The Endocrine System: Definition The endocrine system refers to the collection of glands of an organism that secrete hormones directly into the circulatory system to be carried toward a distant target organ. A hormone is a chemical that is produced by the body and has a specific regulatory effect on a target cell or organ.

The Endocrine System: Definition The major endocrine glands include the pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. The endocrine system is in contrast to the exocrine system, which secretes its products using ducts. Examples of exocrine glands include the sweat glands, salivary glands, mammary glands, and liver.

Anatomical loci of the principal endocrine glands and tissues of the body

Coordination of Body Functions by Chemical Messengers The multiple activities of the cells, tissues, and organs of the body are coordinated by the interplay of several types of chemical messenger systems: Neurotransmitters are released by axon terminals of neurons into the synaptic junctions and act locally to control nerve cell functions. Endocrine hormones are released by glands or specialized cells into the circulating blood and influence the function of cells at another location in the body. Neuroendocrine hormones are secreted by neurons into the circulating blood and influence the function of cells at another location in the body.

Coordination of Body Functions by Chemical Messengers Paracrine communication involves cells that secrete chemical transmitters locally into the surrounding interstitial fluid; the target cells are near “neighbors” and are reached by diffusion of the hormone rather than by its transport in the blood. Autocrine signaling occurs when a cell regulates itself by the release of a chemical messenger. Cytokines are peptides secreted by cells into the extracellular fluid and can function as autocrines, paracrines, or endocrine hormones.

Classes of Hormones Most hormones can be grouped into one of three major chemical classes: Peptides Amines and Steroids.

Classes of Hormones: Peptides Peptides are the largest group of hormones. Peptide hormones are synthesized in the rough endoplasmic reticulum of endocrine cells, typically as inactive preprohormones. A series of cleavage steps occurs in the endoplasmic reticulum and during passage of the prohormones through the Golgi apparatus into the secretory vesicles. Peptide-secreting endocrine cells store active hormones in intracellular vesicles until a stimulus triggers hormone secretion by exocytosis. Peptide hormones are generally water soluble and do not require carrier molecules in the blood.

Classes of Hormones: Amines Amines are a small group of hormones that includes the catecholamines (dopamine, epinephrine, and norepinephrine) and the thyroid hormones. Catecholamines are synthesized from tyrosine and stored in preformed vesicles, awaiting release by exocytosis. Catecholamines are water-soluble hormones that do not require carrier proteins in the plasma. Thyroid hormones are also derived from the amino acid tyrosine but are poorly soluble in water and do require carrier proteins in the blood.

Classes of Hormones: Steroid Steroid hormones are synthesized from cholesterol and include cortisol, aldosterone, testosterone, estrogen, and progesterone. Steroid hormones are not stored in vesicles and rapidly diffuse out of the cell once synthesized due to their high lipid solubility. Steroids generally require carrier proteins in the blood due to their low water solubility.

Peptide Hormone VS Steroid Hormone

Hormones Produced by the Major Endocrine Organs

Hormones Produced by the Major Endocrine Organs Adrenal Cortex: Present in Kidney; Cortisol: Mediates gluconeogenesis/ Stress management; Androgen: Male sex hormone; Sertoli cell: 'nurse' cell of the testes that is part of a seminiferous tubule; Leydig cells: Found adjacent to the seminiferous tubules in the testicle; Corpus luteum: Temporary endocrine structure in female mammals that is involved in the production of relatively high levels of progesterone;

Hormones Produced by the Major Endocrine Organs

Hormones Produced by the Major Endocrine Organs

Hormones Produced by the Major Endocrine Organs Fetoplacental unit: the fetus and the placenta as a single physiological unit;

Hormones Produced by the Major Endocrine Organs Angiotensin is a peptide hormone that causes vasoconstriction and a subsequent increase in blood pressure; Renin-angiotensin-aldosterone system (RAAS) is a hormone system that regulates blood pressure and water (fluid) balance;

