Lining epithelia are specialized in covering the free surfaces (internal and external) of organs and form the boundary with the external environment. They typically consist of epithelial cells arranged in layers and closely bound to each other.
Lining epithelia exhibit clear apicobasal polarity, where the apical pole forms the free surface of the epithelium, in contact with the external or internal environment or body cavity. Meanwhile, the basal pole rests on the underlying connective tissue, separated from it by the basement membrane.
Lining epithelia are classified based on the number of cell layers they contain, the morphology of the cells in the most superficial layer, and other characteristics. These characteristics include whether the apical surface is moist (mucous or cuticular) or dry (cuticular or cornified), as well as the presence of various specializations of epithelial cells. Thus, they can be distinguished as follows:
- Simple epithelia, which consist of a single layer of epithelial cells, all of which extend from the basement membrane to the surface.
- Stratified epithelia, which contain two or more layers of cells. In these epithelia, at least a basal (or germinative) layer is located next to the basement membrane, and a superficial apical layer. In many cases, an intermediate region is also present, often with multiple layers of thickness, known as the intermediate layer (or stratum intermedium).
A specific variation of this type is the transitional epithelium, which forms the lining of the urinary bladder lumen. The number of layers and the cell morphology in this epithelium varies depending on the degree of extension of the epithelium as the bladder distends or relaxes.
- Pseudostratified epithelia include those in which, under light microscopy, it is uncertain whether there is more than a single layer of cells. This is because the nuclei of the epithelial cells appear at different heights, and not all cells seem to extend from the basement membrane to the apical surface.
Currently, from electron microscopy studies, it is known that in most cases, epithelia typically identified as pseudostratified, such as the tracheal epithelium in mammals and the olfactory epithelium in vertebrates, are actually stratified. This is because they contain basal cells that do not reach the apical edge of the epithelium. These basal cells are germinative and form a discontinuous basal layer.
However, in other pseudostratified-looking epithelia, with nuclei located at different heights, all cells extend from the basement membrane to the apical surface, indicating that they are actually simple epithelia.
This refers to the shape of the cells in the superficial layer in the case of stratified epithelia.
- Squamous epithelia in which all cells or those in the apical layer have a flattened appearance.
- Cuboidal epithelia, where all cells have the same height and width in simple epithelia or the apical cells in stratified epithelia.
- Columnar epithelia, where cells are taller than wide. In the case of pseudostratified epithelia with multiple strata, such as the mamalian tracheal epithelium, the columnar appearance refers to cells that extend throughout the thickness of the epithelium.
- Sunken or submerged epithelia, found in some invertebrates (turbellarians, trematodes, and cestodes), where the apical surface is formed by flattened edges of the epithelial cells. These cells have a perikaryon (cell body that houses the nucleus and most organelles) located in the underlying connective tissue. Sunken epithelia can be syncytial, where adjacent apical edges fuse together.
- Mucous epithelia, where the surface remains moist, and cells throughout the thickness of the epithelium remain alive. The surface moisture is due to the presence of a layer of mucus or an external cuticle secreted by the epithelial cells themselves or by exocrine glands.
- Non-mucous epithelia, with a dry and more or less hardened surface, due to the presence of an extracellular cuticle in invertebrates or a layer of dead cornified epithelial cells in terrestrial vertebrates.
- Syncytial epithelia, typical in some invertebrates, where cells fuse together, either completely or only in the apical edge forming the surface.
- Epithelia with intracellular armor or lorica, found in rotifers and acanthocephalans, where the apical border of epithelial cells has an intracellular reinforcement of protein fibrils.
- Cuticular epithelia, where the apical pole is covered by a more or less permanent layer of extracellular material, secreted by the epithelial cells themselves or by exocrine glands. The cuticle can be mucous, continuously renewed, or composed of secreted proteins forming more permanent layers. In the latter case, the cuticle can be flexible, non-chitinized or calcified, as in annelids, or hardened into an exoskeleton, with the cuticle being chitinized (insects) or calcified (crustaceans and mollusks).
- Ciliated epithelia, where most of the superficial cells have cilia.
- Closed-mucous pole epithelia, also referred to as closed-mucosecretory lining epithelia, consisting of cells whose apical pole has a thick mucous layer, formed by continuous mucus secretion from each epithelial cell.
- Epithelia with striated or brush border, formed by superficial cells whose apical pole contains a high number of short microvilli arranged in parallel.
- Epithelia with stereocilia, where the apical cells present long wavy microvilli, often grouped in tufts.
- Epithelia with striated basal pole, where cells show striations associated to a acidophilic cytoplasm in their basal pole. These characteristics correspond ultrastructurally to numerous infoldings of the basal cell membrane, as well as some in the basolateral membrane, comprising folds of the cytoplasm with abundant elongated mitochondria oriented perpendicular to the basement membrane.
- Keratinized epithelia (or cornified), always stratified and forming the epidermis of vertebrates, in which epithelial cells, called keratinocytes, progressively accumulate intermediate filaments of cytokeratins as they mature and move toward the upper layers. The apical layer, called the stratum corneum, is formed by one or more layers of dead and flattened epithelial cells that form scales, either microscopic (microscales) or macroscopic.
To histologically name a given epithelium, a combination of the different properties mentioned earlier is considered (simple or stratified, mucous or non-mucous, shape of apical cells, etc.). For example, an epithelium labeled as simple squamous mucous is one formed by a single layer of flattened cells with a wet surface.
Another example is the stratified squamous mucous epithelium consisting of several cellular layers, the topmost of which contains mucous-secreting cells.
Some epithelia have specific names, such as the epidermis, which forms the outer layer of the integument of animals, and which may consist of different types of epithelia, depending on the species. For example, the epidermis of tetrapod vertebrates (terrestrial) consists of a stratified squamous keratinized (or cornified) epithelium, whereas in invertebrates the epidermis is composed of simple epithelia.
Specific names are also given to the endothelia, which line the lumens of blood vessels in the circulatory system, consisting of a simple squamous epithelium, and mesothelia, composed of simple, squamous, or cuboidal epithelia, and sometimes ciliated, lining body cavities and the surfaces of the organs contained within them.
On the other hand, the term mucosa is used to to designate the whole of a mucous epithelium (of any type) and its lamina propria (the underlying connective tissue) of the internal organs.