Kharkov National Medical University Department of histology cytology

Kharkov National Medical University Department of histology, cytology and embryology

Lecture 3 Tissues 1. Epithelial tissue

Definition • Tissues -- groups of cells organised to perform one or more functions. • Note!!(but do not write) Cell is the basic structural unit of the body. But in the multicellular organism there are tissues that are responsible for maintaining body functions due to collaborative efforts of their individual cells.

4 basic types of tissues: • • Epithelial Tissue (Epithelium). Connective tissue. Muscular Tissue (Muscle). Nervous tissue (Nerve).

Epithelial Tissue - covers body surfaces, - - lines body cavities, and - - - forms glands. There are 2 main types of ET: • Covering and lining (covers outer body surfaces (skin) and inner body surfaces (lumen of hollow organs (intestine) and cavities (peritoneal, pericardial, pleural), blood vessels, ducts, tubules). They separate the underlying connective tissue from the external environment or environment of internal cavities • Glandular - forms glands

Main function : • Epithelium creates a selective barrier between the organism and its external environment. • This barrier facilitates or inhibit the passage of specific substances. • Any substances that enter the body as a metabolites or discharges from the body as a waste products must pass through the epithelial cell, not between them.

Other functions • protection • diffusion or absorption or excretion = exchange • transport (along free surface) • secretion • sensation.

Characteristics • 1. There is no extracellular substance between the cells, • cells are tightly apposed in sheets and strongly attached to one other by means of special junctions.

Characteristics. • 2. Their basal surface is attached to an underlying basement membrane – extracellular protein-polysaccharide-rich layer. • 3. polarity - cells have 2 surfaces : the apical or free side (surface) - is the side towards the lumen or outside medium the basal side (surface) - is the side closest to the basement membrane

Epithelial Tissues and Their Basement Membrane

The Polarity of Epithelial Cells

Characteristics. • 4. Epithelia are avascular : • epithelium does not contain blood vessels but is nourished by diffusion of substances from capillaries of underlying connective tissue. • 5. It is frequently mitotically active

Morphological classification of Epithelial Tissues is based on combination of the number of cell layers (first name) and the shape of surface cell (second name). E. can be called simple or stratified on the basis of the number of cell layers which comprise it: • Epithelium may be simple, when it is one cell layer thick and stratified when it is two or more cell layer thick.

The second name describes the shape of the cells: • Squamous, where the width of the cell is greater than its height. • Cuboidal, where the width, depth and height are approximately the same. • Columnar, where the height appreciably exceeded the width. • !!! In case of stratified epithelia the second name describes the shape of the top layer of cells.

Simple Squamous Epithelium • Single layer of flat cells (very thin) – lines blood vessels (endothelium), closed body cavities (mesothelium), alveoli in the lungs – Function: controls diffusion, osmosis and filtration

Simple Cuboidal Epithelium • Single layer of cube-shaped cells, lines tubules and ducts

Simple Columnar • Lines stomach, intestine. Usuallly has microvilli = finger-like projections of the cell membrane • Function – absorption.

Pseudostratified Columnar • 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 are flat 2 types: Keratinized = surface cells are 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 Lines sweat gland ducts

Stratified Columnar Epithelium • Surface cells are columnar • Lines very large ducts of gland

Transitional Epithelium • Multilayered • Surface cells vary in shape from round to flat if stretched • Lines hollow organs of the urinary tract that expand from within

!! The morphology of an epithelium often correlates with its function: • - Epithelia involved in secretion or absorption are typically simple. • -- Stratification of the epithelium usually correlates with impermeability and protection • --- The height of the cell often reflects the level of secretory or absorptive activity.

GLANDULAR EPITHELIA • Secretion is the main function of the glandular epithelia. • Secretion – cyclic process. 4 phases: • 1. diffusion of metabolites into the cell • 2. synthesis proper • 3. releasing of substances from the cell • 4. cell restoration

There are two types of glands in the body: exocrine and endocrine. • Exocrine glands secrete through ducts or directly onto the surface (skin or cavity of inner organs). • Without ducts - surface mucous cells of the stomach; • With ducts - most exocrine glands; Ex. : salivary glands. • Endocrine secrete hormones into the bloodstream.

Classification of glands by cell number • Unicellular gland Ex. : Goblet cell secrete mucus and lubricate small and large intestine, respiratory tract • Multicellular gland

2 portions of gland: Parenchyma and Stroma • Parenchyma - the epithelial, secreting cells of the gland which perform the gland's essential function. • Stroma - supporting cellular framework of connective tissue which contains blood vessels and nerves and gives structure to the gland.

Classification of Exocrine Glands by type of secretion • 1. Mucous - viscous, slimy mucus Ex. : glands of esophagus, Brunner's glands of duodenum • 2. Serous - watery serum Produce wide variety of proteins (enzymes). Ex. : exocrine pancreas, parotid salivary glands, uterine glands.

Classification of Exocrine Glands by type of secretion • 3. Mixed (Sero-mucous) Ex. : Submandibular and sublingual salivary glands; mixed glands of the nasal cavity, paranasal sinuses, nasopharynx, larynx, trachea and bronchi.

Secretion mechanism – 3 types • • • Merocrine (= eccrine) Apocrine Holocrine

Merocrine (eccrine) • • • ( (note!) This is the most common type of secretion mechanism). Secretory granules are formed in the cells and accumulated in the apex. The granules fuse with the apical plasma membrane and are secreted into the lumen of the gland by exocytosis.

Apocrine • • (ex. : lactating mammary gland (fat droplets)) - secretion of product plus a small portion of pinched off apical cytoplasm, enclosed by a membrane derived from the plasma membrane.

Holocrine • • - ex: Sebaceous glands of the skin the cell get filled with its secretory product while the nucleus becomes smaller. maturing cells filled with lipid move to the center, then toward the opening of the acinus. the cell dies and the entire cell is released and disintegrates, releasing its product.

Mechanisms of Glandular Secretion

A Structural Classification of Exocrine Glands • By branching of the duct exocrine glands may be: simple or compound • By the shape of secretory ends – tubular or alveolar (acinar) • By branching of secretory ends exocrine glands may be: branched or non-branched

A Structural Classification of Exocrine Glands