Cytology, embryology, General histology Lecture 3 Tissues. 1. Epithelial tissue.
Definition • Tissues -- groups of cells organised to perform one or more functions.
Please, note and write down: 4 basic types of tissues: • Epithelial (Epithelium) • Connective • Muscular • Nervous
Please, note and write down: There are two main types of Epithelial Tissue: 1. Covering and lining 2. Glandular
Please, note and write down: Covering and lining epithelia lie on the free surface: - cover outer body surfaces (i. e. skin) and - line inner body surfaces: -- lumen of hollow organs (i. e. intestine) -- cavities (i. e. peritoneal cavity), -- blood vessels, -- ducts (in glands), -- tubules (in kidneys).
Characteristics 1. Consists of cells only (there is no matrix). 2. Cells tightly appose in sheets. 3. Cells rest on the basement membrane. Note: connective tissue lie under the basement membrane
Importance : • Epithelium create a selective barrier between the organism and its external environment: any substances must pass through the epithelial cell, not between them.
Please, note and write down: • • Two types of surface epithelia: epithelium is simple, when it is one cell layer thick and stratified when it is two or more cell layer.
Maine functions • protection • diffusion or absorption or excretion = exchange
Characteristics. 3. polarity - cells have 2 surfaces : the apical or free surface - towards the lumen or outside world the basal surface, closest to the basement membrane 4. Epithelia are avascular, do not contain blood vessels, but are nourished by diffusion of substances from capillaries in underlying connective tissue. 5. It is frequently mitotically active
• • • Shape of cells: Squamous. Cuboidal. Columnar.
Please, note and write down: The morphology of the epithelium correlates with its function: • - Epithelia involved in secretion or absorption are typically simple. • -- Stratified epithelia usually impermeable and protective
Simple Squamous Epithelium – lines blood vessels (endothelium), closed body cavities (mesothelium), alveoli in the lungs – Function: controls diffusion, osmosis and filtration
Simple Cuboidal Epithelium • lines tubules and ducts
Simple Columnar • Lines stomach, intestine. • Usuallly has microvilli = finger-like projections of cell membrane • Function – absorption.
Pseudostratified • Single cell layer • All cells attach to basement membrane but not all reach free surface • Nuclei at varying depths
Stratified squamous Epithelium • • Several cell layers thick, Surface cells flat 2 types: Keratinized = surface cells dead and filled with keratin – Example - Skin • Nonkeratinized = no keratin in moist, living cells at apical surface – Example - Cornea
Stratified Cuboidal Epithelium • Surface cells cuboidal Lie in sweat gland ducts, male urethra
Stratified Columnar Epithelium • Surface cells columnar • Line very large ducts of gland, male urethra
Transitional Epithelium • Multilayered • Surface cells varying in shape from round to flat if stretched • Lines hollow organs of the urinary tract that expand from within
GLANDULAR EPITHELIA • - form glands. • Function – secretion = synthesis and releasing of substances.
There are two types of gland in the body: exocrine and endocrine. • Exocrine glands secrete through ducts or directly onto an surface (skin or cavity of inner organs). • Endocrine secrete hormones into the bloodstream.
By cell number • Unicellular Ex. : Goblet cell secrete mucus and lubricate small and large intestine, respiratory tract • Multicellular
Two type of secretion of Exocrine Glands • Mucous - viscous, slimy (mucus lines and lubricates cavities of all inner hollow organs) • Serous - watery Ex. : exocrine pancreas (enzymes), sweet gland, Fluid in serous cavities, joints.
3 types of secretion mechanism: • • • Merocrine (= eccrine) Apocrine Holocrine
Merocrine (most common): Secretory granules fuse with the apical plasma membrane and are secreted into the lumen of the gland.
Apocrine • (ex. : lactating mammary gland (fat droplet)) • small portion of apical cytoplasm with secretory granules is pinched off
Holocrine - ex: Sebaceous glands of skin • • the cell fills with its secretory product (lipids), dies and disintegrates, releasing its product.
Mechanisms of Glandular Secretion
A Structural Classification of Exocrine Glands
A Structural Classification of Exocrine Glands • By branching of duct exocrine glands may be: simple (b-) or compound (b+) • By shape of secretory ends – tubular or alveolar (acinar) • By branching of secretory ends exocrine glands may be: branched (b+) or nonbranched (b-)
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