Unit 2

• The four basic tissue types are epithelial, connective, muscle and nervous tissue. These are again made up of cells and ECM
• Epithelial tissue is made up of aggregated polyhedral cells, with little ECM. They absorb substances such as in the small intestine, Secrete such as parenchymal cells of glands and cover or line surfaces such as epidermis
• Epithelial cells have elliptic, spherical or flattened nuclei. Nuclei are an indicator of cell shape, density and number of layers. Usually adjacent to connective tissue containing blood vessels, however don't actually contain blood vessels themself.
• The connective tissue that underlies the​ epithelia is called the lamina propria. Area of contact btw the two tissues may be increased by small evaginations called papillae which are most frequently in epithelial tissues subject to friction (skin or tongue)
• Epithelial cells show polarity leading to a basal pole and a apical pole
• Basal membrane: A thin extracellular, felt like sheet of macromolecules where the basal surface of all epithelia rests. This is a semipermeable filter for substances reaching epithelial cells from below. It gives support to epithelial cells and attach epithelia to underlying connective tissue
• The Basement membrane is made up of basal and reticular laminae: The basal lamina is a network of fine fibrils nearest the epithelial cells. Reticular laminae is more diffuse and fibrous layer beneath the basal lamina
• There are 5 (intracellular) junctions between cells: Tight junction, Adherent junction, desmosome, hemidesmosome and gap junction
• Tight junction: Seals adjacent cells together, controlling the passage of molecules between them. It divides the apical and basal ends of the cell laterally
• Adherent Junction: provides points linking the cytoskeletons of adjacent cells. Strengthens and stabilises nearby tight junctions. Found under tight junctions
• Desmosome: Provides points of strong intermediate filament coupling between adjacent cells strengthening the tissue. The filaments are attached to the membrane intracellularly then to nucleus. Also joins adjacent cells.
• Hemidesmosome: A protein that anchors the cell cytoskeleton to the basal lamina. Also uses intermediate filaments
• Gap junction: Allows direct transfer of small molecules and ions from one cell to another
• The apical cell surface can have microvilli which help with absorption
• Stereocilia can be on some apical cell surface but it is uncommon. Plentiful on absorptive epithelial cells lining the male reproductive system. Help with absorbance and are much longer and less motile than microvilli. In some case they can branch distally
• Cilia is a variation of the apical surface of a cell. Cilia are long and highly motile. They aid with absorption but are longer than microvilli. numerously found lining the respiratory tract
• Epithelia by function can either be covering / lining epithelia or secretory / glandular epithelia
• Lining epithelia: cover or line the surfaces and cavities of organs. Classified according to the number of cell layers and the cell morphology in the outer layer.
• Based on the number of cell layers on lining epithelia, they can be simple or stratified. Simple is one layer of cells and stratified is multiple layers of cells
• Based on the cell shape of the lining epithelia they can be classified as squamous (thin cells), cuboidal (where cell thickness and width are relatively similar) and Columnar (where the cell is taller than wider)
• Epithelial cells are named in the order of: layers (simple or stratified), cell shape (squamous, cuboidal or columnar), cell specialisation (if applies eg ciliated, keratinized...) and finish with epithelium
• Secretory epithelia: Organs mostly made up of these cells are called glands. Scattered secretory cells or unicellular glands are called goblet cells. Goblet cells secrete mucus in the lining of the intestine and respiratory system
• Glands can be formed in two ways. Both of these begin during foetal development where the epithelium invaginates into connective tissue. In one case, the gland is formed by the formation of a duct, the epithelium proliferates down and forms an exocrine gland. In the other case the invagination breaks off from the surface epithelium and forms an endocrine gland, surrounded by capillaries and missing a secretory duct. Exocrine gland release substances through a duct while endocrine gland secretions are absorbed by capillaries and transported through the circulatory system
• Exocrine glands are made up of a duct which is the conducting portion and an acinus which is the secretory portion. The acinus contain secretory vesicles, which release their contents into the acinus and then these contents are excreted by the duct
• There are many kinds of exocrine glands. There are 5 simple ducts with no branches and there are 3 compound glands which have several secretory units.
• The three simple glands are the simple tubular, branched tubular, coiled tubular, acinar/ alveolar and branched acinar
• Simple tubular glands have a long secretory portion and almost no duct. These are the mucous glands in colon and intestine
• Branched tubular glands have several long secretory parts which drain into one duct. Can be seen in uterus and stomach
• Coiled tubular glands have a very long secretory portion which is coiled. An example is sweat glands
• Acinar or alveolar glands have a rounded secretory portion which resemble sacks. Found in urethra as small mucous glands
• Branched acinar glands have multiple sack like secretory portions which secrete into the same duct. Found in skin as sebaceous glands
• The three types of compound glands are tubular, acinar/ alveolar and tubuloacinar
• A distinct difference between simple and compound exocrine gland is that simple have short gland portions while compound have long ducts
• Compound tubular glands have several elongated coiled secretory units which converge into one duct. Found in duodenum as submucosal mucous (Brunner) glands.
• Compound acinar/ alveolar glands are made up of several sack like secretory units which come together to one large duct. Found in the pancreas as exocrine glands
• Compound tubuloacinar glands are made up of both tubular and acinar secretory units which converge to one larger gland. Found in salivary glands
• The mechanisms of exocrine glands are: merocrine, holocrine and apocrine. Merocrine glands rely on vesicles that release their (protein) contents into the acini and is then released by the gland Eg salivary glands. Holocrine glands contain a build up of lipid cells within their acini, these cells are disintegrated and secreted through the gland Eg sebaceous and follicular gland. Apocrine glands contain pinching off of apical portions of the secretory cells causing the release of a protein and lipid rich secretion Eg mamillary glands
• Merocrine glands can again be categorised into mucous, serous and mixed. Mucous merocrine glands secrete heavily glycosylated proteins called mucins. Serous merocrine glands secrete not glycosylated proteins such as enzymes. Mixed merocrine glands secrete mucus and enzymes