Pituitary, hypothalamus and pineal gland
Describe the two major components of the pituitary gland and their unusual embryological origin.
The posterior pituitary, also called the pars nervosa or the neurohypophysis, is of neurectodermal origin (i.e. it comes from the brain during development). The neurohypophysis retains its connection to the brain via the infundibular stalk. The anterior pituitary, also called the adenohypophysis, is of oral ectoderm origin during development. Rathke's pouch pinches off during development, and eventually is completely separated from the oral mucosa.
Compare and contrast the histological organization of endocrine and exocrine glands.
Endocrine glands have a rich vascular supply, and many secretory cells. The secretory cells release their hormone products directly into the rich capillary network. The capillaries are fenestrated to allow for easy uptake of the hormone into the blood. There are no ducts in endocrine glands. Exocrine glands also have many secretory cells, but they also have ductal cells that are continuous with an epithelial surface, into which the secretory cells secrete their product.
Identify the hormones released from the adenohypophysis and their target tissues.
GPA: GH, PRL, Acidophils B-FLAT: basophils, FSH, LH, ACTH, TSH Follicle Stimulating Hormone (FSH) Released by gonadotrophs. Releasing hormone is GnRH. Acts on the testicles and ovaries to cause the release of sex hormones (estrogen and testosterone) Luteinizing Hormone (LH) Released by gonadotrophs. Releasing hormone is GnRH. Acts on the ovaries to cause ovulation, and acts on the testicle to maintain the IC cell Growth Hormone (GH) Released by somatotrophs. Releasing hormone is GHRH. Growth hormone acts on various tissues in the body. It causes bone growth among other things. Causes release of insulin-like growth factor I (IGF-I). Adrenocorticotropic hormone (ACTH) Released by corticotrophs. Releasing hormone is CRH. Acts on the adrenal cortex (zona fasciculata) to cause release of- corticosteroids (cortisol) Thyroid stimulating hormone (TSH) Released by thyrotrophs. Releasing hormone is TRH. TSH acts on the thyroid to stimulate release of T3 and T4. Prolactin-Released by mammotrophs- Prolactin acts on mammary glands to stimulate milk production.
Describe two mechanisms by which hormones exert their actions on target tissues.
Hormones can either act via surface receptors (as do hydrophilic hormones), or they can diffuse into the cell and interact with intracellular receptors (as is the case with steroids and lipophilic hormones). Hormones that act via membrane receptors use second messengers to cause intracellular change. Steroid hormones interact with the steroid receptor, that in itself can change the activity inside the cell.
Discuss two mechanisms through which the hypothalamus controls the pituitary gland.
The hypothalamus controls the pituitary gland via two mechanisms: It releases releasing hormones into the small pituitary portal system; these travel to the anterior pituitary and exert effects on the cells there (to release hormones systemically) The hypothalamus has neurons whose axons extend into the posterior pituitary. These neuroendocrine cells produce (and release) hormones into the systemic circulation as their neurotransmitter. The axons terminate onto capillaries in the posterior pituitary.
Describe the two major hormones released from the neurohypophysis and their target tissues.
The neurohypophysis releases ADH (vasopressin), which acts on collecting ducts in the kidney to absorb more water. The osmoreceptors in the hypothalamus detect when blood osmolarity has gone up, and thus controls the release of ADH such that ADH will be released in order to increase water reabsorption and lower blood osmolarity. The osmoreceptors fire onto the ADH cells, causing them to fire and then release their neurotransmitter (i.e. ADH). The neurohypophysis also releases oxytocin, which acts on the uterus to cause contraction, and acts on the myoepithelial cells of the mammary glands to cause milk letdown. The release of oxytocin is in part controlled by the suckling stimulus; when a baby tries to breastfeed, oxytocin will be released to cause milk to be let out.
Describe the anatomical and physiological influences of the visual system on pineal gland function.
The visual system regulates the output from the pineal gland. In darkness, the pineal gland releases melatonin. Melatonin is involved in the inhibition of gonadotropin synthesis by the anterior pituitary. The pineal gland sits just superior to the optic tectum (superior colliculus) The pineal gland receives visual input indirectly, via the superior cervical ganglion.
3. Differentiate the two major components of the pituitary gland and their unusual embryological origin
a. Adenohypophysis: Also known as the anterior pituitary or the pars distalis. Develops from an invagination of oral ectoderm called rathke's pouch. The adenohypophysis is further divided into a very large pars distalis, pars intermedia and pars tuberalis b. Neurohypophesis: Also known as the pars nervosa or posterior pituitary. Arises from the outcropping of the neuroectoderm at the floor of the diencephalon. Further divides into the pars nervosa and infundibulum.
