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16. pituitary gland
 

16. pituitary gland

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    16. pituitary gland 16. pituitary gland Presentation Transcript

    • The Hypothalamic-Pituitary Axis and Pituitary Hormones
    • The hypothalamus , a region of the brain that controls an immense number of bodily functions, is located at the base of the brain. The pituitary gland , also known As hypophysis , lies immediately below the hypothalamus. It rests in a depression at The base of the skull, called sella turcica . Pituitary gland
      • Anterior pituitary (adenohypophysis)
      • - cells secrete peptide/protein hormones
      • Posterior pituitary (neurohypophysis)
      • - is an extension of the hypothalamus
      • - is composed largely of the axons of the hypothalamic neurons
      • In many mammals, there is also an intermediate lobe (pars intermedia) between the
      • anterior and posterior pituitary
      • Based on histologic features, both the adenohypophysis and neurohypophysis are subdivided:
      • - Adenohypophysis
      • - pars distalis
      • - pars tuberalis (contains hypophyseal vessels)
      • - pars intermedia (secretes melanocyte-stimulating hormone [MSH])
      • - Neurohypophysis
      • - pars nervosa
      • - median eminence (the upper section of neurohypophysis)
      • - infundibular stalk (the “stem” that connects the pars nervosa to the bas of the brain)
      • Secretion of hormones from anterior pituitary is under strict control of the hypothalamic
      • releasing/inhibiting hormones which reach the anterior pituitary through
      • Hypothalamic-hypophyseal portal vessels that branch again into a series of capillaries
      • within the adenohypophysis.
    •  
    •  
      • Three distinct cell types are seen in the adenohypophysis:
      • Acidophils (40%)
      • - somatotrophs – produce growth hormone (GH)
      • - lactotrophs – produce prolactin (PRL)
      • Basophils (10%)
      • - thyrotrophs - produce thyroid stimulating hormone (TSH) – a glycoprotein
      • - gonadotrophs – produce luteinizing hormone (LH) & follicle – stimulation
      • hormone (FSH) – glycoproteins
      • - corticotrophs – produce adrenocorticotropic hormone (ACTH) – polypeptide
      • Chromophobes (50%)
      • The major portion of the neurohypophysis contains unmyelinated axons from
      • hypothalamic neurosecretory neurons:
      • Supraoptic nuclei – mainly produce antidiuretic hormone (ADH)/vasopressin (AVP)
      • + neurophysin
      • Paraventricular nuclei – mainly produce oxytocine + neurophysin
    • Anterior pituitary hormones (six peptide hormones)
      • Growth hormone (GH/somatotropin)
      • Synthesized and secreted by somatotrophs
      • Stimulates body growth
      • - GH liver/other tissues IGF 1 (insulin-like growth factor-1)
      • chondrocytes (cartilage cells) long bone growth
      • - IGF 1 also differentiation & prolifiration of myoblasts muscle growth
      • Metabolic effects (on protein, lipid & carbohydrate metabolism)
      • - IGF 1 amino acid uptake protein synthesis + oxidation of protein
      • - IGF 1 glucose synthesis in the liver
      • - GH (direct action) adipocytes (fat cells) free fatty acids
      • -GH (anti-insulin/direct action) uptake of glucose in peripheral tissues
      • glucose in plasma insulin in plasma (hyperinsulinemia)
    • Control of GH secretion
      • GHRH (growth hormone-releasing hormone), a hypothalamic peptide synthesis + secretion of GH
      • Stress, exercise, hypoglycemia, sleep GH
      • Ghrelin , a peptide secreted y stomach GH
      • Somatostatin (hypothalamic growth hormone inhibiting hormone) GH synthesis+release
      • IGF 1 somatostatin GH
      • GH GH ( autocrine negative feedback )
      • Release of hormone is pulsatile
    • Clinical (hyposecretion/hypersecretion)
      • Site of lesion may be at the level of:
      • - the hypothalamus
      • - the pituitary
      • - target cells
      • Deficiency in GH/receptor defects growth retardation or dwarfism
      • GH secretion in young children/adolescents (from tumor of somatotrophs)
      • giantism
      • GH secretion in adults acromegaly
      • Thyroid-stimulating hormone (thyrotropin/TSH)
      • Secreted by “thyrotrophs” of the anterior pituitary
      • - a glycoprotein hormone
      • - a compound of α and β subunits
      • Thyrotropin-releasing hormone (TRH) from the hypothalamus
      • thyrotrophs thyrotropin (TSH)
      • TSH thyroid thyroid hormones (T 3 & T 4 )
      • T 3 and T 4 in plasma TRH action on thyrotrophs T 3 & T 4 levels
      • negative feedback at anterior pituitary thyrotrophs ) + hypothalamus
      • Stress/dopamine/cortisol/somatostatin TSH
    •  
    • Adrenocorticotropic hormone (ACTH/corticotropin)
      • Secreted by the anterior pituitary “corticotrophs”
      • Corticotropin releasing factor (CRF) from the hypothalamus corticotrophs
      • corticotropin ( ACTH )
      • CRF proopiomelanocortin ACTH + β-lipotropin + α-MSH
      • ACTH adrenal glucocorticoids (cortisol)
      • cortisol CRF ACTH ( negative feedback )
      • β -lipotropin – precursor of β-endorphin
      • β -endorphin – opioid peptide that alleviates pain
      • MSH – controls melanin pigmentation in the skin
      • Corticotrophs adenomas ACTH cortisol secondary Cushing’s
