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- Epithelial tissue derivatives, specialized for secretion
- Exocrine or endocrine
Exocrine gland
- Has ducts (simple or complex)
- Secretion flows: secretory portion -> duct -> body surface/cavity
- E.g. sweat, salivary, mammary glands, exocrine pancreas
Endocrine gland
- Ductless, well vascularized, pdt hormones
- Passage of hormones: secretory portion -> extracellular space -> blood (typically, not always) -> target cells (have specific receptors)
Endocrine gland vs tissue vs cell examples
- Thyroid, pituitary etc
- Islet of Langerhans in endocrine pancreas, granulosa and theca cells of ovarian follicles
- Found in stomach, small intest lining, heart, kidney, Leydig cells of testes
- Secreted by endocrine glands
- Intrxt w/ receptors on/in target cells
- Regulates growth, development, repdtn, metabolism, contraction of cardiac/smooth muscles, glandular secretions etc
- Single hormone can affect more than one function, and each function controlled by more than one hormones
Classes of hormones
- Protein hormones
- Steroid hormones
- Amines
- Eicosanoids
Protein hormones
- Hydrophilic, common hormones
- Can be small (3 aa e.g. TRH)~large (200 aa e.g. LH)
- Pdt by endocrine cells
- Pdt in hypothalamus, pituitary, endocrine pancreas
Protein hormone synthesis
- Synth'd as preprohormone in RER
- Processed to prohormone/hormone in Golgi and packaged
- Stored in cytoplasmic secretory granules until specific stimulus induces release
Steroid hormone
- Lipid soluble, synth'd from cholesterol (4 aromatic rings fused together)
- Synth req specific enzymes found only w/in steroidogenic organs
- Diffuse to extracellular fluid and blood immediately following synth i.e. NOT stored in vesicle
Steroidogenic organs
- Gonads (testis/ovaries) pdt testosterone, estrogen (both in male/fem)
- Adrenal cortex pdt aldosterone, cortisol
Amine hormones
- Small molcs derived from tyrosine or tryptophan
- Tyrosine= precursor of catecholamine (epinephrine, NE pdtd in adrenal medulla) and thyroid hormone in thyroid gland (by conjugating 2 tyrosines)
- Tryptophan precursor of serotonin (impt NT in brain, controls emo, appetite, sleep), and melatonin (circadian rhythm?)
Eicosanoid hormones
- Small molc synthd from lipids and phospholipids
- Most derived from arachidonic acid (component of plasma and memb phospholipids)
- Act as local chemical mediators w/ powerful biologic activities e.g. prostaglandins, involved in inflammatino, inhibition of labor, airway constriction)
Negative feedback of hormone secretion
- Usually regulated by neg feedback
- E.g. ant pit secretes TSH -> inc thyroid secretion of thyroid hormones -> decreases gland ant pit secretion
- Contributes to maintaining homeostasis
- Ability of body to maintain rltvly constant enviro despite constantly changing external enviro
- If factor becomes excessive/deficient then neg feedback to bring it back w/in normal range 
- In rare cases pos feedback of hormone induces additional release of that hormone = disrupts homeostasis, can lead to disease/death
- Most common; endocrine cell -> blood -> target tissue (has receptor)
- Specialized neurons (e.g. hypothalamus) -> blood -> target tissue (e.g. ant pit)
- Endocrine cell -> extracellular space -> target tissue
- Endocrine cells -> ECS -> target tissue (same type of cell)
- Hormone acts w/in cell of origin, doesn't leave cell
- So blood not involved for transport
- Local functions, often provided by eicosanoids and growth factors
Hormone receptor complex
- Hormone binds to receptor at target tissue to form hormone receptor complex to exert bio function
- e.g. GnRH+receptor -> GnRHR complex
Locations of receptors
1. Cell memb - protein hormones, catecholamine
2. Cell cytoplasm - some steroid hormones
3. Cell nucleus - some steroid and thyroid hormones
G protein membrane receptor
- Many receptors are G protein-linked
- All have 7 trans-memb segments i.e. spans memb 7 times
- Hormone/protein binding site at each end
G protein
- Member of superfamily of GTP-binding protines which hydrolyze GTP and switch btwn active (GTP bound) and inactive (GDP bound) states
- Have α, β, γ subunits
When hormone binds to receptor
1. Formation of hormone/receptor complex
2. Confrmational change in receptor
3. Activation of G protein by exchanging GDP for GTP at α subunit
4. Dissociation of α subunit, now intrxt w/ memb-bound enzyme (effector) to induce intracellular signal-transduction sys that transfer info from cell surface to intracellular targets
- Enzyme that activates the release of a 2nd messenger
- e.g. Phospholipase C, adenyl cyclase
cAMP second messenger system
1. Hormone (catecholamines, LH, glucagon, TSh etc) binds to receptor
2. Conformation change, G-protein activated
3. Intrxt w/ memb-bound effector= adenyl cyclase
4. Adenyl cyclase convert ATP to cAMP which acts as 2nd messenger to affect intracellular functions
Phospholipase C (PLC) membrane phospholipids second messenger sys
- For hormones like GnRH, oxytocin, TRH use cell memb phospholipids and intracellular Ca as 2nd messengers
- Effector is PLC which inc DAG, IP3, and Ca2+ (IP3 release Ca from ER) that all act as second messengers
Nuclear and plasma receptors
- Are ligand induced transcription factors, regulate gene expression via association of hormone-receptor complex w/ specific DNA regions= hormone-responsive elements (HRE)
- Highly specific and high degree of structural conservation
- Have DNA-binding domains near center, hormone binding domain near carboxyl end
Nuclear/ plasma protein structure

N end               C end
N [trans activation domain 1][DNA binding domain][Blah][hormone binding][transactivation domain 2]C
Steroid and thyroid hormones
- Steroid and thyroid hormones bind to either nuc or plasma receptors to activate transcription of specific genes
- Many steroids e.g. estronge, cortisol also inc w/ specific cell-surface receptors for more rapid responses
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