A 3-year-old boy from rural Maharashtra presents with a 6-month history of progressive café-au-lait spots over his trunk and limbs, along with multiple firm subcutaneous nodules. On examination, he has 8 discrete hyperpigmented macules (each >5 mm), axillary freckling, and palpable neurofibromas. His parents deny any family history of similar lesions. Genetic testing confirms a heterozygous NF1 mutation. Which of the following embryologic structures is primarily affected in this condition?