Final answer:
The cell type most abundant in the rough endoplasmic reticulum (RER) is the cell type that secretes proteins, such as liver cells.
Step-by-step explanation:
The most abundant cell type in the rough endoplasmic reticulum (RER) is the cell type that secretes proteins. RER, also known as rough ER, is characterized by the presence of ribosomes on its surface. These ribosomes are responsible for synthesizing proteins.
Cells that secrete proteins, such as liver cells, have a high concentration of RER due to the need for protein synthesis and modification before secretion. RER is involved in the folding and modification of proteins that will be released from the cell.
For example, liver cells produce and secrete enzymes, which require extensive protein modification in the RER. Therefore, the cell type most abundant in the rough ER is the cell type that secretes proteins.