Background

In the Introduction to the Glossary of Biochemistry and Molecular Biology, Glick (2006) states that “In the sciences, an essential aspect of recognizing, recalling and communicating something, be it a substance, relationship or method, is the naming of it. To create names for new concepts, methods or items, we invent new words, often using the roots of a Classical language (apoptosis, glycocalyx, isosbestic), and we recruit familiar words and invest them with new meanings (chaperone, kringle, library).” In either case, dictionaries and glossaries have become more important than ever in helping scientists understand what they are studying and to communicate new discoveries in rapidly evolving fields. Workshop participants realized early in the session that being able to communicate across coral biology, pathology, and histology might require reconsideration of pertinent terms and their definitions to make them applicable to coral histopathology and understandable to wider audiences.

The definitions for the terms in this glossary have been derived from diverse sources, which are cited below and in the references at the end of this section. During and following the workshop, participants discussed the utility of terms currently used to describe the microscopic anatomy and pathology as it pertains to hexacorals and octocorals (= corals) and suggested including a glossary in the report to assist in explaining the histopathological findings from the coral sections reviewed by the participants.

Most of the anthozoan anatomical terms were developed from dissections and histological studies used to distinguish taxa. Polyp features have been largely based on studies of sea anemones and octocorals, and primarily skeletal features have been used to distinguish taxa among scleractinians. Histological examinations of corals have expanded to include the study of diseases only since the late 1970s. The participants agreed that most descriptors commonly used in the evaluation of vertebrate disease processes could be adapted to the study of coral diseases; however, some new terms might be needed to facilitate interpreting the histological observations being made of coral specimens and making morphological diagnoses.

Some terms were confusing to veterinary and comparative histopathologists and some anatomical terms did not appear to adequately distinguish regions that consist of similar cell types but have different functions and might show different reactions when diseased. For example, “column wall” fits the cylindrical body of an anemone polyp, but should this be used for the colonial polyps embedded in mesoglea or skeleton? Is there a term that could be used to specifically identify the location of lesions in the part of the polyp in contact only with the skeleton or only with the seawater? Are other modifiers needed with “epidermis” and “calicoblastic epidermis”? Participants debated using the term “integument” to identify the unit of the three layers of cells and mesoglea forming the polyp, with modifiers of “paralithic” or “paramaric” to mean “beside the stone (skeleton)” or “beside the sea.” The term “calicoblastic layer” has been used to mean the unit composed of calicoblastic epidermis-mesoglea-gastrodermis, but “layer” means only a single thickness of cells or of a homogeneous substance. Participants proposed writing a white paper to justify the rationale for an internationally recognized terminology for coral histopathology in consultation with experts in the scientific nomenclature of coral anatomy and histology.

During preparation of the Coral Disease and Health Workshop: Coral Histopathology II report (Galloway et al. 2006), proposed and confusingly defined terms were presented to four outside experts in anthozoan and coral anatomy and histology: Stephen Cairns, National Museum of Natural History, Smithsonian Institution; Daphne Fautin, University of Kansas; Walter Goldberg, Florida International University; and Jaroslaw Stolarski, Intytut Paleobiologii PAN. The experts had diverse opinions as to the appropriateness and need for these terms, but all urged caution in developing new terms. Dr. Stolarski noted that separating human bones into those that are completely inside the body and those that extrude as limbs might be “useful for casualty ward but awkward for perfectly homologous structures.”

But we are talking about the “casualty ward” for corals here and finding pathological changes in just the epidermis in contact with seawater rather than the epidermis that aids in skeletal deposition mean differences in organism function and prognosis for the animal, much as a broken ulna versus a broken pelvis do in humans. After more consideration, thought, and much research in publications, glossaries, and dictionaries, one new term is presented in this proposed glossary (“calicodermis,” marked with an “*” below). Other terms that have been in long use for anthozoans were found to be appropriate for the corals.

