The objective was to obtain basic information on the growth and development of early cattle embryos and their environment in the female reproductive tract. Such information is essential to address the problem of early embryo loss, to improve the quality of embryos produced in vitro and ultimately, to enable genome mapping results be placed in a functional context. The main focus was on signal transduction systems, the second messengers which mediate the action of growth factors essential for normal development. Protein content was measured to provide basic data on embryo growth. The energy metabolism of early embryos and the provision of nutrients in oviduct and uterine fluids were also examined since both are crucial for embryo survival.
There was a dramatic increase in embryo size and protein content; length increased by 500-fold and protein content by 2,600-fold between blastocyst formation (day 8) and elongation (day 16). The signal transduction system involving cyclic AMP (cAMP) and cyclic GMP (cGMP), was present, and capable of activation, from the 2-cell to the elongated stage, with a marked decrease in cGMP content between days 13 and 16. Surprisingly, embryos at these stages exported both cAMP and cGMP, possibly as a means of embryo-maternal signalling. Components of the system involving phosphatidylinositol (PI) were present and active and were affected by growth factors. Significantly, the uptake of inositol and its incorporation into phosphoinositides and inositol phosphates was dramatically increased at blastocyst formation and elongation. Epidermal Growth Factor (EGF) stimulated PI turnover in day 16 but not in day 14 embryos. The tyrosine kinase growth factor receptor system was present in 7 to 16 day-old embryos, with a shift in localisation from the apical, at day 13 and 14, to the basal, at day 15 and 16, sides of the plasma membrane of the embryo. The receptor for all members of the EGF family was expressed from day 13 -16. Intracellular calcium [Ca(2+)]I was localised in embryos but no effects of external growth factors were demonstrated, making a role for [Ca(2+)]I unlikely at these early stages. An ATP- stimulated [Ca(2+)]I signalling system was, however, demonstrated in bovine oviduct and uterine epithelia.
The results provide comprehensive, basic information, most of it for the first time, on the cellular mechanisms controlling early cattle embryo growth and development. Significant, qualitative and quantitative shifts in growth factor expression and signal transduction systems were apparent at the time of greatest embryo loss in vivo.
The first, comprehensive profile of energy metabolism in early cattle embryos was obtained, in terms of glucose, pyruvate, amino acid and oxygen consumption and lactate production. Blastocyst formation was associated with a dramatic increase in energy turnover.
This new information on protein content, signal transduction systems and embryo metabolism will allow major questions to be addressed, such as how environmental factors like nutrition, energy balance and metabolism affect embryonic biochemical and physiological mechanisms and in turn, embryo quality and/or embryo loss.
Funding SchemeCSC - Cost-sharing contracts
YO1 5DD York