Dr. Heba Kalbouneh. Dr. Heba Kalbouneh. Dr. Heba Kalbouneh

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1 Dr. Heba Kalbouneh Dr. Heba Kalbouneh Dr. Heba Kalbouneh

2 Tissue: is a group of cells that serve the same function, they are surrounded by extra cellular matrix. The 4 basic types of tissue: 1. epithelial 2.connective 3.nervous 4.muscular Epithelium: cellular sheets that line cavities of organs and cover the surfaces, closely aggregated polyhedral cells with strong adhesion between each other, and separated by a small amount of ECM -Epithelium has 3 main functions: 1. Covering external surfaces: Epithelium has a direct contact with the outer environment (outer surface of the skin "Epidermis"). 2. Lining cavities inside our bodies: ex. lining the oral cavity and the esophagus. 3. Forming glands Why? 1. Protection of underlining structures: Epithelium acts as the first barrier in our bodies. The protection might be: a- mechanical from abrasional stresses or forces / from biological samples like bacteria, viruses or other organisms b- Protection from dehydration (loss of body fluids or water), it's too dangerous condition like in massive burns; the epithelium is lost so our body will lose a large amount of water. 2. trans-cellular transport Cells eat up materials by phagocytosis & that form phagosomes. The ingested material could be used inside the cell or could be transported outside the cell Transcytosis: The movement of a material across epithelial cells, it enters the cell from a surface and leaves from another surface, in this case the materials will not be used inside the cells. 3. Secretion: Epithelial cells are able to produce & secrete substances into the luminal surfaces or outer surfaces "like epithelial cells of digestive tract"

3 4. Absorption: Epithelial cells determine which materials could come in & which should stay out (selective permeability same concept) 5. Sensation: We know that only nerve cells are involved in sensation, but epithelium could have a neurological function. For example, the olfactory epithelium is a specialized epithelial tissue inside the nasal cavity (nose) that is involved in smell sensation (olfaction: smell) Characteristics of Epithelial tissue: 1- Epithelial cells are tightly packed in order to protect the underlying structures. 2-Epithelium has a very small amount of extra cellular matrix = intercellular space, unlike connective tissue that has a large amount of extra cellular matrix - Intercellular means between the cells - Intracellular means within the cells The extra cellular matrix "ECM": non cellular material, looks like spaces between the cells, it's composed of proteins (glycoprotein, sugars attached to main protein core, or proteoglycan which is mainly small protein attached to sugar) & collagen fibers 3- Epithelium tissue rests on basement membrane & its cells have four surfaces: a. apical surface (towards the apex of the cell), it could be named luminal surface (towards the lumen of a tube when it lines a cavity) b. basal surface (faces the basement membrane that separates connective tissue & epithelium) **There's always connective tissue under epithelium c. lateral surfaces 4. Epithelium is derived from all embryonic layers (endoderm like the lining of digestive tract / ectoderm like outer layer of skin / mesoderm like lining of blood vessels) Germ layers (embryonic): 3 layers that appear in early embryo. (Endoderm/ectoderm/mesoderm) which will give all the future tissues of the body 5. Avascular (has no blood vessels & it gets its nutrition from the underlining connective tissue. oxygen & nutrients pass from the blood vessels in the connective tissue to epithelium by diffusion) 6. Its highly innervated "it has a lot of nerve endings in the intercellular

4 spaces between the cells so we could feel pain, temperature changes & other sensations 6. It has a high regenerative power Epithelial cells can undergo continuous mitosis to form new cells & heal an injured skin for example. 7. Polarized cells (because they have 2 surfaces so they have 2 poles) Apical pole + Basal pole Concentration of organelles on basal surface is different from the apical surface Basal surface with the 2 lateral surfaces are considered one domain & the apical surface is the other domain So we have 2 domains apical & baso-lateral 8. Modified to perform special function like glands that are specialized in secretion Classification of epithelial cells According to the shape: Squamous: flat cell in histological section Cuboidal: squared in histological section The height is equal to the width rounded central nucleus Columnar: columnar in histological/longitudinal section, usually with basal nucleus

5 According to the number of the cells layers Simple: one layer of cells/ all the cells rest on the basement membrane Stratified: more than one layer of cells / Just the basal layer of the cells rest on the basement membrane. But we should know that each cell has apical surface & basal surface. Apical cells: are the cells that face the apical surface. Basal cells: are the cells that face the basement membrane We should always look at the shape of apical cells to determine the type of the stratified epithelium: If it is Squamous we name it: stratified Squamous epithelium If it is columnar we name it: stratified columnar epithelium If it is cuboidal we name it: stratified cuboidal epithelium

