While most of the lining and glandular epithelia are primarily composed of epithelial cells, in vertebrates, it is common to find a certain number of specialized cells among them, some of which are non-epithelial in nature.
- Sensory Neurons (neuroepithelial cells), such as those located in the epithelium of the respiratory tract, including olfactory neurons.
These neurons have a sensory apical pole, generally ciliated, and a basal axonal extension that exits the epithelium and projects to nervous centers.
- Secondary sensory cells, of epithelial character, with a sensory pole but lacking an axonal extension. These secondary sensory cells receive sensory innervation from sensory neurons.
Examples of this cell type include Merkel cells and similar ones in the epidermis of many vertebrates, sensory cells of vertebrate taste buds, which have microvilli on their apical surface, and the ciliated sensory cells in the neuromasts of fish and amphibians. Similarly, secondary sensory cells include those in the sensory epithelia of the statoacoustic organ (Organ of Corti and ampullary maculae).
Although, typically, they are resident cells of the connective tissue proper, at the base of many epithelia and within their thickness, pigment cells can be found, which synthesize and accumulate various types of pigments or products that contribute to the coloration of the epithelium.
Melanocytes are an example of these pigment cells, which are located in the dermis or epidermis of vertebrates. These cells synthesize melanins, which they accumulate in melanosomes in their cytoplasm. Melanocytes have branching processes that extend at the base of the epidermis or among epithelial cells.
Through interactions with the cytoskeleton, melanocytes can alter the extent of their branching processes and transport melanosomes within them from the cell body to their ends, or vice versa, thus regulating the lighter or darker color of the skin.
These cells migrate to epithelia from the connective tissue and belong to the local immune system. Normally, there are three types:
- Intraepithelial lymphocytes, which migrate from the underlying connective tissue and penetrate between epithelial cells, where they are clearly visible due to their rounded shape and heterochromatic nucleus.
These intraepithelial lymphocytes are a subpopulation of T-lymphocytes (γδ) responsible for local immunological surveillance of epithelia, both against potential pathogens and against cancerous alterations of epithelial cells.
- Intraepithelial macrophages, which can sometimes be recognized with the light microscope due to the presence of residual bodies in the cytoplasm.
- Antigen-presenting cells, such as Langerhans cells in the epidermis of mammals and similar cells in other vertebrates, which belong to the population of interdigitating dendritic cells. They are named for their dendritic branches, which have cytoplasm with few organelles. Langerhans cells also have Birbeck granules, derived from the endosome and which are a characteristic structural marker of this cell type.
Antigen-presenting cells originate from blood monocytes and are specialized in the endocytosis of molecules from pathogens or transformed cells. After intracellular processing, these cells present antigenic molecules (usually peptides) to T-lymphocytes, thus participating in antigen-specific immune responses.