Implantation & Development of the Embryo & Placenta

Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

We all know human pregnancy takes 9 months, but what exactly goes on during those 9 months? Learn about some of the things that occur during the early stages of development in this lesson on the embryo and placenta.

Embryonic Phases

Ah, life! It's pretty amazing isn't it? When you actually take time to think about how delicate, organized, and complicated living things are. So many things have to be just right for a living being to develop. In humans, the early stages of development are spent inside the mother's body where she can protect and nourish her developing baby.

Pregnancy can be divided into three phases:

  1. The early embryonic phase (weeks 1-2) is the very beginning. The future baby is so tiny you can't even see it. This phase is characterized by lots and lots of cellular divisions.
  2. The second phase is the embryonic phase (weeks 3-8). Now the tiny ball of cells begins to look like a super tiny baby. During this phase, the embryo starts out the size of a poppy seed and ends up the size of a raspberry.
  3. The fetal phase (week 9 - birth) is where the majority of growth takes place. The baby starts out the size of a large grape and ends with a baby ranging between 6-10 pounds - about the size of a watermelon!

Early Embryonic Phase

It all starts out with a tiny cell you can only see with a microscope. After fertilization, the egg, called a zygote, starts the process of cleavage. Cleavage is when the zygote starts to divide. First it divides into two, then four, then eight. . . well, you get the idea. With all these divisions the zygote must increase in size right? Actually, the overall size of the zygote stays the same! Just the number of cells changes.

Following fertilization the single cell zygote cleaves, or divides every 12-24 hours until it reaches the 64 cell stage.
Process of cell cleavage

By the time the zygote reaches 64 cells, it is called a blastocyst. This stage occurs about 5 days after fertilization. The next step is for the blastocyst to implant into the uterine wall of the mother. How will it do that exactly? It's not like it's carrying around a glue gun, right?


Each of the blastocyst's 64 cells is either an inner or an outer cell.

  • The outer cells make up the perimeter of the zygote and are called trophoblasts.
  • The inner cells stick together and become the inner cell mass or ICM. The ICM will develop into the embryo.

The different cell layers of the blastocyst. The trophoblast cells make up the perimeter of the zygote, while the ICM is the cluster on the inside.

Trophoblasts secrete an enzyme or fluid that actually digests the cells of the uterus so that the blastocyst can bury itself into the uterine wall. I know it sounds a little weird, but believe it or not, the mother doesn't feel a thing.

As implantation continues, the trophoblast cells secrete a hormone called human chorionic gonadotropin or hCG. hCG helps to maintain pregnancy until the placenta can take over. Therefore, levels are naturally highest when a woman is pregnant. This is what a pregnancy test looks for. Detectable levels of hCG can be found in urine or blood, which is why pregnancies are confirmed through both urine test strips or blood tests.

Inner Cell Mass Development

So as those outer cells are concentrating on implantation, the inner cell mass is doing its own preparation. As the zygote passes the 2 week mark it becomes an embryo and enters the embryonic phase. The ICM are now dividing themselves up into groups, each with its own job. This early designation is essential, as it determines the future pathways of those cells.

Some of the ICM cells will form the yolk sac, a source of nutrition for the developing embryo, and will later become part of the respiratory or digestive systems. Other cells are designated as endoderm cells, mesoderm, or ectoderm cells.

The ectoderm, endoderm and mesoderm all have different potential courses of development
Fates of cells from each of the 3 germ layers

  1. Cells from the ectoderm (ecto- means 'outer') are located on the outer portions of the embryo and become part of the nervous system, eyes, teeth and others.
  2. The endoderm cells (endo- means 'inner') are in the inner part of the embryo. They become parts of the lungs, endocrine system, liver, and inner ear.
  3. The mesoderm cells (meso- means 'middle') so these cells are in the middle of the embryo. They become part of the skin, skeletal system, muscles and other organs.

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