40 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
Figure 2.12: (Scanning electron micrograph.) The ciliateDidinium(2285×magnification), a single-cell animal
found in still fresh water. Didinium’s “mouth” is at the end of a small projection, surrounded by a ring of cilia.
Chapter 5 will discuss how cilia drive fluid flow. [From (Shih & Kessel, 1982).] [Copyrighted figure; permission
pending.]
atrail of protein which then forms connective tissue. Other crawling cells include the “osteoblasts,”
which lay down mineral material to make bones, and the “Schwann cells” and “oligodendroglia,”
which wrap themselves around nerve axons, creating layers of electrical insulation.
The fibroblast shown in the figure has many protrusions on its leading edge. Some of these
protrusions, called “filopodia,” are fingerlike, about 0. 1 μmin diameter and several micrometers
long. Others, the “lamellipodia,” are sheetlike. Other sorts of cells, such asamœba,push out
thicker protrusions called “pseudopodia.” All of these protrusions form and retract rapidly, for
example searching for other cells with appropriate signaling molecules on their surfaces. When
such a surface is found, the crawling cell adheres to it, pulling the rest of its body along. In this
way,cell crawling can lead to the construction of complex multicellular tissues: Each cell searches
for a proper neighbor, then sticks to it.
Other specialized cells, such as the ones lining the intestine, may have hundreds of tiny fingerlike
projections, called “microvilli,” to increase their surface area for fast absorption of food. Other cells
have similarly shaped projections (cilia and eukaryotic flagella) that actively beat back and forth
(Figure 2.12). For example, the protozoanParameciumhas cilia that propel it through fluid;
conversely, the stationary cells lining your lungs wash themselves by constantly transporting a layer
of mucus upward. Chapter 5 will discuss this process. Figure 2.12 shows yet another use for cilia:
These ones bring food particles to the mouth of a single-celled animal.
Another class of small anatomical features includes the fine structure of the dendrite on a neuron.
The actual synapse frequently involves not the main body of the dendrite, but a tinydendritic spine
projecting from it (fine bumps in Figure 2.22).