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  • 1. The Endocrine System Chapter 25
  • 2. The Endocrine System
    • A system of ductless glands that secrete hormones (‘messenger’) molecules
    • - secrete hormones directly into the blood or lymph
    • - hormones trigger physiological changes in target cells
    • Controls and integrates the functions of other body systems
    • - closely interacts with the nervous system
    • Endrocrinology
    • - the study of hormones and endocrine glands
  • 3.
    • Nervous System
    • Controls homeostsis rapidly
    • Anatomically continuous:
    • nerve impluse conducted along axons from one neuron to the next
    • Neurotransmitters (NTs)
    • Brief effect (muscle contraction)
    • Endocrine System
    • Controls growth and metabolism slowly
    • Scattered:
    • messenger molecules released into the EC space will immediately enter adjacent capillaries
    • Hormones (‘to excite’)
    • Longer lasting effect with feedback loops
    Together these systems interact to coordinate and integrate activity of our cells
  • 4. Endocrine Organs
    • Series of ductless glands
    • - small and scattered throughout the body
    • - some may be both endocrine and exocrine
    • ‘ Pure’ endocrine glands
    • - pituitary, pineal, thyroid, parathyroid, and adrenal
    • Organs containing endocrine cells
    • - pancreas, thymus, gonads, and hypothalamus
    • Most endocrine cells are of epithelial origin
    • - others include hormone-secreting neurons, muscle cells, and fibroblast-like cells
    • Highly vascularized – blood and lymph vessels
  • 5. Location of the Major Endocrine Glands
    • Endocrine cells also
    • occur in the heart,
    • alimentary canal,
    • kidney, skin, placenta,
    • and elsewhere
    Fig 25.1
  • 6. Endocrine System Overview
    • Endocrine glands may be stimulated by the nervous system or chemical changes in the body
    • - respond by secreting hormones into the circulation
    • Hormones travel through the bloodstream but affect only specific tissues called target tissues
    • Hormones secreted by cells regulate the metabolic function of other cells in the body
    • - effects result from pre-programmed responses of target cells
  • 7. Endocrine Functions
    • Regulation of:
    • - Internal environment (adjust fluid/volume ratio): aldosterone
    • - Metabolism and energy balance: thyroid hormones
    • - Cardiac and smooth muscle contraction: epinephrine and norepinephrine
    • - Immune system: cytokines
    • - Glandular secretions: hypothalamus and pituitary hormones
    • Maintainance and assistance of:
    • - Homeostasis despite disruptions: pancreatic hormones
    • - Growth and development: growth hormones
    • - Reproduction: hormones that influence oogenesis and spermatogenesis
  • 8. Classes of Hormones
    • The body produces many different kinds of hormones with distinct chemical structures
    • Two broad molecular categories:
    • 1. Amino acid-based hormones - modified amino acids (amines), peptides (short chains of amino acids), and proteins (long chains of amino acids)
    • 2. Steroid hormones - lipid molecules derived from cholesterol
  • 9. Basic Hormone Action
    • Hormones circulate throughout the body in BVs
    • - leave the bloodstream at capillaries encountering all body tissues
    • - influences only specific tissue cells or target cells
    • - same hormone can have different effects on different target cells
    • - similar molecular structures can have very different functions
    • Cells have receptors on their surface that bind only specific types of hormones
    • - receptor binding initiates a response/reaction
    • - hormones are just molecular triggers and do not carry any coded information
  • 10. Control of Hormone Secretion
    • Secretion is triggered by three major types of stimuli: humoral, neural, and hormonal stimuli
    • Humoral (‘body fluids’) - simplest of endocrine control mechanisms
    • - secretion is in direct response to changing critical ion or nutrient levels in the blood
    • - parathyroid monitors calcium: responds to decline be secreting hormone to reverse decline
    • Neural
    • - a few glands secrete their hormones in response to stimuli by the nervous system to induce physiological changes
    • - sympathetic nerve fibers stimulate cells in the adrenal medulla
    • - induces release of epinephrine and norepinephrine
  • 11. Control of Hormone Secretion
    • Hormonal
    • - many endocrine glands secrete their hormones in response to hormonal stimuli received from other endocrine glands
    • - certain hormones signal secretion of other hormones
