Cell populations and morphogenetic movements underlying formation of the avian primitive streak and organizer

The cell populations and morphogenetic movements that contribute to the for mation of the avian primitive streak and organizer-Hensen's node-are poorly understood, We labeled selected groups of cells with fluorescent dyes and then followed them over time during formation and progression of the primit ive streak and formation of Hensen's node. We show that (1) the primitive s treak arises from a localized population of epiblast cells spanning the cau dal midline of Koller's sickle, with the mid-dorsal cells of the primitive streak arising from the midline of the epiblast overlying Koller's sickle a nd the deeper and more lateral primitive streak cells arising more laterall y within the epiblast overlying the sickle, from an arch subtending about 3 0 degrees; (2) convergent extension movements of cells in the epiblast over lying Koller's sickle contribute to formation of the initial primitive stre ak; and (3) Hensen's node is derived from a mixture of cells originating bo th from the epiblast just rostral to the incipient (stage 2) primitive stre ak and later from the epiblast just rostral to the elongating (stage 3a/b) primitive streak, as well as from the rostral tip of the progressing streak itself. Collectively, these results provide new information on the formati on of the avian primitive streak and organizer, increasing our understandin g of these important events of early development of amniotes, (C) 2001 Wile y-Liss, Inc.