Development Time Line Up to Gastrulation 낱말 카드

Zygote

Day 0; single cell formed after fertilization in the ampulla of the uterine tube.

Cleavage

First division usually occurs within 24 hours (range: 6-36 hours); rapid cell divisions produce 2 cells by about day 1, 4 by day 2, then 8, 16, and more, while the overall structure remains about the original size within the zona pellucida.

Morula

By about day 5; a solid ball of 12 or more cells, still approximately the size of the original cell.

Blastocyst

By about day 6; after hatching from the zona pellucida, fluid creates a cavity (blastocoele), with an outer trophoblast layer and an inner cell mass; enters the uterine cavity.

Implantation

Begins around day 7; the blastocyst attaches to and embeds in the endometrium, with the inner cell mass oriented toward the uterine wall. During days 8-12, trophoblast cells differentiate and invade the uterine wall.

Bilaminar embryonic disk

Around days 12-13; the inner cell mass organizes into epiblast (forms the embryo) and hypoblast (contributes to extraembryonic tissues), with the amniotic cavity above and yolk sac below.

Gastrulation

Around days 13-14; epiblast cells proliferate and form the primitive streak, then migrate through it to form endoderm and mesoderm; remaining epiblast becomes ectoderm, creating the trilaminar embryonic disk.