    • - the hypothalamus secretes hormones  stimulates the pituitary  stimulates other glands (the thyroid, adrenal cortex, and the gonads)
    • Note: the hypothalamus is called the master gland
    • - controls many functions of the endocrine system, through hormonal and other mechanisms
  • 12. Control of Hormone Release: 3 Mechanisms Fig 25.2
  • 13. Control of Hormone Secretion
    • Always controlled by feedback loops
    • - ensures that hormone concentrations stay within a narrow ‘desirable’ range in the blood
    • Negative feedback loop:
    • - Hormonal blood concentration declines below a minimum set point more hormone is secreted
    • - Blood concentration exceeds a maximum set point hormone production is halted
    • Positive feedback loop:
    • - as blood concentrations of a certain hormone increase the response of the effector organ stimulates further secretion
    • - progression of labor in childbirth by oxytocin
  • 14. The Pituitary Gland (Hypophysis)
    • The pituitary gland (hypophysis - undergrowth):
    • - secretes 9 major hormone
    • - sits in the hypophyseal fossa of the sella turcica
    • - resembles a golf club: the gland forms the head of the club, and the stalk, called the infundibulum (funnel), forms the shaft
    • - the infundibulum connects superiorly to the hypothalamus
    • 2 basic divisions of the pituitary gland:
    • - Anterior adenohypophysis (adeno = glandular)
    • - Posterior neurohypophysis (neuro = neural)
    • Blood supply: 2 branches of the internal carotid artery
    • - superior hypophyseal artery supplies the adenohypophysis
    • - inferior hypophyseal artery supplies the pars nervosa
  • 15. The Pituitary Gland Figure 25.3a–c
  • 16. The Adenohypophysis
    • Hormone release is controlled by the hypothalamus
    • - stimulated and inhibited by releasing and inhibiting hormones
    • 3 subdivisions: pars distalis, pars intermedia, pars tuberalis
    • - pars distalis the largest division has 5 different endocrine cell types
    • - secrete protein hormones, have many secretory granules and a well-developed RER and Golgi apparatus
    • Somatotropic cells
    • - secrete growth hormone (GH)
    • Mammotropic cells
    • - secrete prolactin (PRL)
  • 17. Pars Distalis Endocrine Cells
    • Thyrotropic cells
    • - secrete thyroid-stimulating hormone (TSH)
    • Corticotropic cells
    • - secrete adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH)
    • Gonadotropic cells
    • - secrete follicle-stimulating hormone (FSH), luteinizing hormone (LH)
    • Tropic (‘changing’) hormones – TSH, ACTH, FSH, LH
    • - regulate secretion of hormones by other endocrine glands
    • - GH, PRL, and MSH act directly on nonendocrine target tissues
    • 5 cell types group into 3 categories when stained: acidophils, basophils, chromophobes
  • 18.
    • GH or somatotropin - stimulates growth of body tissues, especially in muscle and skeleton
    • TSH - stimulates the thyroid to produce and release T4 and T3 (influences metabolism)
    • PRL – initiates and maintains milk production (lactation) by mammary glands in the breasts
    • ACTH – influences production and secretion of hormones by the adrenal cortex (helps us deal with stress)
    • MSH - stimulates melanocytes of the epidermis to produce more melanin, thus darkening the skin
    • FSH - stimulates ova maturation and estrogen production in ovaries and sperm production in the testes
    • LH or ICSH – stimulates ovulation and progesterone secretion in ovaries and testosterone secretion in the testes
  • 19. Hypothalamic Control of Hormone Secretion from the Adenohypophysis
    • The hypothalamus have neurons that produce and release hormones much like NTs are released
    • - secretes releasing factors to release hormones
    • - secretes inhibiting hormones to turn off hormone secretion
    • - travels through the hypophyseal portal system into the anterior pituitary to stimulate its hormone secretion
    • - the hypophysial portal system involves two beds of capillaries connected by a vein
    • - allows a high level of hormone concentration within a small region
    • - designed so that the hormones released by the hypothalamus travel directly to the anterior pituitary
    • - in turn the anterior pituitary releases hormones into systemic circulation
  • 20. Hypothalamic Control of Hormone Secretion from the Adenohypophysis Fig 25.4
  • 21. The Neurohypophysis
    • Stores and releases hormones produced by the hypothalamus
    • - structurally part of the brain
    • - composed of nervous tissue (unmyelinated axons and neuroglial cells)
