Enigmatic human tails: A review of their history, embryology, classification, and clinical manifestations

Never-ending story of Brachyury: From short-tailed mice to tailless primates

The history of developmental biology starts from the almost simultaneous discoveries of the Organizer of axial structures in amphibians by Spemann and Mangold in Freiburg and of the Brachyury mutant in mammals by the Dobrovolskaya-Zavadskaya laboratory at the Curie Institute and its follow-up studies in the Leslie Dunn laboratory at Columbia University. Following the Organizer's discovery, the inductive activity of several other embryonic tissues was found, including that of the ear primordium by Boris Balinsky in Kiev. Initially, the experimental embryological and genetic lines of research existed independently of each other, but after they met at the bench of Salome Gluecksohn, they strengthened and cross-fertilized each other, eventually leading to developmental genetics, which later became known as developmental biology. It appears that the regulatory activities of Brachyury and related T-box proteins in general are at the heart of the development of all vertebrates. These activities are fundamental and have been discovered in several model organisms subjected to mutagenesis, exemplified by the story of George Streisinger's discovery of the no tail mutant in zebrafish. This essay describes the history of Brachyury studies, their connection to an idea of embryonic induction by Organizer, and an impact of Brachyury and related genes on various fields of research from embryology and cell biology to medical genetics and evolutionary theory.

On the genetic basis of tail-loss evolution in humans and apes

The loss of the tail is among the most notable anatomical changes to have occurred along the evolutionary lineage leading to humans and to the 'anthropomorphous apes'1-3, with a proposed role in contributing to human bipedalism4-6. Yet, the genetic mechanism that facilitated tail-loss evolution in hominoids remains unknown. Here we present evidence that an individual insertion of an Alu element in the genome of the hominoid ancestor may have contributed to tail-loss evolution. We demonstrate that this Alu element-inserted into an intron of the TBXT gene7-9-pairs with a neighbouring ancestral Alu element encoded in the reverse genomic orientation and leads to a hominoid-specific alternative splicing event. To study the effect of this splicing event, we generated multiple mouse models that express both full-length and exon-skipped isoforms of Tbxt, mimicking the expression pattern of its hominoid orthologue TBXT. Mice expressing both Tbxt isoforms exhibit a complete absence of the tail or a shortened tail depending on the relative abundance of Tbxt isoforms expressed at the embryonic tail bud. These results support the notion that the exon-skipped transcript is sufficient to induce a tail-loss phenotype. Moreover, mice expressing the exon-skipped Tbxt isoform develop neural tube defects, a condition that affects approximately 1 in 1,000 neonates in humans10. Thus, tail-loss evolution may have been associated with an adaptive cost of the potential for neural tube defects, which continue to affect human health today.

Atavistic and vestigial anatomical structures in the head, neck, and spine: an overview

Organisms may retain nonfunctional anatomical features as a consequence of evolutionary natural selection. Resultant atavistic and vestigial anatomical structures have long been a source of perplexity. Atavism is when an ancestral trait reappears after loss through an evolutionary change in previous generations, whereas vestigial structures are remnants that are largely or entirely functionless relative to their original roles. While physicians are cognizant of their existence, atavistic and vestigial structures are rarely emphasized in anatomical curricula and can, therefore, be puzzling when discovered incidentally. In addition, the literature is replete with examples of the terms atavistic and vestigial being used interchangeably without careful distinction between them. We provide an overview of important atavistic and vestigial structures in the head, neck, and spine that can serve as a reference for anatomists and clinical neuroscientists. We review the literature on atavistic and vestigial anatomical structures of the head, neck, and spine that may be encountered in clinical practice. We define atavistic and vestigial structures and employ these definitions consistently when classifying anatomical structures. Pertinent anatomical structures are numerous and include human tails, plica semilunaris, the vomeronasal organ, levator claviculae, and external ear muscles, to name a few. Atavistic and vestigial structures are found throughout the head, neck, and spine. Some, such as human tails and branchial cysts may be clinically symptomatic. Literature reports indicate that their prevalence varies across populations. Knowledge of atavistic and vestigial anatomical structures can inform diagnoses, prevent misrecognition of variation for pathology, and guide clinical interventions.

Damaged Dickinsonia specimens provide clues to Ediacaran vendobiont biology

Recently reported specimens of the enigmatic Ediacaran fossil Dickinsonia from Russia show damage and repair that provides evidence of how they grew, and of their biological affinities. Marginal and terminal areas of wilting deformation are necrotic zones separating regenerated growth, sometimes on two divergent axes, rather than a single axis. Necrotic zones of damage to Dickinsonia are not a thick scar or callus, like a wound or amputation. Nor are they smooth transitions to a regenerated tail or arm. The wilted necrotic zone is most like damage by freezing, salt, or sunburn of leaves and lichens, compatible with evidence of terrestrial habitat from associated frigid and gypsic paleosols. Dickinsonia did not regrow by postembryonic addition of modules from a subterminal or patterned growth zone as in earthworms, myriapods, trilobites, crustaceans, and lizards. Rather Dickinsonia postembryonic regrowth from sublethal damage was from microscopic apical and lateral meristems, as in plants and lichens. Considered as fungal, Dickinsonia, and perhaps others of Class Vendobionta, were more likely Glomeromycota or Mucoromycotina, rather than Ascomycota or Basidiomycota.

