Tissues of the Body and Epithelial Tissue

Dr Dalia El Marakby TISSUES OF THE BODY 1- Epithelial tissue: (Ectoderm, Mesoderm, Endoderm) 2- Connective tissue: mesoderm. 3- Muscular tissue: mesoderm. 4- Nervous tissue: ectoderm. EPITHELIAL TISSUE Epithelium is the tissue that covers surfaces or lines the cavities of the body. General characteristics: 1-It is formed of crowded cells with minimal intercellular substance in-between. 2- Upper surface of epithelium is free. The basal surface rests on a basement membrane, which may be clear or non-clear. 3- Avascular, i.e. blood or lymph vessels cannot penetrated between epithelial cells but nerve fibers can penetrate it. It receives nutrition from the underlining C.T. 4-It has a high power of regeneration (renewal). Origin of epithelium: - it may be ectodermal (as skin), mesodermal (serous membranes) or endodermal gastrointestinal tract (GIT). Epithelial tissue is classified into: - Surface epithelium: covers surfaces or lines cavities. - Glandular epithelium: It modify to form secretions. - Neuroepithelium: It modify to act as receptors. - Myoepithelium: It contract. 1 Dr Dalia El Marakby Classification of epithelium: I-Surface epithelium II-Glandular epithelium. III- Neuroepithelium. IV- Myoepithelium A-Simple epithelium B- Stratified epithelium (Only one layer of cells). (More than one layer). I-Surface Epithelium A- Simple Epithelium Simple epithelium is formed of one layer of cells resting on the basement membrane. 1- Simple squamous epithelium: *It is formed of one layer of, flat cells with flat nuclei resting on the basement membrane. Functions: it gives smooth surface →important for easy passage of fluids & easy movement of organs. it gives thin membrane→ help in gas exchange. Sites: 1- Lines heart & blood vessels 2- Lines lung alveoli 3- Lines serous membranes (pleura, peritoneum, and pericardium) 4- Lines Bowman's capsule of kidney gives smooth surface. give a thin membrane for gas exchange gives smooth surface for easy movement of organs. filtration of blood. called endothelium. called pneumocytes. called mesothelium. 2- Simple cubical epithelium: *One layer of cube like cells, with central rounded nuclei. Functions: secretion & reabsorption. Sites: 1- convoluted tubules of Kidney. 2- Secretory acini of glands e.g. follicles of thyroid gland. 2 Dr Dalia El Marakby 3-Simple columnar epithelium: *One layer of tall columnar cells with basal, oval nuclei. Functions: secretion & absorption. Sites: 1- Stomach 2- Small & large intestine 4-Simple columnar ciliated epithelium: One layer of columnar cells with basal, oval nuclei. Cells have cilia. Functions: cilia can transport fluid or particles in one direction. Sites: Female genital tract: uterus & to move ovum. fallopian tube The lower part of the respiratory to move secretion tract (Lung bronchioles) upwards. 5- Pseudo stratified columnar epithelium: (pseudo = false) * It is formed of one layer of columnar cells (actually simple epithelium) *All cells rest on the basement membrane. *Some cells cannot reach the surface. *Cells are crowded so nuclei are present at more than one level giving the epithelium the false appearance of being stratified. Types: a) Pseudo-stratified columnar ciliated epithelium with motile cilia & goblet cells: upper respiratory tract & called respiratory epithelium (nose, trachea & bronchi). b) Pseudo-stratified columnar ciliated epithelium with non-motile cilia (stereocilia) (not true cilia but long microvilli): c) Pseudostratified columnar nonciliated epithelium: Present In epididymis. Present in male genital tract as in the vas deferens & membranous part of male urethra. B- Stratified Epithelium • It is formed of more than one layer of cell. • Function: is protection. • It is classified according to the shape of the superficial cells into: 1- Stratified squamous epithelium. 2- Transitional epithelium (stratified cuboidal). 3- Stratified cubical epithelium. 4- Stratified columnar epithelium. 