Blastocoel forms

on days 5-6

Blastocyst

Made up of embryoblast, blastocoel, and trophoblast cells

Cytotrophoblast cells

Differentiated trophoblast cells surrounding the blastocystic cavity

Syncytiotrophoblast cells

Differentiated trophoblast cells located at the embryonic pole in contact with the endometrium lining of the uterus

How does the blastocyst enter the endometrium?

Syncytiotrophoblast cells produce compounds that erode the uterine endometrium. These compouds allow the syncytiotrophoblasts to grow into and proliferate within the endometrium.

Syncytium

Continuous multinucleated mass with no internal cell boundaries.

Formation: fusion of syncytiotrophoblast cell membranes

Ten days after fertilization

the blastocyst is completely buried in the endometrium

Lacunae

Isolated cavities or spaces in the syncytiotrophoblast; appear by day ten.

Lacunar networks

Larger spaces that have formed as a result of the fusion of lacunae. These form on day twelve.

Maternal blood flow to developing embryo

Endometrial arteries --> lacular networks --> endometrial veins

Primary Chorionic Villi

Nub like extensions into the syncytiotrophoblast, protruding out into the lacunar network space where they are bathed by maternal blood. Form on day thirteen.

Function: increases the fetal vascular surface area.

Normal Implantation

Occurs on the anterior or posterior walls of the superior body of the eterus.

Ectopic Pregnancy

Implantation outside the uterus. Can occur if the zygote does not fully traverse the uterine tube or if the uterine tube has any blockage or scarring.

Location of MOST ectopic implantations

95% occur in the uterine tube.

Result: the uterine tube cannot expand as the embryo grows --> rupturing & embryonic death --> significant maternal pain, hemorrhaging, and possibly maternal death.

Locations of ectopic pregnancies from MOST to LEAST common

1. Uterine tube (95%)

2.Abdominal

3. Ovaries

*Cervical pregnancies are often considered to be ectopic, however, it is appropriately included with uterine pregnancy sites.

Hypoblast

Differentiated embryoblast cells located adjacent to the blastocystic cavity

Epiblast

Remaining embryoblast cells following the differentiation of some to form the hypoblast.

Amniotic Cavity

A cavity that forms within the mass of epiblast cells. As the cavity enlarges, the epiblast spreads into a single layer of cells enclosing the hemispheric amniotic cavity.

Amniotic Membrane

Also known as the amnion, this is formed by the thin layer of epiblast cells comprising the arch of the dome that become adherent to the cytotrophoblast layer.

Bilaminar embryonic disc

Epiblast layer + Hypoblast layer

This forms the embryo

Exocoelomic Membrane

Lining in the blastocoel cavity formed by cells from the marginal edge of the hypoblast disc which have grown over the cells of the cytotrophoblast.

Exocoelomic Cavity

Name for the blastocoel cavity following the formation of the exocoelomic membrane.

AKA - primary yolk sac / primary umbilical vesicle

Exocoelomic Membrane becomes

the endoderm lining the exocoelomic cavity

Extraembryonic Mesoderm

New layer of cells derived from the exocoelomic membrane formed between the exocoelomic membrane and the cytotrophoblast. 

Eventually grows around the outside of the exocoelomic cavity, the epiblast, and amniotic cavity.

Extraembryonic Coelomic Cavity

Spaces formed in the extraembryonic mesoderm layer. Forms a continuous chorionic cavity.

Divides extraembryonic mesoderm into two distinct sheets - somatic and visceral mesoderm.

Extraembryonic Somatic Mesoderm

Lines the cytotrophoblast cells on the outside of the chorionic cavity and covering the amniotic membrane.

Extraembryonic Visceral Mesoderm

Covering the exocoelomic cavity (primary yolk sac).

When can a pregnancy be detected?

By the end of the second week.

How is a pregnancy detected?

Human Chorionic Gonadotropin (hCG) homone detected in maternal blood and urine two weeks after conception. 

hCG is produced by syncytiotrophoblast cells.

Connecting Stalk

Thick band of extraembryonic somatic mesoderm. Eventually becomes umbilicus.

Secondary Yolk Sac

Portion of primary yolk sac that pinched off as the chorionic cavity enlarged. Much smaller than the primary yolk sac.