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Epithelial Tissue Lecturer: Dr.Firdous M. Jaafar Department of Anatomy/Histology section Lecture 1 Objectives • 1- Define epithelial tissue, and describe its cell types. • 2- Identify the intercellular adhesion and junctions, and enumerate their types. • 3- Enumerate the types of epithelial tissue. • 4- Define basal lamina and basement membrane, and recognize their functions. • 5- Describe the specialization of epithelial cell surface; • a- Define microvilli. • b- Define stereocilia. Introduction • the human body is composed of only four basic types of tissue: • Epithelial, connective, muscular, and nervous. • These tissues, which are formed by cells and molecules of the extracellular matrix, exist not as isolated units but rather in association with one another and in variable proportions, forming different organs and systems of the body. Epithelial tissues • are composed of closely aggregated polyhedral cells with very little extracellular substance. These cells have strong adhesions, membrane interdigitations, and intercellular junctions. These features allow the cells to form cellular sheets that cover the surface of the body and line its cavities or are arranged as three-dimensional secretory units. • They have different shapes; columnar, cuboidal, or squamous. • They rest on basal lamina (basement membrane) • The lateral membranes between adjacent cells exhibit intercellular junctions. Epithelial tissues • Epithelial cells are polyhedral in shape, ranging from squamous, cuboidal, and columnar. • Nuclear shape corresponds to the cell shape; ranging from flat, cuboidal, to elongated, and their long axis is parallel to the long axis of the cell. • All epithelial cells rests on connective tissue layer to support these cells. • The lower part of epithelial cell is called basal pole, while the upper part is called apical pole or free surface. Epithelial tissues • The principal functions of epithelial tissues are • 1- the covering and lining of surfaces (eg, skin, intestines). • 2-absorption (eg, intestines). • 3- secretion (eg, glands), • 4- sensation (eg, gustative and olfactory neuroepithelium). • 5- contractility (eg, myoepithelial cells). Basal Lamina & Basement Membrane • Most epithelial cells are separated from the connective tissue by a sheet of extracellular material called the basal lamina. This structure is visible only with the electron microscope, and it is called basement membrane when seen by LM Structure of BM • it appears as a dense layer, 20–100 nm thick, consisting of a delicate network of very fine fibrils (lamina densa), and an electron-lucent layer on one or both sides of the lamina densa, called lamina rara or lamina lucida. Functions of basal lamina • 1- structural functions as supporting the cells. • 2- provide a barrier that limits or regulates the exchange of macromolecules between connective tissue and cells of other tissues. • 3- influence cell polarity, regulate cell proliferation and differentiation by binding with growth factors, influence cell metabolism, and serve as pathways for cell migration. • 4- contain the information necessary for certain cell-to-cell interactions. • 5-establishment of new neuromuscular junctions around muscle cells. Intercellular Adhesion & Junctions • Several membrane-associated structures contribute to cohesion and communication between cells. Types of intercellular junctions • 1- Tight junctions, or zonulae occludens (singular, zonula occludens). EM photo cryofracture Types of intercellular junctions • 2- zonula adherens. Types of intercellular junctions • 3- Gap or communicating junctions. Types of intercellular junctions • 4- Desmosomes(macula adherens ). Types of intercellular junctions • 5- Hemidesmosomes. Specialization of cell surface • 1- Microvilli. are fingerlike cytoplasmic extensions measuring about 1μm high and 0.08 μm wide. They are found mainly on the free cell surface of absorptive cells. They are called brush or striated border when seen by light microscope. Specialization of cell surface • 2- Stereocilia. • Stereocilia are long, nonmotile extensions of cells that are actually long and branched microvilli. Summery • 1- there are four types of epithelia in the body • 2- epithelial tissue is classified into covering and glandular epithelia. • 3- all epithelial cells rest on basal lamia. • 4- there are four types of intercellular junctions • 4- apical surface of epithelial cells has certain cell specializations