A full-term female newborn is noted to have a 7-cm mass arising from the sacrococcygeal region. Surgical excision is performed shortly after birth. Histologic examination demonstrates mature neural tissue, respiratory epithelium, cartilage, skeletal muscle, intestinal glands, and adipose tissue within the same lesion. The surgeon explains to the parents that this tumor arose from remnants of an embryonic structure that normally regresses after establishing the three primary germ layers.
Which of the following embryologic processes is most directly associated with formation of this structure?
The correct answer is:
B) Gastrulation
Gastrulation begins during the third week of development with the appearance of the primitive streak on the dorsal surface of the epiblast. Epiblast cells migrate through the primitive streak to form the ectoderm, mesoderm, and endoderm, establishing the basic body plan. The primitive streak normally regresses after gastrulation is complete. Failure of regression can result in a sacrococcygeal teratoma, a tumor containing tissues derived from all three germ layers, as demonstrated in this newborn.
Answer choice A: Cleavage of the zygote, is incorrect. Cleavage consists of rapid mitotic divisions that produce the morula and blastocyst before implantation. The primitive streak has not yet formed during this stage.
Answer choice C: Implantation, is incorrect. Implantation occurs approximately 6–7 days after fertilization when the blastocyst attaches to the endometrium. Although implantation precedes gastrulation, it does not establish the germ layers or involve formation of the primitive streak.
Answer choice D: Neurulation, is incorrect. Neurulation is the process by which the neural tube forms from ectoderm after gastrulation. This answer may be appealing because neural tissue is present within the tumor, but the defining feature of a teratoma is the presence of tissues from all three germ layers, implicating persistence of the primitive streak rather than abnormal neural tube formation.
Answer choice E: Organogenesis, is incorrect. Organogenesis refers to the differentiation of organs from the established germ layers. It occurs after gastrulation and depends on the germ layers that were formed during primitive streak development.
Key Learning Point
Gastrulation establishes the ectoderm, mesoderm, and endoderm through migration of epiblast cells via the primitive streak. Persistence of the primitive streak can give rise to a sacrococcygeal teratoma, which contains tissues derived from all three germ layers.
