The Endocrine System

EFE Veterinary Science

Anatomy and Physiology

Endocrine Glands

• Ductless: deliver peptides (hormones) into blood, lymph or tissue fluid

• Produce hormones at a site distant from effected organ/tissue

• Regulate most of body functions

Peptide-Target Systems

The various ways in which peptides reach their targets. A, Neuroendocrine; B, endocrine; C, neurotransmitter, neuromodulator (action on postsynaptic membrane); D, paracrine (localized hormone action). 1, Bloodstream; 2, target cell; 3, synapse.

Hypophysis/Pituitary/Master Gland

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Median sections of the hypophysis of the horse (A), ox (B), pig (C), and dog (D). The rostral extremity of the gland is to the left. 1, Adenohypophysis; 2, intermediate part; 3, neurohypophysis; 4, hypophysial stalk; 5, recess of third ventricle.

Posterior Lobe (Neurohypophysis)

• Part of the hypothalamus (brain = neuro-)• Stores and releases

– Oxytocin (contraction of smooth muscle of uterus and udder myoepithelial cells)

– Vasopression (Vasoconstriction, promotes fluid reabsorption by the kidneys)

– These are produced by the hypothalamus

• Very vascular

Anterior Lobe (Adenohypophysis)

• Grows up from the developing dorsal mouth• Products regulated by the hypothalamus

– Follicle Stimulating Hormone (FSH)– Luteinizing Hormone (LH)– Adrenocorticotropic Hormone (ACTH)– Thyroid Stimulating Hormone (TSH)– Alpha-Melanocyte Stimulating Hormone (MSH)– Prolactin

Pars Intermedia (Intermediate Lobe)

• Lies between anterior and posterior lobes• Doesn’t really do much

Brain-Pituitary-Organ Axis

Organization of the brain–pituitary–peripheral organ axis. TRH, thyrotropin-releasing hormone; CRH, corticotropin-releasing hormone; DA, dopamine; PIF, prolactin-inhibiting factor; GnRH, gonadotropin-releasing hormone; SS, somatostatin; GRH, growth hormone-releasing hormone; ACTH, adrenocorticotropic hormone; TSH, thyroid-stimulating hormone; GH, growth hormone; LH, luteinizing hormone; FSH, follicle-stimulating hormone; PRL, prolactin. 1, Adrenal cortex; 2, thyroid; 3, liver; 4, ovary; 5, testis; 6, mammary gland; 7, median eminence; 8, anterior lobe of pituitary; 9, intermediate lobe of pituitary; 10, neural lobe of pituitary.

Pineal Gland

• Caudal Dorsal brain (in mammals)• Secretes melatonin

– Circadian rhythms– Taken supplementally for sleep and jet lag

• More dorsal and external in reptiles

Thyroid Gland

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The thyroid gland of the dog (A), horse (B), cattle (C), and pig (D). The inset to D illustrates the subtracheal connection in transverse section in the pig. 1, Isthmus; 2, trachea; 3, cricopharyngeus.

Thyroid Gland

• Located adherent to ventral trachea• Respond to Thyroid Stimulating

Hormone (TSH) produced by anterior lobe of pituitary

• Releases Thyroid hormone (thyroxine)– Regulates metabolism and growth

• Small release of Calcitonin (antagonist to parathormone)

Thyroid Gland• Utilizes iodine to make thyroid hormone

– Iodine deficiency causes goiter

• Dogs are prone to hypothyroidism• Cats are prone to hyperthyroidism

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Parathyroid

• Located near, attached to or embedded in the thyroid glands

• Set of 4 (typically)• Regulate Calcium metabolism

– Absorbtion from the gut– Mobilization from the skeleton– Excretion in the urine

• Governed by plasma calcium concentration

Adrenal Glands

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The topography of the canine adrenal glands. 1, 1 , Right and left adrenal glands; ′2, left kidney; 3, aorta; 4, caudal vena cava; 5, phrenicoabdominal vessels; 6, renal vessels; 7, ovarian vein; 8, ureter; 9, bladder.

Adrenal Gland

• Craniomedial to kidneys– Left wraps around aorta– Right wraps around vena cava

• Cortex and Medulla• Cortex produces mineralocorticoids,

glucocorticoids and some sex steroids• Medulla produces epinephrine and

noreprinephrine (“fight or flight”)

Pancreatic Islet Cells

• Located diffusely throughout the pancreas• Produce insulin and glucagon• Insulin drives glucose and potassium into cells• Glucagon also affects carbohydrate

metabolism• Also produce somatostatin, pancreatic

polypeptides, and gastrin

Testicles

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Testis (horse). 1, Head of epididymis; 2, body of epididymis; 3, pampiniform plexus.

Testis (dog) (140×). 1, seminiferous tubules (showing spermatogenesis); 2, interstitial tissue with androgen-producing (Leydig) cells.

Testicles

• Affected by LH and FSH• Interstitial (Leydig) cells make androgens

– Male sexual functioning– Accessory sex glands– Secondary characteristics– behavior

• Sustenacular (Sertoli) cells make inhibin and activin, which affects FSH synthesis and release

Ovaries

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Specific and functional variations in ovarian morphology. A, Ovary of a cow (monotocous). 1, Mature follicle.

Specific and functional variations in ovarian morphology. B, Ovary of a bitch in a quiet stage.

Ovaries

• Located in dorsal abdomen• Outer layer contains follicles

– Each follicle contains one egg– Follicle development produces estrogen– Follicle ruptures and releases egg

• “scar” where follicle was becomes corpus luteum• Corpora lutea produce progesterone

Placenta

• Present only during pregnancy• Significant species variation• Source of

– Lactogen (mammary development)– Relaxin (prepare pelvis for parturition, helps oxytocin with

expulsion of fetal membranes)

• Prosteglandin produced by empty uterus; (stimulated by oxytocin) promotes regression of CL and initiating next cycle– In pregnancy, fetus produces factor blocking receptivity to

oxytocin, CL remains and pregnancy persisis