A 3-year-old boy from rural Maharashtra presents with café-au-lait spots on his trunk and extremities, along with multiple nodular swellings along the course of peripheral nerves. His mother reports that similar lesions appeared in infancy and have been slowly enlarging. On examination, he has axillary and inguinal freckling. Genetic testing confirms a mutation in the NF1 gene. Which of the following embryological derivatives is primarily affected in this condition?