Epithelium

The epitheliums are fabrics of which the cell S are jointed and interdependent from/to each other thanks to intercellular Jonctions. They are not vascularized except for the vascular scratches (cochlea, production of the endolymphe). The contribution of the Nutrient S and the export of waste are made in relation to the conjunctive Tissu subjacent via a basal Membrane, on which any epithelium rests (if a fabric has jointed cells but not the basal one, it is not an epithelium, example: épendymocyte S). In this type of fabric, the cells are often polarized (the two opposite ends are different morphologiquement and biochimiquement), although there exist some epitheliums where this polarity is difficult to distinguish (hepatic fabric). One distinguishes the apical area from the cell and the basal area which is coupled with the basal blade.

They fulfill two nonexclusive physiological functions one of the other: function of coating and glandular function.

The specific presence of intermediate filaments of Cytokératine allows their identification by immunohistochimic methods.

Epitheliums of coating

They mark a edge between a fabric, often a conjunctive Tissu, and

  • the ambient air (example: the skin)
  • an interior cavity. In these cavities one distinguishes:
    • cavities opened outside; the epithelium rests then on a conjunctive fabric called Chorion, and the body formed by these two fabrics is called Muqueuse.
    • closed cavities. The séreuses cavities are papered of an epithelium called Mésothélium; the circulatory cavity is papered of an epithelium called Endothélium.

Tissue organization

Criteria of classification

  • Form of the cells : the cells of the same layer are always of the same form

suitable for paving: broader than high
cubic: as broad as high
prismatic or cylindrical: higher than broad
polymorphic: when the cells can change form (according to, for example, the state of vacuity or repletion of the bladder in the case of urothélium)

  • Many cellular layers

simple: a
layer laminated: several layers. In this case, one characterizes only the shape of the cells of the surface layer.
pseudostratifié: the cells appear laminated because the cores all are not on the same level, but in fact they all are in contact with the basal one.

  • cellular Specialization

intercellular Junctions, Microvillosité S, cilia, basal Labyrinth, surface Condensation of the cytoplasm, mucous Cells with mucous pole closed, caliciform Cells…

  • Function of the epithelium

movement: Certain epithelial cells are provided with lashes which beat in a synchronous way and make it possible for example to make go up the Mucus towards the higher air routes of the breathing apparatus.
exchanges: The presence of striated plates, banal microvillosities or Stéréocils (different from the lashes because not mobiles) make it possible certain epithelial cells to multiply their surface of contact. It is the case in the intestinal epithelium which thus reaches 200 m ² of surface thus facilitating the collecting of the Nutriment S by the epithelial cells.
chemical protection: The Mucus which recover certain epitheliums and the Kératine which fills the cytoplasm of the keratinized cells confer both a chemical function of protection on the epithelium.
mechanical protection: The intercellular Jonctions which bind the epithelial cells between them and them bind in the same way to the basal blade confer on the epitheliums a great resistance to the mechanical stresses.

Examples of epitheliums

  • the skin is a epithelium malpighien (or epithelium suitable for paving laminate) keratinized composed of cells called kératinocytes. The deepest layer in contact with the basal blade contains cells in Mitose. The generated cells are pushed back towards the peripheral layers and die by accumulating keratin, creating a protective coating.
  • the gastric epithelium is a simple prismatic epithelium with mucous cells with closed mucous pole playing a part of protection against the Gastric juice. They secrete Hydrochloric acid in the light of the stomach.
  • the intestinal epithelium is a simple prismatic epithelium, comprising cells with striated plate which absorb the nutrients, and of the caliciform Cellules secreting a mucus which limes the microvillosities and helps with the absorption of certain substances. The epithelium of the colon has a role in the reabsorption of liquids. A deregulation of this function, by bacterial infestation for example, involves the appearance of Diarrhée S, first cause of infant mortality in the countries in the process of development.
  • the epithelium of the respiratory tracts is a pseudostratifié prismatic epithelium, comprising cells with cilia and caliciform Cellules, secreting a mucus where dust and bacteria are trapped and evacuated by the lashes during the phenomenon of clearance muco-ciliaire. This mucus thick because is imperfectly hydrated among patients reached of Mucoviscidose. The bacteria are not evacuated any more, causing a Inflammation of this fabric. One of the consequences of this ignition is the closing of the air cells and the difficulty of breathing which follows.
  • the epithelium of the Bladder, the urothélium, is a polymorphic pseudostratifié epithelium. It presents cells to the characteristic shape in racket which are in contact with the light and the basal membrane, able to be flattened to increase volume available in the bladder as well as cells of renewal, which they, do not level on the surface.

Renewal of the epitheliums

The simple epitheliums renew by slip along the basal Membrane starting from a germinative zone containing of the original cells. The laminated epitheliums renew in the layer in contact with the basal Membrane which one calls germinative layer.

Glandular epitheliums

Definition

The glandular function is the fact that a cell is able to secrete a substance which will be excreted out of the body where it is produced: either in the Blood (endocrine gland), or apart from the organization (gland exocrine like the glands sudoripares, the glands mammaires…).

A gland can be amphicrine , i.e. at the same time exocrine, and endocrine. It is the case of the Pancréas (the pancreas exocrine responsible for the pancreatic secretion of Suc, and the endocrine pancreas, with the small islands of Langerhans). The pancreas is thus a heterotypical gland amphicrine , made up of two cellular types having each one a different function. The liver is also a homotypic gland amphicrine, but . The same cell has at the same time an endocrine secretion (Glycogène, etc) and exocrine (Bile). This function requires two or three stages: secretion (made work out a substance, property of any alive cell), possibly put in load, then excretion (" livraison" substance)

Modes of excretion

  • Mode mérocrine : most frequent. The substance is put in load in blisters; when the organization needs some, it is excreted by diffusion or Exocytose. There is maintenance of the cellular integrity. One finds this type of secretion in the Pancréas for example.

  • Mode holocrine : the whole cell is the product of delivery: it accumulates the substance in its Cytoplasme, then dies and is detached from the epithelium. One finds this type of secretion in the sébacée Glande.
  • apocrine Mode or holomérocrine : the cell has two well separated poles. The basal pole gathers the organoids necessary to secretion, the apical pole is formed gradually, as the substance is produced. At the time of the excretion, the apical pole splits up. The cell can then take again a cycle of secretion. One finds this type of secretion in the Glande mammaire

Notice on the glands exocrines

The epithelium of the Glande S exocrines releases its substance in a cavity of the body or about the external middle; it is thus obligatorily an epithelium of coating.

External bonds

  • digital Atlas of microscopic morphology - Medical college of Namur (FUNDP - LabCeTi)
  • www.mhhe.com

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