How tissues self-construct

April 5, 2011 § Leave a comment

The process of development is an astounding journey from simplicity to complexity.  You start with a single cell, the fertilized egg, and you end up with a complete multicellular organism, made up of tissues that self-organize from many individual cells of different types.  The question of how cells know who to be and where to go has many layers to it, starting with the question of how you lay down the basic body plan (head here, tail there, which side is left and where does the heart go?) and continuing on down to microscopic structures, with questions such as how and where to form the small tubes that will allow blood to permeate through apparently solid tissues.  This kind of self-organizing behavior is deeply interesting to robotics researchers (who would love to copy it) and tissue engineers (who would like to manipulate it).

A recent paper (Parsa et al. 2011. Uncovering the behaviors of individual cells within a multicellular microvascular community.  PNAS doi:10.1073/pnas.1007508108) takes a close look at self-organization on the micro level.  It turns out that if you take human endothelial cells and put them in a soft gel, they will spontaneously move around and form small tubes.  Parsa et al. tracked the behavior and morphology of individual cells from the moment they were seeded into the gel to the point when they have formed a connected network that will eventually turn into capillary-like structures.  This wasn’t an easy task: the cells move in three dimensions in the gel, the gel itself can shrink over time, and each cell is making many contacts with other cells; in many cases the cells are literally crawling over each other. To help track individual cells, they made a mixed population of cells that were labeled with 6 different combinations of fluorescent dyes, so that they had a good chance of being able to distinguish two neighboring cells using color.  And to shape the gel in a way that gives reasonable optical imaging they designed a PDMS mold with a removable cap that was used to flatten the gel’s top surface.

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Teflon bacteria

January 25, 2011 § Leave a comment

This post was chosen as an Editor's Selection for
Over the last 20 years or so we’ve been realizing that even bacteria don’t always live alone.  Instead, individual cells often stick to each other to make a biofilm, secreting a substance colloquially known as slime (also known to biologists as extracellular matrix).  Slime sounds wet, or sticky; but new work from Joanna Aizenberg’s lab (Epstein et al. 2011 Bacterial biofilm shows persistent resistance to liquid wetting and gas penetration.  PNAS doi: 10.1073/pnas.1011033108) now shows that biofilms have the least wettable surfaces known on earth: they’re considerably more resistant to wetting than Teflon.

How do you quantitate wettability?  Wettability is a function both of the liquid and of the surface.  What you do is take a drop of the liquid, put it on the surface, and measure the angle of contact between the drop and the surface.  If the surface is highly wettable by that liquid, the drop spreads and the angle is low — less than 90°.  If not, i.e. if it’s energetically unfavorable for that particular liquid to spread on that particular surface, the drop stays round and the angle is large.  The theoretical limit of non-wettability is a contact angle of 180°.

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News on transgenic silks

October 1, 2010 § Leave a comment

This just in from Oscillator:

There was some big news yesterday in transgenic silk from Notre Dame and the University of Wyoming, where scientists have genetically engineered silkworms to produce silk that is a mixture of spider silk and the regular silkworm stuff.

Now read on

see also yesterday’s post for a recently discovered extra-extra-strong spider silk.

Spider silk: the new Kevlar?

September 30, 2010 § Leave a comment

A recent article in PLoS One (Agnarsson et al. 2010. Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider. PLoS One 5 e11234 doi:10.1371/journal.pone.0011234) claims to have discovered the toughest known biomaterial: silk from “Darwin’s bark spider”, Caerostris darwini.  The reason they looked at the properties of silk from this particular spider was the extraordinary size and shape of the webs it builds.  C. darwini likes to build its webs across streams, rivers and even small lakes, and this means that the lines supporting the webs have to be long enough to bridge from one bank to another.  The authors measured bridgelines 10-14 meters long, and webs of 2.8 m(2).  [How the spider gets across the river in the first place is an area of active investigation.]

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