Terms that help us understand what kinds of tissues we are identifying: Terms referring to the layers. Simple = one layer. Stratified = more than. Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world and lining the outside of organs. Is avascular tissue (without blood supply – cells receive nourishment by diffusion from a highly vascular area of loose connective tissue just below the basement.
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General Considerations. ➢ Classification of epithelial tissues. ❖ Lining and covering epithelia. ▫ Form the boundary between external environment and body. Types of Tissue. • Epithelial Tissue. • Connective Tissue. • Muscle Tissue. • Nervous Tissue. 1. Properties of Epithelia. • Cellularity. • Polarity. • Avascularity. (1) COVERING EPITHELIUM. • It is one type of the epithelial tissue which covers the body surface from outside and lines its cavities from inside. • It can be.
Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes. Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles.
A glandular epithelium contains many secretory cells. The Epithelial Cell Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.
Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat.
It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach. Cell to Cell Junctions Cells of epithelia are closely connected and are not separated by intracellular material.
Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions Figure 1. Figure 1.
Types of Cell Junctions. The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.
At one end of the spectrum is the tight junction, which separates the cells into apical and basal compartments.
An anchoring junction includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections.
There are three types of anchoring junctions: desmosomes, hemidesmosomes, and adherens.
Desmosomes occur in patches on the membranes of cells. The adhesion molecule, cadherin, is embedded in these patches and projects through the cell membrane to link with the cadherin molecules of adjacent cells.
These connections are especially important in holding cells together. Hemidesmosomes, which look like half a desmosome, link cells to the extracellular matrix, for example, the basal lamina.
While similar in appearance to desmosomes, they include the adhesion proteins called integrins rather than cadherins. Adherens junctions use either cadherins or integrins depending on whether they are linking to other cells or matrix.
The junctions are characterized by the presence of the contractile protein actin located on the cytoplasmic surface of the cell membrane.
The actin can connect isolated patches or form a belt-like structure inside the cell. These junctions influence the shape and folding of the epithelial tissue. In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells.
These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells. Classification of Epithelial Tissues Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed Figure 2.
Cell shapes can be squamous flattened and thin , cuboidal boxy, as wide as it is tall , or columnar rectangular, taller than it is wide. Similarly, the number of cell layers in the tissue can be one—where every cell rests on the basal lamina—which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina.
Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary. Figure 2. Cells of Epithelial Tissue. Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells. Simple Epithelium The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in simple squamous epithelium have the appearance of thin scales.
Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The endothelium is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells.
Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue.
The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs.
Its primary function is to provide a smooth and protective surface. Mesothelial cells are squamous epithelial cells that secrete a fluid that lubricates the mesothelium. It is during proestrus that E2 and luteinizing hormone levels peak, restarting the cycle. Nucleated epithelial cells are the dominant cell populations observed in vaginal smears obtained from animals in proestrus Importantly, these different stages are accompanied by changes in vaginal secretory proteins such as cytokines, chemoattractants, and antimicrobial molecules E2 signaling in the vagina induces the formation of a physical barrier by maintaining the thickness of the vaginal epithelium and increasing the secretion of antimicrobial peptides, cytokines and chemokines which recruit and activate immune cells 13 , 14 , The epithelial lining of the vagina is also covered by a layer of glycoprotein-containing mucus, namely mucins, that protects the vagina from infectious agents 16 , Vaginal epithelial cells are stimulated by E2 signaling to produce glycogen, a substrate that is metabolized by the native microflora In postmenopausal women, a reduction in E2 levels results in the production of fewer epithelial cells and a reduced glycogen content.
With vaginal epithelial cell atrophy and a lack of lubrication, the vaginal tissue becomes vulnerable to physical damage, e. Additionally, the composition of the vaginal microbiota often changes in postmenopausal women due to the reduction in the nutritional substrate for the microbiota 19 , These changes in the vaginal environment are often alleviated by treatments with a topical synthetic E2 21 , suggesting that E2 signaling within the vagina plays a role in the maintenance of the immune response and vaginal homeostasis.
Although E2 supplementation is the current course of treatment for most postmenopausal symptoms in women, alternative non-hormonal treatment options are often desired.
A better understanding of the E2-regulated actions in the vaginal epithelium is critical to the development of alternative therapeutic options for postmenopausal women. The effects of E2 in the female reproductive tract are primarily mediated through its receptor ESR1.
Using this model, the authors showed a reduced thickness in the vaginal epithelium, thereby suggesting a role for ESR1 in controlling vaginal epithelial cell differentiation 4. Although this data is informative, the changes imparted on the vaginal epithelium in the absence of ESR1 following physical stresses have never been investigated. In the absence of ESR1, physical stresses, especially those induced by mating, could exacerbate the observed effects on the vaginal epithelium.
A compromised epithelial layer is likely to alter vaginal homeostasis, eliciting an observable immune response that differs from an individual with normal ESR1 function.
Generalized Functions of Epithelial Tissue
Therefore, we hypothesized that ESR1 activity within the vaginal epithelial cells is not only required for the maintenance of vaginal tissue integrity, but is also involved in the regulation of the immune response following a physical stress. Results Loss of ESR1 function in the epithelium induces vaginal tissue laceration after mating The physiological impact of mating in animals lacking ESR1 in vaginal epithelial cells has yet to be explored.
