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Thyroid Hormones

by: Brittany Woody

Thyroid Hormones PSB4504

Marketplace > University of Florida > PSB4504 > Thyroid Hormones
Brittany Woody

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Lecture and PowerPoint notes for PowerPoint 12
Developmental Psychobiology
Dr. Donald J. Stehouwer
Class Notes
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This 4 page Class Notes was uploaded by Brittany Woody on Monday April 11, 2016. The Class Notes belongs to PSB4504 at University of Florida taught by Dr. Donald J. Stehouwer in Spring 2016. Since its upload, it has received 9 views.

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Date Created: 04/11/16
Friday, April 1, 2016 PowerPoint 12 Thyroid Hormones - Growth refers to increase in the size of a tissue, organ, or organism - Maturation refers to emergence of a characteristic through growth or differentiation - Growth hormone excess causes giantism; deficiency results in dwarfism; maturation is not affected in either case; puberty and other milestones are met correctly - Larval stages of bull frogs (tadpole, stage 5- 17) growth is independent of thyroid hormones; get bigger but do not change much, some growth of hind limbs - At stage 17, there is a surge in thyroid hormones and limbs grow; limbs grow inside tadpole then are released in later stages; apoptosis of tail tissues - Thyroxine dependent changes in bull frog: mitosis, protein synthesis in limbs, CNS neurogenesis and apoptosis, teeth grow, eyes move laterally, intestines shorten (adult frogs are carnivores, tadpoles eat vegetation) frogs do not eat during transition from tadpole to frog because mouth and intestines • are changing; they absorb their tail and use the protein as energy; thyroxine triggers apoptosis of tail; frog tail in a beaker with thyroxine will disappear due to apoptosis - Thyroidectomy of frog would result in a tadpole getting larger but never metamorphosing into a frog - Gills degenerate and lungs form; putting thyroxine in tadpole’s environment would result in death because the gills would degenerate before lungs were formed; normally lungs differentiate before gills degenerate, reliant on concentration of thyroxine - If food is scarce, tadpoles will develop faster so that they can leave the water - Iodine is sequestered in the thyroid gland by a Na-I pump, whose activity in controlled by TSH - T4 is converted to T3 through de-iodination (removal of iodine) by tissue - Causes of hypothyroidism: • dietary deficiency (iodine): iodized salt in USA 1 Friday, April 1, 2016 • pituitary defect • enzyme deficiency: do not produce thyroid peroxidase, which is necessary to produce thyroxine • thyroid autoimmunity • exposure to I 131: released by nuclear power pants; potassium iodide will counteract effects - Thyroid hormone actions: • direct effects on gene expression and protein synthesis • indirect effects via receptor-activated second messenger systems • products affected during sensitive period for thyroid hormones include neurotrophins and their receptors (BDNF) • NGF, NT family, BDNF, Trk receptors, P75 receptor are all modulated by TH during the sensitive period - Cerebellum of hypothyroid rats: • retarded, prolonged proliferation of granule cells • normal number finally attained but is 25% below normal # on day 14 • shorter parallel fibers; thus, contacts with fewer Purkinje cells; greater cell death during proliferation; secondary migration retarded • proliferation and growth of stellate and basket cells is also retarded • retarded maturation of Purkinje cells: reduced number; reduced dendritic growth; fewer targets for parallel fibers; could be factor in greater death of granule cells along with shorter parallel fibers (“ripple effects”) - In rats, proliferation after birth accounts for • 50% of forebrain cells (mostly glia) • 80% of olfactory bulb • 97% of cerebellum - granule cells : birth-15 days postnatal; secondary migration day 15-30 2 Friday, April 1, 2016 - microneurons: 4-15 days postnatal - basket cells: 4-7 days postnatal - stellate cells: 8-15 days postnatal - Sensitive period for thyroid hormones: birth to day 10 to 12 in the rat; first trimester through 6 month postnatally in humans - Humans: • T4 release begins 10-12 weeks gestation • Increases from 20 weeks to term; a period of development normally associated with: - proliferation - apoptosis - rapid myelination - growth of neuronal processes - synaptogenesis - proliferation in cerebellum (prolonged by hypothyroidism) - Neocortex of Hypothyroid Rats • pyramidal cells: small, densely packed (fewer glia, less neuropil), concentric ring analysis reveals shorter dendrites with fewer high-order branches, fewer dendritic spines • number of potential synaptic contacts reduced by 85%; less spatial summation and neural integration; reliability of processing reduced; range of effective stimuli reduced • axons: myelination greatly reduced; slower conduction velocity; with low metabolic rate, accounts for behavioral; sluggishness • these are same features that distinguish the mature from the immature brain; thus, it is as though development is arrested, much like that of the a-thyroid tadpole - Gross characteristics of a “cretinoid” rat: face foreshortened (brachycephaly), lethargic, hair is thin, learning and memory impaired, brain weight reduced, growth 3 Friday, April 1, 2016 retarded after 12 to 15 days of age (after several months in humans; apparent by 18 months, but by then CNS damage has occurred), O consumpt2on (metabolism) lowered, cold intolerance, deafness • delayed somatic maturation as indicated by delayed eye, ear, vaginal opening; delayed endochondral ossification - Behavior of “cretinoid” rat: fits of activity when stimulated; susceptible to seizures; startle response, righting reflex, placing responses retarded; persistence of mass- action wriggling to noxious stimulus; deficits in both learning and memory  - Human development of cretin: “The mental capacity varies within narrow limits; an intelligent adult cretin may reach the intellectual development of a child 3-4 years of age, though more often the standard attained is even below this. The child cretin learns neither to walk nor talk at the usual time. Often it is unable even to sit without support. Some years later a certain power of movement is acquired, but the gait is waddling and clumsy. Speech is long delayed, or in bad cases may be almost entirely lacking. The voice is usually harsh and unpleasant. Of the senses, smell and taste are but slightly developed, more or less deafness is generally present, and only the sight is fairly normal. In the adult the genital organs remain undeveloped. If the cretin is untreated he rarely has a long life, thirty years being an exceptional age. Death results from some intercurrent disease.” - Effects of thyroid hormones: • organizational effects: structural, sensitive period, irreversible in adulthood • activational effects: metabolic, no sensitive period, reversible throughout life 4


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