Lec . 2 Dr. Shaimaa Munther • The ( adenohypophysis) is derived embryonically from glandular tissue, as an invagination of the pharynx called (rathke's pouch). • It then migrates toward the embryonic nervous tissue destined to form the neurohypophysis. • When these two tissues come into contact, the pituitary gland is formed. • Unlike the neurohypophysis, which releases originally synthesized in the hypothalamus, the adenohypophysis synthesizes its own hormones in specialized groups of cells. • Similar to the neurohypophysis, however, the release of these hormones into the blood is regulated by the hypothalamus

• The anterior pituitary secretes: 1. -stimulating (TSH, Thyrotropin), 2. Adrenocorticotropic Hormone (ACTH, or called Adrenocorticotropin,or Corticotropin ) 3. Gonadotropines ( (LH), & Follicle-Stimulating Hormone (FSH)) 4. Prolactin (Lacto tropes) 5. Growth hormone (Somatotropin) • Of the listed hormones, prolactin acts on the breast. the remaining are tropic hormones ; that is, they stimulate secretion of hormonally active substances by other endocrine glands. • Thyroid-stimulating hormone (TSH or Thyrotropin) 1. Regulates the growth and metabolism of the thyroid gland. 2. Stimulates synthesis and release of the thyroid hormones, T3 and T4. • Adrenocorticotropic hormone (ACTH) 1. Stimulates growth of the adrenal cortex 2. Stimulates steroid hormones production in the adrenal cortex. specifically, it stimulates secretion of cortisol and other corticosteroids. • Growth hormone (GH, Somatotropin) 1. Is one of the few hormones that exerts its effects on organs and tissues throughout the body. 2. It is essential for normal growth and development of the skeleton as well as visceral, or soft, tissues from birth until young adulthood. 3. Growth of the skeleton involves : a. An increase in bone thickness and an increase in bone length. through stimulation of osteoblast (bone-forming cell) activity and proliferation of the epiphyseal cartilage in the ends of the long bones. b. The growth of visceral tissues occurs by hyperplasia (increasing the number of cells) and hypertrophy (increasing the size of cells). • The Gonadotropins , Follicle-stimulating hormone and Luteinizing hormone • Exert their effects on the gonads (ovaries in the female and testes in the male). 1- produce gametes (ova and sperm) 2- secrete sex hormones (estrogen, progesterone, and testosterone)

• Follicle-stimulating Hormone (FSH) 1. Females: stimulates growth & development of ovarian follicles & promotes secretion of estrogen by ovaries. 2. Males: Act on sertoli cells for sperm production • Luteinizing Hormone (LH): 1. Females: responsible for ovulation, formation of corpus luteum in the ovary, and regulation of ovarian secretion of female sex hormones. 2. Males: stimulates leydig cell in the testis to secrete testosterone • Prolactin: 1. Females: stimulates breast development and milk production. 2. Males: involved in testicular function • Hypothalamic hormones regulate anterior pituitary trophic hormones that, in turn, determine target gland secretion.

• There is a peripheral hormones feedback which regulates hypothalamic and pituitary hormones. • The hypothalamus and pituitary gland form a unit that exerts control over the function of several endocrine glands (thyroid, adrenals, and gonads), as well as a wide range of physiologic activities • This unit constitutes an example of —brain-endocrine interactions • Due to their embryonic origins, the neurohypophysis and the adenohypophysis are regulated by the hypothalamus, using two very different mechanisms: 1. Neuronal signals 2. Hormonal signals • Action potentials generated by the neurosecretory cells originating in the hypothalamus are transmitted down the neuronal axons to the nerve terminals in the neurohypophysis and stimulate the release of the hormones into the blood. • Specific forms of sensory input that regulate the release of ADH and oxytocin • The adenohypophysis does not have a direct anatomical connection with the hypothalamus; therefore, regulation of hormone secretion by way of neuronal signals is not possible, instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus

• The neurosecretory cells synthesize two types of hormones: 1. Releasing hormones 2. Inhibiting hormones

• Each of these hormones helps to regulate the release of a particular hormone from the adenohypophysis. For example, thyrotropin-releasing hormone produced by the neurosecretory cells of the hypothalamus stimulates secretion of thyrotropin from the adenohypophysis. • There are three levels 1. Hypothalamic stimulation—from CNS 2. Pituitary stimulation—from hypothalamic trophic hormones 3. Endocrine gland stimulation—from pituitary trophic hormones Seven releasing hormones are made in the hypothalamus • Thyrotropin-Releasing Hormone (TRH) • Corticotropin-Releasing Hormone (CRH) • Gonadotropin-Releasing Hormone (GNRH) • Growth Hormone-Releasing Hormone (GHRH) • Growth Hormone-Release Inhibiting Hormone (GHIH) • Prolactin-Releasing Factor (PRF) • Prolactin-Inhibiting Hormone (PIH) • Hypothalamic hormones regulate anterior pituitary trophic hormones that, in turn, determine target gland secretion.

• There is a peripheral hormones feedback which regulates hypothalamic and pituitary hormones. • The trophic hormone from the adenohypophysis stimulates the release of a hormone from another endocrine gland.

• This final endocrine gland hormone not only carries out its effects on its target tissues, it may also exert a negative feedback effect on the release of the hypothalamic and/or adenohypophyseal hormones. • Secretion of the anterior pituitary hormones is regulated by: 1. The central nervous system provides the primary drive for secretion 2. Hormones produced in peripheral target glands and peripheral input plays a secondary, though vital, role in modulating secretory rates

• Secretion of all the anterior pituitary hormones except PRL declines severely in the absence of stimulation from the hypothalamus , PRL secretion is normally under tonic inhibitory control by the hypothalamus

16 • Environmental factors may increase or decrease pituitary activity by increasing or decreasing hormone secretions from hypothalamus

• Pituitary secretions increase the secretion of target gland hormones, which may inhibit further secretion by acting at either the hypothalamus or the pituitary

• Pituitary hormones may also inhibit their own secretion by a short feedback loop 17 • LONG-LOOP FEEDBACK

• SHORT-LOOP FEEDBACK Figure 7-15 • Growth Hormone (GH), also called somatotropic hormone (STH) • Somatotropes (GH producing cells) are by far the most abundant anterior pituitary cells, and account for at least half the cells • Target: Most tissues • Major actions in humans : • Promotes growth in stature and mass; • Stimulates production of insulin-like growth factor (IGF-I); • Stimulates protein synthesis; • Usually inhibits glucose utilization and promotes fat utilization

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• The hypothalamus secretes GHRH • GHRH acts on anterior pituitary to produce GH • GH acts on liver to produce Somatomedins peptides (Insulin-like growth factor-1 (IGF-1) called Somatomedin C is the major factor produced) • Ghrelin, from stomach also stimulates GH secretion • High levels of IGF-1 and GH stimulate production of Somatostatin from the hypothalamus • Somatostatin inhibits the secretion of GH • High plasma levels of IGF-1 exert negative feedback on anterior pituitary to modify action of GHRH and to inhibit secretion of GH

• In some species there is a well-developed intermediate lobe of the pituitary, whereas in humans it is rudimentary, nevertheless, the intermediate lobe, contain hormonally active derivatives of the pro opiomelanocortin molecule that regulate skin pigmentation, among other functions