A female harbor seal pup was collected from Pebble Beach, Monterey County, California on 16 June 1999 and brought to The Marine Mammal Center in Sausalito, California for treatment of a massive swelling around its neck. The pup was female, weighed 11.4 kg, was 75 cm long (nose to tail), and had a swelling 30 cm in diameter on the ventral aspect of its neck. The skin over the ventral surface of the swelling was ulcerated, presumably due to abrasion on land during locomotion. The seal was bright, alert and vocal, but had an elevated respiration rate of 8 breaths/min. Hematology results revealed a neutropenia (total leukocyte count 3.0×109/L, neutrophils 80%, bands 3%, eosinophils 2%, lymphocytes 15%) with erythrocyte and serum biochemical parameters within normal limits for this species. Aspiration of the mass-produced blood, while radiography revealed amorphous calcified masses.
The swelling was then explored surgically following anesthesia of the seal pup with propofol and isoflurane. Surgical exploration revealed a mass consisting of hair, skin, fat and recognizable internal organs. The mass was completely contained within the subcutis of the host seal and extended along the ventral midline from the intra-mandibular area to the sternum. Subcutaneous blood vessels were enlarged to form a highly vascular mesh around the mass with branches extending into it. The mass was completely removed by blunt dissection. Following removal of the mass it became apparent that the skull of the host seal pup was flattened dorso-ventrally and the mandibles were deviated to the left, presumably due to pressure from the mass. The host seal did not recover from surgery.
Grossly the tissue consisted of a multinodular mass weighing 1.3 kg, and measuring 20×16×10 cm, covered almost entirely by haired skin. A few segments of the mass exhibited varying degrees of differentiation, and were identifiable as palpebrum, primitive jaw and tooth buds, and a tail complete with a vertebra filled by marrow. On incision the mass was composed of variably sized, alternating cystic and solid areas of differing consistencies. Histologic evaluation revealed the presence of well differentiated haired and glabrous skin overlying various types of connective tissue including collagen, bone, cartilage and adipose. Additionally, there were islands of neural tissue, and a rudimentary bronchus adjacent to a structure suggestive of tongue.
A variety of anomalous growths have been reported in human neonates, the most common of which is the extra-gonadal teratoma. These tumors arise along the midline from nests of undifferentiated, pleuripotential stem cells. Teratomas usually contain tissues derived from all three germ cell lines,3 ectoderm, mesoderm and endoderm. They also vary in their organization and level of differentiation, from the solid immature teratomas containing immature epithelial, mesenchymal and neural elements4 to mature teratomas exhibiting a higher degree of differentiation and containing cystic structures2. Much rarer anomalous growths include asymmetric parasitic twins that may develop inside the autosite (fetus in fetu) or externally attached (parasitic twin). These structures may arise from fusion of two embryos during gestation, incomplete separation of blastomeres or may develop from primitive germ cells.1,3 In general, these growths exhibit a much greater degree of differentiation and structural organization than do teratomas and possess segments of vertebrae or limbs.3 However, the exact dividing line between a well differentiated teratoma and a poorly differentiated parasitic twin or fetus in fetu is controversial.1,3
Pathogenesis of this case in a harbor seal is unknown. It is likely that such monsters are rare in pinnipeds, as considerable numbers of neonatal pinnipeds have been observed during field work on rookeries in addition to the less healthy pups observed in rehabilitation centers following stranding, and this is the first report of such a monster.
1. Anandakesavan TM. Craniopharyngeal parasitic twin. Ind. Pediatr. 1998;35:1027.
2. Ferlito A, Devaney K. Developmental lesions of the head and neck: terminology and biologic behavior. Ann Otol Rhinol Laryngol. 1995;1104:913–918.
3. Magnus KG, Millar AJW, Sinclair-Smith CC, Rode H. Intrahepatic fetus-in-fetus: a case report and review of the literature. J Pediatr Surg. 1999;34:1861–1864.
4. Weidner N. Germ cell tumors of the mediastinum. Sem Diagn Pathol. 1999;16:42–50.