Chapter 18 The Endocrine System

Endocrine system glands

Hormone chemistry and Action Are chemically composed of either: (p. 516 in Saladin) –Ring structures = steroids –Polypeptides = ACTH, TSH, FSH, LH, oxytocin, insulin, etc. –Monoamines = dopamine, thryoxine (T3/T4) At their target cell, they may diffuse through the cell membrane and bind to a receptor site in the cytoplasm or nucleus (steroid hormones), or they may they may bind to a receptor site on the cell membrane (water- soluble hormones) and activate a first messenger (e.g. adenylate cyclase) which, in turn, activates a second messenger (cyclic AMP).

Endocrine System vs Autonomic Nervous System 1. The endocrine system releases chemical messengers (hormones) into the blood. The autonomic nervous system communicates by nerve impulses with effectors. 2. The endocrine system acts relatively slowly as compared to the autonomic nervous system.

Target cells Endocrine cells Neuron Nerve impulse Neurotransmitter Hormone in bloodstream Endocrine System vs Autonomic Nervous System

Comparisons of Nervous and Endocrine Systems

Types of Endocrine Glands Three types of glands: 1. Pure endocrine glands – thyroid, parathyroid, adrenal cortex, thymus and pineal. 2. Endocrine/exocrine glands – pancreas, ovaries and testes 3. Neuroendocrine glands – adrenal medulla and hypothalamus (supraoptic nuclei and paraventricular nuclei) to posterior pituitary.

Endocrine Organs Hypothalamus- neuroendocrine gland Anterior pituitary gland- endocrine gland Posterior pituitary gland- neuroendocrine gland Thyroid gland- endocrine gland Parathyroid glands- endocrine gland Adrenal gland (cortex and medulla)- endocrine/neuroendocrine gland Pancreatic islets- endocrine/exocrine gland Gonads- Ovaries in females; Testes in males- endocrine/exocrine glands

The Hypothalamus Location: directly below the thalamus in the diencephalon of the brain. It lies between the optic chiasm anteriorly and the mammillary bodies posteriorly and is inferior to the third ventricle. Structure: Composed of several groups of nuclei, the hypothalamus controls the endocrine system as well as the autonomic nervous system and produces regulatory hormones that regulate the release of numerous pituitary hormones. It also produces the hormones of the posterior pituitary.

The Hypothalamus

The Pituitary Gland or Hypophysis Location: Sits in the sella turcica of the sphenoid bone Attached to the hypothalamus by the infundibulum Consists of two lobes: 1.Adenohypophysis Releases 7 different hormones Consists of 3 divisions: pars tuberalis, pars intermedia and pars distalis (anterior lobe). 2. Neurohypophysis Releases 2 different hormones Consists of 3 divisions: median eminence, infundibular stalk and pars nervosa (posterior lobe)

Pituitary gland Adenohypophysis Pars tuberalis Pars intermedia Pars distalis Neurohypophysis Median eminence Infindibular stalk Pars nervosa

Anterior Pituitary Hormones There are seven anterior pituitary hormones: –Thyroid stimulating hormone (TSH)* –Growth hormone (GH) –Adrenocorticotropic hormone (ACTH)* –Melanocyte stimulating hormone (MSH) –Follicle stimulating hormone (FSH)* –Luteinizing hormone (LH) = ICSH in males* –Prolactin (PRL) * indicate trophic hormones

Hypothalamic releasing hormones Release of anterior pituitary hormones is directed by specific releasing hormones (factors) from the hypothalamic nuclei. All of these are polypeptide molecules. TRH – thyrotropin releasing hormone → (TSH and PRL) TRH – thyrotropin releasing hormone → (TSH and PRL) GHRH – growth hormone releasing hormone → (GH) Somatostatin – inhibits release of growth hormone CRH – corticotrophin releasing hormone → (ACTH) MRH- melanocyte releasing hormone → (MSH) MIF- inhibits release of MSH GnRH – gonadotropin releasing hormone → (FSH/LH) PRH – prolactin releasing hormone → (PRL) PIH – prolactin inhibiting hormone (dopamine)

Anterior/Posterior Pituitary Circulation Blood flow to pituitary gland is via a portal circulation the hypophyseal portal. Arterial flow is via superior and inferior hypophyseal artery into capillary beds in series

Posterior Pituitary Hormones ADH an Oxytocin are secreted by neurosecretory cells in the paraventricular and supraoptic nuclei of the hypothalamus and are transported to posterior pituitary via hypothalamohypophyseal tract.

Neurohypophyseal Hormones Antidiuretic hormone (ADH) – produced by supraoptic nuclei in the hypothalamus. –Consists of 9 amino acids –Reduces the excretion of water by kidney collecting ducts; increases cuddling and grooming behavior. Oxytocin – produced by the paraventricular nuclei in the hypothalamus –Consists of 9 amino acids, but differs from ADH. –Induces smooth muscle contraction; increases cuddling and grooming behavior.

Adenohypophyseal cell types Thyrotropic cells secrete TSH Somatotropic cells secrete GH Corticotropic cells secrete ACTH and MSH Gonadotropic cells secrete FSH and LH –Tropic hormones regulate the release of other hormones from the glands that they stimulate (TSH, ACTH, FSH and LH). MSH, PRL and GH all act directly on non-endocrine target tissues.

