Protein aggregate formations are essential processes to regulate biochemical networks in the cell, while anomalously formed aggregates such as amyloid fibrils cause serious neuronal diseases. It has been discussed for a quarter century that protein crowding milieus, such as micelle-like aggregates, promote the formation of growth nuclei, fibril-growth competent aggregates which trigger rapid growth of pathogenic amyloid fibrils, but the mechanisms are still elusive, in particular at microscopic level. In this study, we examined the long-standing problem by employing atomistic molecular dynamics simulations for amyloid &b(1-42) (A&b42), the paradigmatic amyloid-forming peptide. First, we constructed an atomistic model of A&b42 growth nuclei in A&b42 aggregate milieu, the pentameric A&b42 protomer dimer surrounded by 40 A&b42 monomers. Next, we simulated A&b42 monomer dissociation from the A&b42 growth nuclei and examined the effect of A&b42 aggregate milieu on the process. A&b42 aggregates spatially restrict A&b42 monomer dissociation pathways, while such spatial restriction itself does not significantly suppress A&b42 monomer dissociation from the growth nuclei. Rather, A&b42 aggregate milieus thermodynamically stabilize an A&b42 monomer binding to the growth edge by making atomic contacts with the monomer and contributes to stable formation of growth nuclei. A part of the aggregate milieu anchors dissociating monomer to the remaining part of growth nuclei, suggesting cooperative suppression of A&b42 monomer dissociation from A&b42 growth nuclei. Since the A&b42 aggregate milieu does not take a micelle-like configuration, we here discuss a new mechanism for stable formation of A&b42 growth nuclei in the presence of aggregate milieu.