A blastocyst is a structure that formed in early development and possesses an inner cell mass and outer cell mass. The inner cell mass of the blastocyst gives rise to the actual embryo. The outer cell mass is called trophoblast and surrounds the inner cell mass. The outer cell mass gives rise to the placenta.

In humans, the formation of blastocyst starts about 5 days after fertilization. the diameter of blastocyst ranges from 0.1 to 0.2mm and consists of 200 to 300 cells in which rapid cleavage occurs. the formation of the blastocyst starts when the fluid-filled cavity opens up in the morula.  The blastocyst formation completes in the uterus after which it embeds in the endometrium of the uterine wall followed by the further developmental process that includes gastrulation.

To embed in the endometrium of the uterine wall the blastocyst hatches from the zona pellucida. The function of the zona pellucida is to prevent adherence of pre-embryo to the fallopian tube.

A blastocyst is also used in a reproductive technique called in Vitro fertilization that involves the culturing of a fertilized egg for five days before implanting into the uterus. The use of blastocyst in vitro fertilization can be a more viable method for fertility treatment than a tradition in vitro fertilization.


The process of blastocyst formation occurs in two stages that are the cleavage and blastocyst formation.

The cleavage is the process in which repeated mitotic divisions occur in the zygote and smaller units are formed. The cleavage results in an instant increase in the number of cells. The cells that are formed after the divisions are called the blastomeres. The blastomeres become smaller and smaller with each successive cleavage division. The divisions of zygote begin just after the fertilization and continuous as the zygote moves along the fallopian tube.

During the process of cleavage, the zygote is surrounded by a fairly thick layer called the zona pellucida. The zygote divided into two cells in which one is smaller and the other is large. The larger one first divides and gives rise to a three cell stage followed by the division of a smaller one that gives rise to a four-cell stage.

The four cells divide to creates eight cells which then divides and form 16 cell. The 16 cells embryo is called morula. In the morula, the blastomeres are very small and contain hardly any cytoplasm to survive.

The blastomeres are surrounded by the layer of cells called zona pellucida. After the formation of the morula, the cells are organized into two groups. Cells that are present in the center of the morula are called inner cell mass or embryoblast while the cells that are present at the periphery of the morula are called trophoblast or outer cell mass.

The inner cell mass or embryoblast gives rise to the actual embryo while the outer cell mass gives rise to the placenta.

Blastocyst formation
After the formation of the morula, it enters the uterine cavity. The endometrial fluid penetrates the zona pellucida and enters the intercellular areas of the morula after which the individual cavity is produced.

The cavity that is produced due to the endometrial fluid is called the blastocele and the embryo at this time is called the blastocyst. the blastocyst is filled with liquid that is rich in nutrition. The nutrition present in the blastocele is secreted by the endometrium of the uterus.

The fluid is also called uterine milk. With the passage of time, the cavity becomes large and the external cells of the trophoblast become flattened.

After all these processes the blastocyst now consists of two groups of cells that are the inner cell mass and outer cell mass. The inner cell mass is called the embryo last while the outer cell mass is called the trophoblast.

The embryoblast or inner cell mass forms the actual embryo while the outer cell mass or the trophoblast gives rise to the placenta.

The blastocyst enlarges and the zona pellucida disappear that makes the blastocyst ready for implantation.

Role of zona pellucida
The trophoblast has a high tendency to stick with the endothelium that it touches. Zona pellucida stops the sticking of the trophoblast with the endothelium through the trip of fertilizing oocyte from the ampullary region of the fallopian tube to the uterine cavity.

The main function of the zona pellucida is to prevent the implantation of the blastocyst at sites other than the uterus and this stops the ectopic pregnancy.

Hydatidiform mole

The abnormalities of the blastocysts are common in females. A blighted blastocyst can cause the death of the embryo. This is followed by extreme trophoblast proliferation to create a vesicular or polycystic mass called the hydatidiform mole.

Actually when the embryo dies the developing villi could not develop further due to insufficient blood circulation. The degeneration process starts in the villi and cystic swelling develops in the degenerating villi. Moles secrete more human chorionic gonadotrophins.

The symptoms of the hydatidiform mole are
1. preeclampsia through the first trimester
2.  An elevated level of human chorionic gonadotrophins.
3.  Enlarged uterus
4.  Blood loss
5.  Presence of vesicles in the urine
6.  No fetal movement.
7.  No fetal heart sound
8.  Ultrasound reveals snows storm appearance of the uterine cavity.

Around three to five percent of Hydatidiform mole undergo malignant changes and develop choriocarcinoma.

Types of hydatidiform mole 
there are two types of hydatidiform mole

1. Complete type in which there is no existence of the embryo at all.
2. Partial type in which part of the embryo is seen.

The majority of the hydatidiform moles are monospermic.

What is ectopic pregnancy?

Ectopic pregnancy is a condition in which the embryo attached outside of the uterus in some other parts of the female reproductive system. Some of the symptoms are abdominal pain and vaginal bleeding.

The pain that occurs due to the ectopic pregnancy may spread into the shoulder if the bleeding into the abdomen occurred.

With very few exceptions the fetus develops as a result of ectopic pregnancy is unable to survive.

Assisted reproductive technology, tobacco smoking, chlamydia infection, and pelvic inflammatory diseases are the risk factors for ectopic pregnancy.

Ectopic pregnancy mostly occurs in the fallopian tube that is called tubal pregnancy.