The Question of Functional Homology of Hatschek's Pit of Amphioxus (Branchiostoma belcheri) and the Vertebrate Adenohypophysis

Abstract

Using antibodies to the beta subunit of human luteinizing hormone (hLHβ) and human chorionic gonadotropin, immunocytochemical evidence was obtained for gonadotropin activity in Hatschek's pit of amphioxus, Branchiostoma belcheri. This confirms the claim by C. Y. Chang et al. [1, 2] of vertebrate-like gonadotropin in this structure, an open groove in the dorsal part of the oral cavity. If this evidence is accepted at face value, a scenario can be constructed for the evolutionary pattern of the vertebrate adenohypophysis from the protochordate Hatschek's pit (cephalochordates) or neural gland (ascidians). Both of these structures are open to water currents in the mouth cavity. Thus, they may be able to sample thermal, chemical or pheromonal seasonally cycling clues and by gonadotropic stimulation, synchronize reproductive activity with such seasonal clues. Additional support for the idea that the early vertebrate adenohypophysis was a chemoreceptive organ comes from the fact that in cyclostomes and elasmobranchs it develops as part of the same epithelial layer and is directly contiguous with the olfactory organ. Advancement from the protochordate to vertebrate type of reproductive control involves the eventual use of sense organs and the nervous system to sample environmental changes, and the linkage of adenohypophysial function to central nervous control. The adenohypophysis then can be closed off from the mouth and direct environmental contact.