The terms that generated the most discussion were “ectoderm,” “endoderm,” “epidermis,” and “gastrodermis.” Workshop participants were familiar with the use of the first two terms in developmental biology to refer to the outer and inner layers of cells that form during development of an embryo from the blastula into the gastrula. The cnidarian literature presented references to “ectoderm” and “epidermis” as the outer layer of cells of a polyp and “endoderm” or “entoderm” and “gastrodermis” as the inner layer of cells of a polyp. Libbie Hyman assigned the term “epidermis” to the outer adult cnidarian epithelium and proposed the term “gastrodermis” for the inner adult cnidarian epithelium, noting that “ectoderm” and “endoderm” were embryological terms (Hyman 1940), her first volume in The Invertebrates series). Fautin and Mariscal (1991) used “epidermis” and “gastrodermis” in their chapter on Anthozoa in Volume 2 of the Microscopic Anatomy of Invertebrates series. Because of this and usage of “epidermis” and “gastrodermis” in other recent anthozoan published works, including the Illustrated Trilingual Glossary of Morphological and Anatomical Terms Applied to Octocorallia (Bayer et al. 1983), the workshop participants included these four terms in the glossary, noting their application to the embryological or adult epithelial layers.

In her review of the draft glossary, Dr. Fautin stated that “ectoderm” and “endoderm” refer to the embryological development of the inner and outer layers of cells only in triploblastic animals. She noted that the editor of Volume 2, Jane Westfall, had insisted that Fautin and Mariscal use the terms “epidermis” and “gastrodermis” despite their protests that “ectoderm” and “endoderm” were originally assigned to identify the outer and inner epithelia of adult cnidarians and were later appropriated for embryonic cell layers in the context of ontogenetic recapitulation of the adult layers of Cnidaria. She further reported that Hyman, in her final volume VI (1967), wrote “In a project of such magnitude some errors of fact and judgment are inevitable. It was a mistake on my part to replace the terms ectoderm and entoderm on the grounds that they are embryological terms. They were in fact created by Allman for the two body layers of coelenterates [cnidarians]. I advise that gastrodermis be dropped and regret having introduced it.” In conclusion, she stated that Westfall now regrets having enforced its use (D. Fautin, pers. comm.).

The workshop participants discussed these arguments. Although some sources use “ectoderm” or “ectodermis” to refer to the outer cell layer and “endoderm” or “endodermis” to refer to the inner cell layer in adult Cnidaria, these epithelia change in composition and function as the animal grows. The simple endoderm and ectoderm established during gastrulation are not the same as fully differentiated adult tissues in any eumetazoan, whether it is diploblastic or triploblastic. In the Cnidaria, the cells within each layer differentiate first into ciliated supporting cells, and then other cell types differentiate (e.g., mucocytes or nematocytes) or other functions are expressed (e.g., cells of the outer polyp layer develop microvilli for nutrient uptake or secrete an organic matrix for calcification; stem cells form amoebocytes or germ cells as needed for defense or reproduction; cells lining the gastrovascular cavity develop the ability to maintain zooxanthellae without harming them). Thus, the adult epithelia are not the same as the layers in the gastrula.

After much debate among participants and reviewers, “epidermis” and “gastrodermis” were included in the glossary as the terms for the adult epithelia that are derived from the embryonic ectoderm and endoderm, respectively. “Calicodermis” is the ectoderm that assists in building the calcified exoskeleton of scleractinians after the planula settles on the substratum. This term was proposed to shorten the currently used “calicoblastic epidermis” and to correct the misunderstanding that cells here are “calicoblasts” because they do not secrete the crystals of aragonite. Instead, the organic matrix, secreted by these cells, aids in the deposition of the crystals within the space between the cells and the skeleton.

As noted by one of the participants, most terms and descriptors of microscopic anatomy and pathological processes in human and veterinary medicine have evolved over the past centuries through a process of proposal, use, modification, incorporation into the published literature, and general acceptance. The vocabulary and nomenclature for coral histopathology will evolve similarly in time.

The glossary below presents our best compilation of terms found in the literature to date. Sources of the definitions are cited; however, most definitions have been clarified to better explain their pertinence to usage in coral studies. Not all anthozoan anatomical terms are included here as we have focused on those terms that should be most commonly needed for light microscopic histopathological examinations of these organisms, not those that are additionally needed for taxonomic or ultrastructural descriptions. The literature cited should be consulted as needed for other coral anatomical terms, particularly for specific taxa, as well as for terms to describe pathological changes, as necessary. Note also that new discoveries in coral anatomy and histology probably remain, as recently found by Goldberg (2002a; 2002b) for Mycetophyllia reesi, and might lead to adoption of new terms in this field.