morula

A morula is an early stage in embryonic development, representing a solid ball of cells that forms after several mitotic divisions of the fertilized egg (the zygote). In humans, it typically forms around day 3 after fertilization and contains roughly 16 to 32 cells, called blastomeres. During cleavage, the cells become tightly compacted, a process that establishes distinct outer and inner cell populations.

The morula is surrounded by the zona pellucida and lacks a fluid-filled cavity. The outer cells contribute to the trophoblast, which will form part of the placenta, while the inner cell mass gives rise to the embryo proper. The hallmark of this stage is the transition from a loose cluster of cells to a compact, organized structure, though the embryo remains a solid sphere.

As development proceeds, a fluid-filled cavity called the blastocoel forms within the embryo, marking the transition to the blastocyst stage. The blastocyst then differentiates into the trophoblast and the inner cell mass and eventually implants into the uterine lining, typically around day 5 to 6 in humans. The morula ultimately hatches from the zona pellucida as implantation nears.

Clinical and research relevance: The morula stage is a critical point in preimplantation development used in reproductive technologies. Embryos can be cultured to the morula stage for assessment, transferred at the morula or later blastocyst stage, or cryopreserved, with decisions guided by morphology and developmental timing.