Overview of mammalian vaginal anatomy and the estrous cycle
(technical problems with the illustrations for this post--sorry! I should have it fixed by Sunday morning. UPDATE: sometime Monday, instead--the DSL here and the firewall are fighting, and I need to get some real help, which means Monday.)
1. Cross-sectional view of vagina
These photographs show the physical cross-section of the vagina at two different stages of the estrous cycle. At the top of the photographs are the more superficial layers; at the bottom of the photographs are the deeper layers.
(generic image from Wisconsin here, until I can get my images to work)
Distinct kinds of cells that appear quite differently can be seen within these layers. These different kinds of cells will be discussed in more detail in these posts
The first photograph shows the vagina with its epithelial cell lining during the proestrus stage. The second photograph shows the vagina with its epithelial cell lining during the estrus stage. Note the layer of cornified (keratinized) cells which lines the vagina during the estrus stage.
Figure: cross-sectional view of vagina
2. Description of the estrous cycle
This diagram is a schematic after Schutte 1967, representing the same view of the physical vaginal cross-section shown in the previous photographs. Under the influence of estrogens, the cells proceed from the proliferation stage to the differentiation stage to the exfoliation stage. The cell types which are representative of each stage are magnified in projection, and shown along with their names and descriptions.
The correlation between the types of cells and the physical vaginal layer is shown on the left (deep <--> superficial), and the correlation between the types of cells and cell age is shown on the right (immature <--> mature).
Note that there are significant differences between Schutte's classification scheme (indicated in black) and the one we will follow (indicated in red). We distinguish anucleate squamous cells from superficials (with pyknotic nuclei), both of which Schutte classifies as superficials. In turn, he distinguished between large intermediate cells and small intermediate cells, but we do not---we put all intermediate cells in the same category.
Figure: Description of the estrous cycle
3. Estrous cycle across time
This schematic (after Schutte 1967) shows temporal changes in cell type across the estrous cycle. For clarity of presentation, only the proestrus, estrus, and metestrus stages are shown. The cells are shown in pink and blue, because color is not definitive for cell classification---the same type of cell can appear in different colors, depending on pH. Shape is a much more reliable criterion classification than is color.
The quantity of parabasal cells at the beginning of the proestrus stage yields to a mix of more intermediate cells, which in turn are joined by superficial cells and anucleate squamous cells as the stage progresses into estrus. As the estrus stage turns into the metestrus stage, the number of intermediates, superficials, and anucleate squamous cells decreases, and the number of parabasal cells increases again.
This schematic also shows the hormonal (estrogen) levels through the different stages. Estrogen peaks during the estrus stage, and then falls off sharply. Additionally, leukocytes (white blood cells) proliferate during the metestrus stage, and then decline during the proestrus and estrus stages.