Presentation on theme: "Dep. Of Histology & Embryology"— Presentation transcript:
1Dep. Of Histology & Embryology Epithelial tissueDep. Of Histology & Embryology
2Emphases of today’s lecture 1. Classification of epithelial tissue2. Membrane specializations ofepithelial cells
3What 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.
4Functions 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 co2Receptor function
5Characteristics 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.
62. They exhibit functional as well morphologic polarity. Free surface (Apical domain)Basal surface (Basal domain)Lateral surface (Lateral domain)
73. Epithelium is avascular, their basal surface is attached to an underlying basement membrane. nourished by diffusion.
9Covering epitheliumCovering 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.
10Classification of Covering Epithelium The traditional classification of epithelium is descriptive and based on two factors:Simple- single layerStratified- multi layer1. Number of cell layersSquamous- flat2. Shape of the component cellsCuboidal- squareColumnar- rectangular
23Transitional 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.
26Summary 1 Epithelium: covering epithelium and glands Classification of covering epitheliumsimple squamousstratified cuboidalcolumnar
27Specialization of Epithelium 1. Modification of free surfaceMicrovilliStereociliaCilia
28Microvilli:Microvilli are finger-like cytoplasmic projection on the free surface of most epithelial cells.1.4 μm longFunction: increase the surface areasDistribution: striated border: intestinal epi. cellbrush border: proximal renal tubuleGoblet cellAB
31StereociliaLong, nonmotile process of cells of epididymis and ductus deferens
32CiliaCilia are mobile projections of cell membrane protruding from free surface μm longFunction: moving fluid and particles along epithelial cellsDistribution: epithelial cells of respiratory tract and female reproductive system.AB
38Tight 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.
39Tight junction (zonula occludens) Function: seal the cells
40Intermediate junction (zonula adherens) Just below the tight junctionOpposing membrane are separated by a gap of nm filled with glycoproteinElectron dense material embedding numerous filaments radiating awayThought to hold adjacent cells firmly together
41Intermediate junction (zonula adherens) Function:adherens, keep the cell shape, transfer cell contract force
42Desmosome (macula adherens) plate shape below intermediate junction.Opposed cell membrane diverge slightly with a space nmOn the inner surface of cell membrane adjacent to desmosomes, there is punctate electron dense material.
43· Desmosome (macula adherens) Function: firmly connection, spot-weld type of cell-to-cell attachment
44Gap junction (communication junction) Deep part of lat. Cell surfaceNot visible under LMOuter leaflets comes as 2-3 nmBridging structure termed connexonsProvide a low resistance pathways between cells.Transfer of impulses, ions and small molecules.
45Gap junction (communication junction) Function: allow communication between cells
463. Specialization in cell to extracellular matrix adhension (basal surface)Basement membrane:-between E. & CT-all type of E rest on itBest shown with silver technique and PAS (+ ve)nm thick
47Composed of 2 layers Basal lamina - produced by epithelial cells Reticular lamina – composed of reticularfibers
49· Hemidesmosomes Function: bind the epithelial cells to the subjacent basal lamina
50Plasma 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.
51Summary 2Specializations of apical domainlateral domainbasal domain
52Glandular epitheliumare 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.
53Formation 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.
54On 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 cell2) multicellular glands- are composed of cell clusters. Most glands are multicellular.
55Exocrine 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.
56Classification of exocrine glands A) Simple or compoundsimple- connection with the surface is unbranched. E.g. sweat gland2) compound- connection with the surface is branched. E.g. parotid gland
57Principal 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.
58The 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.
60On 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.
61Mucous 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.
62Mixed 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.
63Secretory MechanismsThe 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.
642) 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.
653) Holocrine secretion designates the breakdown and discharge of the entire secretory cell. It is only seen in the sebaceous glands of the skin.