 |
II./1.4.: Teratoma sacrale
One of most characteristic and frequent teratomas, the teratoma sacrale (sacral teratoma) has an incidence of 1:35 000 of all neonates. Often extended to the coccyx, this form is also termed sacrococcygeal teratoma. This developmental disorder occurs three times more frequently in girls than in boys. The origin of sacral teratomas is not fully understood; suggestions for primordial germ cells or the primitive streak have been put forward. According to a third theory, a deficiency of programmed cell death (apoptosis) regulating regression of the tail bud would lead to the persistence of certain stem cells, from which sacrococcygeal teratomas may develop. By the most widely accepted theory, sacral teratomas likely derive from the caudal rudiment of the primitive streak, regulating the process of gastrulation. Toward the end of gastrulation the primitive streak regresses in a caudal direction but its remnants may persist in the future sacral and coccygeal regions. By intense mitotic division, such persisting cell groups may generate teratomas of the sacral region, comprising an irregular mixture of ectoderm, mesoderm and endoderm. The fact that these tumors are always situated in the body axis, or at least paraxially, further supports the theory of primitive streak origin.
Often the size of a child's head, sacral teratomas are associated in most cases (47 %) with the dorsal osseous tissues of sacrum, or the pelvic surface (34 %). Nine to ten percent of all sacral teratomas have been observed to spread into the abdomen. Typically, teratomas have a diameter of 2-30 cm. Larger teratomas may appear as a saccular mass suspended between the lower limbs, and may constitute as much as 30-40 % of the newborn's total body mass.
The histological composition of this type of teratoma is also rather miscellaneous, including many different tissues and organic primordia such as squamous epithelium, nervous tissue, adipose tissue, striated skeletal and smooth muscle types, choroid plexus, sebaceous glands, cartilage and, sometimes, bone tissues. There are data on the occurrence of cells forming the lens of the eye; rare reports have even mentioned fully developed eye. The tissue types occurring in sacral teratomas represent, in most cases, fully differentiated tissues, hence the predominantly (90 %) benign character of sacral teratomas.
Undifferentiated cell groups can be identified by markers specific for primordial stem cells. As evidenced by immunocytochemical studies, nanog and Oct4 (transcription factors regulating division of undifferentiated stem cells) ; SSEA-4 (stem cell specific surface antigen) and nestin (intermediate filament characteristic for stem cells) are always present in sacral teratomas. Such stem cells occur in undifferentiated areas of teratomas, localized primarily as individual cells in undifferentiated vessels or the stratum germinativum of epithelial tissue.
Sacral teratomas are characteristic tumors of mainly the sacrococcygeal region, however, tumors of similar histological appearance may develop also in the pineal gland, retroperitoneum, testis, mediastinum, brain tissue or the craniocervical region (occipital teratoma). Similar to those of the sacral region, occipital teratomas may develop from the stem cells detached from the cranial part of the primitive streak.
Nearly 25 % of all sacral teratomas occur together with other developmental disorders, mostly those of the rectum and genital organs. In the fourth week of embryonic development, the caudal part of the hindgut (future rectum) is separated by a mesenchymal septum (septum urorectale) from the allantois, which later gets transformed into the urogenital sinus to form the urinary bladder.
The growing sacral teratoma may get wedged into the tissue of septum urorectale, perturbing separation and further development of the anlages of rectum and bladder. The enlarged tumor may cause atresia of the rectum and anus, or the teratoma, once developed, may prevent fusion of the genital folds (urogenital folds, labioscrotal swellings), leading to abnormal development of scrotum or to hypospadiasis (abnormal opening of urethra on penis). Further anomalies such as malformation of the hip, gastrointestinal disorders (anus imperforatus), lack of sacrum, meningocele or, rarely, ventricular septal defect may also be associated with the incidence of sacral teratomas.
|
|