Blastocyst implantation into the uterine endometrium establishes early pregnancy. This event is regulated by blastocyst‐ and/or endometrium‐derived molecular factors which include hormones, growth factors, cell adhesion molecules, cytokines and proteases. Their coordinated expression and function are critical for a viable pregnancy. A rate‐limiting event that immediately precedes implantation is the hatching of blastocyst. Ironically, blastocyst hatching is tacitly linked to peri‐implantation events, although it is a distinct developmental phenomenon. The exact molecular network regulating hatching is still unclear. A number of implantation‐associated molecular factors are expressed in the pre‐implanting blastocyst. Among others, cytokines, expressed by peri‐implantation blastocysts, are thought to be important for hatching, making blastocysts implantation competent. Pro‐inflammatory (IL‐6, LIF, GM‐CSF) and anti‐inflammatory (IL‐11, CSF‐1) cytokines improve hatching rates; they modulate proteases (MMPs, tPAs, cathepsins and ISP1). However, functional involvement of cytokines and their specific mediation of hatching‐associated proteases are unclear. There is a need to understand mechanistic roles of cytokines and proteases in blastocyst hatching. This review will assess the available knowledge on blastocyst‐derived pro‐inflammatory and anti‐inflammatory cytokines and their role in potentially regulating blastocyst hatching. They have implications in our understanding of early embryonic loss and infertility in mammals, including humans.

Mammalian blastocyst hatching is critically an indispensable process for successful implantation. One of the major challenges in IVF clinics is to achieve superior embryonic development with intrinsically potent hatching-competent blastocyst. However, the molecular regulation of hatching phenomenon is poorly understood. In this study, we examined the expression and function of one of the cytokines, IL-1β during blastocyst hatching in the mouse. In particular, the expression of IL-1β (Interleukin-1β), IL-1ra (Interleukin-1 receptor antagonist) and their functional receptor IL-1rt1 (Interleukin 1 receptor type-1) in morulae, zona intact- and hatched- blastocysts was studied. Supplementation of IL-1β to cultured embryos accelerated blastocyst development with improved hatching (treated: 89.6 ± 3.6% vs untreated: 65.4 ± 4.1%). When embryos were treated with IL-1ra, blastocyst hatching was decreased (treated: 28.8 ± 3.1% vs untreated: 67.5 ± 3.8%). Moreover, IL-1β and IL-1ra influenced the expression of hatching enzymes viz., implantation serine proteases (ISP 1 and ISP 2). While IL-1β increased the embryonic mRNA expression of ISPs (ISP1: 2-4; ISP2: 9-11 fold), IL-1ra decreased expression. The protein localization studies revealed increased nuclear presence predominantly of ISP 2 in IL-1β treated blastocysts. This is the first report to show the functional significance of embryonic IL-1β in regulating hatching-associated proteases, particularly ISP2. These findings have implications in our understanding of molecular regulation of blastocyst hatching and implantation failure in other species including humans.

Recent studies show that pre-follicular mouse oogenesis takes place in germline cysts, highly conserved groups of oogonial cells connected by intercellular bridges that develop as nurse cells as well as an oocyte. Long studied in Drosophila and insect gametogenesis, female germline cysts acquire cytoskeletal polarity and traffic centrosomes and organelles between nurse cells and the oocyte to form the Balbiani body, a conserved marker of polarity. Mouse oocyte development and nurse cell dumping are supported by dynamic, cell-specific programs of germline gene expression. High levels of perinatal germ cell death in this species primarily result from programmed nurse cell turnover after transfer rather than defective oocyte production. The striking evolutionary conservation of early oogenesis mechanisms between distant animal groups strongly suggests that gametogenesis and early embryonic development in vertebrates and invertebrates share even more in common than currently believed.

In mammals, the phenomenon of blastocyst hatching is an essential prerequisite for successful implantation. Blastocyst hatching is regulated by various molecules. Of them, cytokines, expressed by preimplantation embryos, are thought to be functionally important in blastocyst development and hatching, but their mechanistic roles are not clearly understood. Here, we examined the involvement of two cytokines, namely, interleukin‐1β (IL‐1β) and its natural antagonist, IL‐1ra, in blastocyst hatching in the golden hamster. Blastocysts expressed both cytokines and their receptor, IL‐1rt1. Supplementation of IL‐1β to cultured eight‐cell embryos improved blastocyst hatching (84.1% ± 4.2% vs. 66.6% ± 6.8%; treated vs. control). This improvement was diminished by IL‐1ra treatment (23.6% ± 12.9% vs. 76.4% ± 12.9%; treated vs. control). Interestingly, IL‐1β‐treated embryos showed increased messenger RNA expression of zonalytic proteases, that is, cathepsin‐L and ‐B by 1.9 ± 0.5‐ and 3.5 ± 0.1‐folds, respectively. This was accompanied by their increased enzyme activities; cathepsin‐L by 2.8 ± 0.7 fold and ‐B by 2.3 ± 0.7‐fold. Strikingly, proteases and IL‐1β were intensely colocalized to trophectodermal projections of hatching blastocysts. This is the first report to show the involvement of embryonic IL‐1β in regulating hatching‐associated proteases required for blastocyst hatching.