3 Dr Dalia El Marakby 1- Stratified squamous epithelium: 2- Transitional epithelium(Stratified cuboidal): Empty bladder • Number of layers • Basement membrane * Basal layer: *Intermediate layers: 5-30. clear & wavy. *Columnar cells with basal oval nuclei *Polyhedral cells with central rounded nuclei. *Cell joined together with desmosomes. *Cells gradually become smaller towards the surface. Superficial layer: * Flat cells with flat nuclei. Sites: Functions a) Non-keratinized stratified b) Keratinized stratified squamous epithelium: squamous epithelium * No keratin. * Covered with keratin. -Cornea. *Epidermis of the skin. -oral cavity - Esophagus. -Vagina, tip of urethra, anal canal. Protection Full bladder 6-8 layers. -3-4 layers non-clear, non-wavy cuboidal cells with central rounded nuclei. *Many layers of *Cells are pushed laterally polyhedral cells with to decrease epithelial rounded nuclei. thickness & accommodate more urine. *Dome shaped cells with during full bladder cells are upper convex surface. flat. *They have central rounded nuclei. *Cells are connected by junction complex *It is covered with rigid plaques act as a barrier • lines the urinary tract (is called Uroepithelium), e.g. prostatic urethra, pelvis of the ureter, ureter & urinary bladder. Distensibility & protective against cytotoxic effect of urine. 3-Stratified cubical epithelium: • It is rare type of epithelium formed of few layers of cells usually two layers of cubical cells as in ducts of sweat glands. 4-Stratified columnar epithelium: • It is similar to stratified squamous but: - Layers are fewer. -Superficial cells are columnar, which may be ciliated or non-ciliated. 4 Dr Dalia El Marakby • Sites: a) Non-Ciliated: found in: b) Ciliated: rare type found in fetal esophagus. - Large ducts of glands. - Male urethra (penile part). - Recto-anal junction. N.B. under abnormal conditions, one type of epithelium may transform into another type (metaplasia). e.g in heavy cigarette smokers, the pseudostratified columnar ciliated epithelium lining bronchi may transform into stratified squamous epithelium. Also, in Bilharziasis, epithelium of urinary bladder may change from transitional into stratified squamous. II- Glandular Epithelium Definition: it is type of epithelium modified to act as a gland & produce secretion. Glands are classified into Exocrine gland Endocrine glands, ductless glands • Formed of: • Formed of: -Secretory portions which form secretion. -Group of secretory cells, their secretion -Excretory portion or duct system which (hormones) is carried by blood to different carry secretion to the outside of the gland. parts of the body. Mixed glands • Formed of: - Exocrine part which has duct system. - Endocrine part which has no duct system. Classification of Exocrine Glands: 1- According the number of cells forming the gland: a) Unicellular glands: formed of one cell. b) Multicellular glands: formed of more than one cell. 2-According to the type (nature) of secretion: a) watery secretion b) Serous secretion (watery with enzymes) 3- According to the mode (mechanism) of secretion: 5 c) Mucous secretion Dr Dalia El Marakby a) Merocrine secretion: *No changes occur in the secretory cells (the secretion discharged by exocytosis). *The most common mechanism. 4- According to the shape of the secretory part: b) Apocrine secretion: the apex of the cell come out with the secretion. c) Holocrine secretion: -Secretion accumulates in the cell. -Cells swell and rupture. - whole cell is lost with the secretion Tubular→ the secretory unit is tube. Alveolar (acinar) →the secretory unit is rounded. Tubuloalveolar→ the secretory unit is flask shape. 