Endocrine functions and Hypothalamic-pituitary axis

Endocrinology

Branch of biology dealing with hormones, their glands and effects

Hormones

Chemical messengers released by endocrine glands

Enter the bloodstream and affect target tissues at very low concentrations

Help maintain homeostasis

Endocrine system

Hormones released in bloodstream

Slower response time but has long lasting effects

Widespread systemic influence (many organs/cells can respond)

Hormone roles

Developmental coordination - controls processes like puberty, pregnancy and sex determination

Environmental response - helps reaction to stress or sexual stimuli

What does classification by site of action mean

How far the hormone travles to reach its target and where it exerts its effects

Classification by site of action - endocrine signaling

Hormones released in bloodstream and travels to distant target organs

e.g. insulin from pancreas acts on muscles and liver

Classification by site of action - paracrine signaling

Hormones act on neighbouring cells (local signaling, not via blood)

e.g. growth factors released by one cell affect nearby ones

Classification by site of action - autocrine signaling

The cell that secretes the hormone is also its own target

e.g. immune cells release cytokines that act back on themselves

Classification by site of action - neuroendocrine signaling

Neurons release hormones (called neurohormones) into the blood, not into synapse

e.g. hypothalamic neurons releasing oxytocin or ADH into circulation

What does classification by chemical structure mean

Molecular makeup of the hormone

Determines how the hormone is stored, transported and acts on target cells

Liposoluble (fat soluble) hormones

Hormones that dissolve in fat, not water (pass through cell membranes, which are made of fat (lipid layer)

They can bind to receptors inside the cell and change gene expression

Liposuble - Steroid hormones

Made from cholesterol, which is a type of fat

Steroid hormones - Adrenocortical hormones

Glucocorticoids - like cortisol (stress and metabolism)

Mineralocorticoids - like aldosterone (salt/water balance)

Androgens - like DHEA (precursor to sex hormones)

Steroid hormones - Sexual hormones

Adrogens, estrogens, progesterone, testosterone…

Liposoluble hormones - Thyroid hormones

Made from an amino acid called tyrosine

Thyroid hormones

T3 - Triiodothyronine (metabolism)

T4 - Thyroxine (metabolism, growth, development)

Hydrosoluble hormones

Dissolve in water, not fat

They cannot cross cell membranes so they work outside the cell

Hydrosoluble hormones - peptide hormones

Made of chains of amino acids

Includes - insulin (blood sugar), oxytocin (birth/milk), growth hormone, LH FSH TSH (reproductive and thyroid reg)

Hydrosoluble hormones - amine hormones

Small and made from amino acids like tyrosine or tryptophan

Includes - adrenaline, dopamine, melatonin…

Classification by site of synthesis

Which gland or tissue produces the hormone

Peptide hormones synthesis

Preprohormone - synthesis in RER, contains extra amino acid segments (signal peptide)

Prohormone - after signal peptide is removed, transferred to golgi apparatus

Active hormone - final process occuring in secretory vesicles

Peptide hormones secretion

Released by exocytosis when cell is stimulated

Requires ATP and calcium

Amine derived hormones synethesis and storage

Adrenal medulla (water soluble) - made in adrenal medulla, stored in vesicles and released by exocytosis

Thyroid hormones (fat soluble) - made in thyroid follicles, stored as part of thyroglobulin and released by diffusion when TSH stimulates thyroid

Melatonin - made in pineal gland, stored and secreted rhythmically

Steroid hormones synthesis, storage and secretion

All made from cholesterol, conversion involves specific enzymes found in mitochondria and SER

Not stores, synthesised on demand

Diffuse through the cell membrane into bloodstream

Liposoluble hormones

Cholesterol enters cell and is converted into hormone by specific enzymes

Hormones not stored so they diffuse directly out of the cell

Hydrosoluble hormones

Hormone made in nucleus and RER

Goes to golgi aparatus and is packaged into secretory vessicles

Stored until signal arrives and exocytosis

Negative feedback regulation

When desired effect is achieved the hormone production shuts down

Negative feedback regulation example

Hypothalamus tells pituitary to make TSH

TSH tells thyroid to make T3 and T4 which control metabolism

When there is enough T3 and T4 in blood, they go back to tell the pituitary and hypothalamus there is enough

TSH and TRH production slows down (inhibited)

