Microtubule alignments in osteoblasts are strikingly correlated with the arrangement
of osseous circuli on Brachydanio and Hyphessobrycon scales. There is the
intriguing possibility that the groupings of microtubules that run closely alongside
circuli form part of a mechanism that specifies and maintains the orientation of circuli
and determines where these ridge-shaped th^kenings will be secreted onto the outer
surface of an osseous layer. Microtubule portions that run at right angles to circuli
often span the inter-circulus spaces. Are these microtubules used as intracellular
rulers to measure out inter-circulus spacings? Admittedly, well-defined patterns of
microtubule alignment were not detected in Salmo osteoblasts. However, the
possibility that microtubule alignment does also occur in these osteoblasts, but is a
transient phenomenon that is only associated with the initial establishment of circuli,
cannot be discounted.