by Mary Curtis

Created on: November 22, 2011   Last Updated: November 23, 2011

Human embryo development begins with fertilization and continues until the embryo becomes a fetus.  This change from embryo to fetus occurs at the end of the 10th week when all organs will be developed.

WEEKS 1 – 2:  This period is calculated from the onset of the last menstrual period.  This is a theoretical estimate of embryonic age since the exact time of fertilization is not known.

WEEK 3:  Fertilization occurs now, if not earlier during the 14-day period after the onset of the last menstrual period.  Fertilization usually takes place in the fallopian tube.  The fertilized egg is now a single-cell embryo called a zygote which begins to travel down the fallopian tube to the uterus.  This trip to the uterus will take five days.

The zygote divides several times to become a ball of cells called a morula.  The embryo is still in the fallopian tube.

A small cavity develops between the cells which are continuing to divide.  The embryo is now called a blastocyst once this small cavity develops.

As the embryo travels down the fallopian tube, its size does not get any larger even though the number of cells continues to grow.  The reason the size does not get any bigger is because the embryo is confined by a shell.  This shell is called a zona pellucida.  The shell causes the cells to become smaller with each division. 

The embryo – still a blastocyst – will reach the uterus sometime during week three, if not earlier.  Typically, the embryo reaches the uterus on day five after fertilization.  Since the actual time of fertilization can only be estimated within a 14-day period, this is why it is included in week three.  But, as noted earlier, the entry of the embryo into the uterus could have actually occurred in week one or week two.

After the embryo reaches the uterus, it essentially hatches from the zona pellucida shell.  Having emerged from its confining shell, the embryo will now implant into the endometrium which is the membrane lining of the uterus.  The special cells which actually do the implanting are called trophectoderm cells and these cells will also form the placenta.

Still called a blastocyst, the embryo contains a rim of cells – called trophoblast cells - which provide nutrition to the growing embryo which continues to embed itself into the uterine lining.  At one end is a clump of cells – which is called an embryonic pole that

• 1
• 2
• 3
• Next Page >>