# Teratoma: Tumors with Hair and Teeth A teratoma is a germ-cell neoplasm that contains tissues derived from all three embryonic germ layers: ectoderm, mesoderm, and endoderm. Because those layers normally build skin, bone, muscle, and glandular organs, a teratoma can produce recognizable structures such as hair, teeth, sebaceous material, cartilage, and even a formed lens inside a single tumor mass. These tumors matter because they force a clinician to think about embryology, not just oncology. A teratoma tumor in the ovary of a dog can grow large enough to mimic pregnancy or pyometra, and a mature teratoma in the testis or midline can be benign while an immature teratoma behaves aggressively. The distinction between mature and immature histology drives prognosis and treatment, and that distinction is made on a glass slide, not on a radiograph. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## What Defines a Teratoma The word teratoma comes from the Greek *teras*, meaning monster, a reference to the bizarre mixture of tissues found inside. A teratoma is classified as a germ-cell tumor because it arises from primordial germ cells, the cells that normally become ova or sperm. When those cells escape normal developmental control, they can differentiate along multiple lineages at once. The defining feature is the presence of all three germ layers in one neoplasm. A tumor with only one or two layers is not a teratoma. A dermoid cyst, for example, is a related lesion that is predominantly ectodermal, and many older veterinary reports use "dermoid cyst" and "mature teratoma" almost interchangeably. Modern pathology reserves the term teratoma for tumors with tridermal differentiation. ### The Three Germ Layers The germ layers are the three primary cell layers that form during gastrulation in the embryo. Each layer gives rise to a defined set of adult tissues, and a teratoma can contain any of them. | Germ Layer | Normal Embryonic Fate | Tissues Commonly Seen in Teratoma | |--|--|--| | Ectoderm | Skin, nervous system, sensory organs | Keratinizing squamous epithelium, hair follicles, sebaceous glands, neural tissue, retina, lens, teeth (enamel) | | Mesoderm | Connective tissue, muscle, bone, blood, urogenital tract | Adipose tissue, cartilage, bone, smooth and skeletal muscle, fibrous connective tissue, blood vessels | | Endoderm | Lining of gut, respiratory tract, and glands | Respiratory epithelium, intestinal-type epithelium, mucous glands, thyroid-type follicles, hepatic-type tissue | This table is the single most useful thing to memorize for a pathology exam. If you can name one tissue from each column, you can defend a diagnosis of teratoma. ### Mature Versus Immature Teratoma The mature versus immature distinction is the most important prognostic split. A mature teratoma is composed of well-differentiated tissues that resemble normal adult structures. Hair, teeth, sebaceous material, and organized neural tissue are typical. Mature teratomas are considered benign in most species and most anatomic sites. The canine ovarian teratoma described by Makovicky and colleagues was a benign tumor with predominantly nervous tissue, and the dog remained clinically healthy with no distant metastases one year after ovariohysterectomy [1]. An immature teratoma contains primitive, embryonic-appearing tissues, especially immature neuroepithelium. Immature teratomas are graded, usually 1 to 3, based on the amount of immature neural tissue. Higher grade means more aggressive behavior. In a Japanese registry study of malignant ovarian germ cell tumors, immature teratoma accounted for 56.6 percent of cases, and age 50 years or older was independently associated with poorer overall survival in immature teratoma grade 3 [2]. That human registry is cited here for the principle that immature histology and grade carry prognostic weight, not as a management protocol for animals. A teratoma can also undergo somatic-type malignant transformation, in which one of its tissue components acquires the features of a carcinoma or sarcoma. A case report described pulmonary adenocarcinoma arising from the teratomatous component of a recurrent nonseminomatous germ cell tumor [3]. This phenomenon is recognized in postchemotherapy germ cell tumor specimens, where growing teratoma syndrome, somatic-type malignancies, and sarcomatoid