Early ontogeny of the olfactory organ in a basal actinopterygian fish: polypterus

Abstract

The present study employed light and electron microscopic methods to investigate the ontogenetic origin of the olfactory organ in bichirs (Cladistia: Polypteridae) and explore its evolution among osteichthyans. In former studies we demonstrated that in teleosts a subepidermal layer gives rise to the olfactory placode which in turn builds all types of olfactory cells (basal, receptor, supporting, ciliated non-sensory cells). In contrast, the olfactory placodes in sturgeons (Chondrostei: Acipenseridae) as well as in the clawed frog Xenopus laevis (Anura: Pipidae) originate from two different layers. Receptor neurons derive from cells of the subepidermal (sensory) layer and supporting cells from epidermal cells. As sturgeons and amphibians in some characters show a more primitive condition than teleosts, we extended our study to Polypterus to allow for an approach at the basic osteichthyan pattern. In Polypterus, an internal lumen occurs in early ontogenetic stages surrounded by the epithelium of the olfactory placode. Two different populations of supporting cells follow one another: a primary population derives from the subepidermal layer. Later supporting cells develop from epidermal cells by transdifferentiation. The primary opening of the internal lumen to the exterior develops by invagination from the epidermal surface and simultaneously by a counter-directed process of cell dissociation and fragmentation inside the olfactory placode. Our results indicate the following features to be plesiomorphic actinopterygian character states: The primary olfactory pit (prospective olfactory cavity) is formed by invagination of the epidermal and the subepidermal layer (as in Acipenser and Xenopus). The incurrent and excurrent nostrils derive from a single primary opening which elongates and is then separated by an epidermal bridge into the two external openings (as in Acipenser and many teleosts). The olfactory epithelium derives from an epidermal and a subepidermal layer (as in Acipenser and Xenopus). Apomorphic (derived actinopterygian) features are: (1) an internal lumen as primordium of the future olfactory chamber; (2) a subepidermal layer gives rise to the olfactory epithelium and its constituents (Polypterus and teleosts). As to the origin of the olfactory supporting cells in Polypterus we assume a combination of plesiomorphic and apomorphic characters. We conclude that Acipenser and Xenopus exhibit the most widely distributed features among basal osteognathostomes and thus ancestral character states in the development of the olfactory organs.

Fig. 1

Early development of the olfactory placode in P. senegalus. Light Microscopy, semi-thin sections. A,B Early embryo 34 h AF, parasagittal section showing early placode formation. C Cone-shaped ingrowth of the thickened placode in 52 h AF (transversal section). D Formation of the primary nasal sac and internal lumen 72 h AF (transversal section). agl- attachment gland, fb — forebrain, opl — olfactory placode, poc — primary olfactory cavity or lumen of the nasal sac.

Fig. 2

Developing olfactory epithelium after hatching in P. senegalus (A — 96 h AF, B — 98 h AF, C,D — 120 h AF), TEM. A Olfactory epithelium with primary (placodal) supporting cells (psc) and developing olfactory receptor neurons (orn) with intracellular primary cilia (ipc). B Extension of the primary olfactory cavity of the nasal sac (poc).C Dendritic ending of ciliated olfactory receptor neuron (corn) with projecting cilium. D Disintegrating junctional complex of the olfactory epithelium close to the primary lumen.

Fig. 3

Development of the olfactory organ in P. ornatipinnis and P. senegalus, SEM. A P. ornatipinnis, stage 28, embryo just before hatching. Formation of the primary olfactory pit by epidermal invagination. B,C P. senegalus, stage 34 (8d 23h AF, yolk-sac larva). Process of division of the primary olfactory opening. D P. senegalus, stage 35 (9d 22h AF, latest yolk-sac larval stage before onset of extrinsic feeding). Anterior (incurrent) and larger, posterior (excurrent) olfactory opening formed by complete epidermal bridge. agl — attachment glands, an — anterior nostril, nce — non-sensory ciliated cell, no — neuromasts organs of cephalic lateral lines, pn — posterior nostril, arrows — primary olfactory pit.

Fig. 4

Advanced development of the olfactory epithelium in P. senegalus, TEM. A Stage 32 (168 h AF). Formation of the primary olfactory cavity (poc) and its open connection with the primary olfactory pit. B Ciliated olfactory receptor neurons (corn) accompanied by secondary (epidermal) supporting cells (esc) in the yolk-sac larval olfactory epithelium (Stage 33, 190 h AF) in cross-section. C, D Apical region of the olfactory epithelium close to the placodal lumen and its duct to the exterior (Stage 33, 190 h AF). Horizontal sections through two different levels. Olfactory knobs (ok) (in C) and dendrites (de) (in D) of olfactory neurons surrounded by secondary (epidermal) supporting cells (esc).

Fig. 5

Receptor types of the mature olfactory epithelium in P. senegalus (larvae and juveniles older than 30 days). A Ciliated (or “mixed”) receptor neurons (corn) accompanied by supporting cells (sc). Arrow indicates basal body and rootlet of a cilium. TEM. B Juvenile olfactory epithelium showing the olfactory knobs of ciliated olfactory neurons (corn). SEM. C Microvillous receptor neuron (morn) among supporting cells (sc) and ciliated receptor neurons (corn). TEM. D Rare crypt olfactory receptor neuron in a cross section through the juvenile olfactory epithelium. Cilia (cc) shown included in “crypt” spaces of the cell body.

Fig. 6

Schematic diagram of major steps in early development of the olfactory organ in Polypterus. A Early embryonic stage with epidermis (ep) and sole subepidermal (se) component of the olfactory placode. B Formation of the primary olfactory pit (pop) and cavity (poc), fibroblasts of the loose connective tissue (fbl) and first forebrain (fb) connections. C Opening of the internal lumen to the exterior and shaping of the olfactory cavity. Ciliated olfactory receptor neurons (corn) and primary (placodal) supporting cells (psc) of the subepidermal placode shaded. Epidermal supporting cells (esc).