Epithelium [epithelial tissue]

Epithelial tissue also referred to as epithelium, refers to the sheets of cells that cover exterior surfaces of the body, line internal cavities, and passageways, and form certain glands.

Epithelial tissue is made up of cells that are tightly bound together by specialized junctions in order to fulfill the main function of this tissue: to create a barrier with the external environment. In addition to performing the task of protection, epithelia perform absorption, secretion, transport, gas exchange, gliding (of two surfaces) and sensory functions. This results in a change in tissue morphology depending on the different function.

At the base of all epithelia is a basal lamina that separates the epithelium from the underlying connective tissue. Below the basal lamina is a reticular lamina of connective nature composed of type III collagen. The two laminae are well evidenced as a single basal membrane by staining with periodic acid Schiff’s reagent (PAS staining).

Epithelial tissues or epithelia generally have three basic characteristics in common:

  1. they are composed of rather regular geometrically shaped, polarized cells, i.e., in which it is possible to distinguish a basal or proximal part (facing the basal lamina, below) and an apical or distal part (facing the free surface of the epithelium);
  2. the cells are adjacent, closely adherent to each other by means of cellular junctions, with the interposition of very little amorphous intercellular substance;
  3. epithelia are not vascularized (oxygen and metabolites reach the cells by diffusion from the underlying connective tissue).

Epithelial membranes

The epithelial membrane is composed of epithelium attached to a layer of connective tissue, for example, your skin. The mucous membrane is also a composite of connective and epithelial tissues. Sometimes called mucosae, these epithelial membranes line the body cavities and hollow passageways that open to the external environment, and include the digestive, respiratory, excretory, and reproductive tracts. Mucous, produced by the epithelial exocrine glands, covers the epithelial layer. The underlying connective tissue called the lamina propria (literally “own layer”), help support the fragile epithelial layer.

serous membrane is an epithelial membrane composed of a mesodermally derived epithelium called the mesothelium that is supported by connective tissue. These membranes line the coelomic cavities of the body, that is, those cavities that do not open to the outside, and they cover the organs located within those cavities. They are essentially membranous bags, with mesothelium lining the inside and connective tissue on the outside. Serous fluid secreted by the cells of the thin squamous mesothelium lubricates the membrane and reduces abrasion and friction between organs. Serous membranes are identified according to locations. Three serous membranes line the thoracic cavity; the two pleura that cover the lungs and the pericardium that covers the heart. A fourth, the peritoneum, is the serous membrane in the abdominal cavity that covers abdominal organs and forms double sheets of mesenteries that suspend many of the digestive organs.

The skin is an epithelial membrane also called the cutaneous membrane. It is a stratified squamous epithelial membrane resting on top of connective tissue. The apical surface of this membrane is exposed to the external environment and is covered with dead, keratinized cells that help protect the body from desiccation and pathogens.[1]

Types and classification of epithelial tissues

Epithelia, depending on function, can be classified into:

  • lining epithelia, with a protective function (from dehydration, from physical and chemical trauma, from the entry of microorganisms, bacteria, viruses);
  • glandular or secretory epithelia, with secretory function;
  • sensory epithelia, specialized in the reception of stimuli (not to be confused with neuroepithelia).

Lining epithelium

The lining epithelia are a type of epithelial tissue that delimit the body surface and the cavities of the organism in communication with the external environment. The internal cavities not communicating with the outside are lined by particular types of epithelium, simple pavimentosum, of mesodermal origin such as connectives: the endothelium (which lines the blood vessels, lymphatics, heart) and mesothelium, which lines the serous cavities such as pleura, pericardium, peritoneum and constitutes a large part of the epithelium of the urinary tract, genitals, ovary, cortex of the adrenal gland.

The lining epithelia are classified according to:

  • the number of cell layers: simple or monostratified epithelia (a single layer of cells) and compound or multilayered epithelia (multiple layers of cells). Pseudostratified or pluriseriated epithelia appear to be multilayered: in reality, their cells are all implanted on the same base but have different sizes, so that their nuclei are at different heights, simulating stratification;
  • the shape of the cells: pavement epithelia (flattened cells), cubic or isoprismatic (cuboidal-shaped cells), cylindrical or prismatic (cylindrical or prism-shaped cells). In multilayered epithelia, the shape of the cells of the surface layer is considered (the shape is different in the different layers). A further criterion for classification of multilayered epithelia is the presence/absence of the superficial stratum corneum;
  • the specializations of the free surface: cilia, stereocilia, flagella.

The following epithelia are monostratified epithelia:

  • simple pavimentous epithelium,
  • simple cubic epithelium,
  • simple cylindrical epithelium,
  • pseudostratified epithelium.