    • Hormones made in the neuron cell bodies, transported along the axons, and stored in dilated axon terminals ( Herring bodies ) – secrete 2 hormones
    • - antidiuretic hormone (ADH) or vasopressin (‘vessel constrictor’): targets the kidney to resorb more water from the urine and return it to the blood
    • - secretes oxytocin induces contractions of smooth muscle of reproductive organs
  • 22. The Neurohypophysis
    • When the neurons
    • fire, they release
    • stored hormones
    • into a capillary bed
    • in the pars nervosa
    • for distribution
    Fig 25.5
  • 23. Table 25.1
  • 24. The Thyroid Gland
    • Largest pure endocrine gland
    • - located in the anterior neck
    • Internally, is composed of hollow follicles
    • - separated by areolar CT rich in capillaries
    • - walls are formed of cuboidal or squamous epithelial cells (follicular cells)
    • - lying within the epithelium are parafollicular (C) cells
    • - central lumen filled with colloid (‘gluelike’) consisting of thyroglobulin (protein precursor to thyroid hormone)
    • Produces 2 hormones: amino-based and protein
    • - Thyroid hormone (TH): thyroxine (T 3 ); tri-iodothyronine (T 4 )
    • - Calcitonin: lowers blood levels of Ca 2+ , mostly during childhood
  • 25. The Thyroid Gland
    • T4 and T3, consists of 2 amino acids and iodine
    • Main function is to increase metabolic rate
    Fig 25.6
  • 26. Histology of the Thyroid Gland
    • Follicle cells continuously synthesize thyroglobulin and secrete it into the follicle lumen for iodination and storage
    • TSH (pituitary gland) signals the follicle cells to release TH
    Fig 25.6
  • 27. The Parathyroid Glands
    • Lie on the posterior surface of the thyroid gland surrounded by CT capsules (number varies)
    • Contains thick branching cords composed of 2 types of endocrine cells
    • - small abundant chief cells and rare larger oxyphil cells
    • Chief cells produce a small protein hormone, PTH
    • - PTH increases calcium levels and is essential to life:
    • 1) stimulates osteoclasts to release calcium from bones
    • 2) decreases secretion of calcium by the kidney
    • 3) activates vit D, which stimulates uptake of Ca by the intestine
  • 28. The Parathyroid Glands
    • Posterior view of the pharynx a
    • and trachea showing the location
    • of the parathyroid glands on the
    • posterior aspect of the thyroid
    Fig 25.7
  • 29. Histology of the Parathyroid Gland
    • The function of oxyphil (‘acid-loving’) cells is unknown
    • PTH is essential to life - low Ca 2+ levels lead to lethal neuromuscular disorders
    • What is the antagonist of PTH?
    Fig 25.7
  • 30. The Adrenal (Suprarenal) Glands
    • Paired pyramidal organs on the superior surface of the kidneys – highly vascularized
    • 3 groups of 60 small suprarenal arteries supply each gland
    • - the superior suprarenal arteries from the inferior phrenic artery;
    • - middle suprarenal arteries from the aorta;
    • - inferior suprarenal arteries from the renal artery
    • Veins
    • - left suprarenal vein drains into the renal vein and the right suprarenal vein drains into the inferior vena cava
  • 31.
    • Nerve supply is almost entirely sympathetic fibers
    • 2 endocrine glands in one - internal and external:
    • - Adrenal medulla: a cluster of neurons derived from the neural crest, acts as part of the sympathetic NS
    • - Adrenal cortex: bulk of the adrenal gland derived from mesoderm
    The Adrenal (Suprarenal) Glands
  • 32. The Adrenal Medulla
    • Part of the autonomic nervous system (Ch 15)
    • Chromaffin (‘affinity for chromium’) cells
    • - arranged in spherical clusters with some branching cords
    • - modified ganglionic sympathetic neurons
    • - secrete catecholamines: the amine hormones epinephrine and norepinephrine
    • - active in the ‘fight, flight, and fright’ (fight or flight) response
    • - hormones stored in secretory vesicles
  • 33. The Adrenal Cortex
    • Secretes a variety of corticosteroid hormones
    • - all are lipid-based steroids
    • Cortex is composed of 3 layers or zones:
    • - Zona glomerulosa (‘ball of yarn’): cell clusters
    • - Zona fasciculata: cells arranged in bundles
    • - Zona reticularis (‘network’): cells arranged in a branching network
  • 34. The Adrenal Gland Figure 25.8a, b
  • 35. Adrenal Corticosteroids
    • 2 main classes: mineralocorticoid & glucocorticoid
    • Main mineralocorticoid is aldosterone
    • - secreted by the zona glomerulosa
    • - in response to a decline in blood volume or BP
    • - prompts kidney to resorb more sodium into the blood; water passively follows, increasing blood volume
  • 36.
