A 42-year-old woman presents with a 2-year history of progressive fatigue, weight gain, and cold intolerance. Examination reveals a firm, rubbery, non-tender thyroid enlargement. Serum TSH is elevated at 8.2 mIU/L with low-normal free T4. Anti-TPO antibodies are positive. A thyroid biopsy is performed showing diffuse lymphocytic infiltrate with germinal center formation and atrophic follicles. The cells marked **C** in the diagram demonstrate abundant granular eosinophilic cytoplasm with numerous mitochondria and pyknotic nuclei. What is the pathological significance of these cells marked **C** in Hashimoto thyroiditis?