The following epithelia are pluristratified epithelia:

  • pluristratified pavimentous epithelium,
  • pluristratified cubic epithelium,
  • pluristratified cylindrical epithelium,
  • transitional epithelium.

Simple lining epithelia

These are epithelia consisting of a single layer of cells that are wider than tall, with a strongly flattened nucleus and arranged to form a thin pavement. Examples of simple pavement epithelia are: the endothelium, epithelium that forms the tonaca intima of vessels; the mesothelium that forms the serosae (pericardium, pleura, and peritoneum); Bowman’s capsule of the renal corpuscle; and the epithelium that forms the wall of the pulmonary alveoli. Cells may have different types of free surface specializations, such as cilia, stereocilia, and microvilli.

The shape of the cells defines the type of epithelium:

  • if the cells are flattened it is called simple pavimentous epithelium if the cells have a square profile it is called simple cubic epithelium;
  • if the cells have a rectangular profile, i.e. the prevailing size is the height, we speak of simple cylindrical epithelium; nuclei are elongated and located in basal, central or (rarely) apical position;
  • in the human organism we find simple pavimentous epithelia forming the wall of the membranous labyrinth and the external surface of the tympanic membrane, the lining of the pulmonary alveoli, the “rete testis” and some parts of the nephrons; the mesothelium and the endothelium, both of mesodermal origin, are simple pavimentous epithelia.

Simple cubic or isoprismatic epithelium: it is formed by a layer of cells generally as high as wide, with a nucleus always round and placed centrally to the cell. Under conditions of activation the cells of this epithelium can take a more elongated shape but are distinguishable from the simple cylindrical epithelium because they always have a round nucleus and arranged centrally.

This epithelium has a containment function: it usually lines the excretory ducts of exocrine glands, some tracts of renal tubules, thyroid follicles (normofunctioning), in the lens and in the retina (pigmented epithelium).

Simple cylindrical or bathyprismatic epithelium: it is formed by cells that are much taller than wide arranged to form a palisade and that have an oval nucleus located in the first third of the cell, in the part closest to the basal lamina. This type of epithelium is widespread, has multiple functions and therefore can have different specializations on its surface.

In the intestinal canal, for example, the simple cylindrical epithelium covers the luminal surface of the mucous membrane and the cells that compose it have numerous microvilli on the apical face, that is, digitiform expansions of the cytoplasmic membrane, designed to increase the surface of absorption and facilitate the exchange of substances.

In the salpinx, the simple cylindrical epithelium is ciliated because its function is no longer that of exchange, but that of creating amorphous currents that favor the descent of the oocyte towards the uterus and, at the same time, of hindering the ascent of the spermatozoa along the oviduct. The simple cylindrical epithelium is also found, without any specialization, in excretory ducts, bronchial tracts, renal tubules, in the small intestine (intestinal villi), mucosa of the oviduct, some areas of the uterus.

Multilayered lining epithelia

These are epithelia consisting of multiple layers of cells. The type of epithelium is defined by the shape of the cells of the most superficial layer. In fact, the shape of the cells is different in the various layers constituting the epithelium; generally the cells of the basal layer, in contact with the basal lamina, are cubic.

Multilayered pavimentous epithelium: it is composed of numerous superimposed layers of cells that are flattened towards the free edge of the tissue. It has the function of protection and barrier against external aggressions. It can be not keratinized (or not corneified), or keratinized (or corneified). In the latter case, the surface cells lose their nuclei and the cytoplasm is occupied by a large amount of keratin (scleroprotein); as a result, on the surface there are no viable cells, but horny scales.

A pluristratified keratinized pavimentous epithelium is typical of the epidermis. Non-keratinized multilayered pavimentous epithelia line the cornea, oral cavity, pharynx, and most of the esophagus. The stratum corneum protects against dehydration, when it is very thick (sole of the foot, palm of the hand) it also has a protective function against mechanical insults.

In the epidermis of a mammal can be distinguished several layers of cells. Starting from the deepest (in contact with the basal membrane) we find: the basal or germinative layer, the spinous layer, the granular layer, the stratum corneum. In the epidermis of the palm of the hand and the sole of the foot, between the stratum granulosum and the stratum corneum, there is the stratum lucidum.

Cubic or isoprismatic multilayered epithelium: is formed by two or more layers of cubic cells. The stratified cubic epithelium is, in mammals, very rare and lines only a few large excretory ducts.

