Endometrium with egg cell

Image copyright: University of Oslo, CC BY-NC-ND 4.0. (Tissue stain: H+E).

This section contains mainly endometrium in the secretory phase but you may notice a cyst-like structure. This is an egg.

At this stage, an implanted egg consists of an inner cell mass (not present in the section) and an outer cell mass forming the placenta (seen here).

Secondary villi, as seen at an early stage in this image, appear 15-21 days after fertilization. To our knowledge, the fertilized egg takes about 6 days to reach the uterine cavity. This means that this egg presumably has been developing for 1-2 weeks after implantation.

Published July 19th 2019, updated May 11th 2020.