Comparative embryology of the squamate pancreas: Structural and <scp>3D</scp> studies on the sand lizard (<i>Lacerta agilis</i>) and brown anole (<i>Anolis sagrei</i>)

This study investigates the developmental processes of the pancreas in two squamate species: the sand lizard (Lacerta agilis, Laterata) and the brown anole (Anolis sagrei, Iguania). Utilizing histological serial sections, we generated 3D reconstructions of the pancreas and its associated structures. Through a comparative analysis, we identified shared developmental events, including the emergence of specific structures and the initiation of distinct processes, to elucidate patterns of pancreatic differentiation and morphology. The pancreas of the sand lizard originates from three buds, while that of the brown anole originates from only two. In the sand lizard, the adult-like pancreas has an elongated Y-shape, while in the brown anole, the pancreas resembles the letter T on its side. Despite the differences in shape, in both species, the pancreas consists of four parts: the splenic lobe, the head of the pancreas, and the upper and lower lobes. The distal end of the splenic lobe in both species forms a thickened region called the juxtasplenic body. In the brown anole, endocrine islets are found only in the splenic lobe, and there are no islets in the other parts of the embryonic pancreas. In contrast, two clusters were identified in the pancreas of the sand lizard: one within the juxtasplenic body and the smaller one located where the splenic lobe emerges from the head of the pancreas. Both species differ in the topology of the ducts. In the sand lizard, the cystic and hepatic ducts enter the upper lobe of the pancreas, pass into the head of the pancreas, and form the common bile duct, which drains into the duodenum. In the brown anole, the cystic and hepatic ducts run along the upper lobe of the pancreas, then enter the head of the pancreas and form the common bile duct, which drains into the duodenum. Based on similarities between the brown anole, varanid lizards, and snakes, we propose that there is a common developmental sequence in toxicoferans and the adult pancreas of the brown anole represents an intermediate condition between "typical," elongated three-lobed lizard morphology and the shortened and partially compact pancreas of varanid lizards. This adds another grade of variation within the evolutionary pathway leading to the compact pancreas of snakes. On the other hand, the development and the adult-like morphology of the sand lizard pancreas strongly resemble the "typical", plesiomorphic, lizard condition. This work contributes to a deeper understanding of squamate anatomy and may shed light on the evolution of the pancreas in this clade.