The Endocrine System

1
Chapter 16

• The Endocrine System
• J.F. Thompson, Ph.D. J.R. Schiller, Ph.D.
  G.R. Pitts, Ph.D.

2
Endocrine System An Overview

• The bodys second homeostatic control system
• Uses hormones as control agents
• Hormones chemical messengers released into the
  blood to regulate specific body functions
• Hormones are secreted by endocrine (ductless)
  glands and tissues
• Endocrinology the scientific study of hormones
  and the endocrine organs

3
Hormones Regulate

• Volume chemical composition of the
  extracellular fluid (ECF)
• Metabolism and energy balance
• Contraction of smooth and cardiac muscle fibers
  and many glandular secretions
• Homeostasis during normal and emergency
  conditions
• Some immune system activities
• Coordinated, sequential growth, development, and
  maturation
• Reproduction by regulating
• gamete production
• fertilization
• nourishment of the embryo and fetus
• labor and delivery
• lactation for nourishment of the infant

4
Nervous vs. Endocrine Systems

• rapid
• action potentials (nerve impulses) propagated via
  nerve fibers
• neurotransmitters released at specific
  effector(s)
• nerve impulses are brief (msecs/seconds),
  although control can be sustained
• response of effectors is of relatively short
  duration (seconds/minutes)

• slower
• hormones released into body fluids circulated
  throughout the body in the blood
• all body cells exposed only target cells with
  receptors respond
• hormones persist for seconds/hours/days
• responses of target cells may last
  seconds/hours/days, even weeks/months

5
Endocrine versus Exocrine Glands

• All glands
• have extensive capillary blood supply
• form a discrete structure/organ
• Endocrine glands
• secrete hormones into surrounding tissue fluid by
  exocytosis and the blood transports them to
  target cells
• Exocrine glands
• secrete various compounds by exocytosis into a
  duct system
• Mixed glands
• both endocrine and exocrine functions

6
Six Pure Endocrine Glands

• pineal
• pituitary
• thyroid
• parathyroid
• adrenal cortex/medulla
• thymus

7
Other Endocrine System Components

• mixed glands
• pancreas
• gonads ovaries testes
• other endocrine tissue
• stomach and intestines
• skin and adipose tissue
• heart
• kidneys
• placenta
• neuroendocrine organs
• Hypothalamus/Pituitary gland

8
Types of Chemical Regulators

• Circulating hormones (endocrines) travel via the
  blood to reach all tissues, and may affect
  distant target cells
• Local hormones diffuse into local interstitial
  fluid, reach and affect only local target cells
• paracrine - acts on target cells close to the
  site of release
• autocrine - acts on the same cell which secreted
  it
• for the various immune system local hormones, see
  Chapter 21 (cytokines, lymphokines, etc.)

9
Circulating vs. Local Hormones

• Local hormone molecules are usually short lived,
  and inactivated quickly
• Circulating hormone molecules linger in the
  bloodstream, and exert their effects for minutes
  or hours
• inactivated by enzymes in the target tissues or
  in the bloodstream or in the liver some hormones
  are also eliminated by the kidneys
• kidney or liver disease may cause problems due
  to increased hormone levels

10
The Chemistry of Hormones

• Two main chemical classes of circulating
  hormones
• Amino acid based
• amines - from single amino acids
• peptides short sequences of amino acids
• proteins - long chains of amino acids
• Steroids synthesized from cholesterol
• A third category exists, if local hormones are
  included
• eicosanoids synthesized from a cell membrane
  fatty acid (arachidonic acid)

11
Mechanisms of Hormone Action

• Hormones may alter cell activities and metabolism
  by
• Changing membrane permeability or membrane
  potential by opening or closing gated ion
  channels
• Synthesis of proteins, lipids, or carbohydrates
  or certain regulatory molecules within the cell
• Enzyme activation or deactivation
• Induction or suppression of secretory activities
• Stimulation of mitosis (and meiosis in the stem
  cells in the gonads)

12
Second Messenger Systems

• Most amino acid, peptide and protein hormones
• Are water soluble/lipid insoluble (hydrophilic)
• Cannot cross the cell membrane
• Need a second messenger to exert their effects

