Tiêu đề Structural Organization in Animals 4.0.pdf ✅

2 Structural Organisation in Animals • It has a free surface which faces either a body fluid or the outside environment • Epithelium cells are closely packed with little inter cellular space. • Due to presence of little intercellular, space blood vessels, lymph vessels & capillaries are unable to pierce this tissue so blood circulation is absent in epithelium. Hence cells depend for their nutrients on underlying connective tissue. • Between epithelium and connective tissue, a thin nonliving acellular basement membrane is present which is highly permeable. • Epithelial tissue is Avascular but has nerve supply. Cell Junctions • To provide mechanical support for the tissue, plasma membrane of adjacent epithelium cells are modified to form following structures (Intercellular Junctions): (i) Tight Junction (Zonula Occludens): At some places Plasma membrane of adjacent cells become fused to form tight Junction. It stops substances from leaking across a tissue. (ii) Adhering Junction: Consists of disc - like protein plate with intermediate fiber known as tonofibrils composed of keratin like sclero protein. Function : cementing (iii) Gap Junction: Facilitate the cells to communicate with each other by connecting cytoplasm of adjoining cells for rapid transfer of ions, small molecules and sometimes big molecules. e.g. Smooth muscles and Cardiac muscles. Note : Cell Junctions are plenty in Epithelial tissue but also seen in other tissues. (Skeletal muscles lack cell Junctions) Origin of Epithelium Tissue: It is the only tissue which is originated from all the three primordial germinal layers. e.g. (i) Ectoderm – Epidermis (stratified squamous Epithelium) (ii) Mesoderm – Mesothelium and Endothelium (simple squamous Epithelium) (iii) Endoderm – Inner lining of GIT (Simple columnar epithelium)