yolk-sac tumors can appear years after initial therapy [4]. Veterinary pathologists should be aware of the same principle when examining a teratoma that has been present for a long time or that has been previously treated. ## Where Teratomas Arise Teratomas follow the migration path of primordial germ cells, so they appear along the midline and in the gonads. ### Gonadal Sites The ovary is the most common site in female dogs. Reported cases include a benign ovarian teratoma with predominantly nervous tissue [1], a teratoma coexisting with a granulosa cell tumor and pyometra in an English Bulldog [5], a mixed germ cell tumor with yolk sac and teratomatous components in a Labrador Retriever [6], a teratoma associated with paraneoplastic optic neuritis and meningoencephalitis [7], and a massive ovarian teratoma in a Swiss Mountain Dog that weighed more than 16 percent of the dog's body weight [8]. The testis is the corresponding site in males. In humans, testicular germ cell tumors are the most common solid cancers in young men [9], and mixed germ cell tumors of the testis frequently contain a teratoma component alongside seminoma, embryonal carcinoma, and yolk sac tumor [10]. Testicular teratomas are less commonly reported in dogs and cats than ovarian teratomas, but the same germ-[cell biology](/blog/careers/cell-biology) applies. ### Extragonadal Midline Sites Extragonadal teratomas arise along the path of germ-cell migration: the cranial mediastinum, the sacrococcygeal region, and the intracranial suprasellar region. A canine case report described a teratoma in the ovary that was later followed by a primary cranial mediastinal neuroendocrine carcinoma, though the two tumors were not necessarily related [11]. In children, suprasellar mixed germ cell tumors can produce growing teratoma syndrome, in which the tumor enlarges despite falling tumor markers [12]. Intracranial teratomas are rare in veterinary patients but are described in the comparative literature. ### Comparative Examples in Dogs, Horses, and Cattle Dogs are the best-documented species for ovarian teratoma. The clinical presentations range from an incidental finding during laparoscopic ovariectomy [13] to a large abdominal mass causing distension and mimicking pregnancy [8] or pyometra [5]. A 23 cm ovarian mixed germ cell tumor with yolk sac and teratomatous components metastasized to the peritoneum and diaphragm, and the metastases consisted solely of the yolk sac component [6]. That case illustrates an important principle: in a mixed germ cell tumor, the metastatic behavior is often driven by the non-teratomatous component. Horses can develop teratomas, most often in the ovary, and they are typically mature and benign. Equine ovarian teratomas are usually incidental findings at necropsy or during evaluation for colic or reproductive abnormalities. They contain the same tridermal tissues seen in dogs, including hair, bone, and dental structures. Cattle can develop ovarian teratomas, and they are also usually mature and benign. Bovine teratomas are uncommon but are reported as incidental abattoir findings. The same mature versus immature rule applies across species. The comparative point is that teratomas are rare in all domestic species but share a common embryologic origin. The species differences are in frequency and in the likelihood of a mixed germ cell tumor component, not in the fundamental biology. ## How Teratomas Are Diagnosed Diagnosis is histologic. Imaging can suggest a teratoma, but only microscopy confirms it. ### Imaging Findings Ultrasound often shows a complex ovarian mass with mixed echogenicity, cystic cavities, and mineralized foci. In the Swiss Mountain Dog case, the mass was large enough to cause progressive abdominal enlargement [8]. In the English Bulldog case, ultrasound revealed ovarian neoformations plus cystic endometrial hyperplasia and pyometra [5]. Computed tomography can be more specific: a canine ovarian teratoma imaged with contrast-enhanced CT appeared as a mixed fat to soft tissue attenuating mass with a complex internal mineralized matrix and heterogeneous contrast enhancement, receiving blood from the left ovarian artery [11]. That combination of fat, soft tissue, and mineral is highly suggestive but not definitive. ### Gross Pathology On cut section, a mature teratoma is often multilobular