Rare Accessory Scrotum in Infant Reveals Hidden Cartilage-Structured Anatomy Beneath Skin

A small, raised skin formation adjacent to a 10‑month‑old Japanese boy’s anus turned out to be the sole external manifestation of a complex internal anomaly. Radiologic evaluation disclosed a slender fibrillar stalk situated near the anal region and coursing deep into the perirectal adipose tissue before terminating in a nodular mass composed primarily of joint‑type cartilaginous material and bone marrow.

The entire ectopic construct measured approximately seven centimeters overall. Its architectural composition mirrored tissue profiles described in scarce human tail specimens—except for the critical detail that the development lingered entirely subsurface, never breaching cutaneous coverage. The investigators originally reported this finding in *Case Reports in Pathology*, which appeared online in July; they asserted that, to their best knowledge, no comparable presentations had been documented prior to this instance.

The manuscript’s lead author, Dr Hiroshi Sonobe of the Department of Diagnostic Pathology at Fukuyama National Hospital in Hiroshima, along with colleagues from Gifu University Graduate School of Medicine and Okinawa Prefectural Nanbu Medical Center, provided detailed case insights. The corresponding author led a multidisciplinary team comprising pathologists and pediatric surgeons from the three institutions.

A Bump Near the Anus Led to Something Deeper

A ten‑month‑old Japanese infant presented with an apparently unremarkable family background. The parents had observed the modest raised nodule since birth, locating it roughly at the five‑o’clock position adjacent to the anus. Prior clinical examination classified the growth as an accessory scrotum—a perplexing variant wherein scrotal‑like dermal formations arise from non‑canonical sites within the pelvis.

Advanced neuroimaging expanded the anatomical survey beyond the initial visual observation. An additional MRI revealed a continuous, narrow cord traversing the perirectal peritoneum—specifically the fascial compartment separating the genitalia from the anus—to a sclerotic nodule embedded within the ischiorectal fossa, a fibro‑fatty compartment nestled beside the distal rectum and anal canal. Comprehensive systemic imaging identified no other irregularities throughout the remainder of the body.

Surgical intervention entailed the total excision of the lesion complex. The resection comprised a two‑centimeter raised epidermal projection, a serendipitously preserved four‑millimeter‑thin yet four‑centimeter‑long fibrous stalk, and a firm calcific mass approximately two centimeters in diameter at the distal terminus.

Cartilage, Bone Marrow and Nerves Under the Microscope

The cutaneous component displayed pronounced foldings interspersed with abundant bands of superficial smooth muscle within the dermis. Consequently, identification of the standard dartos muscle layer—which is characteristic of conventional scrotal development—confirmed the diagnosis of an accessory scrotum.

The central stalk, however, exhibited a markedly different interior organization. Internal examinations revealed longitudinally oriented cords of connective tissue interwoven with clusters of peripheral nervous architecture. Special staining validated the presence of muscular fibers and axonal structures, distinguishing this segment from typical scrotal tissue.

The terminal nodule represented the most enigmatic element. Histomorphologically, it contained mature articular cartilage—the smooth, hyaline matrix habitually capping skeletal eminences within synovial joint capsules—superimposed upon a core of hemoprogenitor bone marrow. Collectively, the stalk’s constituents closely resembled those catalogued in human vestigial tails, yet the totality remained concealed beneath dermal epithelium without envelopment by overlying skin.

While accessory scrotums constitute isolated anomalies, epidemiological surveys indicate concomitant presence in sixty to eighty percent of cases, frequently accompanied by a benign perineal lipoma arising within the adjacent adipose depot. A separate 2025 presentation in *Urology Case Reports* characterized a congruent configuration as an “extremely rare congenital anomaly” involving testis‑free, scrotal‑like excretory derivatives in newborns. That subject underwent comprehensive surgical eradication, achieving therapeutic cure.

