Glands Hormones oestrogen glucagon adrenaline insulin testosterone Syllabus

Glands & Hormones oestrogen glucagon adrenaline insulin testosterone

Syllabus reference:

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Glands simple stratified Types of epithelial cells columnar cuboidal squamous Covering and lining most surfaces in the body are epithelial cells. Glands are structures formed from epithelial cells

Some epithelial cells are specialized by having cilia; others are capable of secreting complex molecules.

Glands of the body are classified as either exocrine or exocrine endocrine types. endocrine

Glands; summary so far: Glands are multicellular epithelial structures that specialize in synthesizing and secreting complex molecules including hormones and enzymes. Glands are classified as either exocrine or endocrine glands. Exocrine glands have ducts to body surfaces or body cavities. Endocrine glands are small isolated blocks of tissues with no ducts to surfaces. They are therefore referred to as "ductless" glands. The products of exocrine glands collect in the duct of the gland flow toward the surface through the duct. Since endocrine glands lack ducts, their product is released across the cell membrane into interstitial spaces around the cells. They then diffuse into capillaries.

Types of gland

Exocrine Glands • Most glands of the body are exocrine glands with ducts connecting to body surfaces or body cavities. • For example, your salivary glands open into the oral cavity and sweat glands deposit their product on the skin surface. Types of exocrine gland structure: Simple tube Coiled tube Branched tube

Exocrine Glands Exocrine glands can also be classified by their secretion mechanism 1. Merocrine glands: these secrete substances by the process of exocytosis. Merocrine glands Vesicles fuse with the plasma membrane resulting in the release of their contents into the extracellular space. Examples include most sweat glands, the salivary glands and the goblet cells lining of the respiratory and digestive tracts. secretory product Most exocrine glands release their products in this way.

Example of a merocrine gland: sweat glands Section through a sweat gland of the skin; a coiled tube exocrine gland

Example of a merocrine gland: salivary glands

Exocrine Glands Exocrine glands can also be classified by their secretion mechanism 2. Apocrine glands: Glands that secrete by shedding a portion of their Apocrine glands cytoplasm, containing the secretory product, into a duct pinched off portion of the cell Examples include the mammary glands and sweat glands in the armpits.

Example of an apocrine gland: mammary glands Simplified diagram of a mammary gland Each one is a functional unit [alveolus] of epithelial cells that secretes milk into its centre. Normal alveoli in lactating mammary gland look like bunches of grapes.

Exocrine Glands Exocrine glands can also be classified by their secretion mechanism 3. Holocrine glands: Glands that secrete by shedding entire cells from the Holocrine glands lining of a duct. disintegrating cell and its released contents mitotic divisions to replace lost cells Examples include the sebaceous glands associated with hair follicles.

Example of a holocrine gland: Sebaceous glands Sebaceous gland associated with a hair follicle; a branched exocrine gland

Endocrine Glands NOTE: the pancreas has both exocrine and endocrine gland cells

Endocrine Glands • Endocrine glands are the hormone producing structures of the body. hormone producing structures • Some, like thyroid are large and obvious. • Others, for instance the Islets of Langerhans found in the pancreas, are small islands of endocrine cells embedded within the larger exocrine portion of this organ. • Endocrine cells release their secretory products into the interstitial spaces around the cells. • The hormones then diffuse into nearby capillaries and are carried to all parts of the body. • Only when the hormones encounter a target tissue do they exert an effect. target tissue

Endocrine Glands Only cells in the target tissue or target organ have the correct membrane receptors for this hormone Secretory cell in endocrine gland Hormone diffuses into the blood Cells without the appropriate membrane receptors remain unaffected by this hormone

Key terms: • Exocrine gland: a gland that secretes externally through Exocrine gland a duct • Endocrine gland: a gland that secretes hormones Endocrine gland internally directly into the bloodstream • Hormone: the secretion of an endocrine gland that is Hormone transmitted by the blood to the tissue(s) on which it has a specific effect • Target tissue: cells of an organ that are affected by Target tissue specific hormones

Hormones There are 2 major types of hormones: • Lipid-based Hormones • Protein-based Hormones

Lipid-based hormones • The commonest type of lipid-based hormones are the steroids • Examples include aldosterone and the sex hormones such as oestrogen and testosterone Aldosterone: hormone produced in the adrenal gland, regulating salt and water balance in the kidney

Lipid-based hormones • Lipid based hormones are hydrophobic which means they are lipid soluble. • They can diffuse through the cell membrane without the need for a receptor on the membrane. • These hormones bind to receptor proteins in the cytoplasm or nucleus. • This steroid-protein complex then binds to DNA and either starts or inhibits transcription of a particular gene. • This means it coordinates which genes should be transcribed as m. RNA, to make a protein.

How steroid hormones work: 1. Steroid hormone diffuses across the plasma membrane 2. Steroid hormone binds with protein receptor 3. Steroid-protein complex diffuses into the nucleus 4. Steroid-protein complex binds to a specific gene 5. Gene is transcribed to produce a section of m. RNA 6. m. RNA is translated by ribosomes in the cytoplasm to produce the required protein

Protein-based hormones • These include: – Polypeptides e. g. insulin; ADH – Glycoproteins e. g. FSH; LH – Catecholamines e. g. adrenaline; noradrenaline ADH – anti diuretic hormone FSH – follicle stimulation hormone LH – luteinizing hormone

Protein-based hormones • Protein based hormones are hydrophilic and are not lipid soluble. That means they cannot diffuse across a membrane. • The protein hormone acts as a first messenger by first messenger binding to receptors in the plasma membrane. • This causes an enzyme to be activated. • This enzyme causes the formation of cyclic AMP from cyclic AMP ATP. • Cyclic AMP acts as a second messenger to activate a second messenger particular reaction pathway in the cytoplasm.

How protein-based hormones work – outline of mechanism: hormone first messenger hormone receptor in plasma membrane cyclic AMP ATP second messenger an enzyme activated another enzyme activated or direct effect on a biochemical reaction in the cytoplasm

What is cyclic AMP? adenosine monophosphate [AMP] cyclic adenosine monophosphate [c. AMP]

How protein-based hormones work – detailed mechanism: 1. Protein hormone binds to a protein in the plasma membrane of a target cell 2. G-Protein activated 3. Cyclase enzyme activated. This catalyses the conversion of ATP to cyclic AMP (c. AMP) 4. c. AMP activates an enzyme in the cytoplasm. This enzyme will catalyse the required reaction in the target cell.

How adrenaline affects target tissues: FIRST MESSENGER adenylyl cyclase adrenaline ATP SECOND MESSENGER inactive enzyme c. AMP G protein active enzyme inactive glycogen phosphorylase active enzyme active glycogen phosphorylase receptor protein glycogen glucose phosphate

How adrenaline affects target tissues

The pancreas • Both endocrine and exocrine functions can be found in the same organ. • The pancreas simultaneously produces exocrine secretions like digestive enzymes and endocrine secretions like insulin and glucagon that are released into the blood to exert their effects throughout the body.