The Beginning of the Embryo

The gametes have already come together during fertilization to form a zygote. But how does it form a human? Let's take a look!


The MORULA is a solid ball of about 16 cells. It is still surrounded by the zona pellucida. Soon it becomes a blastocyst, a fluid-filled ball of cells, with the outer cells called the TROPHECTODERM and the INNER CELL MASS attached to one of the inner sides. It has already traveled down the fallopian tube to the uterus, and soon (6 days post fertilization) it will hatch from the zona pellucida and implant in the endometrial lining.


These are the outer cells making up the lining of the blastocyst. They are multipotent, meaning they can differentiate into only the cells of the placenta. As the embryo begins to invade the endometrium, the trophectoderm splits into the SYNCYTIOTROPHOBLAST and the CYTOTROPHOBLAST.


Cells fuse together to form a syncytium, which is a giant multinucleated cell. At about day 10, lacunae (spaces) begin forming, which will eventually help form the "blood lake" that bathes the chorionic villi of the placenta.


This is the layer formerly called the trophoblast. It surrounds the fluid-filled cavities, and eventually becomes lined with amniotic ectoderm dorsally and yolk sac endoderm ventrally.

EXTRAEMBRYONIC MESODERM: So it's not known whether it's produced by the yolk sac endoderm or the cytotrophoblast, but it forms between them. Then it surrounds the entire embryo, lining the whole cytotrophoblast. It splits in two, forming the chorionic cavity. The order (dorsal to ventral) of cavities is amniotic, yolk sac, chorionic. It forms the connecting stalk which attaches the embryo to the trophoblast.


Twins can form a few different ways, we'll go from less to more similar. Dizygotic (fraternal) twins are separate eggs and sperm that happened to ovulate and implant at the same time. They share a womb and as much DNA on average as any other siblings. There are three types of monozygotic (identical) twins:

Inner Cell Mass

These cells will eventually form the embryo along with all the wonderous extra-embryonic membranes. The ICM splits into the EPIBLAST and HYPOBLAST, and is now known as the bilaminar embryo.


This is the ventral layer of the embryo and will form the YOLK SAC ENDODERM. This layer begins to migrate around inside of the cytotrophoblast. The former fluid-filled cavity of the blastocyst (blastocoele) is now called the yolk sac cavity. The hypoblast is replaced by the migration of endoderm generated during gastrulation.


This layer contains the cells that will form the embryo. It produces the amniotic endoderm (which surrounds the inner surface of the cytotrophoblast and forms the amniotic cavity) and the PRIMORDIAL GERM CELLS.

Primordial Germ Cells

Although these cells are formed in the epiblast, they migrate into the yolk sac endoderm. This is probably so that they remain separate from the rest of the cells during further differentiation.

Developmental Mechanisms

Well, we've made our bilaminar embryo. Now we have to add a couple layers, move things around, and grow some organs! Let's see how it's done.

Cell Fate Specification

These processes are used to make new types of cells from undifferentiated types, or converting one type to another. Examples of these signals are Bmp, Fgf, Shh, Ret-A, and Wnt.


These are the processes involved in shaping tissues into the appropriate shapes and positions through movement of cells.

Pattern Formation

By using patterns, the body requires less complex instructions to follow.


It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life.

- Lewis Wolpert

As Dr. Wolpert says, this is the most important process in your life. It is the point where the embryo goes from bilaminar (epiblast and hypoblast) to trilaminar (ECTODERM, MESODERM, and ENDODERM). This process begins at around Day 14. Some important terms:

PRIMARY CILIARY DYSKINESIA: A mutation produces non-functional cilia. These patients usually present with respiratory problems. Upon examination half the patients are found to have SITUS INVERSUS, a condition where the viscera are reversed in the thorax and abdomen! This is because the cilia are not present to create the gradient, so the left-right asymmetry is determined by chance! Some individuals have a combination of normal and reversed organs which will lead to health problems.

What happens to the three layers? Check out ECTODERM, MESODERM, and ENDODERM!