What term is used to describe the process when cells of the embryo begin to change shape and structure into distinct layers?

The process referred to in the question is known as differentiation. This is the biological process through which unspecialized cells, like those in the early stages of an embryo, undergo morphological and functional changes to develop into specialized cells with distinct structures and functions. During differentiation, cells organize into distinct layers or germ layers—ectoderm, mesoderm, and endoderm—each giving rise to various tissues and organs in the developing organism.

Understanding differentiation is crucial, especially in the context of oncology and developmental biology, as it plays a vital role in normal development, and abnormalities in this process can lead to various diseases, including cancer. Differentiation is a fundamental concept that provides insight into how complex multicellular organisms develop from a single fertilized egg.

The other terms mentioned are related to different biological processes: proliferation refers to cell multiplication, apoptosis is the programmed cell death essential for removing damaged or unnecessary cells, and metaplasia describes the process where one differentiated cell type is replaced by another mature cell type, usually in response to chronic irritation or inflammation.