Final answer:
A lipophilic primary message that can cross the plasma membrane to interact with cytosolic receptors is commonly a steroid hormone, which regulates gene expression by acting as a transcription factor.
Step-by-step explanation:
A lipophilic primary message that crosses the plasma membrane and interacts with a cytosolic receptor is typically a lipid-derived or steroid hormone. These hydrophobic signaling molecules can diffuse across the cell membrane due to their fat-soluble nature, allowing them to reach and bind with intracellular receptors located within the cytoplasm. Once inside, these receptors often function as transcription factors and have the ability to regulate gene expression by interacting with DNA in the nucleus.
These pathways of cell communication illustrate the role of hormones as first messengers. When such a hormone enters a target cell, it can lead to changes in the cell's activities by influencing the production of proteins. This regulatory mechanism is crucial for maintaining various physiological processes within the organism and can have lasting effects on the cell's function and structure.