Implantation in Human

Implantation in Humans:

• The blastocyst, formed after fertilization, is implanted or embedded into the uterine endometrium, usually on day 7 after fertilization.  
• By the end of the 10th day, the embryo is completely buried inside the endometrium. The embryo attaches itself to the uterine wall through its embryonic pole, and the trophoblast cells stick to the wall.  
• Rapid division of these cells results in the formation of two distinct layers: syncytiotrophoblast and cytotrophoblast.
• The outer layer, syncytiotrophoblast, is a protoplasm layer with numerous nuclei that invades the endometrium.  
• It ruptures endometrial cells, creating a burrow for embryo implantation. By the 10th day, the entire embryo is deeply embedded in the endometrium.
• The inner layer, cytotrophoblast, has defined membranes and retains cell boundaries.
• The blastocyst becomes embedded in the endometrium and the process is called as implantation.
• The endometrium enlarges and becomes more vascularized. The endometrium is referred as decidua. It is of three types:  

1. Decidua basalis:  Portion of endometrium between the embryo and the stratum basalis of the endometrium. It provides a large amount of glycogen and lipids for the developing embryo and later becomes the maternal part of the placenta. 
2. Decidua capsularis: Portion of the endometrium between the embryo and the uterine cavity.  
3. Decidua parietalis: It is the remaining non- involved part of the endometrium not concerned with the embryo.