Figure 2.1
Development of a mechanosensory bristle from a sensory organ precursor (SOP).
Compass (upper left) gives initial directions (A, anterior; P, posterior). Times (hours: minutes at 23°C) are for microchaete mitoses on the notum but are similar for other bristles [1447].

The SOP arises from an ordinary epithelial cell. It starts to divide (at ~16 h after pupariation) to form IIa and IIb. IIa's daughters will make a socket and shaft. IIb's daughters are IIIb and a glial cell. The glial cell is smaller and buds off basally in the manner of a CNS neuroblast division [1073, 1740]. IIIb divides to form a sheath cell and neuron. Some bristles have a thick hair ('bract') atop their sockets (inset), which is made by a clonally unrelated cell.

Each mitosis obeys stereotyped steps (dashed trapezoid) that comprise a modular subprogram: (1) Numb localizes to one side of the cell cortex (crescent), (2) segregates to one daughter, and (3) alters cell fate. Letting 1 and 0 signify Numb's presence or absence, each cell can acquire a unique code if it 'remembers' its former Numb states. Imaginary memory registers (underlined spaces) are shown for a few cells, with left-to-right order recording successively later states.

For such a binary code to work, IIb must eliminate ('reset') Numb before dividing. When SOPs are prevented from dividing, they become neurons [1743]. This result has been interpreted as a default condition, but it may instead reflect persistence of Numb: the continual presence of Numb should lead to a 'nonsense' code (111) that might be interpreted as 'neuron' (101). The mechanism whereby cells remember former Numb states is unknown.

Timing and branching of the pedigree are as per [1447,3549]. Other details are based on [1449,1741,1808,3579]. See [3195] for lineage comparisons with other sensory organs.

N.B.: Grooves are absent from some bristles (e.g., sex comb teeth [1714]). Epidermal cells are sometimes aligned with this degree of precision [2388], though they need not be. Chemosensory bristles have 4 additional neurons (cf. Fig. 2.8) [4125], and their SOPs obey a different lineage [3529]. See also App. 7.