AN78.1-5 | Second week of development — Glossary

The process by which the blastocyst attaches to and embeds within the uterine endometrium, beginning on day 6-7 and completing by day 10-11 post-fertilisation

The outer cell layer of the blastocyst that forms the embryonic contribution to the placenta, differentiating into cytotrophoblast and syncytiotrophoblast

The inner mitotically active layer of the trophoblast consisting of distinct individual cells that serve as the stem cell population

The outer multinucleated continuous layer of the trophoblast without cell boundaries, responsible for invasion, hormone production, and maternal-foetal exchange

The dorsal layer of the bilaminar embryonic disc consisting of tall columnar cells that will give rise to all three germ layers and the entire embryo proper

The ventral layer of the bilaminar embryonic disc consisting of cuboidal cells that contribute to extraembryonic membranes but not the embryo itself

The two-layered embryonic disc consisting of epiblast and hypoblast, formed during the second week of development

The fluid-filled cavity that forms within the epiblast layer, eventually surrounding the entire embryo as the amnion expands

The extraembryonic membrane derived from hypoblast; the secondary (definitive) yolk sac replaces the primary yolk sac and is the site of early blood cell formation

A layer of loosely arranged cells forming outside the embryonic disc that lines the trophoblast, amnion, and yolk sac, splitting to form the chorionic cavity

The extraembryonic coelom formed by splitting of the extraembryonic mesoderm, in which the embryonic disc is suspended

The bridge of extraembryonic mesoderm connecting the embryonic disc to the trophoblast shell, which will become the umbilical cord

A thickened area at the cranial end of the embryonic disc where epiblast and hypoblast are firmly fused, marking the future site of the oropharyngeal membrane

The transformed endometrium of pregnancy, named by region: basalis (deep to embryo), capsularis (covering embryo), and parietalis (lining rest of uterus)

The transformation of endometrial stromal cells into large glycogen-rich decidual cells at the implantation site, providing nutrition and immunological protection

Human chorionic gonadotrophin — hormone produced by syncytiotrophoblast that maintains the corpus luteum; basis of pregnancy tests

Spaces that form within the syncytiotrophoblast during implantation, which connect with eroded maternal sinusoids to establish primitive uteroplacental circulation

Implantation outside the uterine cavity, most commonly in the fallopian tube (95%); a surgical emergency if ruptured

The brief period of endometrial receptivity (days 20-24 of the menstrual cycle) during which the blastocyst can successfully implant

A small endodermal diverticulum from the yolk sac that extends into the connecting stalk; associated with umbilical vessel development

The exocoelic membrane that lines the inner surface of the cytotrophoblast, forming the wall of the primary yolk sac

Twins derived from a single zygote that splits; the timing of splitting determines the placental membrane arrangement

Abnormal trophoblastic proliferation with swollen chorionic villi and absent or abnormal embryonic development, presenting with markedly elevated hCG