morula

After fertilization in the ampulla of the fallopian tube, the zygote undergoes a series of mitotic divisions called cleavage, without an increase in overall size (cytoplasm is simply divided among smaller cells called blastomeres). By day 3, the embryo consists of approximately 12-16 blastomeres and is called a morula (Latin for 'mulberry,' reflecting its appearance). The morula continues to travel through the fallopian tube toward the uterus, arriving around day 3-4. As the morula enters the uterine cavity, fluid begins to accumulate between the cells, creating a fluid-filled cavity (blastocoele) that transforms the morula into a blastocyst by day 5. The blastocyst then consists of an inner cell mass (embryoblast, which becomes the embryo proper) and an outer cell layer (trophoblast, which contributes to the placenta). Clinically, understanding this timeline is important for assisted reproductive technology (IVF), as embryos are often transferred at the morula or blastocyst stage. Ectopic pregnancy can occur if the morula/blastocyst implants outside the uterine cavity, most commonly in the fallopian tube, due to delayed transport (e.g., from tubal scarring in PID). Compaction, the process by which blastomeres maximize cell-to-cell contact via tight junctions and adherens junctions (E-cadherin dependent), is essential for morula formation and subsequent segregation into inner and outer cell populations.