13
Second Messenger Systems

• Since amino acid based hormones cannot enter
  cells, a 2nd messenger must convey the hormone
  signal to the inside of the cell (the hormone is
  the 1st messenger)
• Molecules that serve as second messengers
  include
• cyclic AMP ? activates protein kinases
• cyclic GMP ? inactivates protein kinases
• IP3 (inositol triphosphate) ? Ca2 ions released
• Ca2 ions that may bind to calmodulin

14
Cyclic AMP (cAMP)

• Hormone A (excitatory) binds membrane receptor,
  activating Gs
• Gs stimulates adenylate cyclase (AC)
• AC forms cAMP from ATP
• cAMP activates Protein Kinase A
• PKA activates/deactivates other enzymes
  stimulates cell secretion opens ion channels,
  etc.

• Hormone B (inhibitory) binds its membrane
  receptor, activating Gi
• Gi inhibits adenylate cyclase
• Antagonistic control

15
Second Messengers (cont.)

• Two second messengers may work together (e.g.,
  IP3 Ca2)
• Twice as much activation
• Activate enzymes and trigger other intracellular
  activities

16
Amplification by Hormones

• Hormones are in very low concentrations in body
  fluids
• They bind reversibly to target cell membrane
  receptors
• Second messengers initiate a cascade of events (a
  snowball effect) because they activate enzymes
  that act on other enzymes
• This cascade effect amplifies the effect of small
  quantities of hormone binding to cells

17
Amplification the Cascade Effect

• For instance, consider a single hormone molecule
  binding to a specific receptor on a cell surface
• It may activate 10 membrane proteins
• Each membrane protein may activate 10 adenylate
  cyclase enzymes to produce 1000 cAMPs
• This produces a total of 100,000 second
  messengers in the cell which act on various
  cytoplasmic enzymes
• Each enzyme may then activate hundreds/thousands
  of other protein molecules

18
Steroid Hormone Action

• Steroid hormones (derived from cholesterol) are
  lipid soluble and penetrate the cell membrane
• Bind to cytoplasmic receptors inside the cell
• Hormone-receptor (h-r) complex enters the
  nucleus, binds to a DNA receptor protein
• This causes transcription of certain genes, and
  thus produces specific proteins
• This direct gene activation is a slower process,
  but with longer lasting effects

19
Target Cell Specificity

• Target cells have specific cell surface or
  cytoplasmic receptors which bind to a specific
  hormone
• A target cell has 2,000 to 100,000 receptors for
  each hormone to which they respond
• down-regulation reduction in the number of
  receptors when a hormone is present in excess so
  target tissues become less sensitive
• up-regulation increase in the number of
  receptors when hormone is deficient so that
  target tissues become more sensitive

20
Hormone Interactions at Targets

• Permissveness one hormone allows another hormone
  to cause an effect
• ex thyroid hormone permits reproductive hormones
  to cause their effects on reproductive
  development
• Synergism effect of two hormones acting
  together is greater than either acting alone
• ex glucagon and epinephrine together cause more
  increase in blood glucose than either alone
• Antagonism one hormone has an opposite effect to
  another hormone
• ex glucagon elevates blood glucose, insulin
  lowers blood glucose

21
Control of Hormone Release

• Humoral Control/Autocontrol levels of substances
  in the blood regulate the release of the hormone,
  e.g.
• Ca2 levels in blood regulate PTH release by the
  parathyroid gland
• Glucose levels in blood regulate insulin and
  glucagon release by the pancreatic islets
• Na and K levels in the blood regulate
  aldosterone release by the adrenal cortex

22
Control of Hormone Release

• Nervous System Control neural input stimulates
  the release of specific hormones, e.g.
• Sympathetic ANS stimulation of the adrenal glands
  cause them to release epinephrine and
  norepinephrine
• Nerve impulses from the hypothalamus cause
  oxytocin release from the posterior pituitary
  during labor or breast feeding
• Nerve impulses from hypothalamus cause ADH
  release from the posterior pituitary when water
  concentration of blood declines

