Dep. Of Histology & Embryology Epithelial tissue Dep. Of Histology & Embryology

Emphases of today’s lecture 1. Classification of epithelial tissue 2. Membrane specializations of epithelial cells

What is epithelium? Conception: Epithelium is a tissue composed of closely aggregated polyhedral cells and very little extracellular matrix. Distribution: Cells cover body surfaces, line body cavities, and constitute glands.

Functions of Epithelium Protection- main function of covering E. Secretion- main function of glandular E. Absorption- intestinal, distal renal tubule, alveolar E. Excretion- urine, sweat and co2 Receptor function

Characteristics of Epithelium 1. They are closely apposed adhere to one another by means of specific cell to cell adhesion molecules that form specialized cell junctions.

2. They exhibit functional as well morphologic polarity. Free surface (Apical domain) Basal surface (Basal domain) Lateral surface (Lateral domain)

3. Epithelium is avascular, their basal surface is attached to an underlying basement membrane. nourished by diffusion.

Classification of Epithelium 1. Covering epithelium 2. Glandular epithelium 3. Sensory epithelium 4. Germinal epithelium 5. myoepithelium

Covering epithelium Covering epithelia appears as members whose boundaries at free and luminal surfaces face either air or fluid. cover the ext. and int. surface of body serve as barrier membranes.

Classification of Covering Epithelium The traditional classification of epithelium is descriptive and based on two factors: Simple- single layer Stratified- multi layer 1. Number of cell layers Squamous- flat 2. Shape of the component cells Cuboidal- square Columnar- rectangular

Simple epithelium Simple squamous E Simple cuboidal E Simple columnar E Pseudostratified ciliated columnar E Endothelium Mesothelium

Simple squamous epithelium B C

Simple squamous epithelium Typical location: Endothelium: epithelium lining the inner surface of vascular system, heart, blood vessel, lymphatic Mesothelium: epithelium lining body cavities, pericardial, pleural, peritoneal Major function: Exchange, lubrication, barrier.

Simple cuboidal epithelium Typical location: exocrine glands, kidney tubules, pancreas, ducts of many glands Major function: Absorption, secretion

Simple columnar epithelium B A Typical location: Small intestine, colon, stomach, gallbladder Major function: Absorption, secretion

Pseudostratified columnar ciliated epithelium Containing different types of cells: columnar, goblet, fusiform and basal cells Typical location: Trachea and bronchial tree, ductus deferens Major function: Secretion, conduit

· Stratified epithelium Stratified squamous epithelium Transitional epithelium Non-keratinised Keratinised

Stratified squamous epithelium 2 types: a) Non-keratinised (mucous type) moist cavities or orifices , sufrace cell are viable and contain nuclei but lack keratin. b) Keratinised (cutaneous type) entire exposed surface of body, cells of surface layer contain keratin but no nucleus.

Stratified squamous epithelium C A B Typical location: Epidermis, oral cavity, esophagus, vagina Major function: Barrier, protection

Keratinised

Transitional epithelium A highly distensible stratified epithelium with large polyploid (nucleus, cell or organism that has more than two haploid sets of chromosomes ) superficial cells that are cuboidal in the relaxed state but broad and squamous in the distended state; occurs in the kidney, ureter, and bladder.

· Transitional epithelium Superficial cells A B Typical location: Urinary tract (renal calyces, ureters, bladder, urethra) Major function: Barrier, distensible property

Summary 1 Epithelium: covering epithelium and glands Classification of covering epithelium simple squamous stratified cuboidal columnar

Specialization of Epithelium 1. Modification of free surface Microvilli Stereocilia Cilia

Microvilli: Microvilli are finger-like cytoplasmic projection on the free surface of most epithelial cells.1.4 μm long Function: increase the surface areas Distribution: striated border: intestinal epi. cell brush border: proximal renal tubule Goblet cell A B

Stereocilia Long, nonmotile process of cells of epididymis and ductus deferens

Cilia Cilia are mobile projections of cell membrane protruding from free surface. 5-10 μm long Function: moving fluid and particles along epithelial cells Distribution: epithelial cells of respiratory tract and female reproductive system. A B

2. Specialization in cell to cell adhesion (lat. Surface) Tight junction (zonula occludens) Intermediate junction (zonula adherens) Desmosome (macula adherens) Gap junction (communication junction)

Tight junction (zonula occludens) -Belt shape, expand around the apex, -Higher magnification EM reveal that membranes are in contact only at this points. -It also serve as barrier to prevent solutes passing out to in or vice versa.

