Uterine endometrium is an extremely dynamic tissue, undergoing sequential developmental changes in preparation for implantation during each estrous cycle. The successful implantation of the blastocyst requires successful development of uterine endometrial receptivity. In many ways, cyclic endometrial development in the adult can be considered analogous to embryonic development. Many of the genes traditionally thought of as regulators of embryonic development are also used to regulate endometrial development. Hox genes, essential regulators of morphogenesis and tissue differentiation in the embryo, are also essential for endometrial development and for endometrial receptivity.
Hox genes encode evolutionarily conserved transcription factors, which are important regulators of embryonic morphogenesis and differentiation. In mammals, 39 Hox genes reside in four separate chromosomal linkage groups, designated Hoxa, Hoxb, Hoxc, and Hoxd, each of which has parallel and overlapping expression domains. Homologous members within the separate linkage groups are separated into 13 sets of paralogous genes, each having two to four members. A distinguishing feature of the Hox genes is colinearity; their linear arrangement along the chromosome from 5′ to 3′ parallels their nested and segmental order of expression along the anterior-posterior body axis of the embryo during development. Typically, paralogous HOX genes have similar or redundant functions and overlapping expression during embryonic development.