17.7: Pre-implantation Embryonic Development

After fertilization, the zygote and its associated structures, collectively referred to as the conceptus, continue their journey toward the uterus, driven by peristaltic movements and the beating cilia. The zygote undergoes five to six rapid mitotic cell divisions during this voyage. These cleavages result in an increased number of cells, although they do not alter the overall size of the conceptus.

Around three days post-fertilization, a 16-cell conceptus arrives in the uterus. The previously loosely arranged cells have now compacted, presenting a more solid appearance. This stage is called the morula (meaning "little mulberry"). Upon entering the uterus, the conceptus remains free-floating for several more days. The cells within it continue to divide, forming a cluster of approximately 100 cells. Simultaneously, the conceptus takes in nutritive endometrial secretions known as uterine milk while the uterine lining undergoes thickening. Subsequently, the closely-knit cell cluster begins secreting fluid and organizes itself around a central fluid-filled cavity called the blastocoel. At this point, the conceptus is referred to as a blastocyst. Within the blastocyst structure, a set of cells differentiates into an inner cell mass destined to become the embryo. The outer layer of cells, called trophoblasts (trophe, meaning "to feed" or "to nourish"), will develop into the chorionic sac and the fetal portion of the placenta. The placenta serves as the primary organ for nutrient, waste, and gas exchange between the mother and the developing offspring. During this stage, the inner mass of embryonic cells retains totipotency, signifying that each cell possesses the potential to differentiate into any cell type within the human body. This totipotent state is relatively short-lived, lasting only a few days before the cells' destinies are determined as precursors to specific cell lineages. Concurrently, as the blastocyst takes form, the trophoblast releases enzymes that initiate the degradation of the zona pellucida.⁴

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