• Complex interaction between embryo and uterine tissues
• Occurs ~6-7d after fertilization
• Hormones have prepared uterus to accept embryo
• Embryo binds to uterine epithelium then penetrates
• These events transforms the embryo into a fetus
  
  A Short Movie of Implantation To get the dynamics of the events of implantation, let's look at a short movie. Then we'll go into some detail about each of the specific stages before focusing in on the development of the embryo. Click on the link to download the movie. Download Movie

1. Apposition: of blastocyst to endometrial epithelium
2. Adherence: via cell adhesion molecules; the glycoproteins involved are unknown
3. Formation of Syncytiotrophoblast: Fusion of cytotrophoblast cells results in giant multinucleate cell that will surround complete embryo
4. Penetration: Syncytiotrophoblast is invasive and works way into uterine tissue ultimately making contact with maternal blood vessels
5. Decidual Reaction: Uterine tissue responds to embryonic invasion by setting up an immunological barrier, the decidua; thus the decidua is derived from maternal tissues

• The maternal body supplies nutrients, oxygen and protection to developing fetus
• Because it has a different genetic makeup than the mother, the fetus is recognized a foreign tissue which normally the body would show an immune response to -- thus there is a need to avoid immune response by the mother's body which would lead to rejection (natural abortion) of the fetus
• The Placenta & chorion provide this function
• The Chorion is one of a group of extraembryonic membranes
  

To understand the origin of the cell lineages that give rise to the embryo and to the extraembryonic tissues, we need to take a step back to the blastocyst stage.

• Blastocyst: Two cell types exist as specific groups of cells
• ICM = Inner Cell Mass; forms embryo plus some extraembryonic membranes
• Trophoblast forms only extraembryonic membranes
• Cell Lineages: These groups (ICM & Trophoblast cell) represent two specific "Cell Lineages"

The following figure reveals the origin of each of these extraembryonic membranes. Thus cleavage results in the blastocyst in which two groups of cells are discernable each of which will contribute to extraembryonic tissues. These thus represent two cell lineages: the inner cell mass which will form cells of the amnion, yolk sac and the allantois. (Of course, the ICM will also contribute to the embryo which will be the focus of the rest of the course and detailed later). The trophoblast cells will generate the chorion and contribute to the placenta. Let's now look at a bit more detail about each of these cell lineages.

• Lower vertebrates: contains yolk; function lost in humans
• Source of PGCs (endodermal lining)
• forms blood islands (embryonic hematopoeisis)
• some blood vessels remain (gut of adult)

• yolk sac remains attached to gastrointestinal tract
• most common malformation of the gastrointestinal tract
• Follows the "Rule of 2's": - it is 2 cm wide; - it occurs 2 feet from ileocecal valve; - it occurs in a male:female ratio of 2:1; - it is present in 2% of the population
• usually asymptomatic but can become infected, rupture and/or secrete excess acid and hormones
• often a massive amount of painless dark red rectal bleeding; associated with abdominal pain

• "surrounds embryo like a fluid filled balloon"
• suspends embryo in amniotic fluid that: a. protects against mechanical injury/shock and adhesions, and b. allows for fetal growth and movement
• amniotic fluid contains 200 proteins: used for assessing status of mother & fetus
• Amniotic fluid contains cells from embryo: used in genetic analysis (e.g., sex, anomalies)

Amniocentesis is carried out when there are medical concerns of one nature or another. Thus, depending on the genetic background of the parents, amniocentesis may be carried out to generated cells and amniotic fluid for the detection of, for example, Tay Sachs disease. A couple may want to know the sex of their child and a simple chromosomal analysis will reveal this. Such tests can also reveal problems such as trisomy. Let's look at the diagram showing how this procedure is carried out and then list some more information about it.

• Removal of small amount of amniotic fluid via needle
• Not done before 13-14wks (not enough fluid)
• Amniotic fluid contains cells that have been sloughed from embryo
• Fluid: examine proteins present (e.g., alpha-fetoprotein signals neural tube defects)
• Cells: sex (e.g., X chromosome; trisomy) or genetic defects (e.g., PCR/enzymes for Tay Sachs)

Twinning occurs in many ways. If two different eggs are fertilized at the same time (obviously by different sperm) then fraternal (non-identical) twins result. The formation of identical twins can occur in different ways and is related to certain developmental defects (e.g., conjoined twins).

• The development of the fetal/maternal interface is a critical step
• The embryo and the fetus interact to construct the placental relationship
• The hypoblast & inner cell mass each form specific extraembryonic membranes
• The extraembryonic membranes serve many critical functions
• The amniotic fluid provides a window into the development of the fetus