23
Control of Hormone Release

• Hormonal Control hormones stimulate the release
  of other hormones
• Neurohormones from the hypothalamus stimulate the
  anterior pituitary to release hormones which, in
  turn, stimulate the thyroid gland, the adrenal
  cortex, and the gonads, respectively, to release
  their hormones

24
What To Know About Every Endocrine Organ For The
Exam

• Name and location of each endocrine gland
• Names and acronyms of hormones secreted by each
  endocrine gland
• Chemical class of the hormone(s) (amine,
  peptide/protein, or steroid)
• Release mechanisms for the hormone(s)
• Antagonistic control to reduce the release of the
  hormone(s)
• Target tissues or cells for each hormone
• Major responses of the target tissues or cells to
  each hormone

25
The Pituitary Gland

• Two structural components with different
  embryological origins

Anterior Lobe (Adenohypophysis)
Posterior Lobe (Neurohypophysis)
26
The Master Gland

• The pituitary gland has two functional components
• Anterior pituitary
• Adenohypophysis
• Primarily glandular tissue
• Synthesizes protein hormones
• Posterior pituitary
• Neurohypophysis
• Primarily neuosecretory cells (their cell bodies
  in the hypothalamus)
• Secretes peptide hormones
• Some support/glial cells

27
The Pituitary Gland

• Connected to the hypothalamus by the infundibulum
• Vascular linkage
• hypothalamus to the anterior pituitary
• two capillary beds the hypophyseal portal
  system
• Nervous linkage
• hypothalamus to the posterior pituitary
• hypothalamic neuron axons

28
Regulation of Pituitary Hormone Release

• Anterior pituitary
• hypothalamic releasing and inhibiting
  hormones/factors
• transported via blood
• in the hypophyseal
• portal system
• Posterior pituitary
• neuroendocrine release from neurosecretory cells
• hormones produced in hypothalamus and
• released from axon end bulbs in the posterior
  lobe

29
Anterior Lobe / Adenohypophysis

• Growth Hormone human growth hormone (hGH)
• Release
• stimulated by GHRH from the hypothalamus
• negative feedback regulation by low blood levels
  of GH
• inhibited by GHIH (somatostatin) from the
  hypothalamus
• Actions
• targets especially liver, muscle, bone,
  cartilage also most tissues
• stimulates growth, mobilizes fats, elevates blood
  glucose (insulin antagonist)

30
Anterior Lobe / Adenohypophysis

• Growth Hormone
• pathologies
• hyposecretion pituitary dwarfism (normal
  trunk/limb proportions)
• hypersecretion
• childhood pituitary gigantism
• adulthood - acromegaly

31
Anterior Lobe / Adenohypophysis

• Thyroid Stimulating Hormone (TSH)
• Release
• stimulated by
• TRH from hypothalamus
• indirectly by pregnancy and body temperature
• inhibited by negative feedback from the thyroid
  hormones and GHIH (somatostatin)
• Actions
• targets thyroid gland
• stimulates thyroid hormone release (T3 and T4)

32
Anterior Lobe / Adenohypophysis

• Thyroid Stimulating Hormone (TSH)
• pathologies
• hyposecretion hypothyroidism
• hypersecretion -- hyperthyroidism

myxedema
thyroid cretinsim
exophthalmia
33
Anterior Lobe / Adenohypophysis

• Adrenocorticotropic Hormone (ACTH)
• Release
• stimulated by corticotropin releasing hormone
  (CRH) from hypothalamus
• inhibited by negative feedback by glucocorticoids
  from adrenal gland (and by chronic use of
  therapeutic anti-inflammatory steroids)
• Actions
• targets adrenal cortex
• stimulates release of glucocorticoids (and to a
  lesser degree -- gonadocorticoids, and
  mineralocorticoids)

