Regeneration and endocrinology in the polychaetePlatynereis dumerilii : An experimental and structural study.

1. In the polychaetePlatynereis dumerilii, the hormone-elaborating portion of the prostomium was determined by means of prostomium transection and implantation experiments. The area in question lies between the two pairs of eyes, extending longitudinally from the posterior border of the anterior eyes to about the posterior border of the posterior eyes. This corresponds approximately with the brain area delimited by the anterior and posterior dorsoventral connective tissue tubes and which is covered ventrally by the infracerebral gland epithelium. 2. The infracerebral gland-complex and neurosecretory neurons within the brain were envisaged as possible sites of hormone synthesis. 3. The infracerebral gland-complex inPl. dumerilii was investigated with light-and electron-microscopical techniques. A leaf-shaped area (measuring 120 by 95 μm at the most) of the pericapsular epithelium at the ventral side of the brain, adjacent to the main blood vessel and to its efferent branches, consists of specialized columnar epithelial cells. Numerousa-cells and scarceb-cells can be distinguished. Fibre tracts with glia fibres and axons (some being neurosecretory axons) descend from the neuropile and in part terminate with prominent end-structures at the inner face of the brain capsule in the gland region. Probably some axons penetrate the capsule and make contact with the gland cells. Neither structural nor experimental findings prove that the infracerebral gland synthesizes the brain hormone. Accessory functions are discussed. 4. Investigations in secretory brain cells ofPl. dumerilii are reported. In agreement with Müller (1973), a lack of correlation between the number of stainable neurosecretory neurons and the hormonal activity of the brain was found: in immature worms (to which high hormonal titers are ascribed) only few or even no neurosecretory brain cells at all were detectable. Prostomium transection and implantation experiments show further that not all regions of the brain which enclose neurosecretory neurons produce brain hormone. The results are discussed with reference to the hypotheses of Müller (1973) which suggest that the appearance of stainable neurosecretory brain cells indicates inactivation of neurons possibly previously involved with hormone synthesis.