Cell classification: Intermediate, superficial and anucleate squamous cells of the vaginal epithelium

Types of more mature cells

As the estrous cycle progresses, the immature parabasals are replaced by more mature cells: the intermediate cells (intermediates) (generic image from ThaiLabOnline for the moment),




the superficial cells (superficials) (generic image from bphealthcare.com for the moment)



and the anucleate squamous cells (anucleates). These are rarely seen anymore in humans, except in cases of infectious inflammation, or the wearing of pessaries, which deserves its own topic. I couldn't find any examples on the Web for that reason, so will illustrate anucleates when my graphics are working again.

All of the attributes introduced for the parabasals undergo the following regular changes as they proceed from parabasals to intermediates to superficials to anucleates:

• their round or oval shape becomes less round and more polygonal or irregularly shaped;
  
  
• their large nuclei become smaller and smaller, in some cases dying or disappearing altogether, so their high nucleus to cytoplasm ratio becomes progressively less;
  
  
• their thick cytoplasm becomes thinner;
  
  
• as their cytoplasm becomes thinner, the dark stain (high saturation) of parabasals is replaced by lighter and less intense color in the more mature types of cells.
  
  

So we can see a progression of attributes as the cells move through the estrous cycle.

Both parabasals and superficials have distinct attributes as previously described. Intermediates are all those cells that fall in between. Once a cell loses its round ovoid shape (increasing the cytoplasm-to-nuclear ratio), it should be classified intermediate. Once the nucleus degrades/compresses, thus becoming pyknotic, it should be classified superficial.

Superficials and intermediates are easily distinguished from anucleates because of the absence of nuclei in anucleates.

Distinguishing between intermediates and superficials is the most subtle distinction to learn to make, since the primary difference between them is the pyknotic nucleus of the superficial. Since the nucleus is already degenerating in the intermediate, learning to determine when the nucleus is actually pyknotic (as opposed to simply becoming pyknotic) is an important skill to master.

Up to this point, all of the examples of cell types have been in isolation. Vaginal cytology slides, however, typically contain a mixture of cell types. The proportion of each cell type is the maturation index, and the value of the maturation index---i.e., what type(s) of cells are in the majority---indicates what stage of estrus the animal is in.