Embryonic stem cells (ESCs) are foundational to mammalian development and regenerative medicine due to their remarkable pluripotent capabilities. Understanding their origin helps clarify developmental processes and improve stem cell research applications.

In mammalian embryos, the blastocyst forms about 5–7 days after fertilization, consisting of a fluid-filled cavity called the blastocoel, an outer cell layer called the trophoectoderm, and a cluster of cells inside known as the inner cell mass (ICM). The ICM is the source of embryonic stem cells.

Unlike the trophoectoderm, which forms extraembryonic tissues such as the placenta, the ICM cells are pluripotent; they can differentiate into all embryonic cell types spanning the three germ layers—ectoderm, mesoderm, and endoderm—forming every tissue and organ in the developing fetus.

This distinction makes the ICM the prime target for isolating ESCs for scientific and therapeutic use. ESCs derived from the inner cell mass retain their pluripotency in culture, enabling extensive study and potential clinical applications.

Therefore, the correct answer is:

(2) Inner cell mass

This knowledge underpins the field of stem cell biology and tissue engineering, paving the way for advances in developmental biology, disease modeling, and regenerative therapies.

Answer: (2) Inner cell mass