278
AUGUST EPPLE
more about its molecular structure, especially with respect to the site of
its biological activity {the spatial arrangement of the three disulfide
bonds? see, e.g., Wilson, 1965; Rieser, 1967) and its possible relation to
proinsulin ( Steiner and Oyer, 1967).
Glucagon, on the other hand, may be a late acquisition of the higher
deuterostomians. Its apparent lack in the cyclostomes is in relatively
good agreement with Weinstein’s conclusion ( 1968) that glucagon was
established in the mesozoic or late paleozoic era. According to his
molecular-genetic calculations, glucagon and secretin, which were originally produced by one gene, became independent of each other at this
time.
B. General Trends in Pancreas Evolution
All fishes have endocrine and exocrine pancreas tissue, yet great
morphological variations and a large number of endocrine cell types are
reflections of phylogenetic distances and of differing evolutionary trends.
In the light of recent findings, it is possible to differentiate four types
of pancreas:
(1) A cyclostome type with total separation of specialized exocrine
and endocrine tissue
( 2 ) A primitive gnathostome type, where, within a compact pancreas,
the ductlike arrangement of the endocrine tissue resembles the early
stages of human islet formation (many selachians and Latimeria)
( 3 ) A tetrapodlike type of more or less compact pancreas with typical
islets ( holocephalians, Protopterus, and some teleosts )
(4) An actinopterygian type, with a tendency to develop a disseminated pancreas which is widely scattered throughout the body cavity
and some of its organs (liver, spleen, and ovary), and with a partial
separation of endocrine and exocrine tissue
However, the islets of the holocephalians differ strongly in their
cytology from those of the tetrapods and show a close topographic
relation to the ducts. The pancreas of Protopterus is (secondarily?) embedded in the intestinal wall.
Among the gnathostomes, it is possible to trace two phylogenetic
trends:
(1) To concentrate the endocrine cells as “islets” within a compact
pancreas. This tendency is evident in some elasmobranchs and in
Latimeria, and it is highly developed in the holocephalians, Protopterus,
actinopterygians, and tetrapodes.
(2) To split the originally compact pancreas of the actinopterygians,
AUGUST EPPLE
more about its molecular structure, especially with respect to the site of
its biological activity {the spatial arrangement of the three disulfide
bonds? see, e.g., Wilson, 1965; Rieser, 1967) and its possible relation to
proinsulin ( Steiner and Oyer, 1967).
Glucagon, on the other hand, may be a late acquisition of the higher
deuterostomians. Its apparent lack in the cyclostomes is in relatively
good agreement with Weinstein’s conclusion ( 1968) that glucagon was
established in the mesozoic or late paleozoic era. According to his
molecular-genetic calculations, glucagon and secretin, which were originally produced by one gene, became independent of each other at this
time.
B. General Trends in Pancreas Evolution
All fishes have endocrine and exocrine pancreas tissue, yet great
morphological variations and a large number of endocrine cell types are
reflections of phylogenetic distances and of differing evolutionary trends.
In the light of recent findings, it is possible to differentiate four types
of pancreas:
(1) A cyclostome type with total separation of specialized exocrine
and endocrine tissue
( 2 ) A primitive gnathostome type, where, within a compact pancreas,
the ductlike arrangement of the endocrine tissue resembles the early
stages of human islet formation (many selachians and Latimeria)
( 3 ) A tetrapodlike type of more or less compact pancreas with typical
islets ( holocephalians, Protopterus, and some teleosts )
(4) An actinopterygian type, with a tendency to develop a disseminated pancreas which is widely scattered throughout the body cavity
and some of its organs (liver, spleen, and ovary), and with a partial
separation of endocrine and exocrine tissue
However, the islets of the holocephalians differ strongly in their
cytology from those of the tetrapods and show a close topographic
relation to the ducts. The pancreas of Protopterus is (secondarily?) embedded in the intestinal wall.
Among the gnathostomes, it is possible to trace two phylogenetic
trends:
(1) To concentrate the endocrine cells as “islets” within a compact
pancreas. This tendency is evident in some elasmobranchs and in
Latimeria, and it is highly developed in the holocephalians, Protopterus,
actinopterygians, and tetrapodes.
(2) To split the originally compact pancreas of the actinopterygians,