Morphogenesis by Orientated Cell Division - Mechanisms of Morphogenesis - page 289

FIGURE 23.3 The increase in number, and change in orientation, of mitoses in rat endometrium when treated
with the oestradiol. The diagram is based on micrographs in Gunin 6 and Gunin et al. 7
the embryo 5 ( Figure 23.2 ). This has a profound effect on the cells, for only those that leave the
plane of the epithelium to form the inner cell mass will contribute directly to the mouse (the
epithelium itself forms the placenta).
Changes in the orientation of cell division can also affect the morphogenesis of mature
epithelia. The morphology of the lining of mammalian uteri is sensitive to hormones, partic-
ularly oestrogens and progesterones, which together prepare the uterus for implantation of
an embryo. In the absence of these hormones, the endometrium (ureterine lining) is thin but it
thickens and acquires more layers of cells following treatment with oestrogen. In ovariecto-
mized rats, which have extremely low levels of oestrogens, the few mitoses that are present
are orientated so that the cleavage plane is parallel to the basement membrane and daughter
cells remain in the same layer ( Figure 23.3 ). If those rats are treated with oestradiol, however,
up to 40 percent of mitoses are orientated so that the cleavage plane is parallel to the base-
ment membrane and one of the daughter cells is therefore pushed upwards to become
a new layer. 6 The molecular connection between hormones and mitotic orientation has not
been established in this system, but this effect of oestrogen is specific to uterine epithelium
and is not seen in irrelevant epithelia such as those of the gut.
The molecular mechanisms that control the positioning of the cleavage plane are still under
investigation, but in the systems that have been studied so far it seems that the position and
FIGURE 23.4 Nomenclature of spindle mitotic apparatus used in this chapter.