Histology is the study of the bodily tissues of organisms and their cellular structure. In histology, animal tissues are conventionally divided into no more than four main types: Muscle Tissues, Connective Tissues, Nervous Tissues, and Epithelial Tissues. Muscle Tissues constitute muscles, which allow an organism to move. Connective Tissues are tissues that connect body parts to other body parts, and include bone, cartilage, and blood. Nervous Tissues constitute the nervous system, including the brain, spinal cord, and peripheral nerves, and are utilized by organisms to sense and be cognizant of their environments. It is often asserted that these four tissue types are natural groupings that arise from common shared characteristics of the tissues grouped within them. While this appears to be the case for Muscle, Nervous, and possibly Connective Tissues, I think it is not true for Epithelial Tissues. I think Epithelial Tissues are an arbitrary and artificial grouping of several disparate tissue types that humans have lumped together, without good cytological or ontogenetic justification. This article will explore Epithelial Tissues in depth, and arrive at an explanation as to why I propose that this unnatural grouping ought to be split into several different tissue types.
To start out, it shall be noted that all tissues in an adult animal are ultimately derived from one of three original germ layers that develop in an embryo during a process known as gastrulation: the Ectoderm, the Mesoderm, and the Endoderm. If two or more tissues in the adult were derived from the same embryonic germ layer, then this furnishes a natural basis for them to be grouped together. Indeed, analogously to phylogeny, if two or more adult tissues share a common ancestor, so to speak, in an embryonic germ layer, this is the ontogenetic equivalent of sharing a common ancestor in phylogenetics, and, thus, provides good reason to group them together, with the resultant tissue group being the equivalent of a monophyletic group in phylogeny.
On the contrary, if two or more adult tissues do not derive from the same embryonic germ layer, then grouping them together would be analagous to grouping together two or more species that do not share a most recent common ancestor together in phylogeny, rendering the resultant group the equivalent of a polyphyletic group in phylogeny. A notable example of such a polyphyletic grouping is Pachydermata, including usually large mammals with thick skin such as rhinoceroses, hippopotamuses, and elephants. Pachydermata, as a group, has now been abandoned by those who study the phylogenetic relationships of these mammals, as it has now been demonstrated that elephants actually share a more recent common ancestor with manatees and hyraxes than with either of the other two, hippopotamuses share a more recent common ancestor with cetaceans than with either of the other two, and rhinoceroses share a more recent common ancestor with horses than with either of the other two.
Now here's the kicker. While all tissues classified as Muscle Tissues are derived from the mesoderm, all tissues classified as Connective Tissues are, likewise, derived from the mesoderm, and all tissues classified as Nervous Tissues are derived from the ectoderm, tissues classified as Epithelial Tissues are derived from all three of the germ layers, endoderm, mesoderm, and ectoderm, with different subcategories of Epithelial Tissues being derived from different germ layers. This makes Epithelial Tissues analogous to a polyphyletic phylogenetic grouping, such as Pachydermata. Just as polyphyletic groupings have now largely fallen by the wayside in favor of the more natural monophyletic groupings in taxonomy, likewise, it makes sense for groupings naturally derived from shared ontogenetic provenance from one of the embryonic germ layers to take precedence over artificially-derived arbitrary groupings of disparate tissues from different embryonic germ layers in histology.
Additionally, it shall be noted that at least Nervous Tissues and Muscle Tissues share common aspects of physical appearance. For example, although the exact specifications may vary between different locations in the nervous system, all Nervous Tissues are composed of the same type of cells, neurons. Meanwhile, while there is variation between striated, smooth, and cardiac types of muscles, all muscle tissue, likewise, is comprised of cells that have an appearance and structure that is, overall, mostly similar.
The same cannot be said for Epithelial Tissues. There are numerous variegated types of Epithelial Tissues, and the cells present wildly varying morphologies. Epithelial Tissues are currently divided into seven subcategories based upon the shape and configuration of their constituent cells: simple squamous, simple cuboidal, simple columnar, stratified squamous, stratified cuboidal, pseudostratified columnar, and transitional. As shown in the juxtapositions of Figure I, Figure II, and Figure III below, these different subcategories of Epithelial Tissues look vastly different, as opposed to the subcategories of Muscle Tissues and Nervous Tissues, which, overall, present a pretty similar appearance.
Additionally, unlike Muscle Tissues, which are all universally internal, and Nervous Tissues, which are all universally internal, as well, Epithelial Tissues are found both externally and internally. The tissue on such widely separated locations in the body as the epidermis of the skin and the lining of the gastrointestinal tract is said to consist of Epithelial Tissues, for example. An often-asserted commonality shared by all Epithelial Tissues is that their job is to protect the body from external substances in the environment. However, this seems like a rather arbitrarily-chosen criterion to me. For example, adipose tissue, or fat, is classified as one of the Connective Tissues, yet it also plays a role in protecting the body from various putative threats in the environment, including trauma from impacts and cold, to name two. Yet it is classified among the Connective Tissues, rather than among the Epithelial Tissues. This shows that this shared characteristic of function is not enough to group the widely differing varieties of tissues grouped under the name of Epithelial Tissues into such a broad, overarching category.
Overall, to recap, Epithelial Tissues are derived from all three of the embryonic germ layers, meaning that they lack common ontogenetic provenance, unlike the other principal tissue types, they present a wide variety of cell structures and configurations, unlike the other principal tissue types, and the proposed criterion of common function is not enough to salvage the grouping, as, if applied logically and consistently, this same criterion would subsume other tissues that are not classified as Epithelial Tissues into the category, as well.
This is why I propose that, since Epithelial Tissues seem to me to be an arbitrary and artificial grouping of several unrelated tissues together by humans, it would be beneficial for histology to drop this grouping, and split it into several different groupings, with the result that there would be more than four principal types of tissues present in animals' bodies, just as phylogeneticists have now dropped arbitrary, artificial polyphyletic groupings in favor of natural monophyletic groupings.
Fig. I: The three primary types of neurons, cells that constitute what is classified as Nervous Tissue.
Fig. II: The three types of Muscle Tissue and their characteristics and functions.
Fig. III: The seven recognized types of tissue currently classified under the label of "Epithelial Tissues", and the characteristic shapes of the cells that comprise them.