Fig. 1.

Epigenetics changes in the endometrium throughout the menstrual cycle. The endometrium cycle is divided into the menstrual phase, the proliferative phase, and the secretory phase. The proliferative phase occurs from day 4 to day 14 of the cycle, according to the length of the 28-days menstrual cycle. This phase promotes the growth (proliferation) of endometrial glands and stromal cells (ESC); also, there is an increase in the depth of the spiral arteries that supply the endometrium. The secretory phase occurs after ovulation from day 14 to 28. During this phase, the endometrium’s proliferation is stopped as this tissue undergoes a series of transformations to achieve a receptive state for implantation in a process called decidualization. Decidualization is the transformation of ESC into decidual stromal cells (DSC) induced by progesterone. If no implantation occurs, constriction of the spiral arteries producing ischemia and cell death of the functional layer causes the endometrium to shed, and this phase is denominated menstrual phase. During the proliferative phase, there is an increase in the DNA methylation levels of hormone-responsive genes and the expression of DNMT3a, GCN5, and EZH2 genes. On the other hand, during the secretory phase, an increase in the DNA methylation levels of genes associated with transcription regulation, the protein content of HDAC-2, and the expression of H19 is observed. The global content of H3K9ac, H2AK5ac, H3K14ac and, H4K8ac is downregulated during the ovulation and menstrual phase. No changes have been reported for the global levels of H3K4me, H3K9me, and H3K27me. Endometrial epithelial cells (EEC)