Thyroid gland Location: largest pure endocrine gland in adults ~ gms. and located adjacent to trachea inferior to larynx. Structure: Butterfly shaped with two lobes joined by an isthmus. ~ 50% of people have a pyramidal lobe growing upward off of isthmus. Gross Anatomy: Bulbous at inferior end and tapers superiorly. - Thyroid is highly vascular via thyroidal arteries. Cellular Anatomy: Composed of sacs of thyroid follicular cells and lined with simple cuboidal or simple squamous epithelium that is filled with protein rich colloid (thyroglobulin).

Thyroid gland Follicular cells produce tri-iodo thyronine (T3) and thyroxine (T4) which are stored in thyroglobulin. –Target cells are every cell and tissue in the body Parafollicular or “C” cells found between follicular cells in the thyroid gland produce calcitonin which keeps blood Ca++ levels within the normal range by depositing excess Ca++ in the bones and teeth. –Target cells are osteoblasts in bone –Has no demonstrable function in adults, most active in fetus, infants and adolescents.

Thyroid gland

Parathyroid gland Located on the posterior lateral margins of the thyroid gland are 4 to 8 small nodules. Structure is small ovoid nodules ~ 2-5 mm x 3-8 mm. Produces parathyroid hormone (PTH) which helps regulate blood Ca++ levels. Target organs of PTH are bone, kidneys and intestines. Histologically it contains numerous small chief cells and rare large oxyphilic cells. –Chief cells secrete PTH. –Oxyphilic cells are probably inactive or immature chief cells.

Parathyroid glands

Adrenal glands Located in the abdominal cavity attached to superior pole of each kidney (suprarenal). Two distinct regions: Cortex and Medulla Adrenal cortex has 3 layers: Zona glomerulosa – outer layer → mineralocorticoids. Zona fasciculata - middle ¾ of cortex → glucocorticoids. Zona reticularis – innermost layer → androgens Adrenal Medulla is neuroendocrine tissue and is part of sympathetic division (postganglionic) of ANS.

Adrenal glands Blood supply is via: Blood supply is via: Superior suprarenal from Inferior phrenic arteries. Middle suprarenal and Inferior suprenal off of aorta.

Adrenal Cortex Histologic features of adrenal cortex: Outer layer is a dense fibrous capsule. Zona glomerulosa (15% ov) looks like little balls or knots densely clustered together. Zona fasiculata (78% ov) looks like cords that radiate toward the medulla. Zona reticularis (7% ov) branching network of pink staining cells between fasciculata and medulla. Adrenal medulla is composed of chromafin cells arranged in spherical clusters.

Adrenal gland histology

Pancreas Location: Just inferior to the stomach and in the first loop of the duodenum approximately in the middle of the abdomen. Structure:- mixed gland (endocrine/exocrine); spongy-like appearance. Exocrine cells produce digestive enzymes. Pancreatic “Islet of Langerhans” are endocrine cells. Hormones produced by 5 classes of islet cells include: –α-cells → Glucagon- a 29 amino acid molecule which targets the liver to breakdown glycogen and release glucose. –β cells → Insulin- a 51 amino acid molecule which targets the liver and most body cells except the brain to take up glucose. –Delta cells → Somatostatin ↓ release of insulin & glucagon. –“F” cells → Pancreatic polypeptide ↓ gall bladder contraction. –“G” cells → Gastrin ↑ acid secretion, gastric motility and stomach emptying.

Pancreas

Ovaries Primary sex organs of females Located retroperitoneal in the abdominal cavity lateral to the uterus and at the proximal end of the uterine tubes (fallopian tubes). Pair of almond shaped organs ~ 3 cm x 1.5 cm x 1 cm. Produce female sex hormones (estrogen and progesterone) and contain ova. More about the ovaries in reproduction.

Ovaries

Testes Primary male sex organs. Located in the scrotum outside of abdominal cavity. Produce sperm and male sex hormones androgens (testosterone and inhibin). Size ~ 4 cm ↑ x 3 cm a/p x2.5 cm →. More about the testes in reproduction

Testes

Thymus Located in mediastinal space of the thoracic cavity deep to sternum and supeficial to the pericardium. Produces several hormones amongst which are thymosin, thymopoietin, and IGF-1. Stimulates the maturation of T- lymphocytes Largest size occurs at puberty and thereafter diminishes in size as one gets older. By the age of 50 it is ~ ¼ its original size. Largest size occurs at puberty and thereafter diminishes in size as one gets older. By the age of 50 it is ~ ¼ its original size.

Thymus

Pineal gland or “epiphysis cerebri” Part of the epithalamus in the brain Contains neurons, glial cells and pinealocytes which produce and secrete melatonin. Melatonin regulates the circadian cycle as well as slows the maturation of sperm and ova by inhibiting FSH and LH release from the adenohypophysis.

Pineal gland

Other endocrine like organs Heart secretes atrial naturetic peptide (ANP). Skin initiates synthesis of calcitrol → vitamin D. Kidneys secrete renin, erythropoietin, and aids in → vitamin D synthesis. Liver secretes erythropoietin and angiotensinogen. The liver also aids in → vitamin D and insulin-like GF. Placenta secretes human gonadotropin. Stomach secretes gastrin and cholecystokinin and other enteroendocrine hormones that affect digestion.

Hypothalamic releasing and inhibiting hormones

Pituitary Hormones