Intestine in the lung
[moderator's note: This gets my vote for ickiest title so
far. Sounds like the aftermath of a (American) football
game. - JAH]
Jonathan MW Slac
Abstract
The phenomenon of metaplasia, in which one tissue type is
converted into another, is beginning to be explained in
molecular terms. The transformation of lung to intestinal
tissue has not previously been described, but it is now
reported that it can be brought about by prolonged Wnt
signaling in late development.
The transformation of one tissue type to another, a process
called metaplasia, is a fascinating aspect of both
pathology and developmental biology. Metaplasias represent
the homeotic transformations of vertebrates, comparable to
the homeotic mutations in Drosophila that can transform
antenna to leg or wing to haltere [1]. Of particular
developmental interest are cases where tissues are
transformed to other types that normally occur elsewhere in
the body, as for example in the formation of intestinal
tissue from the esophageal epithelium in the condition
known as Barrett's esophagus
[2].
Both the intestine and the esophagus arise from the endoderm
of land vertebrates, which starts out as a simple sheet, or
tube, of cells that becomes a series of epithelial tissue
types; these are, arranged roughly from cranial to caudal:
esophagus, trachea, bronchi, lungs, stomach, liver,
pancreas, small intestine, and colon. These epithelia arise
in different places because of distinct inductive signals
from the mesoderm that is associated with the endoderm in
different locations [3]. (The endoderm and mesoderm are the
inner and middle two, respectively, of the three original
cellular layers of the vertebrate embryo from which the
adult tissues arise.) The ultimate source of the positional
information in the mesoderm may be the pattern of expression
of Hox genes, which is set up in early development, but this
is not yet certain [4].
It has become a well-established principle of developmental
biology that the identity of a tissue type is specified by
the combination of transcription factors activated during
embryonic development. Only a small subset of transcription
factors is involved in this process, acting combinatorially
to form developmental 'codes' that correspond to particular
tissue types. If the genes encoding these factors are
turned on or off inappropriately then the code is changed
to that for a different tissue type and a homeotic
transformation results [5]. These genes are thus the ones
usually described as homeotic genes, or selector genes or
master regulator genes (depending on the organism of
interest). For the endoderm-derived epithelia, each
homeotic gene is normally active in only one region, so
there is a correspondence between epithelial tissue type
and body part, and the similarity between metaplasias and
homeosis in Drosophila is quite clear-cut.
Read the rest at the Journal of Biology
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Posted by Robert Karl Stonjek.