Therefore, histological evaluation was performed on vaginal tissues at 0. These included a cornified outer layer containing enucleated cells, Fig. Representative images showed a disrupted epithelial layer with cells infiltrating from the stroma into the vaginal lumen Fig. Figure 1 Loss of tissue integrity and leukocyte infiltration after mating in the absence of vaginal epithelial ESR1. Black arrows indicate a site of cell infiltration from stromal and epithelial layers into the vaginal lumen.
Insets are negative control without primary antibodies. To determine the proliferative index, we evaluated the number of Kipositive cells present in vaginal tissues during estrus as well as at 0. The proliferative index in the epithelial layer was not different at estrus, but was significantly higher at 0.
Generalized Functions of Epithelial Tissue
C The total thickness of the vaginal epithelium at estrus and 0. E Transcription levels of keratin gene family in the whole vaginal tissue at 0. Insets are the higher magnification of apical layer.
Arrows denotes sat cells with MUC1-positive staining.
S1 , while in other areas, the epithelial cell layer was completely absent and the vaginal plug was adhered directly to the stromal cell layer Supplementary Fig. As keratinization strengthens the epithelium we next determined the expression of the keratin family of genes, specifically Krt6a, Krt6b, Krt10, Krt13, and Krt Figure 3 Increased number of leukocytes in the absence of epithelial ESR1.
B Quantification of leukocyte levels through estrous cycle arbitrary unit. Arrows denote multinucleated cells within the semen portion in the vaginal lumen and in the epithelial cell layer.
Full size image The leukocytes present in the upper vaginal lumen appeared to be multi or segmented -nucleated cells, characteristic of neutrophils. To validate whether or not the infiltrating cells were neutrophils, Ly6G was used as a neutrophil marker.
Due to a mating-induced lesion at 0.
Basement membrane (BM)
At estrus, Ly6G-positive cells accounted for 8. Figure 4 Neutrophil infiltration and upregulation of immune gene expression. Each data point from all technical replicates are presented.
In smears from diestrus and proestrus stages, we observed scarce or no bacterial colonies. Histone 3 H3 is a critical mediator involved in the formation of NETs 23 , so an H3 antibody was used to determine the presence of NETs in vaginal smear samples.
Figure 5 A loss of gram-positive bacteria and increased extruding neutrophil nuclei in vaginal smears from female mice lacking ESR1 in the vaginal epithelial cells.
Yellow arrowhead indicates gram-positive bacterial colonies. B Number of mice with gram-positive bacterial colonies in their vaginal smear. White arrows denote neutrophil nuclei with Ly6G-positive staining extruding into the distance. Full size image A loss of epithelial ESR1 in the vagina changes enzymatic activities in the cervical-vaginal fluid Matrix metalloproteinase 9 MMP9 is produced by neutrophils that undergo NET transformation in order to induce tissue remodeling 24 , In addition to MMPs, other proteases could also induce changes in tissue integrity.
Yellow arrow denotes at the date animal was plugged. A and B were cropped from four full zymography images presented in Supplementary Fig. Full size image Discussion Our study revealed new insights regarding the role of epithelial ESR1 in maintaining tissue integrity and immune response in the lower female reproductive tract. We have summarized our findings and proposed a working model in Fig. First, ESR1 in the vaginal epithelium is responsible for E2-induced cell hypertrophy during the estrous cycle.
Without ESR1, epithelial cells become atrophic, similar to what occurs in postmenopausal women. Second, epithelial ESR1 regulates genes involved in maintaining cellular integrity keratins and secretions mucins.
Epithelial Tissue - Overview PDF
Lack of epithelial ESR1 contributes to a loss of cornification, reduced cellular integrity, and excessive glycoprotein production. Third, epithelial ESR1 plays a vital role in immune suppression. Lacking epithelial ESR1 would result in a failure to suppress vaginal leukocytes, leading to excessive MMP activities.
MMPs are proteinases that digest cellular matrix proteins, causing extracellular matrix ECM modification and cellular detachment. ECM breakdown may also create a feedback loop to recruit more neutrophils to clear out cellular debris. This proposed working model illustrates that epithelial ESR1 is necessary for homeostasis of the vagina and potentially facilitates the development of treatments for postmenopausal symptoms in women.
Figure 7 Working model of the function of ESR1 in vaginal epithelial cells. In the presence of epithelial ESR1, E2 action through ESR1 suppresses neutrophil infiltration by minimizing the level of cytokine production Il1b and Il8 in vaginal tissue.In , Long and Evans described the cyclic pattern of the rat vaginal epithelium At estrus, the absence of leukocytes in the vaginal lumen allows the growth of bacteria colonies during this window of vulnerability 6.
Stratified cuboidal epithelium and stratified columnar epithelium can also be found in certain glands and ducts, but are uncommon in the human body. Other areas include the airways, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium.
Metestrus is followed by diestrus, a resting stage of the reproductive cycle, and vaginal smears from animals in diestrus show a reduced number of cell types. However, further investigation is needed to identify the different immune cell types in the presence or absence of epithelial ESR1 in the vagina.
Epub Dec It is found only in the urinary system, specifically the ureters and urinary bladder. The estrous cycle is divided into four distinct stages: estrus, metestrus, diestrus, and proestrus. Nucleated epithelial cells are the dominant cell populations observed in vaginal smears obtained from animals in proestrus
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