicles with neurophysin (carrier protein)

Posterior Pituitary Gland Does not synthesize hormones Consists of axon terminals of hypothalamic neurons Posterior pituitary gland

The posterior lobe is a downgrowth of hypothalamic neural tissue Has a neural connection with the hypothalamus (hypothalamic-hypophyseal tract) Hypothalamic-Pituitary Relationships: Posterior Lobe

Hypothalamic control of pituitary secretions Secretions of the posterior pituitary are controlled by Nervous signals from hypothalamus

(vasopressin) Antidiuretic Hormone (ADH)

Synthesis of ADH It is synthesized as pre-prohormone and processed into a nonapeptide ADH synthesized in the cell bodies of hypothalamic neurons(supraoptic nucleus) ADH is stored in the posterior pituitary

Receptors of ADH (vasopressin) There are 2 types of receptors for ADH: V 1 V 2  V 1 receptors mediate vasoconstriction  V 2 receptors are located in the principle cells in distal convoluted tubule and collecting ducts in the kidneys

Mechanism of action of ADH

Mechanism of action of ADH: Antidiuresis ADH binds to V2 receptors on the principle cells of the distal convoluted tubules and collecting ducts. Via adenylate cyclase/cAMP induces production of specific proteins into the luminal membrane and enhances permeability of cell to water. Increased membrane permeability to water permits back diffusion of free water, resulting in increased urine osmolality (concentrates urine).

The most important function of ADH is to conserve body water by reducing urine output

Control of ADH Release Osmotic pressure: Osmoreceptors in the hypothalamus  osmotic pressure   ADH secretion  osmotic pressure   ADH secretion Blood volume : Baroreceptor in carotid artery and aortic arch, and Stretch receptors in left atrium  blood pressure   ADH secretion  blood pressure   ADH secretion

Function of ADH (vasopressin)

ADH Increased Blood Pressure V1 receptor

Oxytocin

Synthesis of Oxytocin Oxytocin is synthesized in the cell bodies of hypothalamic neurons(paraventricular nucleus) Oxytocin is stored in the posterior pituitary

Functions of oxytocin  Suckling during breast-feeding o Contracts the myoepithelial cells of the alveoli  Childbirth (Parturition) o In late pregnancy, uterine smooth muscle (myometrium) becomes sensitive to oxytocin (positive feedback)

Summary of posterior pituitary hormones actions