Timely mitosis is critically important for the early embryo development. It is regulated by the activity of the ubiquitously conserved CDK1 kinase. The dynamics of CDK1 activation must be precisely controlled to assure physiologic and timely entry into mitosis. Recently, a known S-phase regulator CDC6 emerged as a key player in the mitotic CDK1 activation cascade in early embryonic divisions, operating together with Xic1 as a CDK1 inhibitor upstream of the Aurora A- and PLK1 kinases, both CDK1 activators. Herein we review the molecular mechanisms that underlie the control of the mitotic timing, with special emphasis on how CDC6/Xic1 function impacts CDK1 regulatory network. We focus on the presence of two independent mechanisms inhibiting the dynamics of CDK1 activation: Wee1/Myt1- and CDC6/Xic1-dependent, and how they cooperate with CDK1 activating mechanisms. As a result we propose a comprehensive model integrating CDC6/Xic1-dependent inhibition into the CDK1-activation cascade. The physiological dynamics of CDK1 activation appear to be tuned by the system of multiple inhibitors and activators and their integrated modulation ensures concomitantly both the robustness and certain flexibility of the control of this process. Identification of multiplied activators and inhibitors of CDK1 activation upon M-phase entry allows a better understanding of why cells divide at a specific time, and how the pathways involved in the timely regulation of the cell division are all integrated to precisely tune the control of mitotic events.

Cadherins are a group of membrane proteins responsible for cell adhesion. They are crucial for cell sorting and recognition during the morphogenesis, but also play many other roles such as assuring tissue integrity and resistance to stretching, mechanotransduction, cell signaling, regulation of cell proliferation, apoptosis, survival, carcinogenesis, etc. Within the cadherin superfamily, the E- and N-cadherin have been especially well studied. They are involved in many aspects of sexual development and reproduction, such as germline development and gametogenesis, gonad development and functioning, and fertilization. E-cadherin is expressed in the primordial germ cells, (PGCs) and also participates in PGC migration to the developing gonads where they become enclosed by the N-cadherin-expressing somatic cells. The differential expression of cadherins is also responsible for the establishment of the testis or ovary structure. In the adult testes, the N-cadherin is responsible for the integrity of the seminiferous epithelium, regulation of sperm production, and the establishment of the blood-testis barrier. Sex hormones regulate the expression and turnover of N-cadherin influencing the course of spermatogenesis. In the adult ovaries, E- and N-cadherin assure the integrity of ovarian follicles and the formation of corpora lutea. Cadherins are expressed in the mature gametes, and facilitate the capacitation of sperm in the female reproductive tract, and gamete contact during fertilization. The germ cells and accompanying somatic cells express a series of different cadherins, however, their role in gonads and reproduction is still unknown. In this review, we show what is known and unknown about the role of cadherins in the germline and gonad development, and suggest the topics for future research.