34
Anterior Lobe / Adenohypophysis

• Adrenocorticotropic Hormone (ACTH)
• pathologies
• hyposecretion Addisons Disease
• hypersecretion Cushings Disease (pituitary
  tumor)

hyperpigmentation
Cushings Disease - edema
35
Anterior Lobe / Adenohypophysis

• Follicle Stimulating Hormone (FSH)
• Release
• stimulated by gonadotropin releasing hormone
  (GnRH) from hypothalamus
• inhibited by negative feedback
• estrogen and inhibin in females
• testosterone and inhibin in males
• Actions
• targets ovaries and testes
• female
• stimulates ovarian follicle to mature
• stimulates production of estrogen
• male - stimulates sperm production

36
Anterior Lobe / Adenohypophysis

• Luteinizing Hormone (LH) Interstitial Cell
  Stimulating Hormone (ICSH) in males
• Release
• stimulated by GnRH
• inhibited by negative feedback
• estrogen and progesterone in females (except
  during LH surge)
• testosterone in males
• Actions
• targets ovaries and testes
• stimulates
• females - ovulation and production of estrogen
  and especially progesterone
• males production of androgens, e.g.,
  testosterone

37
Anterior Lobe / Adenohypophysis

• Prolactin
• Release
• stimulated by an unidentified Prolactin Releasing
  Hormone (PRH) from the hypothalamus
• enhanced by estrogens, birth control pills and
  breast feeding
• inhibited by
• dopamine Prolactin Inhibiting Hormone (PIH)
• lack of neural stimulation (no suckling)
• Actions
• targets breast secretory tissue
• stimulates milk production for lactation

Note The seventh anterior pituitary hormone,
Melanocyte Stimulating Hormone MSH is of
limited importance in humans.
38
Posterior Lobe / Neurohypophysis

• Oxytocin
• Release
• positive feedback
• uterine stimulation (stretch) and suckling
  stimulate the hypothalamus to release oxytocin
  from the posterior pituitary
• stimulates uterine contractions (labor) and milk
  letdown
• increases feedback for more oxytocin release
• inhibited by lack of these stimuli
• Actions
• targets smooth muscle of the uterus and the
  breast
• stimulates uterine contractions and milk
  ejection/letdown

39
Posterior Lobe / Neurohypophysis

• Antidiuretic Hormone (ADH) or Vasopressin
• Release
• stimulated by impulses from hypothalamus in
  response to
• increased osmolarity (dehydration)
• decreased blood volume or blood pressure
• stress
• inhibited by adequate hydration or ethanol
  ingestion
• Actions
• (1) targets kidney (ADH effect)
• stimulates kidney tubule cells to reabsorb water
• NaCl (salt) will be conserved passively to some
  degree
• (2) targets vascular smooth muscle to constrict
• elevates blood pressure (vasopressin effect)

40
Thyroid Gland

• Located in the anterior neck inferior to the
  larynx (Adams apple)
• Two lateral lobes connected by isthmus
• The largest pure endocrine gland in the body
• Has a rich blood supply

41
Thyroid gland (continued)

• Structure
• Spherical follicles
• lined with cuboidal follicular cells
• site of production of thyroid hormones
• thyroxine (T4)
• (tetraiodo- thyronine)
• triiodothyronine (T3)
• amine hormones
• Parafollicular (C cells)
• between follicles
• produce calcitonin (thyrocalcitonin)
• a protein hormone

The interior of the follicle contains the thyroid
colloid which is the inactive storage form of
thyroid hormones, called thyroglobulin.
42
Thyroid Gland (continued)

• Thyroid Hormones
• thyroxine (T4) and triiodothyronine (T3)
• amine hormones unusual in penetrating its
  target cells to bind with cytoplasmic receptors
• formed from an amino acid (AA) tyrosine
• two linked tyrosines with iodine atoms covalently
  bound
• 4 iodine atoms - thyroxine (T4)
  tetraiodothyronine
• 3 iodine atoms - triiodothyronine (T3)

43
Thyroid Hormones (continued)

• Actions
• targets all tissues except adult brain, spleen,
  testes, uterus and thyroid gland
• carried in blood attached to a transport protein,
  only active when freed from the transport protein
  to diffuse into the tissues
• stimulates glucose metabolism
• increases basal metabolic rate
• increases body heat thermogenesis
• important regulator of growth and development in
  conjunction with hGH
• Regulation
• decreased levels of thyroid hormones stimulate
  TRH and TSH
• hypothalamic TRH stimulates the anterior
  pituitary to release TSH which stimulates the
  thyroid to release thyroid hormones

