Students’ perception of rudimentary organs - Is the vermiform appendix a vestigial structure?

Introduction

Vestigial anatomical structures or rudimentary organs are considered to have lost much or all of their function through evolution. These non-functional anatomical features (e.g., Plica semilunaris conjunctivae, Darwin´s tubercle in humans) can give insights into the phylogenetic history of species.1, 2, 3 Additionally, vestigial structures can be of clinical importance since these structures might be confused with or be the source of pathologies (e.g., hamartoma of the juxtaoral organ of Chievitz).4 Nevertheless, there is some debate in anatomical literature which structures should be considered as vestigial and which not. While this discussion in several cases is just of academic interest (e.g., function of Mueller´s orbitalis muscle), we were interested in students´ perspective about rudimentary organs.5

Materials and Methods

In this prospective single centre study (University Medical Center Mainz, Germany), we requested a total number of 220 students of human medicine to fill in an online survey. This survey was performed in winter term 2023/2024. The participation was anonymous and voluntary. All participants were in the preclinical phase of medical studies (before first state examination) and after completion of a hands-on dissection course (66 hours of practical training on body donors). Students were asked to define the terms “rudimentary organ or vestigial structure” and name up to three corresponding structures (Table 1).

Results

A number of 46 students completed the survey (20.9%). 39 students were able to define the terms vestigial anatomical structure or rudimentary organ (84.8%). Students should name up to three rudimentary organs or vestigial structures. 46 students named in total 91 structures. Most frequently, the vermiform appendix (appendix vermiformis) was mentioned (21 times, 23.1%). At second place, wisdom teeth (third molars, 17 times, 18.7%) and on third place the coccyx (Os coccygis, 9 times, 9.9%) were mentioned (Figure 1).

Figure 1

Putative vestigial structures named by students (n = 46)

Disscusion

Almost half of the students thought that the vermiform process as a perfect example of a vestigial structure.

The vermiform appendix is a blind-ending tube with a length usually between 6 and 10 cm in adults. It lies at the posteromedial wall of the caecum below the ileocolic junction in the right iliac fossa with the tip lying most commonly retrocaecal, retrocolic or pelvic. The vermiform appendix has a continuous outer layer of longitudinal muscle formed by the fusion of the three taeniae coli.6 The lumen is irregularly narrowed by submucosal lymphoid tissue (Fig. 2 A). It may be widely patent in early childhood but often partially or wholly obliterates by fibrous tissue in the elderly. Agenesis or duplication of the vermiform appendix are exceptionally rare.7, 8

Figure 2

Microscopic anatomy of the vermiform appendix. A: Overview. B: Detail of a lymphoid follicle (LF) with dome epithelium (DE)

The appendix belongs to the so-called GALT (gut-associated lymphoid tissues).9 The submucosa typically contains large lymphoid aggregates that may extend into the mucosa and disrupt the integrity of the muscularis mucosae. The epithelium, which overlies the mucosal lymphoid tissue (follicle associated epithelium, dome epithelium, Fig. 2 B), contains microfold cells (M cells). Lymphoid follicles are absent at birth but accumulate during the first 10 years of life to become prominent. In adults, the lymphoid follicles gradually atrophy.6 The vermiform appendix has an immunoregulatory function (gut tonsil). In addition, it probably serves as a microbiome reservoir (“safe house”) that enables a rapid recolonization of the colon with a physiological bacterial flora familiar to the immune system after diarrhoea.10

Third molars (wisdom teeth) do not play a key role in mastication or speech production in humans. Additionally, there is a high variation of agenesis of third molars in different populations from practically 0% (Tasmania) to nearly 100% (Mexican Indians), suggesting that humans may be on evolutionary track to lose them totally.11 The coccyx is not entirely useless in humans, because it has attachments to various muscles, tendons and ligaments. However, it is doubtful that the coccyx attachments are important to the well-being of humans.

Study Limitations

There are several limitations of this study. First, the number of participants was very low. This is because we only asked students who completed a full practical dissection course on body donors (no restrictions due to corona pandemic). We are convinced that learning on corpses directly shows variability of the human body and helps students to recognize vestigial structures. Nevertheless, this study should be repeated at different universities and countries. Due to the small sample size, we did not take into account sociodemographic characteristics of the participants.

Conclusion

Taken together, anatomists should be (more) careful in discussing and teaching vestigial structures so that students do not get confused in context with highly functional organs.

Source of Funding

This research received no external funding.

Conflicts of Interest

The author declares that there is no conflict of interest.

Ethical Statement

According to the Ethics Committee of Rhineland-Palatine no ethics approval was necessary for the present study, as it is not a biomedical research project in the narrower sense of the Declaration of Helsinki.