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Abstract

Transmission electron microscopy of the spermatogenic stages of the hydroid, Hydractinia echinata, reveals a series of complex structural and positional changes in the centrioles of spermatocytes and spermatids. The newly generated centriolar pairs of spermatocytes form an unusual four-centriole aggregate that persists until cell division. The distal centrioles of this aggregate are shrouded with a very dense matrix that accumulates after centriolar replication. This matrix facilitates the mechanical attachment between distal centrioles and microtubular nucleating satellites, striated rootlets and pericentriolar processes. The association of these accessory structures occurs sequentially and is repeated in spermatocytes and spermatids. An electron dense plaque, which is an extension of distal centriolar matrix, is interposed between centriolar pairs of the aggregate. The plaque structurally maintains the centriolar aggregate and apparently facilitates the orientation of centrioles to prevent spacial interference while satellites, rootlets and pericentriolar processes associate with the distal centrioles. Striated rootlets are also involved in maintaining precise spacing and orientation between centriolar pairs. A single striated rootlet emanates from the base of each distal centriole of the aggregate and attaches with the opposite distal centriole. The attachment of rootlets to distal centrioles changes the spacing and orientation of centriolar pairs during the process of precocial flagellar development seen in Hydractinia spermatogenesis.

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