Chapter 42 Animal Hormones

Chapter 42 Animal Hormones

Biology 102Tri-County Technical College

Pendleton, SC

Two Major Systems

Nervous system involved with high-speed messages

Estes and the Dutch Oven…oh yeah!!!!

Endocrine system is much slower and involves production, release, and movement of chemical messagesAs always, it is the subtle interplay between the two systems that COUNTS…

Endocrine System Components

Most endocrine glands/tissue contain neurosecretory cells that secrete hormones

Chemical signal that communicates regulatory messages within body

Hormones reach all parts of body but only certain types of cells [target cells] are equipped to respondOnly target cells respond because ONLY they have receptors for that hormone

Target Cell Visual

Insect Development Hormones

In insects/crustaceans, molting triggered by ecdysone

Secreted (insects) by prothoracic glands just behind headFavors development of adult characteristicsi.e., from caterpillar to butterfly

Ecdysone production in insects controlled by brain hormone (BH)Balanced by juvenile hormone (JH) secreted by corpora allata (pair of small glands just behind brain)Promotes retention of larval characteristics

General Chemical ClassesPeptide hormones: oxytocin, ADH, calcitonin, PTH, and thymosin Protein hormones: GH, prolactin, insulin, and glucagonGlycoprotein hormones: FSH, LH, TSHAmine hormones: T3 and T4, epinephrine and norepinephrine, and melatoninSteroid hormones: glucocorticoids, mineralocorticoids, androgens, estrogens, and pregesterone

Two General ClassesHormones can be divided into two general classes: steroid and nonsteroidNonsteroid hormones such as amines, peptides, and proteins usually combine with receptors in target cell membrane

First messenger/second messengerSignal transduction pathway: converts extracellular chemical signal to specific intracellular response

Classes, cont.

Steroid hormones are lipid-soluble and easily diffuse into cellsOnce inside target cell, steroid hormone MAY combine with specific protein molecules (the receptor[s]) or they may exert their influence directly Typically, steroid hormone bind with their specific protein receptor

Stirring the Steroids…Steroids easily cross plasma membraneBinding of steroid (signal molecule) with specific receptor (usually in nucleus) initiates signal transduction processSignal initiates cascade of events in which proteins interact with other proteins until final response(s) are achievedIn many cases, signal-receptor complex binds to DNA to modify gene expression

Target Cell SpecificityEach chemical signal has specific shape recognized by that signal’s target cellsSignal’s action begins when it binds to specific receptorReceptor protein may be in PM of target cell or inside the cellDiversity of responses of target cells depends on nature of target cell and the affinity of receptor molecules on or within cellCells are unresponsive to signal if they lack appropriate receptors

HypothalamusHypothalamus is region of lower brainHormone releasing cells are 2 sets of neurosecretory cells whose secretions are stored in posterior pituitary (antidiuretic hormone/oxytocin)Hormones released into capillaries in region at base of hypothalamusCapillaries drain into portal vessels (short blood vessels that subdivide into second capillary bed within anterior pituitary

Hypothalamus, cont.Hypothalamic hormones have direct access to gland they control (releasing/release-inhibiting hormones)

Pituitary: Location/Function

Pituitary located at base of hypothalamusHas 2 discrete parts that develop from 2 separate regions of embryo and have very different functionsAnterior pituitary (adenohypophysis) consists of endocrine cells that synthesize and secrete several hormones directly into blood

Pituitary, cont.

Posterior pituitary (neurohypophysis) is extension of the brainStores and secretes 2 hormones made by hypothalamusOxytocin: contraction of uterus and mammary gland cells; regulated by nervous systemAntidiuretic hormone (ADH): promotes water retention by kidneys; regulated by water/salt balance

ADH/Homeostasis/Neg. FBOsmoreceptors in hypothalamus monitor blood osmolarityPlasma osmolarity >s; osmoreceptors shrink slightly and transmit nerve impulse to certain hypothalamic neurosecretory cellsThese cells respond by releasing ADH into general circulation from their tips in posterior pituitaryTarget cells for ADH are cells lining collecting ducts of nephrons in the kidneys

ADH, cont.

ADH binds to receptors on target-cells and activates signal-transduction pathway that increases water permeability of collecting ductsWater retention is >ed as water exist collecting ducts and enters nearby capillariesOsmoreceptors also simulate thirst drive

ADH IIIAs more dilute blood (lower osmolarity) arrives at brain, hypothalamus responds by reducing ADH secretion and lowering thirst sensationThis prevents overcompensation by stopping hormone secretion and quenching thirstThis negative feedback scheme includes hormonal action and behavioral response

Tropic Hormones

Have other endocrine glands as their targetsFour of hormones secreted by anterior pituitary gland are tropic hormonesThyroid-stimulating hormone (TSH)) stimulates thyroid glandAdrenocorticotropic hormone (ACTH) stimulates adrenal cortex to secrete glucocorticoids

Tropic Hormones, cont.

Follicle-stimulating hormone (FSH) stimulates production of ova and spermLuteinizing hormone (LH) stimulates ovaries and testesAnterior pituitary also secretes other hormones with profound effects on body: Growth hormone (GH), Prolactin; Melanocyte-stimulating hormone; Endorphins and enkephalins

Control of Anterior Pituitary

Neurosecretory cells in hypothalamus exert control over anterior pituitary by secreting 2 kinds of hormones into bloodReleasing hormones: anterior pituitary secretes its hormonesInhibiting hormones: make A.P. stop secreting hormonesIs subtle blend of R and I hormones that control timing and amount of hormonal release by A.P.

