Species: mouse (Mus musculus; mammal)
Technique: haematoxylin-eosin, 8 µm thick section, paraffin embedding.
The pseudostratified epithelium was initially classified as stratified at the end of the XIXth century because it shows nuclei at different heights in the epithelial layer (see also figure 1). Later, it was observed that all epithelial cells make contact with the basal lamina, so that it was only one cell row with nuclei at different distances from the basal lamina. Then, it was renamed as pseudostratified epithelium.
The pseudostratified epithelium is distributed in several parts of the animal body: upper respiratory tracts (trachea and bronchi), epididymus, Eustachian tube, olfactory epithelium, some segments of the urethra, pharynx and larynx. During the embryo development, the pseudostratified epithelium is also found in structures that start to be formed, such as ectoderm placodes and the neural tube, that develops into the nervous system.
Pseudostratified epithelium is composed of at least two types of cells: columnar or fusiform cells, which reach the apical surface, and basal cells, which apparently do not. However, it has been found that basal cells grow thin apical extensions that reach the apical surface of the epithelium. This has been observed in the epididymus and upper respiratory tracts, but can be a general feature of every pseudostratified epithelium (see Shum et al., 2008).
The height of the pseudostratified epithelium is variable, from dozens to hundreds of µm. The thinner usually are less than 30 µm in height and 2 to 3 layers of nuclei. They are observed in the otic vesicles, neural plate and initial expansions of the ectoderm to form organs such as glandular ducts and epididymus. The middle height pseudostratified epithelium, between 30 and 60 µm, and 4 to 6 rows of nuclei, are found in the mammal neural tube. The thicker pseudostratified epithelium are a layer with more than 100 µm in height, with 8 to 9 rows of nuclei, and can be found in the neural tube and in the retina epithelium. The density of nuclei in the epithelial layer is also variable. It is interesting that the position of nuclei can change in the epithelial layer depending on the physiological stage of the cell. These movements are known as interkinetic movements. They are usually related with cell division, when nuclei move upward for mitosis and cytokinesis.
It is common that the cells of pseudostratified epithelium bear cilia, for instance in the respiratory ducts, or stereocilia as in the epididymus. However, the final part of the urethra shows no apical specializations. In the apical domain of the epithelium, there are cellular junctions, such as tight junctions and adherent junctions, to keep cells together. The centrosome is usually found near the apical domain of the cell, where it is thought to be involved in the cilia formation (but not stereocilia). The Golgi apparatus is also located in the apical part of the cell. However, mitochondria and endoplasmic reticulum are distributed through the cell cytoplasm.
There is a high cellular proliferation in the pseudostratified epithelium. During development, pseudostratified epithelium is found in those regions where new organs are emerging. For example, it is found in the otic and optic placodes, and in the expansions that develop into the liver and pancreas. Pseudostratification may be due to the high proliferation rate of columnar epithelia that need to accommodate the new nuclei in the layer at different heights. Pseudostratification may be transient during development. For instance, the expansions that give rise to the liver and respiratory bronchioles during development are initially pseudostratified, but are later transformed into columnar or cuboidal epithelium.
Norden C. 2017. Pseudostratified epithelia – cell biology, diversity and roles in organ formation at a glance . Journal of cell science. 1-5 doi:10.1242/jcs.192997.