"A rare congenital anomaly presented as a seemingly benign skin bump near a baby’s anus, only for investigations to reveal a unique, deep-seated structure composed of tissue reminiscent of human tails and joint components."
This extraordinary medical case, reported by a team of Japanese researchers, highlights the complex and sometimes surprising nature of congenital malformations. What began as a minor external observation led to the discovery of an intricate, dumbbell-shaped structure beneath the skin, raising intriguing questions about developmental biology and the potential persistence of embryonic remnants. The findings challenge existing medical classifications and underscore the importance of thorough investigation for even seemingly minor birth anomalies.
A tiny, raised bump of skin located beside a 10-month-old Japanese boy’s anus was the sole external manifestation of a far more complex and unusual anatomical finding. Beneath this small nodule, physicians in Japan uncovered a slender stalk of tissue extending deep into the subcutaneous fat adjacent to the rectum, culminating in a distinct nodule containing joint-type cartilage and bone marrow. This entire, dumbbell-shaped structure measured approximately 7 centimeters in length. Its histological composition bore a striking resemblance to tissues described in exceptionally rare cases of human tails, yet crucially, it had never breached the skin’s surface. The findings were meticulously detailed in a case report published online in July in the journal Case Reports in Pathology. The authors emphasized that, to their knowledge, no similar documented cases have been previously reported in the medical literature.
The research team was led by Hiroshi Sonobe, of the Department of Diagnostic Pathology at the National Hospital Organization Fukuyama Medical Center in Hiroshima. His co-authors included a distinguished group of pathologists and pediatric surgeons from the Fukuyama Medical Center, the Gifu University Graduate School of Medicine, and the Okinawa Prefectural Nanbu Medical Center and Children’s Medical Center, bringing a multidisciplinary expertise to the investigation.
A Perineal Bump Unveils a Deeper Anomaly
The young patient, a 10-month-old Japanese boy, had no significant family history of congenital abnormalities. His parents had observed the small, raised nodule from birth. Situated at the approximately 5 o’clock position adjacent to his anus, the lesion was initially diagnosed during a clinical examination as an accessory scrotum – a rare anomaly characterized by the development of scrotum-like skin tissue outside its typical anatomical location.
Further diagnostic exploration, specifically an Magnetic Resonance Imaging (MRI) scan, revealed the true extent of the condition. The MRI demonstrated a continuous, narrow, cord-like structure originating from the external bump and traversing the soft tissues of the perineum – the anatomical region situated between the external genitalia and the anus. This cord extended to a calcified nodule located within the ischiorectal fossa, a significant fat-filled space that lies beside the rectum and the anal canal. Comprehensive screening of the boy’s entire body revealed no other congenital abnormalities.
Surgical intervention was undertaken to completely excise the entire anomalous lesion. The removed specimen comprised a 2-centimeter skin nodule, a soft stalk approximately 4 millimeters in thickness and 4 centimeters in length, and a firm nodule measuring 2 centimeters in diameter at its distal end.
Microscopic Examination Reveals Cartilage, Bone Marrow, and Nerves
Under microscopic examination, the skin portion of the lesion exhibited prominent folds and numerous bundles of smooth muscle within the dermis. This histological pattern is consistent with normal scrotal skin, including the presence of the dartos muscle layer, which supported the initial diagnosis of an accessory scrotum.
However, the stalk displayed a distinctly different histological composition. It contained lengthwise strands of connective tissue and nerve bundles embedded within adipose tissue. Special staining techniques confirmed the presence of both smooth muscle and nerve bundles within this stalk.
The nodule at the far end of the stalk proved to be the most intriguing component. It was found to contain mature articular cartilage, the smooth, resilient tissue that typically covers the ends of bones within joints, situated directly above bone marrow tissue. While the stalk’s components bore similarities to tissues described in human tails, the entire structure was located entirely beneath the skin, lacking any superficial epidermal covering of its own.
An accessory scrotum, as an isolated condition, is already considered an unusual congenital anomaly. Medical literature indicates that approximately 70% to 80% of accessory scrotum cases are accompanied by a perineal lipoma, which is a benign fatty tumor. A separate case report published in Urology Case Reports in 2025 characterized this condition as an "extremely rare congenital anomaly," defining it as the presence of scrotal-like tissue outside its normal anatomical position, notably without the presence of testes. In that specific newborn case, imaging studies effectively ruled out a teratoma, a type of germ cell tumor, and complete surgical removal resulted in a curative outcome.
The Enigmatic Connection to Human Tails
The scientific rationale for comparing this structure to a human tail stems from embryological development. Every human embryo transiently possesses a tail-like structure. According to the case report, a cluster of cells known as the tail bud emerges between the fifth and sixth weeks of gestation. These cells are typically destined to undergo programmed cell death, or apoptosis, by approximately the eighth week of embryonic development.
