Introductory Mammalian Physiology (PHOL0002) Notes - Introduction to Endocrinology

Introduction to Endocrinology
Endocrinology
 Endocrine system
o Collection of glands that secrete hormones
 Signals are passed through the blood to arrive at a target organ – which has cells possessing
the appropriate receptor
o Maintains homeostasis and long-term control using chemical signals
o Works in parallel with the nervous system to control homeostasis
o More than fifty human hormones have been identified
 All act by binding to receptor molecules
 Hormones
o Grouped into three classes based on their biochemical structure
 Peptides
 Short chains of amino acids
 Secreted by the pituitary, parathyroid, heart, stomach, liver, kidneys
 Amines
 Derived from the amino acid tyrosine
 Secreted from the thyroid and adrenal medulla
 Steroids
 Lipids derived from cholesterol
o Testosterone = male sex hormone
o Oestradiol – responsible for many female sexual characteristics
 Secreted by the gonads, adrenal cortex, placenta
o Mechanism of hormone action
 Endocrine system acts by
releasing hormones  triggering
actions in specific target cells
 Receptors on target cell
membranes bind only to one
type of hormone
 Binding hormone changes the shape of the receptor = causing the response
o Non-steroidal signalling
 Non-steroid hormones (water soluble) do not enter the cell – but bind to plasma membrane
receptors = generating a chemical signal (second messenger) inside the target cell





 Five different second messenger chemicals (cAMP) have been identified
 Second messengers activate other intracellular chemicals to produce the target cell
response
o Steroid signalling
 Steroid hormones can pass through the plasma membrane –
acting in a two-step process
 Once inside the cell – steroid hormones bind to the
nuclear membrane receptors = producing activated
hormone-receptor complex
 Activated hormone-receptor complex binds to DNA =
activating specific genes – increasing the production of
proteins

, Introduction to Endocrinology
 Endocrine-related problems
o Overproduction of a hormone
o Underproduction of a hormone
o Non-functional receptors = cause target cells to become insensitive to hormones
 Pituitary gland
o Majority of endocrine control comes from pituitary gland
 Protected at base of the brain in the sphenoid bone
– connected by pituitary stalk
o Pituitary gland development
 Forms from a downgrowth of the base of the brain
(diencephalon) and upgrowth from the roof of the
mouth = forming Rathke’s pouch
 Two tissues fold around one another = forming neurohypophysis (posterior pituitary) and
adenohypophysis (anterior pituitary)
 Posterior pituitary
o Elongated nerve fibres
o In the hypothalamus – there is:
 Large bodied neurons = magnocellular neurons – in paraventricular
nucleus
 Send axonal productions to posterior pituitary
o Hormones are made here and transported down
neural pathways to posterior pituitary via pituitary
stalk
o Hormones – synthesised in cell bodies of magnocellular neurons (PVN and
SON) + axonal transported from hypothalamus to posterior pituitary
 Oxytocin
 Vasopressin = anti-diuretic hormone (ADH)
 Anterior pituitary
o Produced a whole array of hormones that target many different tissues in
the body
o Regulated by hypothalamic releasing hormones that stimulate the anterior
pituitary to produce different hormones
 Each hormone has particular signals in the hypothalamus that
regulate their synthesis and secretion
o Work on target tissues that produce other hormones that feedback and
control endocrine cycle
o Hormones
 TSH, LS, FSH
 Heterodimeric glycoproteins
 Common alpha-subunit
 Specific beta-subunits (TSH-beta, LH-beta, FSH-beta)
 ACTH
 39aa fragment of POMC
 GH and PRL
 190aa peptides – internal di-S bonds
 Homologous receptors
 Intermediate lobe
o Closely associated with posterior pituitary
o Separated from the anterior lobe by the hypophyseal cleft
o Shows variation in size among species
 Produces melanocyte-stimulating hormone = involved in skin colour