Final answer:
Cells in a developing organism respond to molecular signals known as morphogens, which guide the developmental processes through gene expression and differentiation.
Step-by-step explanation:
During development, cells respond to molecular signals from other cells that are known as morphogens.
Cells communicate with each other through both direct contact and chemical signaling. These chemical signals orchestrate the expression of genes and ultimately shape the developmental processes. In multicellular organisms, such as humans, specific cells can release morphogens, which diffuse through tissues and provide positional information to guide cell fate decisions and tissue patterning. For instance, growth factors are a type of morphogen that can direct some cells in the ectoderm to form skin cells or neural crest cells, depending on their concentration and the cells' positions.
Hormones are another class of signaling molecules that can regulate gene expression. Unlike morphogens, which usually act locally, hormones can exert effects at a distance, targeting cells throughout the body to coordinate a response. These external molecular cues trigger internal changes within cells, including the rearrangement of chromatin and the expression of different sets of genes, thereby guiding the differentiation process during embryonic development.