Genetics of Atavism

Abstract

Atavisms have attracted people’s attention for a long time. First, atavisms excited their imagination and created fertile ground for myths and superstitions. With the development of science, atavisms became the subject of investigation, which soon provided evidence to support evolutionary theory. However, at the molecular level, the formation of atavisms remained insufficiently understood. Recent progress in comparative genomics and molecular developmental biology has helped in understanding the processes underlying the formation of one of the human atavisms: the vestigial tail.

Experience with human tail and its outcome

There are some common genetic heritages between human and mammals. Human tail, though rare is one of the most noticeable. Till the date, around 60 cases reported in the literature. A true human tail is a benign vestigial caudal cutaneous structure composed of adipose, connective tissue, muscle, vessels and nerves. A true human tail can be distinguished from a pseudotail as the latter is commonly associated with underlying spinal dysraphism, which requires specialised management. We report a case series of four caudal appendages. Two clients were infants, while others two were toddler and presented with cutaneous appendage arising from the lumbosacral region. Out of four, only one had associated spinal dysraphism and neurological manifestation while others did not have spinal dysraphism and neurological manifestation. The appendage was removed by the surgical excision. Clinicians should emphasise the use of 'true tail' and 'pseudo-tail' as the specific disparate terms as the clinical, radiological and histological findings of these conditions differs significantly, along with the management strategies and outcomes.

[Cutaneous signs of occult cranial and spinal dysraphism]

Dysraphism refers to neural tube closure abnormalities and midline closure abnormalities of the skin, paravertebral muscles, vertebrae and meninges. Cranial dysraphism (CD) and occult spinal dysraphism (OSD) may be discovered via evocative skin signs present at birth or appearing later in childhood or even in adulthood. This review describes the various types of skin signs associated with CD and OSD. All congenital midline skin lesions, particularly on the frontonasal area, the vertex or the occipitocervical and low back regions, should prompt suspicion of underlying dysraphism. The main evocative midline skin abnormalities are: (i) for underlying DCEO: a nodule, swelling, skin openings and hair collar sign or hair tuft; (ii) for underlying DSO, localized hypertrichosis, an atypical or complex lower back dimple, a dermoid fistula, infantile haemangioma, caudal appendage and lipoma. In the event of suspected DCEO or DSO, spinal or medullary MRI constitutes the reference examination.

Human tail-like cutaneous appendage with a contiguous stalk of limited dorsal myeloschisis

PURPOSE

Limited dorsal myeloschisis (LDM) is characterized by a fibroneural stalk linking the skin lesion to the underlying spinal cord. On account of the external skin lesion, all LDMs are either flat (nonsaccular) or saccular, and a human tail-like cutaneous appendage has not been reported.

METHODS

In our 14 LDM patients, 2 had tail-like appendages. We retrospectively analyzed the relationship between the appendage and the LDM tract from the clinicopathological findings of these 2 patients.

RESULTS

Preoperative magnetic resonance imaging including three-dimensional heavily T2-weighted images demonstrated an intradural tethering tract, but failed to reveal the precise communication with the appendage. However, surgery revealed the extradural and intradural slender stalk, starting at the base of appendage and running through the myofascial defect. Histological examination demonstrated that there was a tight anatomical relationship between the fibroadipose tissue of the appendage and the fibrocollagenous LDM stalk.

CONCLUSION

When there is potential for an LDM stalk in patients with an appendage, a meticulous exploration of the stalk leading from an appendage is required. Clinicians should be aware of possible morphological variations of skin lesions associated with LDM.

Sonic Hedgehog Signaling Is Required for Cyp26 Expression during Embryonic Development

Deciphering how signaling pathways interact during development is necessary for understanding the etiopathogenesis of congenital malformations and disease. In several embryonic structures, components of the Hedgehog and retinoic acid pathways, two potent players in development and disease are expressed and operate in the same or adjacent tissues and cells. Yet whether and, if so, how these pathways interact during organogenesis is, to a large extent, unclear. Using genetic and experimental approaches in the mouse, we show that during development of ontogenetically different organs, including the tail, genital tubercle, and secondary palate, Sonic hedgehog (SHH) loss-of-function causes anomalies phenocopying those induced by enhanced retinoic acid signaling and that SHH is required to prevent supraphysiological activation of retinoic signaling through maintenance and reinforcement of expression of the Cyp26 genes. Furthermore, in other tissues and organs, disruptions of the Hedgehog or the retinoic acid pathways during development generate similar phenotypes. These findings reveal that rigidly calibrated Hedgehog and retinoic acid activities are required for normal organogenesis and tissue patterning.