44
Thyroid Gland Pathologies

• Hypothyroidism
• adults myxedema
• lethargic, low metabolism, puffy eyes, easily
  chilled, mental impairment
• if due to lack of iodine, then a goiter -
  increased thyroid size
• infants cretinism
• short, thick body, mental retardation
• improper development
• Note the defect may be in the pituitary gland
  or in the thyroid gland itself

45
Thyroid Gland Pathologies

• Hyperthyroidism Graves disease among others
• body produces autoantibodies which bind and
  stimulate the TSH receptor inappropriately
• stimulates excess thyroid hormone production
• causes elevated metabolic rate, sweating, rapid
  heartbeat, high blood pressure, nervousness,
  bulging eyes (exophthalmia)
• Note the defect may be in the pituitary gland
  or in the thyroid gland itself

46
Thyroid Hormones (continued)

• (Thyro)Calcitonin
• A protein hormone
• Release
• from parafollicular (C) cells in thyroid tissue
  (between the follicles)
• triggered by elevated blood calcium levels
• Actions
• targets bones, primarily in childhood
• inhibits osteoclast activity (stops bone
  resorption)
• stimulates osteoblasts for calcium uptake and
  incorporation into hydroxyapatite in the bone
  matrix
• Net effect decreases blood Ca2 levels

47
Parathyroid Glands

• Typically four small glands
• on the posterior surface of the thyroid gland
• Filled with chief (principal) cells which secrete
  parathyroid hormone (PTH or parathormone)
• Oxyphil cells larger cells, function unknown
• PTH is a protein hormone

48
Parathyroid Hormone (PTH)

• Release - negative feedback
• stimulated by low blood Ca2 levels
• inhibited by high blood Ca2 levels
• Targets
• Bone osteoclasts dissolve matrix liberating Ca2
  and PO4- ions
• Intestine absorb Ca2 and PO4- ions
• Kidney reabsorb Ca2 and eliminate PO4- ions
• activates vitamin D to active vitamin D3
  (calcitriol), enhances Ca2 absorption at the
  intestine
• Net effect elevates blood Ca2 levels

49
The Adrenal Glands

• Paired glands near the tops of the kidneys
• Two separate parts
• adrenal medulla
• interior of the gland
• derived from nervous tissue works with the
  sympathetic division of the ANS
• adrenal cortex
• exterior region of gland
• made up of three layers
• zona glomerulosa
• zona fasciculata
• zona reticularis
• glandular epithelial tissue

50
The Adrenal Cortex

• Multi-enzyme pathways convert cholesterol into
  the various steroid hormones
• Synthetic enzymes are organized in the layers of
  the cortex
• zona glomerulosa (outer)
• produces mineralocorticoids (aldosterone)
• controls homeostasis of electrolytes (ions) and
  water
• zona fasciculata (middle)
• produce glucocorticoids (cortisol)
• involved in glucose metabolism and overall
  metabolism
• zona reticularis (inner)
• produce male and female gonadocorticoids in small
  quantities
• insignificant contribution to reproductive
  functions

51
Mineralocorticoids

• Regulate electrolyte (ion) levels, particularly
  Na and K
• movement of other ions (K, H, Cl-, HCO3- ,etc.)
  is linked to Na movement
• an electrostatic equilibrium must be maintained
  therefore if certain positive ions are returned
  to the plasma, other positive ions must move into
  the urine or negative ions must move to the
  plasma to maintain the body fluid electrostatic
  (charge) equilibrium
• water follows Na and Cl- by osmosis
• play an important role in blood pressure
  regulation and regulation of acid-base balance
• Aldosterone
• the primary mineralocorticoid in humans
• causes Na and Cl- reabsorption into the blood
  plasma, by targeting the kidney, and causes K
  excretion into the urine
• water is conserved passively because it follows
  NaCl movement

