1.3 Connective and Supportive Tissues

1.3.Epithelial tissues Surface Epithelia Diverse group of tissues, cover or line external and internal body surfaces, cavities and tubes, thus, function as interfaces bet ween different biological compartments. Epithelia mediate a wide range of activities: Selective diffusion Absorption : Secretion Physical protection : Sensory function O Origin of the epithelial tissues: (corneal epithelium, epidermis of skin, glands) : Ectoderm Entoderm(alimentary tract, intestinal glands, liver, pancreas) Mesoderm (kidney, male and female reproductive system, endothelium, mesothelium) : Blood supply of the epithelial tissue: Normally it is avascular Exception: pharyngeal epithelium of frog : Urinary bladder of rabbit Ner ve supply of the epithelial tissue: is an inner vated tissue containing free ner ve endings & : It ner ve endings associated to specialized cells of sensory function Tissue components of epithelial tissues: Cells (squamous, cuboidal, columnar) closely packed : Intercellular matrix is reduced to the glycocalyx of the adjacent cells THE POLARITY OF THE EPITHELIAL CELLS Membrane modifications of the apical surface of epithelial cells: cilia Membrane specialization of the basal membrane surface of epithelial cells: basal striation + basal lamina (green arrows) CLASSIFICATION OF EPITHELIAL TISSUES FUNCTIONAL BASIS: SURFACE EPITHELIA GLANDULAR EPITHELIA : ABSORPTIVE EPITHELIA EPITHELIA : PIGMENT SENSORY EPITHELIA 680 SURFACE EPITHELIA: Further classification: On the basis of the number of cell layers On the basis of the shape of the cells Simple epithelia: a single layer of cells Simple squamous Simple cuboidal Simple columnar Pseudostratified epithelia: single layer of cells, with nuclei in more than one layer Pseudostratified columnar Transitional Stratified epithelia: several layers of cells on top of each other Stratified squamous (non-keratinized, keratinized) Stratified cuboidal Stratified columnar Simple epithelia: simple squamous epithelium Occurrence of simple squamous epithelium in the body alveoli : Pulmonary Pariétal layer of Bowman’s capsule - kidney Thin segments of Henle’s loop - kidney : Rete testis Inner aspect of the tympanic membrane in the middle ear : Endothelium lining in the blood & lymphatic vessels Serous membranes lining body cavities & internal organs SOME EXAMPLES: O Cross-section of the simple squamous epithelium: Kidney glomerulus, Bowman’s capsule Mesothelial lining of the peritoneal cavity: „fried-egg” appearance Endothelium: lining of the blood vessels & heart (arrows). Note the flattened nuclei The alveoli of the lung are composed of similar cells Simple epithelia: simple cuboidal epithelium Occurrence of simple cuboidal epithelium in the body Lining the follicles in the thyroid gland : The surface of ovary Choroid plexus - brain : The capsule of lens - eye Pigmented epithelium of retina - eye : Ducts of many glands : Secretory acini of many glands Several segments of kidney tubules Kidney: proximal and distal convoluted tubules, collecting tubules are lined by simple cuboidal epithelium Proximal tubules in cross-section in longitudinal section (arrow) The follicles of the thyroid gland are formed by simple cuboidal cells Simple epithelia: simple columnar epithelium Occurrence of simple columnar epithelium in the body Digestive tract from the cardia of the stomach to the anus excretory ducts of some glands : Larger Uterus (ciliated) in human : Oviduct (ciliated) in humans Pulmonary bronchi (ciliated) - lungs : Paranasal sinuses - nose Central canal of spinal cord (ependyma) O Pseudostratified epithelia: pseudostratified columnar epithelium Occurrence of pseudostratified columnar epithelium The male urethra Epididymis - male genital organ Trachea (ciliated) : Primary bronchi (ciliated) - lung The auditory tube Part of the tympanic cavity - ear Lacrimal sag @ • O D e Transitional epithelium, or urothelium Urinary bladder, fixed in empty state: note the thickness of the urothelium: 8-10 cell layers (arrow) Urinary bladder, fixed in full state: note the thickness of the urothelium: 4-5 cell layers (arrow) The luminal surface of the umbrella cells is covered by a specialized cell membrane called calyx, or crusta special membrane Stratified epithelia: stratified squamous non-keratinized epithelium Layers from the basal lamina to the surface: Stratum basale/germinativum: columnar cell layer w/ mitotic division Stratum spinosum: cuboidal poligonal cell layers with many desmosomes Stratum planocellulare: fusiform or flattened cell layers : O Occurrence: cornea, internal body surfaces, e.g. esophagus (see above) Stratified epithelia: stratified squamous keratinized epithelium Layers from the basal lamina to the surface: 1. stratum germinativum, 2. stratum spinosum 3. stratum granulosum (fusiform cells with keratohyalin granules), 4. stratum lucidum (flattened layer with eleidin), 5. stratum corneum (dead cell layer) Occurrence: epidermis of skin, beginning and end-portions of the gastrointestinal tract Stratified cuboidal epithelium Very rare: 2 rows of cuboidal cells on top of each other Duct of the sweat gland (cisterna lactis of teat- not shown) Stratified columnar epithelium Cavernous urethra, : Large excretory ducts of some glands, Larynx (ciliated), : Fetal esophagus (transiently)