Transport of Hormones in the Blood Water-soluble hormones (peptides and catecholamines) are dissolved in the plasma and transported from their sites of synthesis to target tissues, where they diffuse out of the capillaries, into the interstitial fluid, and ultimately to target cells. Steroid and thyroid hormones, in contrast, circulate in the blood mainly bound to plasma proteins. Usually less than 10 per cent of steroid or thyroid hormones in the plasma exist free in solution. For example, more than 99 per cent of the thyroxine in the blood is bound to plasma proteins. However, protein-bound hormones cannot easily diffuse across the capillaries and gain access to their target cells and are therefore biologically inactive until they dissociate from plasma proteins.

Synthesis of Peptide Hormones

Hydrophilic Hormones: Mode of Action cAMP as Second Messenger 1) Hormone binding activates G protein 2) Activates adenylate cyclase 3) Produces cAMP 4) Activates kinases 5) Activates enzymes 6) Metabolic reactions: synthesis, secretion, change membrane potentials

Mechanism of Action: Steroid Hormones

Hydrophobic hormones (steroids and thyroid hormone) penetrate plasma membrane – enter nucleus Hydrophilic hormones (monoamines and peptides) can not pass through membrane so must bind to cell-surface receptors

Hypothalamus and Pituitary Gland The pituitary gland can be viewed as a master gland because it controls the secretion of several target endocrine glands. The secretion of pituitary hormones is controlled by release factors from the hypothalamus.

Anterior Pituitary Gland The anterior pituitary gland secretes the following six major peptide hormones: Growth hormone (GH) Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Prolactin

Anterior Pituitary Gland The following five major cell types are present in the anterior pituitary gland: Somatotropes secrete GH. Thyrotropes secrete TSH. Corticotropes secrete ACTH. Gonadotropes secrete both LH and FSH. Lactotropes secrete prolactin.

Anterior Pituitary Gland In most cases, the anterior pituitary hormones are influenced by the following stimulatory hypothalamic release factors: Secretion of TSH is stimulated by thyrotropin-releasing hormone (TRH). Secretion of ACTH is stimulated by corticotropin-releasing hormone (CRH). Secretion of FSH and LH is stimulated by gonadotropin releasing hormone (GnRH). Secretion of GH is controlled by a balance between the stimulatory factor growth hormone-releasing hormone (GHRH) and the inhibitory factor somatostatin.

The Thyroid Gland The thyroid gland is palpable in the anterior neck in front of the trachea, and consists of the main right and left lobes and a connecting branch, the isthmus. The thyroid hormones thyroxine (T4 ) and triiodothyronine (T3) play a major role in the overall control of the metabolic rate.

Synthesis and Secretion of Thyroid Hormones

Synthesis and Secretion of Thyroid Hormones 1.The thyroid follicle cells secrete thyroglobulin into the lumen of the follicle, forming a colloid that fills an active follicle. Thyroglobulin has 123 tyrosines, but only 4 to 8 of them are destined to become incorporated into thyroid hormone. 2. The follicle cells actively transport iodide ions (I) from the blood into the cell and release them into the lumen. Here, the iodine is immediately oxidized to neutral iodine (I). 3. An iodine is added to a tyrosine of the thyroglobulin molecule, converting it to monoiodotyrosine or MIT. 4. Another iodine may be added, converting MIT to diiodotyrosine, or DIT.

Synthesis and Secretion of Thyroid Hormones 5. DIT can combine either with an MIT or with another DIT. MIT splits away from its thyroglobulin backbone at this stage, but the DIT remains linked to the thyroglobulin. At this point, the hormone T3 is essentially formed. (If two DITs combine, the hormone produced is T4. 6. When stimulated by TSH, the follicle cells take up droplets of colloid by pinocytosis (return to the top of the figure). A lysosome fuses with the pinocytotic vesicle and contributes an enzyme that hydrolyzes the thyroglobulin and liberates the thyroid hormone (step 6 at the bottom). 7. TH is released into the blood as a mixture of about 10% T3 and 90% T4.