Mechanical relationship of filum terminale externum and filum terminale internum: is it possible to detether the spinal cord extradurally?

INTRODUCTION

Intradural transection of the filum terminale (FTI) is often used to treat tethered cord syndrome. Recently, some have proposed that the extradural part of the filum terminale (FTE) can be sectioned with equal results but with fewer complications. Therefore, the present cadaveric study aimed to evaluate the anatomical foundation of such procedures.

METHODS

A posterior lumbosacral approach was performed on five fresh-frozen cadaveric specimens to expose both the FTI and FTE. Tension was then applied to the FTE and observations and measurements made of any movement of the FTI. Other morphological measurements (e.g., length, diameter) of the FTI and FTE were also made.

RESULTS

Although very minimal movement of the FTI was seen in the majority of specimens following tension on the FTE, no specimen was found to have more cranial movement of the conus medullaris or cauda equina. The mean length and diameter of the FTI was 52.2 and 0.38 mm, respectively. The mean length and diameter of the FTE was 77 and 0.60 mm, respectively. The force necessary to move the FTI with tension applied to the FTE had a mean of 0.03 N. The average distance that the FTI moved with distal FTE tension was 1.33 mm. All specimens had a thecal sac that terminated at the S2 vertebral level. And no specimen had a low-lying conus medullaris, cutaneous stigmata of occult spinal dysraphism, or grossly visible adipose tissue in either the FTI or FTE.

CONCLUSIONS

Based on our studies, tension placed on the FTE has very little effect on the FTI and no obvious effect on the conus medullaris or cauda equina. Therefore, isolated transection of the FTE for a patient with tethered cord syndrome is unlikely to have significant effect. To our knowledge, this is the first study to quantitate the distal forces needed on the FTE to move the FTI.

Tail reduction process during human embryonic development

Although the human tail is completely absent at birth, the embryonic tail is formed just as in other tailed amniotes. Since all morphological variations are created from variations in developmental processes, elucidation of the tail reduction process during embryonic development may be necessary to clarify the human evolutionary process. The tail has also been of great interest to the medical community. The congenital anomaly referred to as 'human tail', i.e. the occurrence of a tail-like structure, has been reported and was thought to represent a vestige of the embryonic tail; however, this hypothesis has not been verified. Accordingly, in this study, we aimed to establish a new method to visualize all somites in an embryo. We used sagittal-sectioned embryos from Carnegie Stage (CS) 13 to CS23. All samples were obtained from the Congenital Anomaly Research Center, Kyoto University, Japan. Combining photomicroscopy and three-dimensional reconstruction, we clearly visualized and labeled all somites. We found that the number of somites peaked at CS16 and dramatically decreased by approximately five somites. Tail reduction with a decrease in somites has also been observed in other short-tailed amniotes; thus, this result suggested the possibility that there is a common mechanism for morphogenesis of short tails in amniote species. Additionally, our findings provided important insights into the cause of the congenital anomaly known as 'human tail'.

A Rare Case of "Human Tail" Associated with Lipomyelomeningocele and Tethered Cord

Human tail might be the most interesting cutaneous sign of neural tube defects. From little cutaneous appendixes to 20-cm-long taillike lesions were reported in the literature. They may occur connected to an underlying pathology such as lipoma or teratoma, but most of the time, they conceal an underlying spinal dysraphism. Many classifications about human tails have been suggested in history, but the main approach to these lesions is, independent of the classification, always the same: investigating the possible spinal dysraphism with concomitant pathologies and planning the treatment on the patient basis.

Dinosaurs, Chameleons, Humans, and Evo-Devo Path: Linking Étienne Geoffroy's Teratology, Waddington's Homeorhesis, Alberch's Logic of "Monsters," and Goldschmidt Hopeful "Monsters"

Since the rise of evo-devo (evolutionary developmental biology) in the 1980s, few authors have attempted to combine the increasing knowledge obtained from the study of model organisms and human medicine with data from comparative anatomy and evolutionary biology in order to investigate the links between development, pathology, and macroevolution. Fortunately, this situation is slowly changing, with a renewed interest in evolutionary developmental pathology (evo-devo-path) in the past decades, as evidenced by the idea to publish this special, and very timely, issue on "Developmental Evolution in Biomedical Research." As all of us have recently been involved, independently, in works related in some way or another with evolution and developmental anomalies, we decided to join our different perspectives and backgrounds in the present contribution for this special issue. Specifically, we provide a brief historical account on the study of the links between evolution, development, and pathologies, followed by a review of the recent work done by each of us, and then by a general discussion on the broader developmental and macroevolutionary implications of our studies and works recently done by other authors. Our primary aims are to highlight the strength of studying developmental anomalies within an evolutionary framework to understand morphological diversity and disease by connecting the recent work done by us and others with the research done and broader ideas proposed by authors such as Étienne Geoffroy Saint-Hilaire, Waddington, Goldschmidt, Gould, and Per Alberch, among many others to pave the way for further and much needed work regarding abnormal development and macroevolution.