The occurrence of babies born with visible tails is exceedingly rare. Medical professionals often categorize these phenomena based on a seminal 1984 paper by Anh Dao and Martin Netsky. Following a comprehensive review of 33 prior cases, Dao and Netsky described a "true vestigial tail" as containing fat, connective tissue, striated muscle, blood vessels, and nerves, but critically, lacking bone or spinal cord elements. "Pseudotails," on the other hand, represent a more heterogeneous group of conditions. The most frequent cause of pseudotails is an abnormally elongated tailbone (coccyx), while other causes include lipomas, teratomas, and related lesions. Some fatty growths can remarkably mimic the appearance of tails, as illustrated by a case report detailing a congenital perineal lipoma presenting as a pseudotail.
The hidden structure discovered in this infant boy does not fit neatly into either of these established categories. Its stalk exhibited characteristics suggestive of tail tissue, while the presence of cartilage and bone marrow within its terminal nodule aligns with components found in some pseudotails. However, the entire structure was concealed beneath the skin and never manifested as an external tail. Consequently, the authors aptly described it as an accessory scrotum with a human tail-like structure.
A Tail Bud Theory, with Caveats
The researchers proposed two primary hypotheses to explain the origin of this unusual anomaly. One theory suggests a disruption in cellular signaling pathways during development, which may have prevented certain tail bud cells, destined for regression, from undergoing programmed cell death. These persistent cells could then have remained in the ischiorectal fossa. The second hypothesis posits that pluripotent tail bud cells, possessing the capacity to differentiate into various tissue types, may have migrated aberrantly to the perineum during secondary neurulation, the developmental stage when the caudal portion of the spinal cord forms.
However, the second hypothesis is tempered by a significant caveat. Aberrant cell migrations of this nature are typically associated with concomitant malformations of the spine and spinal cord, a pattern observed in other reports, such as one detailing a vestigial human tail alongside occult spinal dysraphism. The absence of such spinal abnormalities in this patient presents a challenge to this explanation. The research team acknowledged that fully elucidating the formation of this lesion based solely on existing medical literature remains a complex endeavor. Furthermore, they underscored the critical need to differentiate such anomalies from other common benign masses found in the perineal region, including hamartomas, choristomas, lipomas, dermoid cysts, and teratomas.
The outcome for the young patient was highly reassuring. At the one-year follow-up appointment, the boy was reported to be in excellent health, with no evidence of complications or recurrence of the anomaly.
It is crucial to emphasize that this report details a single case. The authors do not suggest that an accessory scrotum typically conceals such complex underlying structures. Nevertheless, the case serves as a vital reminder for parents and healthcare providers. Any unusual bump, skin tag, or appendage observed near a baby’s genitals, anus, or lower back warrants prompt evaluation by a pediatrician. This initial assessment will guide decisions regarding the necessity of further imaging studies or referral to a specialist for a comprehensive evaluation.
Key Questions Addressed
What is an accessory scrotum?
An accessory scrotum is a rare congenital anomaly where skin resembling that of a scrotum develops outside its normal anatomical position, most commonly in the perineal region. It is characterized by the absence of testes within this ectopic tissue and is frequently associated with a benign fatty growth known as a lipoma.
Did this baby have a visible tail?
No, the baby did not have a visible external tail. The tail-like portion of the anomaly was entirely concealed beneath the skin, extending from the accessory scrotum beside the anus to a nodule situated within the fatty tissue adjacent to the rectum. Only a small skin bump was externally apparent.
Why did doctors compare it to a human tail?
The comparison to a human tail was made due to the histological composition of the stalk-like structure. It contained connective tissue and nerve bundles embedded within fat, which are characteristic components found in tissues described in human tails. Additionally, the nodule at the end of the stalk contained mature joint-type cartilage and bone marrow, elements that can also be associated with certain pseudotail formations.
How was the anomaly identified and treated?
The anomaly was initially detected as a small skin bump. An MRI scan revealed a cord-like structure extending from this bump to a calcified nodule. The entire 7-centimeter structure was surgically removed when the boy was 10 months old. A year later, he remained healthy with no signs of recurrence.
Does an extra scrotal bump in a baby usually indicate a similar underlying condition?
No, the authors explicitly state that they are unaware of any other reported cases presenting with such a complex, hidden structure associated with an accessory scrotum. They emphasize that any such bump should be evaluated by a pediatrician, who can determine if further imaging or specialist consultation is necessary.
What remains uncertain about this case?
The precise developmental mechanism by which this complex structure formed remains uncertain. While theories involving the persistence or aberrant migration of tail bud cells are proposed, the authors acknowledge that existing medical literature alone cannot fully account for the unique features of this lesion. Further research may be needed to fully understand such rare developmental processes.