Tight junction (zonula occludens) Function: seal the cells

Intermediate junction (zonula adherens) Just below the tight junction Opposing membrane are separated by a gap of 15-20 nm filled with glycoprotein Electron dense material embedding numerous filaments radiating away Thought to hold adjacent cells firmly together

Intermediate junction (zonula adherens) Function: adherens, keep the cell shape, transfer cell contract force

Desmosome (macula adherens) plate shape below intermediate junction. Opposed cell membrane diverge slightly with a space 25-30 nm On the inner surface of cell membrane adjacent to desmosomes, there is punctate electron dense material.

· Desmosome (macula adherens) Function: firmly connection, spot-weld type of cell-to-cell attachment

Gap junction (communication junction) Deep part of lat. Cell surface Not visible under LM Outer leaflets comes as 2-3 nm Bridging structure termed connexons Provide a low resistance pathways between cells. Transfer of impulses, ions and small molecules.

Gap junction (communication junction) Function: allow communication between cells

3. Specialization in cell to extracellular matrix adhension (basal surface) Basement membrane: -between E. & CT -all type of E rest on it Best shown with silver technique and PAS (+ ve) -50-300 nm thick

Composed of 2 layers Basal lamina - produced by epithelial cells Reticular lamina – composed of reticular fibers

· Hemidesmosomes Function: bind the epithelial cells to the subjacent basal lamina

Plasma membrane infolding This structure is well developed in basal surface of epithelial cells in proximal and distal renal tubules. Function: increase the basal surface areas facilitate the passage of water and ions from lumen to surroundings capillaries.

Summary 2 Specializations of apical domain lateral domain basal domain

Glandular epithelium are cells or aggregations of cells whose function is secretion. Exocrine glands release the secretory product via a system of ducts that opens upon one of the surfaces of the body which are in contact with the external world (skin, gastrointestinal tract etc.). Endocrine glands release their secretory product (typically hormones) into the spaces between the secretory cells (extracellular space) from which it enters the bloodstream.

Formation of glands from covering epithelia Formation of glands from covering epithelia. Epithelial cells proliferate and penetrate connective tissue. They may—or may not—maintain contact with the surface. When contact is maintained, exocrine glands are formed; without contact, endocrine glands are formed. The cells of endocrine glands can be arranged in cords or in follicles. The lumens of the follicles accumulate large quantities of secretions; cells of the cords store only small quantities of secretions in their cytoplasm.

On the basis of the no. of cells in them, exocrine glands can be divided into Unicellular glands- single secretory cell interposed in the epithelium between cells which have other functions. E.g. goblet cell 2) multicellular glands- are composed of cell clusters. Most glands are multicellular.

Exocrine glands have secretory unit or portion (acinus), which contains cells for the secretion process and a duct system which carries the secretion out of the gland.

Classification of exocrine glands A) Simple or compound simple- connection with the surface is unbranched. E.g. sweat gland 2) compound- connection with the surface is branched. E.g. parotid gland

Principal types of exocrine glands Principal types of exocrine glands. The part of the gland formed by secretory cells is shown in black; the remainder shows the ducts. The compound glands have branching ducts.

The secretory portion may have a variety of shapes The secretory portion may have a variety of shapes. Secretory cells may form # acini in acinar glands, # tubes in tubular glands, # alveoli in alveolar glands.

On the basis of secretion, classification of gland: Serous acinus is composed of serous secretory cells. The acinus produces a serous (whey-like) secration, which is clear, watery fluid containing a proteinaceous component. glands containing only serous acini are termed serous glands.

Mucous acinus is composed of mucous secretory cells Mucous acinus is composed of mucous secretory cells. A mucous acinus secretes the glycoprotin mucin ( mucin + water = mucus) glands containing only mucous acini are termed mucous glands.

Mixed acinus is composed of above two types of secretory cells Mixed acinus is composed of above two types of secretory cells. Glands containing both serous and mucous acini or mixed acini are termed mixed glands.

Secretory Mechanisms The secretory cells can release their secretory products by one of three mechanisms. 1) Merocrine secretion corresponds to the process of exocytosis. Vesicles open onto the surface of the cell, and the secretory product is discharged from the cell without any further loss of cell substance.

2) Apocrine secretion a mechanism in which part of the apical cytoplasm of the cells is lost together with the secretory product. The continuity of the plasma membrane is restored by the fusion of the broken edges of the membrane, and the cell is able to accumulate the secretory product anew. This mechanism is used by apocrine sweat glands, the mammary glands and the prostate.

3) Holocrine secretion designates the breakdown and discharge of the entire secretory cell. It is only seen in the sebaceous glands of the skin.

Today’s lecture Classification of Epithelium: distribution and function Membrane specialization of epithelial cell Glandular epithelium

Questions What is epithelium? Functions of Epithelium Characteristics of Epithelium Classification of Epithelium Specialization of Epithelium