Implantation

Step 1

• When the blastocyst starts to attach onto the endometrium lining, it will release enzymes to break down or digest part of the lining (mucosa)

• This allows the blastocyst to embed deeply within the wall of the uterus

Step 2

• Next, the endometrium will wrap around the blastocyst so it is held securely to the lining. The embryo is protected in this way and can receive nutrients for growth.

Step 3

• The implanted embryo will continue to grow within the endometrium (7-8 weeks old from fertilization)

How it attaches

• After the blastocyst successfully implants into the lining, the outer cells of the trophoblast will now merge together.

• This merged ring of cells is called a “synctiotrophoblast”

• These cells will release enzymes to digest the endometrial lining so that the blastocyst firmly stays attached to the wall of the uterus

• In response to this, the cells which were “killed off” by this blastocyst, will now regrow and cover/envelope the blastocyst

Hormones

• The trophoblast is also capable of secreting hormones

• It secretes hCG (human chorionic gonadotropin)

• The role of hCG is to maintain the corpus luteum that is located in the ovary. The structure will be signaled by the hormone to enlarge and continue producing hormones such as progesterone and estrogen so that menses is suppressed (no follicular development or ovulation)

Positive pregnancy test

• Because we want to tell the body to suppress menses, there is a lot of hCG (hormone) that gets released!

• The excess of this will eventually be released into the blood.

• Implantation is completed by the middle of the second week of conception

• By this point, the trophoblast (the outer cell border) has secreted enough hCG to be detected and result a positive pregnancy test

2nd week of pregnancy

• Cells from the upper layer of the embryo: epiblast (right below the trophoblast) will grow and extend around the amniotic cavity. This creates a membranous sac now called the amnion which will surround the embryo to keep it safe.

• The amnion will now fill with fluid and surround the developing embryo

• Within the amniotic fluid, we will be able to see the embryo (soon fetus) floating. This fluid serves the role of protecting it from trauma and sudden temperature changes. The embryo is able to move freely within this fluid and can practice actions like breathing and swallowing so it can live life outside of the uterus. We can say that the epiblast forms the amnion.

yolk sac formation

• On the bottom side of the embryo, (the side which faces the blastocyst cavity) we have the hypoblast. This layering of cells will extend around the blastocyst cavity to form the yolk sac.

• This structure provides nutrients to the embryo which were absorbed from the trophoblast. This structure also helps in providing early blood circulation to embryo in the 2nd and 3rd weeks of development.

• Around week 4, eventually the placenta will take over the role of providing nourishment to the embryo so the yolk sac will significantly decrease in size as it does not need to play this crucial role anymore.

• The yolk sac now serves the role of being a source of blood cells and production of germ cells (the cells which will become egg or sperm).

Transformation of germ layers

• In the 3rd week, we will see that the epiblast and the hypoblast will start to become 3 new germ layers which form different sorts of cells.

Endoderm

• Digestive System

• Lungs (inner layer)

• Pancreas

• Liver

Mesoderm

• Circulatory System

• Muscular System

• Skeletal System

• Lungs (epithelial layer)

Ectoderm

• Hair

• Skin

• Nails

• Nervous System