52
Control of Aldosterone Release

• Aldosterone release from the zona glomerulosa is
  regulated by
• decreasing plasma levels of Na and increasing
  levels of K which trigger aldosterone release
• increasing plasma levels of Na and decreasing
  levels of K inhibit aldosterone release
• ACTH
• usually does not stimulate much mineralocorticoid
  release
• but at high levels, ACTH will stimulate
  aldosterone production

53
The Renin-Angiotensin System

• The kidneys monitor Na levels
• If Na is low, special kidney cells release renin
  (enzyme)
• Renin catalyzes the formation of angiotensin I
  from angiotensinogen
• ACE (angiotensin converting enzyme) catalyzes
  formation of angiotensin II (hormone)
• AII has many functions
• stimulates aldosterone release from adrenal
  cortex
• increases Na reabsorption at the kidney
• potent vasoconstrictor
• stimulates thirst

of lungs
54
Atrial Natriuretic Peptide (ANP)

• Aldosterone is inhibited by Atrial Natriuretic
  Peptide (ANP)
• ANP is released from the hearts atrial walls in
  response to
• increase in blood pressure
• increased stretch of the atrial walls
• ANP actions
• increases Na excretion and K retention at the
  kidney
• inhibits aldosterone release and the
  renin-angiotensin system
• decreases blood pressure

55
Glucocorticoids

• Influence cellular metabolism and respond to
  stress and inflammation
• Cortisol (hydrocortisone), cortisone,
  corticosterone
• Release (from the zona fasciculata)
• regulated by negative feedback
• stimulated by ACTH from the anterior pituitary
• negative feedback inhibition by increasing levels
  of glucocorticoids
• Actions
• targets most tissues
• promotes hyperglycemia (insulin antagonist)
• mobilizes fats for catabolism (energy production)
• mobilizes protein for catabolism (energy
  production)
• resistance to stress by providing nutrient
  building blocks
• depresses inflammatory response and immune system
  as a normal part of immune system regulation

56
Gonadocorticoids

• Production by the adrenal cortex is relatively
  unimportant
• Produced in small amounts at the zona reticularis
• Both males and females produce small quantities
  of both androgens and estrogens, even before
  puberty
• androgens male sex hormones
• primarily androstenedione - a precursor to
  testosterone
• estrogens

57
The Adrenal Medulla

• A modified sympathetic ganglion in which the
  postganglionic neurons have become specialized
  neurosecretory cells
• Produces two very chemically similar amine
  hormones
• Stimulated by the sympathetic nervous system to
  release epiniphrine and norepinephrine (NE) into
  the bloodstream, targeting cells with NE
  receptors
• Causes brief excitatory responses
• the same responses as elicited by the sympathetic
  nervous system stimulation
• these circulating hormones bind to the same
  adrenergic receptors in target organs that are
  stimulated by the ANS

58
Major Endocrine Glands
long term

• The Adrenal Gland and Stress

short term
59
The Pancreas

• a soft, fragile organ in abdomen beneath the
  stomach
• a mixed gland with both exocrine and endocrine
  functions
• acinar cells (exocrine)
• secrete various digestive enzymes
• pancreatic islets of Langerhans (endocrine)
• produces protein hormones
• alpha cells secrete glucagon
• beta cells secrete insulin
• other endocrine cell types present in small
  numbers

60
Glucagon from Alpha Cells

• Release direct assesment of the blood glucose
  (humoral influence)
• triggered by hypoglycemia (decreased blood
  glucose levels)
• also stimulated by increased plasma levels of
  amino acids
• Actions
• primarily targets the liver
• increase release of glucose into blood (insulin
  antagonist)
• stimulates glycogenolysis (breakdown of glycogen
  to glucose)
• stimulates gluconeogenesis (synthesis of
  newglucose from amino acids, lipids and lactic
  acid)

61
Insulin from Beta Cells

• Release - direct assesment of the blood glucose
  (humoral influence)
• triggered by hyperglycemia (increased blood
  glucose levels)
• triggered by increased levels of amino acids and
  fatty acids
• Actions
• targets most cells in the body (except nervous
  tissue) to increase glucose uptake
• increases glucose metabolism
• increases glycogen synthesis
• increases conversion of glucose to fat
• inhibits breakdown of glycogen and gluconeogenesis