    • Glucocorticoids: cortisol is the main type
    • - secreted by zona fasciculata and zona reticularis
    • - helps the body deal with stressful situations by keeping glucose levels high to support the brain
    • - body cells switch to fats and amino acids as energy sources
    • - high amounts depress the inflammatory response
    Adrenal Corticosteroids
  • 37. Adrenal Corticosteroids
    • Hormonal pathway of stress:
    • - hypothalamus sends corticotropin-releasing hormone (CRH)
    • to the adenohypophysis, which secretes ACTH
    • - ATCH travels to the adrenal cortex to signal glucocorticoid secretions
    • - the sympathetic NS can also stimulate glucocorticoid secretions
  • 38.
    • Zona reticularis secretes an androgen hormone, dehydroepiandrosterone (DHEA)
    • - DHEA is converted to testosterone and estrogens in peripheral tissues
    • - proposed beneficial effects include counteracting stress, boosting immunity, and mood
    Adrenal Corticosteroids
  • 39. Structure of Steroid-Secreting Cells
    • Steroid-secreting cells have distinctive features
    • - abundant SER and no secretory granules
    • - mitochondria have unusual cristae shaped like tubes
    • - lipid droplets are abundant in cytoplasm (lipids = raw material of steroids)
    • Characterize cells of the adrenal cortex
    • - also testicular and ovarian cells which secrete steroid sex hormones: the interstitial cells, theca folliculi cells, and cells of the corpus luteum
  • 40. Interstitial cell in the testis Fig 25.9
  • 41. The Pineal Gland
    • Small, pine cone shaped structure at the end of a short stalk on the roof of the diencephalon
    • Pinealocytes are arranged in both spherical clusters and branching cords
    • - star-shaped cells with long, branching cell process
    • - dense particles of calcium lie between the cell clusters, forming the ‘pineal sand’ (which is radiopaque)
    • - in Xrays used as a landmark to identify brain structures
    • - secretes melatonin: a hormone that regulates circadian rhythms (hypothalamus responds to a lack of visual input)
  • 42. The Pineal Gland
  • 43. The Pancreas
    • Located in the posterior abdominal wall
    • Contains endocrine and exocrine cells
    • Exocrine acinar cells, form most of the gland
    • - secrete digestive enzymes into the small intestine
    • Endocrine cells are contained in spherical bodies
    • - pancreatic islets or islets of Langerhans
    • - about 1 million scattered among the exocrine acini
  • 44.
    • Main endocrine cell types:
    • - Alpha cells ( α cells): secrete glucagon signals liver to release glucose from glycogen; raises blood sugar
    • - Beta cells ( β cells): secrete insulin signals most body cells to take up glucose from the blood; promotes glucose storage as glycogen in liver; lowers blood sugar
    The Pancreas
  • 45. Figure 25.10 A Pancreatic Islet
  • 46. The Thymus
    • Located in the lower neck and anterior thorax
    • Important immune organ
    • Site at which T-lymphocytes arise from lymphocyte-precursor cells
    • - transformation stimulated by thymic hormones, secreted by the thymus epithelial reticular cells
    • Thymic hormones – a family of peptide molecules, including thymopoietin and thymosin
  • 47.  
  • 48. The Gonads
    • Main sources of sex hormone – testes and ovaries
    • Male testes
    • - interstitial cells secrete androgens (primarily testosterone)
    • - promotes the formation of sperm
    • - maintains secondary sex characteristics
  • 49.
    • Female ovaries
    • - androgens secreted by the theca folliculi directly converted into estrogens by the follicular granulosa cells & progesterone
    • - estrogens and progesterone secreted by the corpus luteum
    • - estrogens maintain reproductive organs and secondary sex characteristics
    • - progesterone signals uterus to prepare for pregnancy
    The Gonads
  • 50.