      • disease
      Clinical
    •  
    • Gonadotropins (FSH and LH)
      • Secreted by pituitary “gonadotrophs”
      • Hypothalamic GnRH gonadotrophs LH (luteinizing hormone) and
      • FSH (follicle-stimulating hormone)
      • LH sex steroid (testosterone in males and estrogenes + progesterone in females)
      • LH follicle induces ovulation corpus luteum (luteinization) pro-
      • gesterone + estrogenes
      • FSH maturation of ovarian follicles in females and spermatogenesis in males
      • FSH inhibin both in males and females
      • Inhibin FSH secretion at the pituitary level ( negative feedback )
      • Estrogens/testosterone LH secretion ( negative feedback )
      • LH & FSH secretions are pulsatile
      Clinical
      • Attenuated secretion of LH/FSH or both results in hypogonadism
      • - in males low sperm count
      • - in females cessation of reproductive cycles
      • Elevated blood levels of LH and FSH or both usually reflect lack of gonadal steroid
      • negative feedback
    •  
    • Prolactin
      • A single-chain protein hormone closely related to growth hormone
      • It is secreted by “lactotrophs” of the anterior pituitary
      • Also secreted by the “deciduas” of the pregnant uterus
      • Prolactin induces lobuloalveolar growth of the mammary gland ( during pregnancy
      • and lactation )
      • Prolactin stimulates lactogenesis after giving birth to a baby – in the presence of
      • cortisol&insulin , prolactin produces milk proteins
      • In some mammals ( rodents, dogs, skunks ) prolactin is necessary to maintain
      • pregnancy by maintaining their luteal function. In mice , prolactin deficiency
      • blocks ovulation, fertilization, implantation and thus makes the animals infertile
      • Experimental evidences suggest that prolactin may act as autocrine/paracrine
      • modulator of immune activity
      • Prolactin-inhibiting factor (PIF/dopamine) of the hypothalamus tonically
      • suppresses prolactin secretion
      • TRH (thyrotopin-releasing hormone) prolactin secretion
      • Estrogenes prolactin synthesis and release
      • Stimulation of the nipple during breast feeding/nursing, leads to prolactin release
    • Clinical
      • Excessive secretion of prolactin ( hyperprolactinemia ), is a common disorder in
      • humans due to microadenoma of the prolactin secreting cells
      • Common manifestations of hyperprolactinemia:
      • in women:
      • - lack of menstrual cycle
      • - galactorrhea (excessive & spontaneous secretion of milk)
      • in men:
      • - hypogonadism - breast enlargement (gynecomastia)
      • - decreased sex drive - rarely galactorrhea
      • - low sperm count
    • Antidiuretic hormone (ADH)/Vasopressin (AVP)
      • ADH is a nanopeptide, secreted mainly by the supraoptic nuclei of the
      • hypothalamus and released from the posterior pituitary (its carrier protein,
      • neurophysin is also released)
      • ADH conserves body water by reducing the volume of urine ( antidiuresis )
      • It acts primarily on the collecting tubules of the kidney, forms “ water channels ” and
      • promotes reabsorption of water from the glomerular filtrate back into the circulation
      • Reabsorption of this “ solute-free ” water plasma osmolarity and increase osmo-
      • larity of urine
      • ADH/vasopressin widespread vasoconstriction arterial pressure
      • Osmoreceptors in the hypothalamus sense plasma osmolarity
      • plasma osmolarity ( normal ~ 290 mOsm/L ) ADH secretion
      • ADH H 2 O conservation by kidney H 2 O balance
      • osmolarity of plasma thirst sensation ( center in the hypothalamus ) drinking
      • H 2 O balance
      • Decrease in ECF/blood volume volume receptors (in atria) monitor
      • ADH secretion
      • Nausea & vomiting ADH secretion
    • Clinical
      • Diabetes insipidus (excessive loss of body H2O in urine)
      • Types:
      • - Neurogenic (hypothalamic/central)
      • - deficiency in secretion of ADH from the posterior pituitary (due to trauma,
      • infections or tumors)
      • - Nephrogenic (due to ADH receptor deficiency in kidney)
      • Diabetes insipidus urine production (10-16 L/day). However, the loss is
      • compensated by water intake.
    • Oxytocin
      • Oxytocin is a nine amino acid peptide , synthesized mainly by paraventricular
      • nuclei in the hypothalamus and transported down the axons to the posterior
      • pituitary for secretion into blood
      • It is also formed in the ovaries and testes
      • Oxytocin stimulates contraction of myoepithelial cells of mammary gland alveoli,
      • causing milk ejection ( milk letdown )
      • Oxytocin is involved in facilitating sperm transport both in the females and in the
      • males genital tracts
      • Oxytocin is released during labor
      • - fetus stimulates the cervix & vagina stretch reflex oxytocin
      • secretion prostaglandins contraction of uterine smooth
      • muscle parturition ( birth )
      • Physical stimulation of nipples/teats initiates oxytocin release (a neurohormonal
      • reflex )
      • In males, oxytocin is released in pulses during ejaculation
      • Acute stress oxytocin secretion
      • Estrogen oxytocin receptor synthsis
      • Alcohol drink oxytocin secretion
    •