Final answer:
During the Pre-B cell stage, B cells lose markers like TdT and gain others, including the expression of the IgM heavy chain, which signifies B cell development.
Step-by-step explanation:
During the Pre-B cell stage of B cell development, several changes take place, including alterations in the expression of certain surface markers. These markers are proteins that can be detected by specific antibodies and are used to identify and classify the different stages of B cell development. Typically, as the pre-B cell transitions from the pro-B cell stage, it loses expression of certain markers such as TdT (Terminal deoxynucleotidyl transferase) and gains others like the IgM heavy chain, which is part of the B cell receptor complex. It is through these changing markers that the maturation and readiness of B cells for further development into fully functional immune cells can be tracked.