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Saturday, 25 January 2014

Implantation : A miracle begins

Pregnancy starts with implantation of an embryo in the tissue that lines the womb cavity. This is the most intriguing part of conception and this process is still not fully understood. 

Blastocyst formation
Embryo forms when a sperm and an egg combine. Just like the egg, the embryo remains encased in a shell and grows within it. The cell that results then starts to divide. The number of cells increases to 2,4,8 and so on till the a tight ball of cells forms. A fluid filled cavity develops that divides the cells into two groups: one group lines the cavity all around (these give rise to the placenta and help in implantation) while at one pole the rest of the cells remain in the form of a tight pack of cells (these form the baby eventually). This stage is called blastocyst. At this stage the embryo hatches out and is ready for the next stage, the implantation.
The lining of the uterine cavity undergoes cyclical changes in an attempt to get the ground ready for an embryo to stick and when it does not, the lining is shed (during a period) and starts growing all over again. 
In the first part of a menstrual cycle, follicles grow in the ovary and release estrogen hormone. Estrogen causes the lining to grow and become thick. Once ovulation occurs, the follicle from which the egg is released restructures itself and starts secreting progesterone hormone. Progesterone changes the lining such that it is able to provide a nutritious and friendly environment for the embryo. It is the release of progesterone that triggers the opening of the 'implantation window' which is a limited time period during which the endometrium will be in a state of acceptance of an embryo.

Embryo-Endometrium interaction
The interaction of the embryo and the tissue lining the womb cavity is very complex and the successful conclusion depends on the presence of a 'good' embryo, a 'receptive' endometrium and their synchronization.

The embryo first finds a location in a specific area of the endometrium. The cross-talk between the embryo and the endometrium is the key to the initiation of implantation. The embryo snuggles close to this point (called apposition). Then the embryo sticks to the cells of the endometrium (called adhesion). The hatched blastocyst then causes destruction of the surface cells and invades the deeper levels of the endometrium. The cells that form the early placenta invade into the blood vessels that will provide nutrition to the growing pregnancy.

At the time that the next period is expected, some women may get minimal to small amount of bleeding. This is called implantation bleeding. Many (but not all) women also get tenderness in the breast or sore nipples when they become pregnant. However this is not a constant feature and the absence of these symptoms should not cause concern.

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