Pollen Wall.(A) Scheme of the main architectural features of a generalized mature pollen wall. Pollen Wall.(A) Scheme of the main architectural features of a generalized mature pollen wall. The terminology is according to Erdtman (1969).(B) Scanning electron micrograph of a Lilium pollen wall showing a reticulate pattern formed by the fused heads of the columellae. The nexine I is visible through the lacunae. Bar = 5 μm.(C) Model of pollen wall development based on Lilium (after Scott, 1994). Events begin before meiosis and proceed left to right until the mature wall is formed. 1, Transcription of “pattern” genes in the premeiotic microsporocyte nucleus. 2, Insertion of labile pattern information into the plasma membrane via endoplasmic reticulum–derived vesicles, followed by tessellation to produce a negative stencil of plates. 3, First phase of primexine synthesis over the entire microspore surface except areas destined to become apertures. 4, Conversion of primexine to a sporopollenin-receptive state through the activity of a factor(s) secreted from sites between the plates. 5, Second phase of primexine synthesis, more rapid than the first, resulting in limited primexine conversion and specification of columellae. 6, Nascent columellae are apparent as lamellated strands that lack substantial sporopollenin. 7, Consolidation of columellae by the appearance of partially polymerized (proto)sporopollenin on the receptive surfaces. 8, Final phase of primexine synthesis, during which the pattern stencil dissipates or is circumvented to produce the solid nexine I. 9, The callose wall is dissolved. 10, Wall elements are further consolidated by tapetally derived sporopollenin. 11, Nexine II is synthesized without the participation of the primexine. 12, Intine synthesis is initiated. R. J. Scott et al. Plant Cell 2004;16:S46-S60 ©2004 by American Society of Plant Biologists