    • Endocrine cells occur within
    • The heart - the atria contains atrial natriuretic peptide (ANP)
    • - hormone that stimulates the kidneys to produce more urine containing salt
    • - getting rid of the excess fluid and salt reduces excess blood volume and salt levels; reduces blood pressure
    • The GI tract has scattered enteroendocrine cells
    • - release amino acid/peptide hormones chemically similar to neurotransmitters
    • - affect functions related to regulating digestion, blood chemistry, and blood flow
    Other Endocrine Structures
  • 51. Other Endocrine Structures
    • The placenta is produced when conception occurs
    • - secretes hormones that prevent the uterus from getting rid of the nutrient layer to which it is attached
    • - produces other steroid protein hormones: estrogen, progesterone, corticotropin-releasing hormone, and human chorionic gonadotropin
  • 52.
    • The kidneys
    • - cells of the juxtaglomerular apparatus (JGA) secrete rennin which regulates blood pressure
    • - endothelial cells and interstitial CT secrete erythropoietin which stimulates erythrocyte production
    • The skin
    • - when exposed to UV rays produces a steroid hormone precursor to vitamin D essential for calcium metabolism
    Other Endocrine Structures
  • 53. Pituitary Disorders
    • Gigantism
    • - hypersecretion of GH in children
    • Acromegaly
    • - hypersecretion in adults causes
    • Pituitary dwarfism
    • - hyposecretion of GH
    • Diabetes insipidus
    • - pars nervosa does not make enough ADH
  • 54. A Closer Look Potential Uses for Growth Hormone
  • 55. Disorders of the Pancreas
    • Diabetes Mellitus
    • - caused by insufficient secretion of insulin
    • - or resistance of body cells to the effects of insulin
    • Type 1 diabetes
    • - develop suddenly, usually before age 15
    • - T cell-mediated autoimmune response destroys β cells
  • 56. Diabetes Mellitus
    • Type 2 diabetes
    • - Adult onset
    • - Usually occurs after age 40
    • - Cells have lowered sensitivity to insulin
    • - Controlled by dietary changes and regular exercise
  • 57. Disorders of the Thyroid Gland
    • Grave’s Disease
    • - most common type of hyperthyroidism
    • - immune system makes abnormal antibodies
    • - stimulates the oversecretion of TH by follicle cells
    • - leads to nervousness, weight loss, sweating, and rapid heart rate
    • Myxedema
    • - adult hypothyroidism
    • - antibodies attack and destroy thyroid tissue
    • - common symptoms include low metabolic rate and weight gain
  • 58. Disorders of the Thyroid Gland
    • Endemic goiter
    • - due to lack of iodine in the diet
    • Cretinism
    • - hypothyroidism in children
    • - short, disproportionate body, thick tongue and mental retardation
  • 59. Thyroid Disorders Figure 25.11
  • 60. Disorders of the Adrenal Cortex
    • Cushing’s syndrome
    • - caused by hypersecretion of glucocorticoid hormones usually due to a pituitary tumor
    • Addison’s disease
    • - hyposecretory disorder of the adrenal cortex
    • - deficiencies of both mineralocorticoids and glucocorticoids
  • 61. Thyroid Disorders Figure 25.12
  • 62. The Endocrine System Throughout Life
    • Endocrine organs operate effectively until old age
    • Adenohypophysis
    • - increase in CT and lipofuscin
    • - decrease in vascularization and number of hormone-secreting cells
    • Adrenal cortex
    • - normal rates of glucocorticoid secretion continue
    • Adrenal medulla
    • - no age-related changes in catecholamines
  • 63.
    • Pituitary gland – dual origin
    • - adenohypophysis originates from the roof of the mouth
    • - neurohypophysis grows inferiorly from the floor of the brain
    • Thyroid hormones
    • - decrease slightly with age
    • Parathyroid glands
    • - little change with aging
    • GH, DHEA, and the sex hormones
    • - marked drops in secretion with age
    The Endocrine System Throughout Life
  • 64. Embryological Origin of Endocrine Organs
    • Thyroid gland
    • - forms from a thickening of endoderm on the floor of the pharynx
    • Parathyroids and the thymus gland
    • - from endoderm lining the pharyngeal pouches
    • Pineal gland
    • - originates from ependymal cells
    • Adrenal gland – dual origin gland
    • - adrenal medulla from neural crest cells of nearby sympathetic trunk ganglis
    • - adrenal cortex from mesoderm lining the coelom
  • 65. Embryonic Development Figure 25.13
  • 66. Figure 25.b–d