The insulin-like growth factors (IGFs) are broadly distributed in the human conceptus and are thought to play a role in the growth and differentiation of tissues during development. The addition of IGF-I and IGF-II to early cleavage stage mouse embryos in vitro did not influence their growth rate to the blastocyst stage; they neither affected cell numbers nor cell doubling. The addition of basic fibroblast growth factor (bFGF) marginally increased blastocyst hatching, and in serum-free conditions the addition of IGF-I plus bFGF to postimplantation embryos partially compensated for the absence of serum. Using in situ hybridization it has been demonstrated that human blastocysts produced by in vitro fertilization show no IGF-II expression, thus bracketing the time of first accumulation of IGF-II mRNA to between 5 and 18 days postfertilization. The pattern of IGF-II expression shows specific age-related differences in different tissues. In the first trimester kidney, for example, expression is found in the cells of the metanephric blastema and this is dramatically reduced as the blastema differentiates. The reverse pattern of expression is also seen in that there is an increase in expression of IGF-II in the cytotrophoblast layer of the placenta with gestational age. IGF-II expression is most frequently, but not exclusively, associated with areas of high mitotic activity and it is possible that IGF-II functions in an autocrine/paracrine fashion during human development. Apolipoprotein B (apo B) is thought to be involved in mediating the transport of lipids to the growing human conceptus. Apo B expression is detected in the hepatocytes of the fetal liver, in the large endodermal cells of the secondary yolk sac, and in the lining epithelium of the intestine from 6-12 week post-fertilization human conceptuses. The fetal brain shows no detectable hybridization. The cellular distribution of apo B transcripts emphasizes the similar functions of these cells. Unlike the situation seen in the adult, immunoprecipitation experiments demonstrate that only apo B-100 is synthesized and secreted by the liver, yolk sac and intestine in the early stage of human development.