6 We should also remember that epithelium is always found next to a space because it lines & covers this will help us find the tissue under the microscope. Only Basal cells are able to undergo mitosis so they give the epithelium its regenerative power in order to replace dead cells ** Exception for simple epithelium (Pseudo _fake_ stratified Epithelium) - More than one nucleus at different levels (cells have different heights) & all the cells touch the basement membrane but not all of them reach the apical surface - Remember that Cell membrane is not visualized under the light microscope so we have to look at the nuclei -Simple epithelium single layer of nuclei -Stratified epithelium more than one layer of nuclei -Pseudo stratified epithelium different levels of nuclei **Exception of Stratified epithelium Transitional epithelium: - More than one layer of cells & the apical cells are dome shaped - The shape of the apical cells is changeable. This type of epithelium lines the urinary bladder, when the bladder is filled with urine the shape changes to Squamous & when its empty they appear dome- shaped - It's also named urinary epithelium because it's found in urethra & urinary bladder.

7 - The name may also include accessory structures: Goblet cells, Cilia, Keratin - Goblet cells are specialized cells for mucus secretion, they found for example in the lining epithelium of respiratory tract. The mucus released by these cells forms a protective layer that is sticky and cleans the inhaled air from dust particles. The type of epithelium found in trachea is pseudostratified columnar epithelium with goblet cells. (goblet cells are the yellow cells in the figure below) - Cilia are cytoplasmic extensions from the apical side of epithelial cells, they are motile, they produce regular movement in one direction. Cilia are found also in respiratory epithelium, their movement helps propel mucus outside the respiratory tract (either you swallow it or expectorate (spit it)). So the respiratory epithelium is ciliated pseudostratified columnar epithelium with goblet cells (refer to figure above to see cilia) - Keratin: is a protein that protects epithelial cells from damage or stress and it s a waterproof layer that prevents water loss from the underlying tissue. It is found on the outer surface of the skin (epidermis). The epithelium of epidermis is stratified squamous epithelium (stratified for protection). This epithelium is covered by a layer of keratin that is in fact dead squamous cells filled with keratin. We call this epithelium keratinized stratified squamous epithelium

8 So again,,,,for naming epithelium we have two criteria: a) The first is according to the number of layers Single row (layer) of cells: simple epithelium. More than one layer: stratified epithelium. b) The second is according to the shape of cells, they are Squamous.(flat cells) Cuboidal.(square cells) Columnar cells (tall cells). Pseudostratified: it appears under the light microscopy as many layers, but actually all the cells touch the basement,which means that it's composed of a single layer of cells, but because of the different heights of cells, not all of them reach the apical surface.we mentioned that epithelial cells have apical surfaces which are directed toward the luminal/ external surface (e.x epithelium of the skin is exposed to the outer environment), and basal surfaces directed toward the basement membrane. REMEMBER; there are two definitions of simple epithelium - All the cells touch the basement membrane. - It's composed of a single layer of cells. Again, because of the different heights of cells not all of them reach the lumen/surface and also the nuclei appear in many rows (at different levels). NOTE; it is difficult to discriminate pseudostratified epithelium under light microscopy LM (using hematoxylin and eosin) since we can't visualize the cell membrane under LM, and this is due to thickness of this membrane which is equal to 7-10 nm and this is less than the resolution power of the LM. There is a type of stratified epithelium that has cells on the apical layer with changeable shape: Known as transitional epithelium. Lines the urinary bladder (hence the name urinary epithelium). Cells of the apical layer are: i. Dome-like cells: when bladder is empty These cells could be binucleated ii. Squamous cells: the cells will stretch when bladder is full with urine Q: how to discriminate between stratified squamous epithelium from transitional epithelium (stretched state)? Will be discussed later on in the lab. REMEMBER; always and always epithelium is adjacent to a connective tissue, simply because epithelium is avascular (has no blood vessels) and it depends on connective tissue blood vessels to get its nutrients and O2 and to get rid of its waste products. NOTE; there are 2 types of stratified squamous epithelium 1. Keratinized: with keratin layer on its apical surface. 2. Non-keratinized: no keratin layer on the apical surface. Examples on keratinized and non-keratinized epithelium: 1. Skin has a keratinized epithelium. 2. Lining of oral cavity has some regions with keratinized and others with nonkeratinized epithelium. 3. Lining of esophagus is composed of non-keratinized epithelium. In general stratified squamous epithelium is keratinized in dry surfaces while nonkeratinized in wet surfaces. REMEMBER; keratinized layer is a layer of dead cells that have no nuclei and contain keratin on their cytoplasm.