Multilayered cylindrical or bathyprismatic epithelium: is formed by two or more layers of overlapping cells. In the deeper layer, they are small, polyhedral and never reach the surface of the epithelium, while the more superficial layer is formed by true cylindrical cells. In preparations, the nuclei appear to be overlapping. This epithelium has the function of protecting and covering large ducts or particularly irrigated surfaces and is very rare in mammals: we find it only in large excretory ducts, in the penile urethra, in a short stretch of the epiglottis and on the inner surface of the eyelid.

Pseudostratified lining epithelia

In this type of epithelia the nuclei appear at different heights, but in reality there is a single cell layer, because all cells are in contact with the basal lamina but only some reach the free surface. The shape of the cells is varied, predominantly elongated.

Pseudostratified epithelium: it is an epithelium in which all cells are in contact with the basal lamina, but only some reach the surface. In the preparations, the nuclei are on different but staggered planes, never overlapping. The cells constituting this epithelium have a decidedly variable shape. They present a cytoplasmic swelling in which the nucleus is housed: in some cells this swelling is located towards the base in others towards the apex. This organization gives the impression, under the light microscope, of a multilayered epithelium, without being so (hence the name of pseudostratified or pluriseriated epithelium). There are two types of pseudostratified epithelium: ciliated pseudostratified epithelium and pseudostratified epithelium with stereocilia.

Pseudostratified ciliated epithelium: the function of this epithelium is to purify the inhaled air of dust and atmospheric pathogens. The calyciform mucipar cells secrete mucus that has the function of trapping impurities, the mucus is then spread evenly and moved by the high hair cells, as happens for example in the trachea where the mucus is moved by the hair cells from the bottom to the top (muco-ciliary elevator). This epithelium is found in most of the airways, the Eustachian tube, part of the tympanic cavity, and the lacrimal sac.

Transitional (or polymorphic) epithelium: represents a particular example of pseudostratified epithelium, has a very variable morphology because it lines organs, such as the urinary bladder or ureter, subject to strong volume variation during their function. This epithelium is formed by three types of cells: in the deepest layer, in direct contact with the basal lamina, we find cells of cubic or cylindrical shape (basal or germinative layer), immediately above there are some layers of elongated cells (called clavate or piriform), finally the most superficial layer consists of cells, sometimes binucleate, orthogonal with respect to clavate and with the upper surface convex called cupoliform or umbrella.

The possibility of the clavate cells to deform stretching on a horizontal plane, while remaining embedded in the umbrella-shaped, allows this epithelium to increase its surface in accordance with the dilation of the organ it covers. For example, in the released state (empty bladder) the transitional epithelium appears as consisting of 5-6 layers of cells, the most basal ones cubic, the superficial ones globular and sometimes binucleate, often with prominent nucleoli (cupoliform cells). In the distended state (full bladder) the transitional epithelium appears as consisting of only 2-3 layers of cells; the superficial cells are very flattened. Such variations in the number of layers are only apparent, there being always a single layer of cells.

Glandular epithelium

The glandular epithelium is an epithelial tissue that specializes in producing and secreting substances including different types of proteins, enzymes, lipids, polysaccharides, and hormones. Glands are made up of epithelial cells specialized in secretion, derived from epithelial laminae. The secreted substances are of various types: enzymes, substances with a protective function (eg, the secretion of mucipar cells), toxic substances to be eliminated, hormones, etc..

In particular, glands are formed as a result of a proliferation of epithelial cells that deepen into the underlying connective tissue to form cellular structures with a secretory function. If the gland retains any relationship to the free surface by means of secretory portions (adenomeres) or an excretory duct, thereby pouring the product of secretion onto the surface of the epithelium of origin, we have an exocrine gland. If, on the other hand, the duct disappears and the secretion is released into the bloodstream, we have an endocrine gland (the product of these glands is generically called a hormone).

Not all glands are of epithelial derivation, there are some secreting cells of different nature as the interstitial cells of the gonads, the cells of the theca of the ovarian follicle, neurons that secrete hormones (neuroendocrine cells).

Glandular secretion can be continuous or discontinuous. Continuous secretion is when the secretion is expelled from the cell as it is produced. Examples are endocrine cells that secrete steroid hormones and mucipar cells. In discontinuous or rhythmic secretion, on the other hand, the secretion product is accumulated within the cell in the form of granules and only later expelled following hormonal, nervous or chemical stimuli. A typical example is the cells of the pancreas.

Exocrine glandular epithelia

Exocrine glands consist of a secreting part called the adenomere, and a part that drains the secretion to the outside, called the excretory duct. Depending on their topographical location, they may be intraepithelial or extraepithelial (which in turn are divided into parietal and extraparietal).

Exocrine glands can be unicellular or multicellular.

  • Unicellular exocrine glands consist of a single secreting cell; a classic example are the mucipolar calycephalic cells, intercalated with enterocytes in the intestinal mucosa.
  • Pluricellular exocrine glands consist of a secreting portion, the adenomere, and an excretory duct that allows the secretion to be poured out.

