The critical principle is breadth of differentiation, not simply tumor formation. A convincing result requires histological evidence of derivatives representing ectoderm, mesoderm, and endoderm within the growth. Because these three germ layers encompass multiple developmental lineages, their presence supports multilineage potential in the tested cell population rather than demonstrating only one specialized fate.
Teratoma formation assessment provides a broad test of pluripotency, whereas directed differentiation assays examine whether cells efficiently produce a chosen lineage under defined conditions. A cell line may generate representatives of all three germ layers in vivo yet still perform poorly in a particular directed differentiation protocol. The assay therefore supports characterization but does not substitute for lineage-specific performance testing.
Histological analysis converts the in vivo growth into interpretable evidence. Tissue sections are examined for representative derivatives of each embryonic germ layer, allowing investigators to determine whether the candidate line displayed multilineage differentiation. The assessment depends on identifying these tissue types, so an enlarging mass alone is not sufficient evidence of pluripotency.
At a basic workflow level, investigators introduce candidate pluripotent stem cells into immunodeficient animals, allow an in vivo growth to develop, and then analyze the resulting material histologically. The readout is tissue composition rather than growth alone: investigators look for representative ectodermal, mesodermal, and endodermal derivatives to support the assay’s conclusion.
Researchers apply the assay when they need in vivo evidence to help characterize embryonic or induced pluripotent stem cell lines. It is especially relevant when a line’s capacity for broad developmental potential must be assessed across the three germ layers. The resulting evidence can complement other characterization approaches while remaining focused on multilineage potential rather than a specific directed differentiation outcome.
The assay has important interpretive limits. It is time-consuming, and teratoma formation does not fully predict how efficiently the same cells will undergo a directed differentiation protocol. It also does not establish clinical safety. Consequently, a positive result should be treated as evidence of pluripotency-related potential, not as a complete assessment of research or clinical suitability.