Thyroid GlandIn humans/other mammals, thyroid gland consists of 2 lobes located on ventral side of tracheaProduces 2 very similar hormones from tyrosine

Triiodothyronine (T3) and Tetraiodothyronine (T4)/thyroxine

In mammals, T3 is more active form

Thyroid, cont.

Thyroid important in human developmentDeficiency (cretinism) results in markedly retarded skeletal growth/poor mental developmentThyroid hormones important for normal functioning of bone-forming cells and for branching nerve cells during embryonic development of the brainIn adults, help maintain normal BP, heart rate, muscle tone, digestion, and reproductive functions

Thyroid III

Hyperthyroidism (> amounts) produces high body temperature, profuse sweating, weight loss, irritability, and high BPHypothyroidism (< amounts) produces cretinism in infants and symptoms of weight gain, lethargy, and intolerance to cold in adults

Thyroid IV

Deficiency of iodine in diet can lead to shortage of thyroid hormones and cause enlargement of thyroid called a goiterThyroid also produces calcitonin which lowers calcium levels in blood as part of calcium homeostasis

Stimulates osteoblasts to use circulating calcium to deposit new bone

Thyroid Hormones Loop

HypothalamusTRH (thyroid releasing hormone) [some texts call thyrotropin-releasing hormone or TRH] to anterior pituitaryTSH (thyroid stimulating hormone)induces thyroid to manufacture and release T3 and T4

Increasing levels of TSH, T3, and T4 signal hypothalamus to reduce secretion of TRH

Parathyroid Glands

Four parathyroid glands embedded in surface of thyroidFunction in homeostasis of calcium ionsParathyroids secrete parathyroid hormone (PTH)Raises blood levels of calcium [has opposite effect of thyroid hormone calcitonin

Parathyroids, cont.Stimulates Ca2+ reabsorption in kidneys and induces osteoclasts to decompose mineralized matrix of bone and release Ca2+ into bloodVitamin D (synthesized in skin) essential to PTH functionLack of PTH causes blood levels of calcium to drop dramaticallyLeads to convulsive contractions of skeletal musclesIf unchecked, tetany follows which is FATAL!!!

PancreasPancreas has both endocrine and exocrine functionsIslets of Langerhans are cluster of endocrine tissue in the pancreas which secretes 2 hormones directly into circulatory systemAlpha (α) cells secrete peptide hormone called glucagonBeta (β) cells secrete the hormone insulin

Hormones of Pancreas

Glucagon and insulin work together (antagonistic) to regulate [ ] of glucose in bloodIn humans, blood glucose levels must remain about 90 mg/100 mL for proper body functioningAt glucose levels above set point, insulin secreted and lowers blood glucose [ ] by stimulating body cells to take up glucose from blood

Pancreas III

Insulin also slows glycogen breakdown in liver and inhibits conversion of AAs/fatty acids to sugarBlood glucose levels drop below set point, glucagon is secreted and increases blood glucose [ ] by stimulating liver to increase hydrolysis of glycogen, convert AAs/Fas to glucose, and slowly release glucose into blood

Adrenal MedullaAdrenal medulla synthesizes/secretes catecholamines (epinephrine and norepinephrine)Secreted in times of stressNerve cells excited by stressful stimuli release neurotransmitter acetylcholine in medullaAcetylcholine combines with cell receptors and stimulates release of epinephrineNorepinephrine released independently of epinephrine

Adrenal Medulla, cont.

Have rapid/dramatic effect on several targetsGlucose mobilized in skeletal muscle cellsFatty acid released from fat cellsRate and stroke of heartbeat increasedDelivery of blood changedOxygen delivery to body cells increased

Adrenal CortexAdrenal cortex synthesizes and secretes corticosteroidsStressful stimuli cause hypothalamus to secrete releasing hormone that stimulates release of ACTH from anterior pituitaryACTH stimulates release of corticosteriods from adrenal cortexIn humans, 2 primary types are glucocorticoids (cortisol) and mineralocorticoids (aldosterone)

Adrenal Cortex, cont.

Glucocorticoids promote glucose synthesis from noncarbohydrate substances such as proteinsAlso have immunosuppressive effects and are used to treat inflammationMineralocorticoids affect salt and water balanceAldosteone stimulates kidneys to reabsorb sodium ioins and water from filtrate

Adrenal Cortex IIIGlucocorticoids/mineralocorticoids important to maintaining body homeostasis during extended periods of stressShort-term stress responses triggers release of epinephrine and norepinephrine (fight or flight)Long-term stress responses trigger the release of mineralocorticoids and glucocorticoids

Gonadal HormonesIn humans, gonads are ovaries and testesHormones are androgens, estrogens, and progestinsPrimary androgen is testosteronePrimary estrogen is estradiolProgestins include progesterone

Gonadal Hormones, cont.

Gonadotropins from anterior pituitary (FSH and LH) control synthesis of both androgens and estrogensFSH and LH controlled by gonadotropin-releasing hormone (GnRH) from hypothalamus

Wrapping it up…so to speak

Pineal gland is small mass of tissue near center of mammalian brainIt secretes melatonin which regulates functions related to light and to seasons marked by changes in day length (biorhythms)Pineal contains light sensitive cells or has nervous connections from the eyes (depends on species)Some studies indicate it has role in fertility

Wrapping, cont.Thymus is located just posterior to sternumProduces thymosinPromotes development and maturation of lymphocytesImportant in immunityLarger and most active in younger years, by adulthood, has atrophied and is mostly just gone….as is this chapter…