1. Compare and contrast the histological organization of endocrine and exocrine glands.
a. Endocrine organization: A collection of ductless glands throughout the body whose main function is to assist in the maintenance of the internal environment. Endocrine gland structures include the hypothalamus, pituitary, pineal body, thyroid, parathyroid, islets of Langerhans, gonads and the adrenals which produce hormones. Hormones are low molecular weight organic chemicals that are released into the vasculature to target specific tissues. Because of their need for vascular transport, endocrine glands have a rich vascular supply characterized by fenestrated capillaries. Hormones either directly interact with intracellular receptors to instigate action (lipophilic molecules) or they interact with secondary messengers via a exofacial plasma membrane receptor which leads to the synthesis of intracellular second messengers (cAMP or cGMP). Unlike exocrine glands, endocrine glands release their contents directly into the vasculature as opposed to using a duct system. b. Exocrine glands: Include salivary and sweat glands. Maintain their connection to an epithelial surface and secrete their products into ducts.
2. Outline two mechanisms through which the hypothalamus controls the pituitary gland
a. The hypothalamus constitutes the final common pathway for the central neural control of the pituitary. The hypothalamus controls the pituitary gland through two mechansism. A direct neural pathway called the hypothalamo-hypophyseal tract and a vascular route via the hypophyseal portal system. i. Hypothalamo-hypophyseal tract: axons of the supraoptic and paraventricular neurons that terminate on the posterior portion (pars nervosa) of the pituitary. The supraoptic/ PVN generate an action potential that results in the release of ADH and oxytocin into fenestrated capillaries of the pars nervosa where they are carried into general circulation. The axons which generate an action potential to secrete granules are next to the capillaries of the posterior pituitary and will be released in response to adequate stimulus. ADH causes water absorption from the kidney tubules while oxytocin causes contraction of the smooth muscle of the uterus and myoepithelial cells of the mammary gland which results in milk ejection during lactation. There has been some speculation that oxytocin release in men deals with bonding relationships with their mates. ii. Hypophyseal portal system: The anterior pituitary is controlled by peptidergic neruons whose cell bodies lie in the medial basal region of the hypothalamus. These neurons secrete releasing and inhibiting hormones into the vascular system where they are carried to the anterior pituitary via the portal plexus to regulate hormone release by pituitary cells. The superior hypophyseal artery supplies the pars distalis (anterior pituitary) which breaks up into a plexus that will then better interact with the anterior pituitary. The hypothalamus will release hormone releasing factors which will then travel down through the infundibulum to interact with the nuclei of the anterior pituitary to release hormones into circulation.
4. Characterize the two major hormones released from the neurohypophysis and their target tissues.
a. The hypothalamus interacts with the neurohypophysis portion of the pituitary gland via the hypothalamo-hypophyseal tract. The axons of the tract terminate in the pars nervosa adjacent to the fenestrated capillaries. The release of neurohypophyseal hormones is under the control of both humoral and neural control. ADH is released in response to an increase in the osmotic concentration of the blood which is recognized via specialized osmorecepotrs adjacent to hypothalamic tissue. ADH results in increased permeability to water in the collecting duct of the kidney. The neurohypophysis also releases oxytocin which is released in response to a suckling stimulus. Oxytocin causes a contraction of the myoepithelial cells of the alveoli in the lactating breast and contraction of smooth muscle of the uterus.
6. Correlate the anatomical and physiological influences of the visual system on pineal gland function.
a. The pineal gland is not heavily innervated by neurological processes. Also known as the epiphysis cerebri, the pineal gland is joined at the brain by a small stalk. The gland secretes melatonin which is involved in the inhibition of gonadotropin synthesis by the anterior pituitary. Neural input is derived from the visual system and is from postganglionic neurons of the superior cervical ganglion whose axons enter the gland from the stalk. Visual input inhibits the pineal glands ability to release melatonin indirectly through a circuitous route (retina-suprachiasmic nucleus of the hypothalamus- sympethatic neurons of spinal cord-superior cervical ganglion-pineal gland). There is reduced inhibition through this pathway which is linked to an increased release of melatonin from the pineal gland. Both melatonin and serotonin are released in greater abundance at night based on the relationship of the pineal gland to the circadian rhythm. Pineal gland enlargement is linked to cerebral aqueduct pressure which in turn will lead to hydrocephalus.
5. Summarize the hormones released from the adenohypophysis and their target tissues.
a. This portion of the hypophysis has a typical endocrine organ profile consisting of cords and follicles of cells richly supplied by sinusoidal capillaries. i. Cell type: Somatotroph which releases GH, STH, somatoropin- Bone growth ii. Mammotrophy-releases prolactin- milk secretion by mammary glands iii. Gonadotroph LH-Leutinizing hormone- results in ovulation, maintenance of testicular IC cell iv. FSH- released from gonadotrophs- follicular development and spermatogenesis v. Thyrotroph- thyroid stimulating hormone- synthesis and release of thyroid hormone vi. Corticotroph- releases ACTH- Synthesis and release of glucocorticoids from adrenal cortex. vii. Dopamine-negative regulator of prolactin which results in the inhibition of GnRH
