Final answer:
Microglia are not found in loose connective tissue; they belong to the CNS where they serve as active immune defenders. Cells like fibroblasts, plasma cells, and histiocytes are typical in loose connective tissue.
Step-by-step explanation:
The cell type that is not routinely found in loose connective tissue is b. Microglia. While fibroblasts, histiocytes, and plasma cells are all commonly found in connective tissue proper, microglia are specialized cells that are found in the central nervous system (CNS) where they act as the main form of active immune defense. They are not typically present in loose connective tissue.
Cells like fibroblasts produce a viscous ground substance forming the extracellular matrix of connective tissue proper, while plasma cells and other phagocytic cells move in and out in response to immune signals.