and cystic. A classic description is a multilobular tumor with several cystic cavities containing tufts of dark hair in thick creamy-white sebaceous fluid [14]. The presence of hair and sebaceous material at gross examination is a strong clue. Teeth, bone, and cartilage may also be visible. ### Histologic Diagnosis Histologic diagnosis requires identifying tissues from all three germ layers. A representative section might show keratinizing squamous epithelium with hair follicles and sebaceous glands (ectoderm), islands of cartilage and bone with adipose tissue (mesoderm), and respiratory or intestinal-type epithelium (endoderm). Neural tissue is common and can be prominent, as in the canine case with predominantly nervous tissue [1]. Immunohistochemistry can help characterize the components. In a canine mixed germ cell tumor, the yolk sac component was diffusely immunopositive for cytokeratin with scattered cells reactive for alpha-fetoprotein and placental [alkaline phosphatase](/knowledge/molecular-biology/alkaline-phosphatase), while the teratomatous neuropil was immunopositive for S100, neuron-specific enolase, and neurofilaments with a few glial fibrillary acidic protein positive cells [6]. These markers are used to confirm lineage, not to make the initial diagnosis. A formed lens is a striking but uncommon finding. One canine ovarian teratoma contained a crystalline lens along with adipose tissue, central nervous tissue, cartilage, and bone, and the nervous tissue showed nonsuppurative inflammation with glial cells and lymphocytic perivascular cuffing [14]. That case was diagnosed as a mature cystic teratoma. ### Grading Immature Teratomas Immature teratomas are graded by the amount of immature neuroepithelium. Grade 1 has rare foci, grade 2 has moderate foci, and grade 3 has abundant immature neural tissue. The Japanese registry study found that immature teratoma grade 3 in patients aged 50 years or older was associated with poorer overall survival [2]. In [veterinary medicine](/blog/careers/veterinary-medicine-careers-from-clinical-practice-to-public-health), the same grading principle applies, though breed and species-specific survival data are limited. ## Clinical Relevance, Limitations and Common Mistakes The clinical relevance of a teratoma depends on three things: whether it is mature or immature, whether it is pure or mixed with another germ cell component, and whether it is causing mass effect or paraneoplastic disease. A mature pure teratoma is usually cured by surgical resection. The Swiss Mountain Dog recovered fully after removal of a tumor weighing more than 16 percent of its body weight [8]. The dog with a benign ovarian teratoma and predominantly nervous tissue was clinically healthy one year after surgery [1]. The dog with laparoscopic removal of an ovarian teratoma also had a successful outcome [13]. A mixed germ cell tumor is more dangerous because the non-teratomatous component can metastasize. The Labrador Retriever with a mixed germ cell tumor had peritoneal and diaphragmatic metastases composed solely of the yolk sac component [6]. The Doberman Pinscher with an ovarian teratoma and granulosa cell tumor developed intra-abdominal metastasis and was euthanized six weeks postoperatively [15]. These cases show that the teratoma itself may be benign while the overall tumor is not. Paraneoplastic syndromes are a recognized but uncommon complication. A dog with an ovarian teratoma developed bilateral optic neuritis and meningoencephalitis, with neural tissue in the teratoma showing perivascular cuffing and mixed lymphocytic infiltration [7]. After tumor resection and corticosteroid treatment, the pleocytosis resolved and vision was preserved in one eye [7]. This case highlights that a teratoma can provoke an immune response against nervous tissue, a phenomenon also described in human patients with ovarian teratomas. Common mistakes in diagnosis include: 1. Calling every cystic ovarian mass a teratoma. Cystic endometrial hyperplasia, pyometra, granulosa cell tumors, and other ovarian neoplasms are far more common. Histology is required. 2. Assuming a teratoma is benign because it looks mature on imaging. Immature teratomas can appear well-circumscribed. 3. Missing a mixed germ cell tumor component. If the tumor contains yolk sac or embryonal carcinoma elements, the prognosis changes. 