axoneme and basal body
/ AK-soh-neem and BAY-zul BOD-ee /
If a cilium is like a tiny oar, then the axoneme is the wooden shaft inside that lets the oar bend and beat, and the basal body is the oarlock bolted to the boat that holds it in place. These two structures are the internal skeleton and the anchor of every eukaryotic cilium and flagellum.
The axoneme is the bundle of microtubules running the length of a cilium or flagellum, almost always in a nine-plus-two pattern: nine pairs (doublets) of microtubules arranged in a ring around two single central microtubules. Motor proteins called dynein arms reach from one doublet to the next; when they walk, neighboring doublets try to slide past each other, but because they are cross-linked, the sliding is converted into a controlled bend. The basal body sits at the base, just under the cell membrane. It is essentially a centriole repurposed as a foundation: the same nine-fold barrel, which acts as a template that grows the axoneme and locks it to the cell.
Understanding this anatomy explains a whole class of human disorders. In primary ciliary dyskinesia, faulty dynein arms leave cilia unable to beat, causing chronic lung infections, infertility, and sometimes mirror-image organ placement (because cilia help set left-right body asymmetry in the embryo). The nine-plus-two axoneme is one of the most conserved structures in biology, found in organisms from algae to humans, which is why studying a pond alga has taught us a great deal about our own airways.
People with primary ciliary dyskinesia have axonemes missing their dynein arms, so their cilia cannot beat — and about half also have their heart and other organs flipped to the wrong side.
A single missing motor part inside the axoneme can ripple out into whole-body consequences.
The basal body is a centriole reused as an anchor, so a cell must lend a centriole to grow a cilium — one reason most cells cannot both divide and run a primary cilium at the same time.