Biomedical Engineering Reference
In-Depth Information
property, unique to animals, is the ability to form specialized cell junctions.
Specialized cell junctions account for the blastula stage of embryogenesis,
and also accounts for the development of coelomic cavities, a digestive tract,
and complex organs.
Unlike fungal cells (wrapped in chitin) and plant cells (wrapped in cellu-
lose) animal cells have no structural molecule in cell walls to constrain them
into any particular shape. With the exception of cells that produce intracellu-
lar bands of protein that force a spindle or elongated shape (such as actin,
myosin, or desmin), most animal cells are relatively floppy bags of proto-
plasm, somewhat like water-filled balloons. If you press two water-filled bal-
loons, of the same size, together, you will find that their surface of union is
a flat circle. If you crowd together, onto a flat surface, a monolayer of floppy
balloons, you will get a matrix of polygonal forms, wherein each straight
edge of the polygon is bounded by an edge of an adjacent balloon. It is easy
to see how a collection of spheres can, when pushed together, yield a
cuboidal, or polygonal cell matrix, simulating an organ.
If polygonal epithelial cells were simply squeezed spheres, what would
stop fluids from slipping through the spaces between the spheres? What
would keep tissues from falling apart, when the squeezing stopped? The
integrity of the polygonality of animal tissues is accomplished with two spe-
cialized structures that reside on the surface of animal cells: desmosomes
and gap junctions. These structures, unique to Class Animalia, effectively
zip together adjacent cells, creating a leak-proof continuum of epithelial
cells. All animals contain epithelial cells that line the external surface of the
animal (i.e., the skin), the gastrointestinal tract, and most of the internal
organs (e.g. liver, pancreas, salivary glands).
When we look at histologic sections of any organ under a microscope,
we typically see glands. Glands are round or tubular spaces lined by cuboidal
or polygonal cells and filled with fluid or excreted proteins or mucus. When
the cells that line glands are separated from one another with a digestive
enzyme and suspended as single cells, in an aqueous solution, they appear to
be spherical. Within glands, spherical cells transform into polygonal cells
when they are pressed together and zipped up by specialized junctions.
The blastula phase of animal embryos is accomplished with the help of
non-rigid cells connected by junctions. Fluids secreted by the cells accumu-
late in a central cavity, because the junctions are water-tight. An embryo,
with a fluid-filled center, is known as a blastula. Only animals are equipped
to produce a blastula. Presumably, the earliest animals, the gallertoids, were
lined by non-rigid epithelial cells, zipped together with tight junctions. These
junctions constrained the jelly-like fluid, produced by the epithelial cells, to
the center of the gallertoid.
In eumetazoans (all animals other than sponges and placazoans), the
gastrointestinal tract owes its existence to non-rigid epithelial cells with
intercellular junctions [89]. Likewise, complex tissues and organs are built
