الفهرس 
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Abstract
SUMMARY
Implantation is the process by which an embryo attaches to uterine wall and penetrate first the epithelium and circulatory system of the mother to form placenta. This process require a receptive endometrium as implantation can’t occur except during restricted period called implantation window, a normal embryo at blastocyst stage and cross communication between these immunologically and genetically different organism. It composed of 3 phases:
1) Apposition.
2) Adhesion.
3) Invasion which is the longest step in implantation process and can be divided into early invasion and lat invasion.
The surface of the endometrial epithelium plays an important role in the process of implantation. The endometrium undergoes a series of histological and ultrastructural changes under the influence of estrogen and progesterone.
The surface morphology of secretory endometrium consists of two types of cells (A) ciliated cells doesn’t change during menstrual cycle, (B) secretory cells which bear microvilli and undergo hormone dependent changes.
• Pinopods: which is projection of whole apical surface of epithelial cells to uterine cavity, its appearance is related to serum progesterone concentration. They first appear on luteal day 5 corresponding to mid luteal phase increase progesterone.
• Apoptotic cell death: locally regulated apoptosis is vital for successful implantation.
• Tight junction of epithelial cells: junctions undergo a significant decrease in between days 13 and 23 of menstrual cycle which is important for implantation.
Synchronized development of healthy blastocyst and receptive endometrium is vital for implantation. Hence factors affecting implantation can be subdivided into:
(1) Patient factors: like age, Hox genes, previous pregnancies, and body mass index.
(2) Embryo: like number transferred and laboratory manipulation.
(3) Endometrial: like histology, anti-adhesion molecules, cytokines, adhesion molecules and growth factors
(4) Embryo / endometrial interface: like cytokines, matrix metalloproteinases, and apoptosis control.
The initiation and progression of implantation is the result of coordinated integration of variant signaling pathway between the embryo and the uterus. Since the process implantation is considered a proinflanimatory reaction, it is accompanied with an increased endometrial vascular permeability at the sites of blastocysts, it is envisioned that prostaglandins (PGs), by virtue of their vasoactive properties, are involved in implantation and deciduallzation (McMaster et al., 1993). In addition, an emerging concept in implantation is the role of endocannabinoids, a group of lipid mediators that are ligands for the cannabinoid receptors (Paria and Dey, 2000).
Ovarian steroid hormone exert their effect on the endometrium after they have bound to their specific nuclear receptors in endometrial gland and stroma. The expression of endometrial steroid receptors varies during the menstrual cycle, this variability indicates that they may play a role in mediating the endometrial response to steroid from corpus luteum around the time of implanation.
Also, the researcher found that the uterine concentration of L.H receptors and their occupancy by L.H increase in pre-implantation period.
Recent evidence suggests that preparation of the endometrium for implantation is not merely a question of adequate hormonal stimulation but that implantation also depends on the interaction between the blastocyst and the endometrium and is mediated by cytokines, growth factors, and adhesion molecules, which are produced and secreted by the endometrium and the blastocyst (Cross et al., 1994 and Giudice, 1999). Several biochemical factors of presumed importance to implantation have recently been discovered, and new ones are constantly being revealed. We have selected some of the most important factors for reviewing.
Also, we discus imaging assessment of implantation by using:
- Ultrasonography.
- 3D ultrasonography.
- Doppler ultrasound of uterine arteries.
- MRI.
Lastly, there are four strategies that can be utilized for improving endometrial receptivity:
1) To develop ovarian stimulation protocols that cause a minimum reduction in endometrial receptivity or may even increase it.
2) To avoid the endometrium during stimulated cycles altogether by freezing the embryos and replacing them in subsequent natural cycles.
3) To improve uterine vascularization.
4) To treat the pathological conditions.