Final answer:
The basement membrane is the structure that acts as a barrier to stop epithelial cells from becoming metastatic, preventing cancer cells from spreading to other parts of the body.
Step-by-step explanation:
Carcinomas are cancers that develop from epithelial cells, which can become metastatic, meaning they are capable of migrating and forming new tumors elsewhere in the body. The structure that acts as a barrier to prevent epithelial cells from becoming metastatic is the basement membrane. This crucial membrane separates epithelial cells from underlying connective tissue and comprises two layers: the basal lamina and the reticular lamina. The basement membrane's integrity is essential for maintaining cellular organization and function, thus hindering the ability of cancer cells to penetrate and spread (metastasize) to other parts of the body. In contrast to other cellular structures like the nucleus, mitochondria, or Golgi apparatus, it is the basement membrane that acts as a physical barrier and plays a pivotal role in both normal cell architecture and in the body's defense against metastatic cancer.