Positive feedback regulation

The hormone stimulates more of itself until an external loop happens

Positive feedback regulation example

Baby stretches cervix → triggers oxytocin release

Oxytocin causes stronger uterine contractions

More stretching → more oxytocin

Ends when the baby is delivered

Circadian rhythms regulation

Hormone release follows a 24-hour cycle

Regulated by supraschiasmatic nucleus in the hypothalamus

Melatonin peaks at night and cortisol peaks in the morning

Neuroendocrine reflex regulation

Nervous system triggers hormonal release under stress of stimuli

Transport of hormones in blood

Hydrosoluble - circulate freely

Liposoluble - carrier protein needed

Liposoluble hormones mechanism

Diffuse through plasma membrane, bind to intracellular receptors

Hormone-receptor complex interatcs with DNA in nucleus and acts as a transcription factor that regulates gene expression

Hydrosoluble → cell membrane action

1 - hormone binds to receptor on cell surface (specific to hormone)

2 - receptor activates G-protein (molecular switch that has subunits)

3 - G-protein activates enzyme adenynyl cyclase which converts ATP into cAMP (second messenger)

4 - cAMP activates protein kinase A (PKA) which adds phosphate groups to proteins (phosphorylation) which activates/inactivates enzymes, ion channels or transcription factors

5 - final outcome depends on the target cell

Liposoluble → steroid hormone action

1 - enters cell by diffusion through membrane

2 - inside cytoplasm, binds to intracellular receptor

3 - hormone receptor complex enters nucleus

4 - binds to specific DNA region called ‘acceptor site’ on chromatin

5 - activates transcription, forming mRNA → new protein synthesis

Liposoluble → thyroid hormone action

1 - enter the cell easily through diffusion

2 - go straight to nucleus without binding in cytoplasm

3 - inside nucleus, bind to intranuclear receptor already attached to DNA

4 - T3-receptor complex activates transcription → mRNA → protein synthesis

How are hormones eliminated

Metabolism - mainly in liver and kidney, hormones chemically altered (broken down or into inactive form) → no longer fits in the receptor

Excretion - mainly through urine (kidneys) but also bile (liver), hormones or broken down products are filtered by kidney and excreted, some fat-soluble hormones may be excreted in bile

Enzymatic degradation - enzymes at target tissues break down hormone after binding to receptor

Function of pituitary and hypothalamic axis

Critical component of endocrine system

Coordinates hormonal signals between hypothalamus and pituitary gland

Pituitary gland

Located at the base of the brain in region called Sellaturcica

Posterior - neurohypophysis, develops from nervous tissue and is connected to hypothalamus via nerve fibers

Anterior - adenohypophysis, develops from epithelial tissue, grows upwards from roof of the mouth (Rathke’s pouch)

Posterior pituitary (neurohypophysis) and hypothalamus

Hypothalamus sends nerve impulses to posterior

Hormones are produced in the hypothalamus but stored and released from the posterior pituitary when hypothalamus sends nerve impulses

For instance oxytocin (OXT) and ADH (antidiuretic hormone) are stored and later released in the bloodstream

Anterior pituitary (adenohypophysis) and hypothalamus

Hypothalamus released chemical messengers called ‘releasing or inhibiting hormones’ into blood supply between the anterior and hypothalamus

Travel through a portal blood system where they stimulate or inhibit the release of other hormones called ‘tropic hormones’ into the bloodstream

Tropic hormones act on other endocrine glands to release their own hormones like the thyroid, adrenal glands, gonads…

Neurohypophysis

Oxytocin and Antidiuretic hormone are stored in nerve terminals

When hypothalamus receives signal, it sends nerve impulses down axon terminal → triggers release of hormones in bloodstream → neurosecretion

Paraventricular nucleus - produces mainly oxytocin, PNV axons extend into posterior and release hormones in bloodstream

Supraoptic nucleus - produces mostly ADHand releases hormones the same way as PNV

Tropic hormones

Hormones that target other endocrine glands to produce own hormones:

FSH (follicle stim) - targets gonads, helps regulate reproductive functions

LH (luteinizing) - works with FSH in regulating reproductive functions (ovulation or testosterone prod)

TSH (thryoid stim) - stim thyroid gland, thyroid hormones, metabolism

ACTH (adrenocorticotropic) - adrenal glands to produce cortisol, used for body stress response

Non tropic hormones

Directly affect target cells without stimulating other glands:

GH (growth) - promotes growth, cell reproduction and repair, bones+muscles

PRL (prolactin) - milk production in females after childbirth

MSH (melanocyte-stim) - skin pigmentation by stimulation the production of melanin in melanocytes

Hypothalamus hormones

Travel to anterior and tells it to ‘release’ or ‘hold back’ certain hormones

Adenohypophysis hormones

Produces and releases own hormones in response to signals from hypo

Neurohypophysis hormones

Does not make its own hormones, but stores and releases from hypo

Overview

Overview