Ultrastructural patterns of secretory activity in poison cutaneous glands of larval and juvenile Dendrobates auratus (Amphibia, Anura) R. Angel a , G. Delfino b, * , G.J. Parra a a Instituto Colombiano de Medicina Tropical, KRA 43A n8 52S99, Sabaneta-Antioquia, Colombia b Dipartimento di Biologia Animale e Genetica dell’, Universita ` di Firenze, via Romana n8 17, I-50125, Firenze, Italy Received 7 March 2002; accepted 16 July 2002 Abstract A transmission electron-microscope study has been performed on larval and juvenile skin of the Central American arrow- frog Dendrobates auratus to investigate early secretory processes and maturational changes in the serous (poison) glands. Poison biosynthesis involves the endoplasmic reticulum (both smooth and rough types), as well as Golgi stacks which release early serous product as secretory vesicles (or pre-granules). These vesicles contain fine-grained material, along with single electron-opaque bodies, spheroidal in shape, that accompany the grained product throughout its post-Gogian, maturational change. The first steps of this process involve condensation and lead to the formation of secretory granules with a glomerular- like substructure, resulting from a thick, random aggregation of rods (secretory granule subunits). Advanced maturational activity causes the loss of peculiar granule substructure: the dense bodies split into fragments, whereas the thick glomerular arrangement becomes looser, until the secretory product changes into a dispersed material. This ultrastructural study revealed biosynthesis and maturation processes in close sequence, suggesting the poison of D. auratus contains proteins and/or peptides as well as lipophilic compounds. Molecules of both these classes are known to perform several roles relevant to survival strategies in extant anurans. Furthermore, the ephemeral granules with a glomerular- like substructure detected in tadpoles and froglets exhibit the complex patterns of mature poisons in adult specimens of other anurans: Hylidae and related families. This agrees with current trends in the taxonomy of these advanced frogs and underlines the pertinence of an ontogenetic approach in investigating anuran phylogenesis. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Dendrobates auratus; Poison biosynthesis; Ultrastructure 1. Introduction Although serous (or poison) cutaneous glands in adult anurans produce secretions that are heterogeneous in composition, the transmission electron microscope (TEM) reveals that they exhibit monotonous secretory features. This consistent trait largely depends on the slow activity rate of the secretory units in adult glands, which are syncytial in structure (secretory syncytia), and involved in serous storage rather than the biosynthesis. The only functional performance during the storage phase is the maturational process which affects secretory granules through complex relationships with the syncytial cytoplasm holding them (Delfino et al., 2001b); however, maturation only marginally involves the biosynthesis machinery: rough endoplasmic reticulum (rer), smooth endoplasmic reticulum (ser) and Golgi stacks. Two main procedures have been developed in order to depict the role of biosynthesis organelles in anuran serous glands: (a) observation of poison neosynthesis during gland rehabilitation after secretory discharge, and (b) 0041-0101/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0041-0101(02)00206-4 Toxicon 41 (2003) 29–39 www.elsevier.com/locate/toxicon * Corresponding author. Tel.: þ39-55-2288-295; fax: þ 39-55- 2288-299. E-mail address: delfino@dbag.unifi.it (G. Delfino).