The endometrium is the mucous membrane that internally lines the uterus. It is a very vascularised layer which main function is to receive and sustain the embryo at blastocyst stage.
The process known as implantation is a complex one where several factors play a role and a synchronisation between the embryo and the endometrium must take place for this process to be possible.
If the egg is not fertilized or there is no embryo implantation in the uterus, the uterine wall shreds (endometrium) at the end of the menstrual cycle, thus producing what is known as period or menstruation.
In order for the endometrium to be able to carry out its functions, it must be subject to morphological and functional alterations associated with the secretion of sexual hormones by the ovaries; these hormones are the oestrogen and the progesterone.
Endometrium growth phases
The development of the endometrium has two main stages:
- Proliferation phase: this phase is influenced by the oestrogen, and it starts from the very first day of the cycle, that is to say, from the menstruation to the ovulation. It is at this stage that the endometrium keeps on increasing its thickness. At this time two layers, one of which one is superficial while the other is deeper, are distinguishable. By the end of this phase, the endometrium can measure up to 6-10mm.
- Secreting phase: the development continues under the influence of the progesterone. One of progesterone’s main functions is to mature the endometrium. At this stage, the endometrium glands increase in size and start secreting mucus and a nutritional substance, which is a rich source of glycogen. Glycogen serves to prepare the endometrium in the event of implantation.
If neither fertilisation nor implantation has happened by the end of the secreting phase, the shedding of the uterine wall is produced 26 to 28 after the start of the cycle and the woman has her period. Simultaneously, and following the process already described, a new endometrium is being regenerated for the next ovulation.
For implantation to occur, the endometrium must be receptive. The endometrial receptivity phase lasts about 4 days, and is known as implantation window. At this point, both the embryo at blastocyst stage and the endometrium have to interact and synchronise themselves.
Endometrium thickness and implantation potential
A successful implantation not only depends on the hormonal preparation of the endometrium, but also on the existence of a reciprocal dialogue between the blastocyst tissues and the mucous membrane, where several growth factors and cytokines are decesive.
If fertilization of the egg and implantation of the embryo take place, the endometrium becomes a more specialized endometrium called decidua. The decidua will give place to the placenta and will be involved in the process of gas and nutrient exchange between the mother and the embryo.
There has been many different studies about what the optimal endometrium thickness for the implantation to be completed should be. Within assisted reproduction, and given these results, it is advisable not to make transfers below 7 mm, since it may lead to implantation failures. However, it is difficult to evaluate what the adequate thicknesses, but it is considered that if it is equal to or higher than 8 mm, it would be appropriate for the implantation to take effect.
Endometrial alterations: endometriosis
Endometriosis is a disorder frequently linked to the endometrium. (10-20%).
Endometriosis is defined as the endometrial growth outside the uterine cavity, e.g. the ovaries, the Fallopian tubes, the pelvic cavity or even the bladder.
The most common symptom of this disorder is abdominal and lower pack or pelvis pain, which ranges from mild to very severe.