INTRODUCTION
Tissue : A organised group of cells similar in structure, function and origin.
In a tissue cells my be dissimilar in structure and function but they are always similar in origin.
–Word animal tissue was coined by – Bichat
– N. Grew coined the term for Plant Anatomy.
– Study of tissue – Histology
– Histology word was given by – Mayer
– Father of Histology – Bichat
– Study of tissue is also called Microscopic anatomy.
– Founder of microscopic anatomy – Marcello Malpighi
Based on functions & location tissues are classified into four types :
EPITHELIAL TISSUE
Word epithelium is composed of two words Epi – Upon
Thelio – grows
– A tissue which grows upon another tissue is called Epithelium.
– Cover external or internal surface.
– Cells are either single layered or multilayered.
– Cells are compactly arranged and there is no intercellular matrix.
– Cells of lowermost layer always rest on a non living basement membrane.
– Cells are capable of division and regeneration throughout the life.
– Free surface of the cells may have fine hair cilia or microvilli or may be smooth.
– Epithelial tissue is non-vascularised.
Due to absence/less of intercellular spaces blood vessels, lymph vessels 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 & connective tissue, a thin non living acellular basement membrane is present which is highly permeable.
Basement membrane consist of 2 layers.
(a) Basal lamina : made up of glycoprotein, and secreted by epithelium cells.
(b) Fibrous lamina : Formed of collagen and reticular fibres suspended in mucopoly-saccharide which is matrix of connective tissue.
– So basement membrance is secreted by both epithelium and connective tissue.
Mucopolysaccharide is present in the form of Hyaluronic acid which is composed of 2 components–N acetylglucosamine & glucuronic acid. Both these components are found in alternate form.
– NAG – GA – NAG –
– Specialized junctions between epithelial cells : -
– To provide mechanical support for the tissue plasma membrane of adjacent epithelial cells modified to form following structures called as Intercellular Junctions.
Tight junctions (Zonula occludens) : help to prevent substances from leaking across the tissue.
Plasma membranes in the apical parts become tightly packed together or are even fused.
Interdigitations : These are interfitting, finger like processes of the cell membranes of the adjacent cells.
Intercellular Bridges : These are minute projections that arise from adjacent cell membrances.
They make contact with one anther.
→Gap Junctions : Facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes big molecules.
→ Intermediate Junctions (= Zonula adherens) : These usually occur just below tight junctions.
→There is no intercellular filaments between the adjacent cell membranes.
→ They probably serve anchoring functions.
→Desmosomes ( =Macula adherens) : Perform cementing to keep the neighbouring cells together.
These are like zonula adherens but are thicker and stronger and are disc like junctions. They have intercellular protein. The microfilaments which extend from Plasma membrane are called
tonofibrils.
→Hemidesmosomes (single sided desmosomes) are similar to desmosomes, but the thickening of cell membrane is seen only on one side.
→Hemidesmosomes join epithelial cells to basal lamina (outer layer of basement membrane).
– Specialised functional structures shown by Epthelial Cells : -
Plasma membrane of free end get modified to form 3 types of functional structures.
Microvilli .
– Minute protoplasmic process which are non motile, non contractile.
– Help in absorption, secretion, excretion
–Increase surface more than 20 times.
Present in the wall of Intestine, Gall bladder, Proximal convoluted tubule etc.
Cillia or Kinocilia
– Long cylindrical protoplasmic process.
– Motile and contractile
–Movement of cilia is always in uniform direction.
– Originated from basal granule or kinetosome.
– Diameter of cilia is same from base to apex.
– In internal structure of cilia 9 + 2 arrangement of microtubules is present.
– They helps in conduction
e.g. – Fallopian tube.
– Trachea.
– Fallopian tube
– Uterus.
– Ependymal epithelium : (Inner lining of ventricles of brain & central canal of spinal cord. Function of cilia is to conduct substances in CSF.)
Steriocilia
– Long cytoplasmic process
– Non motile, non contractile
– Basal granule is absent
– Plasma membrane is thick & rigid.
– Base of stereocilia is broad & apical part is narrow so they are conical in shape.
– They increase surface area .
eg. Epididymis
Vasadeferens
