|
|
|
|
|
|
|
|
|||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | |||||
|
Fig. 3. Elongation, pseudostratification and cleavage precision of APs. The apical plasma membrane and corresponding cortical domains are represented by blue lines, and the basolateral plasma membrane and corresponding cortical domains are represented by red lines. Interphase nuclei are shown in grey and representative sister chromatids are in dark blue. Black rectangles represent junctional complexes and yellow dots indicate centrosomes or mitotic-spindle poles. For clarity, only the astral microtubules of the mitotic spindle are depicted (black lines). (A) Cell elongation enables more progenitors to inhabit each unit of epithelial surface area. (Ai) An example of a cuboidal epithelial progenitor generating two similar progenitors, which require twice the area of apical surface (bars). (Aii) An example of a cuboidal epithelial progenitor generating two elongated progenitors, which can be accommodated without an increase in apical surface area. (B) Pseudo-stratification enables more apical mitoses to occur per unit of ventricular space. (Bi) In a cuboidal epithelium, progenitors in interphase (G1, S, G2) and in mitosis (M) occupy approximately the same space. Hence, as the duration of M-phase typically constitutes only a small fraction of the total length of the cell cycle, only a minor proportion of the apical surface area is used for progenitors that are engaged in mitosis. (Bii) In a pseudostratified epithelium, interphase nuclei translocate away from the apical surface, so a much greater proportion of the space at the apical surface can be filled by progenitors that are engaged in mitosis. Hence, the proliferative potential of a pseudostratified epithelium increases until the progenitor interphase nuclei that can be accommodated in the cylindrical space basal to an apical mitotic progenitor have reached a number that is equal to the length of the cell cycle divided by the duration of mitosis. According to Smart (Smart, 1972a; Smart, 1972b), in the mammalian neuroepithelium, APs in mitosis occupy three times more space beneath the ventricular surface than AP interphase nuclei. Thus, as an example, if the cell-cycle length of APs is 12 hours and mitosis takes 30 minutes, proliferation is maximized when 24 (12/0.5) AP interphase nuclei reside basal to an apical mitotic progenitor, which is equivalent to a pseudostratified neuroepithelium with eight (24/3) nuclear layers basal to the apical mitotic layer. (C) In elongated apical progenitors, the relative frequency of symmetric and asymmetric division can be controlled by the regulation of cleavage precision rather than by mitotic-spindle rotation. (Ci) In a cuboidal epithelial progenitor with apicobasal polarity, symmetric cell division (the inheritance of apical plasma membrane and junctional complexes by both daughter cells) can result from both a perfectly vertical and a slightly non-vertical cleavage plane (top and middle, respectively; broken lines). Asymmetric cell division is typically achieved by a 90° rotation of the mitotic spindle, which leads to a horizontal cleavage plane (bottom). (Cii) With increasing pseudostratification and AP elongation, the apical domain shrinks correspondingly. Consequently, asymmetric cell division can result from a slight deviation of the cleavage plane away from the vertical apicobasal axis (bottom) (Huttner and Brand, 1997), and symmetric cell division requires a cleavage-precision machinery that ensures ingression of the cleavage furrow precisely along the apicobasal axis (top) (Fish et al., 2006).
|
