Endocrine System

A. Introduction 1. endocrine organs -

hypophysis, thyroid gland, adrenal gland, parathyroid gland, pineal gland

3. Typical secretions: hormones which affect distant targets and/or many tissues a. peptide hormones - proteins, glycoproteins, or peptides which bind to receptors on cell surfaces; work via second messengers, water soluble; e.g. insulin, glucagon, follicle stimulating hormone b. steroid hormones - lipid soluble and bind to intracellular targets, e.g. to DNA or hormone receptors; derived from cholesterol, e.g. progesterone, estradiol, testosterone c. amino acid derived hormones - water soluble, catecholamines, thyroxin and epinephrine

B. Hypophysis, aka pituitary gland 1. Basic structure and origin:

• direct connection between nervous system and the other endocrine organs• lies in the sella turcica, a depression in the sphenoid bone (hypophyseal fossa), with two ridges cranial and caudal to it (supposed to look like a Turkish saddle....)

Canine, dorsal view, calvarium removed.

Hypophysis

Ventral view, canine brain

B. Hypophysis, aka pituitary gland 1. Basic structure and origin:

•divided into two main regions, neurohypophysis and adenohypophysis

Neuro-hypophysis

Adeno-hypophysis

Brain, diencephalon

Cranial Caudal

Sagittal section

Arcuate n

MammillaryBody

Median eminencewith primary capillary bed

SuperiorHypophyseal a

Neurohypophysis

Adeno-hypophysis

These factors are transported to the adenohypophysis by small portal veins where they leave the capillaries to act on the secretory cells there.

Adenohypophysis

Portal v

Releasing or inhibiting factors bind to receptors in or on the secretory cells in the adenohypophysis to affect their secretion rates of a DIFFERENT hormone. E.g. Corticotropin releasing hormone stimulates the release of corticotropin (ACTH)

Releasing hormone

Secondary hormone

C. Adrenal Glands 1. General morphology

• Embedded in the adipose tissue on top of kidneys.

•Triangular shape(sometimes)

Capsule

Cortex

Medulla

3. Cortex Divided into 3 visibly different zones, all of which are steroid-secreting and all of which synthesize hormones from cholesterol, but in different regions different types of hormones are produced.

3. Cortex:a. zona glomerulosa

outermost layer, cells look clustered

Secretes mineralocorticoids (e.g. aldosterone, which is involved in salt balance via the kidney), and deoxycorticosterone.

Secretion is stimulated by angiotensin II from juxtaglomerular apparatus and ACTH from hypophysis.

3. Cortexb. zona fasciculata

• middle layer, longitudinal cords of cells• secretes glucocorticoids (e.g. cortisol, corticosterone) and adrenal androgens• secretion stimulated by ACTH

3. Cortexc. zona reticularis

• innermost layer, much thinner than other layers, network of cell cords• cells secrete androgens and small amounts of glucocorticoids• secretion stimulated by ACTH

4. Medulla• Secretion is triggered separately by preganglionic axons (norepinephrine release is associated with fear, epinephrine with pain), the fight or flight responses.• Portal system glucocorticoids stimulate conversion/synthesis of norepinephrine to epinephrine. The cell distribution is related to capillary distribution, so that the epinephrine secreting cells are nearest to capillaries from cortex.

Adrenal gland, medulla

Chromaffin cells (virtually every cell)

Ganglion cell