Dynamics in the stratified epithelium.

<p>A) Basal and parabasal cells can divide either asymmetrically (1 − <i>p</i><sub>1</sub> − <i>p</i><sub>2</sub> and 1 − <i>q</i><sub>1</sub> − <i>q</i><sub>2</sub> respectively) or symmetrically, which result in two daughter cells of either the same (<i>p</i><sub>1</sub> or <i>q</i><sub>1</sub>) or different phenotype as the mother cell (<i>p</i><sub>2</sub> or <i>q</i><sub>2</sub>). B) The squamous epithelium is abstracted into a basal, a parabasal, a mid-upper and a surface layer. Proliferation (<i>ρ</i>) and maturation (<i>ν</i>) rates determine the movement of cells up the layers. Cells die and are shed (<i>μ</i>). <i><b>Chlamydia trachomatis</b></i> (in green) infects the most superficial live cells underneath the mucus and surface dying cells. Once inside a cell, the elementary bodies (EB) change into reticulate bodies, which go through several rounds of replication, and then change back into EBs that are released upon cell death. <i><b>Human papillomaviruses</b></i> (in purple) must infect basal cells to establish an infection, thus usually requiring a microabrasion. The virus is non-lytic and replicates in host cells as they follow their natural life-cycle up the epithelium column. Progeny virions are released once the cell dies at the surface. Immune cells (in blue) enter the epithelium from the basal layer.</p>