The prochordal plate is a circular, midline thickening of hypoblast cells that are firmly
attached to the overlying epiblast cells. The plate will eventually develop into a membrane
called the oropharyngeal membrane at the site of the future mouth. It is interesting to note that
at this early stage of development the cranial versus caudal region of the embryo is established
by the prochordal plate, and since the prochordal plate is located in the midline, bilateral
symmetry is also established.

The extraembryonic mesoderm is derived from the epiblast and is located between the
exocoelomic membrane and the cytotrophoblast. The overall effect is to completely separate
the embryoblast from the trophoblast, with the extraembryonic mesoderm serving as a
conduit (connection) between them.

The embryoblast consists of the two distinct cell layers (epiblast and hypoblast) at the end of
development week 2 (day 14) and forms the bilaminar embryonic disk.

The syncytiotrophoblast plays the most active role in invading the endometrium of the
mother’s uterus. During the invasion, endometrial blood vessels and endometrial glands are
eroded and a lacunar network is formed

Oncofetal antigens are normally expressed during embryonic development, remain unexpressed in normal adult cells, but are re-expressed on transformation to malignant neoplastic
tissue. CEA is associated with colorectal carcinoma.

The amniotic cavity develops within the epiblast, and it is a cavity that contains the embryo
and amniotic fluid.

During week 2 of development, the embryoblast receives its nutrients from endometrial
blood vessels, endometrial glands, and decidual cells via diffusion. Diffusion of nutrients does
not pose a problem, given the small size of the blastocyst during week 2. Although the beginnings of a uteroplacental circulation are established during week 2, no blood vessels have yet
formed in the extraembryonic mesoderm to carry nutrients directly to the embryoblast (this
occurs in week 3).

The definitive chorion consists of three components: extraembryonic somatic mesoderm,
cytotrophoblast, and syncytiotrophoblast. The chorion defines the chorionic cavity in which the
embryoblast is suspended and is vital in the formation of the placenta.

After a hydatidiform mole, it is very important to assure that all the invasive trophoblastic tissue is removed. High levels of hCG are a good indicator of retained trophoblastic tissue because
such tissue produces this hormone. In this case, the trophoblastic tissue has developed into a
malignant choriocarcinoma and metastasized to the brain, causing her symptoms of headache,
blurred vision, and so on.

Human chorionic gonadotropin (hCG) can be assayed in maternal serum at day 8 of development and in urine at day 10. If this teenager is pregnant, the blastocyst would be in week 2 of
development (day 10). Laboratory assay of hCG in either the serum or urine can be completed;
however, serum hCG might be more reliable. It is important to note that if she is pregnant, she
will not miss a menstrual period until May 15, at which time the embryo will be entering week 3
of development