62
Insulin Pathologies - Diabetes

• Diabetes mellitus
• insulin problems result in sustained increased
  blood glucose levels
• physiological changes
• polyuria - excessive urination and resulting
  dehydration
• polydypsia - excessive thirst
• polyphagia - excessive hunger despite
  hyperglycemia
• often, weight loss over time
• increased susceptibility to injuries and
  infections
• ketoacidosis - fat metabolism yields ketone
  bodies including acetone which can be smelled
• cardiovascular and neurological problems

63
Types of Diabetes Mellitus

• Type I - insulin-dependent diabetes mellitus
  (IDDM)
• rapid onset of symptoms prior to age 15
• old name juvenile onset
• lack of insulin activity - insulin production
  problems
• beta cells destroyed by the immune system
• daily, frequent dosages of insulin
• Type II - non-insulin-dependent diabetes mellitus
  (NIDDM)
• old name adult onset
• usually in overweight individuals
• some insulin is produced by islets but body cells
  do not respond adequately to the insulin a lack
  of sensitivity
• insulin receptors do not respond to insulin
• management by diet and exercise or by oral
  antihyperglycemic drugs

64
The Gonads

• Male Testes
• Female Ovaries
• A Preview of Chapters 27 28

65
The Ovarian Cycle

• Controlled by FSH and LH from the adenohypophysis
• The target organ is the ovary, which becomes
  responsive at puberty
• The ovary releases estrogens and progesterone in
  varying proportions depending on the mix of FSH
  and LH during the 28 day cycle
• A midcycle pulse of LH triggers ovulation

ovulation
66
The Menstrual Cycle

• Is controlled by estrogens and progesterone from
  the ovary
• The target organ is the uterus, which becomes
  responsive at puberty
• The uterine lining increases in anticipation of
  the arrival of a developing embryo, if
  fertilization occurred at the right time during
  the 28 day cycle

• If there is no pregnancy, the uterine lining
  will be sloughed producing a discharge of tissue
  and blood, the menses

67
Pregnancy

• Placental human chorionic gonadotropin (hCG)
  provides the positive feedback loop between
  placenta and ovaries and the anterior pituitary
  during pregnancy
• Continued growth of the placenta in support of
  the developing embryo is controlled by estrogens
  and progesterone supplied by both the ovaries and
  the placenta

68
Endocrine Control of Female Cycles
69
The Testes

• Structure
• seminiferous tubules with interstitial cells
  between the tubules
• seminiferous tubules are the site of sperm
  production
• interstitial cells between the tubules secrete
  male hormones

70
Brain-Testicular Axis in Males

• Anterior pituitary activity changes during
  puberty for males (and females)
• begins to secrete FSH, LH
• controlled by GnRH from hypothalamus
• LH stimulates the interstitial endocrinocytes
• results in testosterone production
• negative feedback regulates the levels
• FSH stimulates sustentacular cells to produce
• androgen-binding protein (ABP)
• inhibin

71
Testosterone and Other Androgens

• Secondary sex characteristics
• muscular and skeletal growth
• heavier, thicker muscle and bones in men than in
  women
• contributes to epiphyseal closure
• pubic, axillary, facial and chest hair patterns
• oil gland secretion
• laryngeal enlargement deepens the tone of voice
• Sexual functions
• male sexual behavior and aggression
• spermatogenesis
• sex drive in both male and female
• Metabolism - stimulates (anabolic) protein
  synthesis

72
Other Endocrine Tissues

• Heart
• the atria walls have special endocrine cells that
  secrete Atrial Natriuretic Peptide (ANP)
• ANP increases urine output and inhibits
  Aldosterone release in response to increased
  blood volume
• GI tract
• enteroendocrine cells scattered through digestive
  tract
• several amine and protein hormones which function
  to increase or decrease GI secretions and
  motility
• Kidney
• secretes protein hormone Erythropoietin to target
  bone marrow for red blood cell (RBC) production
• secreted in response to low RBC numbers

73
End Chapter 16