5- According to branching of the duct: a) Simple: the duct is single & not branched with non-branching secretory part.. b) Simple branched, the duct is single & not branched, and the secretory part is branched. c) Compound the ducts are branched like a tree and each duct branch has a secretory part. N.B. since the gland formed of secretory part & a duct system so they classified into: A) Tubular: • Simple tubular glands. • Simple coiled tubular glands. • Simple branched tubular glands. • Compound tubular glands. B) Alveolar: C) Tubulo-alveolar: • Simple alveolar glands. • Simple branched alveolar. • Compound alveolar glands. • Simple tubulo-alveolar glands: • Simple branched tubulo-alveolar glands: • Compound tubulo-alveolar glands: III- Neuroepithelium - Special type of epithelium. - its epithelial cells (called sensory cells) modified to act as a receptor (receive stimuli) for some sensation. Sites: - Taste buds in tongue for taste sensation. IV. Myoepithelium •Definition: Epithelium modified to contract. Site: • Myo epithelium present between the base of the secretory cell & their basement membrane. • They contract squeezing the secretory cells to discharge their secretion into the ducts. • e.g. around acini of salivary glands, mammary glands and sweat glands. 6 Dr Dalia El Marakby Polarity and Membrane Specialization of Epithelia I- Apical Specializations Cilia, flagella, microvilli & stereocilia (see cytology) II. Intercellular or lateral Specializations (Cell Junctions) Type of junction 1-Tight Junctions or Occluding Junctions (Zonula Occludens): 2- Adherens Junctions (Zonula Adherens): 3- Desmosomes (Macula Adherens): (macula = spot) At the site of junction Encircles the apex of the cell like a belt. *The 2 adjacent cell membranes fuse at certain points, via transmembrane proteins in each cell. Encircles the cell as a belt (zonula). *The 2 cell membranes are separated by a wide space. *In the intercellular space, extracellular components of transmembrane protein of the two adjacent cells are connected by Ca ions *At the cytoplasmic side, condensed protein molecules bind transmembrane protein to actin filaments (cytoskeleton) Do not encircle the cell but scattered as circular spots (maculae). *The 2 cell membranes are separated by wide space. * In the intercellular space, extracellular components of transmembrane protein of the two adjacent cells are connected by Ca ions. 4-Gap Junctions or nexus = communicating junctions *The 2 cell membranes are separated by a narrow gap but connected by arrow channels. *Each channel is formed of 6 symmetrical transmembrane protein subunits. *The cytoplasmic side is thickened by attachment plaque that is composed of several proteins in which intermediate filaments are inserted *Present between cells especially in stratified squamous epithelium. Function: 7 Prevent passage of substance between adjacent cells. Lateral adhesion between the epithelial cells It is the strongest type of junction in between epithelial cells to support cells subjected to friction 1- Exchange of small molecules and ions between adjacent epithelial cells. 2- Passage of impulses between muscle cells. Dr Dalia El Marakby Ill- Basal Specializations 1- Hemidesmosomes: It takes the shape of 1/2 a desmosome. Present at the basal part of epithelial cells Functions: fix the epithelium to the underlying basement membrane & C.T. 2- Basement Membrane: • A amorphous dense layer present under the epithelia • LM: seen when stained with Ag brown. PAS red. It may be: - Clear (thick) as skin. - Non-clear (thin) as in transitional epithelium. • EM: is formed of 2 components: A) Basal lamina =epithelial components. B) Reticular lamina = C.T. components Electron dense layer formed of: Formed of: *Collagen granules (type IV). * Collagen fibers type III (reticular fibers) *Glycoproteins. * Ground substance (glycoproteins & proteoglycan). Both layers are fixed to C.T. by collagen fibers type VII (anchoring fibrils). Functions of the basement membrane: 1- Support epithelium. & fix it to the C.T. 2- Controls the passage of nutrients and ions e.g. kidney capillaries and lung alveoli. 3- Basal Infoldings: In ion transporting cells as Kidney tubules: *The basal surface of cell membrane shows invaginations 8 in-between invaginations there are vertical mitochondria to supply energy for the active transport Dr Dalia El Marakby 9