Why Doctors Compared It to a Human Tail

Human embryogenesis includes a transient caudal projection during weeks five through six. Cells within the so‑called tail bud undergo pristine apoptosis by approximately the eighth prenatal week, eliminating the nascent tail. Birth participants bearing overtly recognizable tails are exquisitely uncommon; clinicians typically reference seminal literature from the 1984 work of Anh Dao and Martin Netsky to classify these anomalies.

The current case departs distinctly from textbook tail phenotypes. Although its stalk morphology mimics tail tissue, the accompanying chondrocortical matrix aligns rather more closely with compositions identified in certain pseudotail constructs. Importantly, the structure remained entirely subsurface, concealing itself behind redundant dermatolytic layers absent of integumentary coating.

Pathologists proposed two principal mechanisms underlying the persistence of this embryonic remnant. First, deficient signaling possibly permitted aberrant tail‑bud progeny to remain viable and encroach upon the ischiorectal fossa. Second, pluripotent early‑stage tail‑bud cells—which possess multipotent developmental potential—might have undergone secondary neurulation migration toward the perineal territory during spinal column formation.

The latter hypothesis encounters considerable constraint. Migratory pleomorphism of stem‑derived cells is characteristically linked to concurrent vertebral defect manifestations; analogous reports involving vestigial human tails coinciding with occult spinal dysraphism have documented such syndromic associations. This particular infant exhibited no residual spinal anomalies. Accordingly, the authors acknowledged persistent methodological gaps in elucidating definitive pathogenesis. Distinguishing this entity from alternatively benign accumulations—such as hamartomas, choristomas, lipomatous proliferations, or neoplastic cystic masses—remains a clinical responsibility of the treating physician.

Long‑term follow‑up demonstrated favorable natural history. At the twelve‑month post‑operative benchmark, the child maintained optimal health status without sequelae or regenerative recurrence.

This represents a singular case record, and the contributors eschewed normative generalization regarding accessory scrotal behavior. Any parent observing an anomalous papule, skin nodule, or ectopic appendage proximal to a juvenile’s anus, perianal region, or lumbar spine warrants prompt pediatric assessment to determine appropriate imaging protocols and specialist referral pathways.

Key Questions Answered

What constitutes an accessory scrotum? It denotes a rare congenital variation wherein scrotal‑type cutaneous differentiation manifests extrascopularly—typically within the pelvic perineum—without intraperitoneal testicular homology. The syndrome commonly co‑exists with a somatic fatty proliferation termed a lipoma.

Was there a visible tail present in this infant? The extrasomatic elevation harbored external evidence in the form of a minor skin dimple adjoining the posterior anal district. No externally apparent tail structure existed; the cartilaginous continuation resided undercut by adipose sheathing within the perirectal compartment.

Why was the lesion likened to a human tail? Stalk cytology yielded collagenous stromal fascicles integrated with autonomic neural plexuses dispersed amid adipose matrices. These microanatomic signatures paralleled those historically described in vestigial human appendices. Meanwhile, the distal calcareous tuber housed mature articular gelatinaceous tissue ensconced by primitive skeletal condensate.

How was the anomaly detected and mitigated? Magnetic resonance imaging delineated a linear, filamentous conduit traversings from the ectopic nodule through the perineal fascia to a mineralized aggregate nested within the ischiorectal lacuna. Surgical resection eliminated the entire septimal assembly at a chronological age of ten months, leaving the child free of complications or subsequent recurrence following routine surveillance.

Does the presence of an extra scrotal bump in infants routinely signal a condition resembling the reported case? The authors affirm that no parallel incidents precede their narrative—a peerless occurrence until now. Pediatricians retain authority to assess whether such blemishes merit radiographic interrogation or redirection toward subspecialty consultants depending on clinical course.

What aspects remain unexplained? The authors could not definitively ascertain the ontogenetic pathway. Whether survival of embryonic tail buds persisted intracellularly or whether transitory neural crest intermediates transited during secondary neurulation constitutes theory pending further investigative models.

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