What is considered the oldest of all stains used in histopathology?

Iodine is considered one of the oldest stains used in histopathology due to its historical significance and simplicity of preparation and application. It has been utilized since the early days of microscopy, particularly for highlighting carbohydrates and some cellular structures. Iodine can form complexes with starches, which was particularly important in the luminal context of early histological examinations.

While other stains such as Gram stain, methylene blue, and hematoxylin are indeed important in histology, iodine’s use predates these techniques, making it a foundational stain in the development of histopathological practices. Its application in staining tissues helped pave the way for more advanced staining methods that are commonly used today.