4. Relying on serum markers alone. Alpha-fetoprotein and beta-human chorionic gonadotropin can be elevated in mixed germ cell tumors, but they are not specific for teratoma and are not validated in all veterinary species. In growing teratoma syndrome, markers can fall while the tumor enlarges [12]. 5. Forgetting that teratomas can occur outside the gonad. A midline mass in the mediastinum or sacrococcygeal region should raise the possibility. Limitations of current knowledge include the rarity of these tumors in animals, which means most evidence comes from case reports and small case series. Treatment decisions should be made with a veterinarian who can assess the individual patient. ## Quick Review - A teratoma contains tissues from all three germ layers: ectoderm, mesoderm, and endoderm. - Mature teratomas are benign and often contain hair, teeth, and sebaceous material. - Immature teratomas contain primitive neuroepithelium and are graded 1 to 3. - Common sites are the ovary, testis, and extragonadal midline. - Dogs are the best-documented species, with ovarian teratomas reported in many breeds. - Mixed germ cell tumors can metastasize via the non-teratomatous component. - Diagnosis is histologic, supported by imaging and immunohistochemistry. ## Frequently Asked Questions ### What is a teratoma tumor? A teratoma tumor is a germ-cell neoplasm that contains tissues from all three embryonic germ layers. It can produce hair, teeth, sebaceous material, cartilage, and neural tissue inside a single mass. ### Can a dog survive a teratoma? Yes. Mature teratomas are usually cured by surgical removal. Dogs in reported cases recovered fully after ovariohysterectomy, including one with a tumor weighing more than 16 percent of its body weight [8]. ### Are teratomas cancerous? Mature teratomas are generally benign. Immature teratomas and mixed germ cell tumors can be malignant because of their primitive or non-teratomatous components. ### Why do teratomas have hair and teeth? Teratomas arise from primordial germ cells, which retain the ability to differentiate into any tissue. When those cells differentiate in an uncontrolled way, they can form ectodermal structures like hair follicles and teeth. ### Where do teratomas usually occur in animals? The ovary is the most common site in female dogs. Testicular and extragonadal midline sites also occur. Horses and cattle can develop ovarian teratomas, usually mature and benign. ### How is a teratoma diagnosed? Diagnosis is made by histologic examination of the resected mass. Imaging can suggest the diagnosis by showing fat, soft tissue, and mineralized components, but microscopy is required to confirm tridermal differentiation. ## Related Articles - [Molecular Basis of Cancer: From Oncogenes to Tumor Suppressors](/knowledge/molecular-biology/molecular-basis-of-cancer) - [Telomerase and Cancer: How It Drives Tumor Formation](/knowledge/molecular-biology/telomerase-cause-cancer) - [Tumor DNA in Blood: Liquid Biopsy for Cancer Detection](/knowledge/molecular-biology/tumor-dna-in-blood) - [Oncogene Activation and Tumor Suppressor Silencing: A Mechanistic Review of Genetic and Epigenetic Drivers in Cancer](/knowledge/molecular-biology/oncogene-activation-and-tumor-suppressor-silencing-a-mechanistic-review-of-genetic-and-epigenetic-dr) - [Detecting Somatic Structural Variants in Cancer Genomes: Challenges and Best Practices for Tumor-Normal Analysis](/knowledge/bioinformatics/detecting-somatic-structural-variants-in-cancer-genomes-challenges-and-best-practices-for-tumor-norm) - [RNA Therapeutics for Gene Regulation: A Comparison of ASOs, siRNAs, and mRNA-Based Approaches for Targeting Oncogenes and Tumor Suppressors](/knowledge/molecular-biology/rna-therapeutics-for-gene-regulation-a-comparison-of-asos-sirnas-and-mrna-based-approaches-for-targe) - [Primary Dentition: Deciduous Teeth Explained](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/primary-dentition-deciduous-teeth-explained) - [Wolf Teeth in Horses: Extraction and Management](/knowledge/veterinary-medicine/equine-care/wolf-teeth-in-horses-extraction-and-management) - [Teeth Diagram: Types, Numbers and Animal Comparison](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/teeth-diagram-types-numbers-and-animal-comparison) ## Sources 1. 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