Based on analysis of available genome sequences, five gene lineages of MHC class I molecules (MHC I-U, -Z, -S, -L and -P) and one gene lineage of MHC class II molecules (MHC II-D) have been identified in Osteichthyes. In the latter lineage, three MHC II molecule sublineages have been identified (MHC II-A, -B and -E). As regards MHC class I molecules in Osteichthyes, it is important to take note of the fact that the lineages U and Z in MHC I genes have been identified in almost all fish species examined so far. Phylogenetic studies into MHC II molecule genes of sublineages A and B suggest that they may be descended from the genes of the sublineage named A/B that have been identified in spotted gar (Lepisosteus oculatus). The sublineage E genes of MHC II molecules, which represent the group of non-polymorphic genes with poor expression in the tissues connected with the immune system, are present in primitive fish, i.e. in paddlefish, sturgeons and spotted gar (Lepisosteus oculatus), as well as in cyprinids (Cyprinidae), Atlantic salmon (Salmo salar), and rainbow trout (Oncorhynchus mykiss). Full elucidation of the details relating to the organisation and functioning of the particular components of the major histocompatibility complex in Osteichthyes can advance the understanding of the evolution of the MHC molecule genes and the immune mechanism.

Olfaction is the major sense of smell in teleost involved in many physiological response and habitat acclimatization including food searching, migration for spawning, predator avoidance, reproduction behavior, as well as identification of fish of the same species. Our study illustrates the ultrastructure of the olfactory rosette of the grass carp Ctenopharyngodon idella (Cuvier and Valenciennes, 1844) by using Scanning Electron Microscope (SEM) and transmission Electron Microscope (TEM). Herein, the peripheral olfactory organs are represented by two olfactory rosettes lying in two nasal chambers, one on each side of fish snout. Each nasal chambers opens to the exterior by two nostrils a narrow inlet and wider outlet, segregated by a somewhat elevated, nasal bridge. The two nostrils are somewhat faraway as far as length of rosette’s length to permit entering and leave water flow bearing odorant molecules to the nasal cavities. The SEM revealed that each olfactory rosette is elongated oval-shaped and made up of 48-50 foliar lamellae transversely arranged on both sides of a narrow median raphe. Alongside, the magnitude of lamellae differs in relation to their location on the raphe, since the larger are in the middle whereas their dimensions gradually reduced towards both ends of the rosette indicating that the number and magnitude of lamellae increase as the fish grow. Moreover, the lamellar surface comprises sensory and non-sensory areas concealed in a mucous layer and not distributed uniformly within the epithelial surface of the olfactory lamellae. Accordingly, The TEM observations indicated that the sensory areas holds four main receptor neurons, two  are major including ciliated and microvillous receptor cells bearing either cilia or microvilli, respectively emitted from a dendritic knob. Additionally, two other minor rod-tipped and crypt cells bearing a compound rod cilium, or few microvilli and occult cilia emitted also from dendritic knob were rarely observed. From the other side, the non-sensory area comprises cylindrical flat top surface; ciliated non-sensory cells with motile long kinocilium and nonciliated stratified epithelial cells with fingerprint-like microridges both are mainly of supporting in addition of and ovoid goblet mucous cells in-between and stem basal cells. Collectively, our study revealed the general organization and ultrastructure of an important economic teleost fish that affects feeding habitat and has an important influence on the fish food intake.