Pluricellular exocrine glands can be classified according to several criteria:

  1. according to the shape of the adenomere: if the adenomere is elongated with a fairly obvious lumen we have tubular glands. A particular case of tubular glands are the tubulo-glomerular glands (the adenomere has the shape of a curled tubule) represented only by sweat glands. If the adenomere is roundish with a small lumen and therefore very little evident we have an acinous gland. If the adenomere is round, large and with a very evident lumen we have an alveolar gland. In the case of branched or compound glands we can have a combination of tubular adenomers with acinar adenomers (tubulo-acinar glands) or with alveolar adenomers (tubulo-alveolar glands);
  2. according to the complexity of the organization: if a single adenomere is drained by a single excretory duct we have a simple gland. If, on the other hand, several adenomeres are drained by a single excretory duct we have a branched gland. If the draining excretory ducts are more than one and converging into each other and then into a common excretory duct we have a compound gland;
  3. based on the mode of secretion:
    • merocrine secretion: the release of the secretion occurs through the cytoplasmic membrane leaving the cell perfectly intact (exocytosis), examples are the parotid, the exocrine pancreas, salivary glands.
    • apocrine secretion: with the secretion there is a loss of part of the cytoplasm, surrounded by plasma membrane, which thus becomes an integral part of the product of secretion. This group includes the mammary gland (limited to the mechanism of secretion of lipids), the sweat glands with large lumen.
    • holocrine secretion: it occurs when the secretion is expelled in the excretory duct through the disintegration of the cell itself, sebaceous glands are an example of this type of secretion.
  4. according to the type of secretion (only for merocrine glands): serous (if the secretion is formed mainly by proteins and water), mucous (if the secretion has a mucopolysaccharide nature and is formed mainly by glycoproteins and water) and serum-mucous (mixed, they are made up of mucous cells and serous cells).

Endocrine glandular epithelia

The endocrine glands are devoid of excretory ducts and pour their secretions (hormones) directly into the bloodstream to specific organs (called targets) in order to regulate their activity, thanks to the presence of a rich network of capillaries in the connective tissue support that surrounds the glands themselves. Hormones can be of amino acid or steroid origin.

The endocrine glands are generally formed by epithelial cells arranged to form cords or islands; in the case of the thyroid gland they form follicles instead. The endocrine glands constitute separate organs (e.g. pituitary, thyroid, parathyroid, adrenal glands) or alternatively are contained within other organs (e.g. islets of Langerhans in the pancreas).

There are pure endocrine glands, i.e. formed exclusively by endocrine glandular tissue, and amphiphrine glands, i.e. composed of both endocrine and exocrine glandular tissue (typical example is the pancreas, whose parenchyma, mostly exocrine secretion, has clusters of cells with endocrine activity, the pancreatic islets or Langerhans islets).

The endocrine glands are classified into:

  • endocrine glands with solid epithelial cords: the secreting cells form cellular cords differently arranged in space. Most endocrine glands include: pituitary, parathyroids, adrenals, epiphysis, placenta, corpus luteum;
  • endocrine glands with closed follicles: the secreting part is formed by follicles that contain within them the precursor of thyroid hormones (thyroglobulin), the only example is the thyroid;
  • islet endocrine gland: the islets of Langerhans of the pancreas;
  • interstitial endocrine gland: found in the interstitial spaces between the seminiferous tubules of the testis, in groups of six to eight or more cells placed to surround a capillary, into which they input the secretion (testosterone) and into the ovary.

Sensory epithelium

Sensory epithelia are made up of true epithelial cells, around which end extensions of nerve cells located in the cerebro-spinal ganglia. They are highly specialized epithelia for the reception of stimuli and for the transmission of impulses to the nerve endings with which they are in functional contact (synapses).

Sensory epithelia are the taste buds (in Mammals located at the level of the taste buds of the tongue), the neuromasts (mechanoreceptors) of the lateral line of Fish and Amphibians, the ampullary ridges of the semicircular canals of the inner ear.

Separately, the neuroepithelia should be considered. These are not actually epithelia, because they are not composed of epithelial cells but of neurons that, for the location (peripheral to the central nervous system) and the arrangement (epithelioid), resemble an epithelium; typical examples are the olfactory epithelium (formed by olfactory neurons alternating with supporting cells) and the retina (formed by cones and rods specialized in photoreception and pigmented epithelial cells).

References

  1. Anatomy and Physiology. OpenStax. Authors: J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. https://openstax.org/books/anatomy-and-physiology/pages/1-introduction

Leave a Comment