Transmissions- und rasterelektronen-mikroskopische Untersuchung an den Sinnesknospen der Tentakeln von Myxine glutinosa L. (Cyclostomata)

Evolutionary origins of taste buds: phylogenetic analysis of purinergic neurotransmission in epithelial chemosensors

Taste buds are gustatory endorgans which use an uncommon purinergic signalling system to transmit information to afferent gustatory nerve fibres. In mammals, ATP is a crucial neurotransmitter released by the taste cells to activate the afferent nerve fibres. Taste buds in mammals display a characteristic, highly specific ecto-ATPase (NTPDase2) activity, suggesting a role in inactivation of the neurotransmitter. The purpose of this study was to test whether the presence of markers of purinergic signalling characterize taste buds in anamniote vertebrates and to test whether similar purinergic systems are employed by other exteroceptive chemosensory systems. The species examined include several teleosts, elasmobranchs, lampreys and hagfish, the last of which lacks vertebrate-type taste buds. For comparison, Schreiner organs of hagfish and solitary chemosensory cells (SCCs) of teleosts, both of which are epidermal chemosensory end organs, were also examined because they might be evolutionarily related to taste buds. Ecto-ATPase activity was evident in elongate cells in all fish taste buds, including teleosts, elasmobranchs and lampreys. Neither SCCs nor Schreiner organs show specific ecto-ATPase activity, suggesting that purinergic signalling is not crucial in those systems as it is for taste buds. These findings suggest that the taste system did not originate from SCCs but arose independently in early vertebrates.

Schreiner organs: a new craniate chemosensory modality in hagfishes

An extensive system of sensory organs resembling taste buds was previously known in the skin of hagfishes. These sensory organs, called here Schreiner organs, are found throughout the epidermis of both Eptatretus stoutii and Myxine glutinosa. They are found also at high densities in the prenasal sinus, nasopharyngeal duct, and pharynx, and at lower densities in the oral and velar chambers. Schreiner organs are multicellular aggregates composed of acetylated tubulin-immunoreactive receptor cells and nonimmunoreactive cells. A considerable range of variation was found in Schreiner organ morphology, but discrete classes of organs could not be recognized. Schreiner organs are innervated by all sensory trigeminal rami, the glossopharyngeal/vagal nerve, and cutaneous rami of spinal nerves, but not by the facial nerve. The central projections of these rami form a continuous tract in the trigeminal sensory zone and the dorsolateral funiculus of the spinal cord. Some Schreiner organs may be represented in the nucleus of the solitary tract, but this structure is certainly not the primary recipient zone of Schreiner organ afferents. In light of these systemic differences between vertebrate taste systems and the Schreiner organ system of hagfishes, it is concluded that Schreiner organs are not homologous to taste buds. This sensory modality of hagfishes has no direct homolog in vertebrates and appears to be a specialization of hagfishes, perhaps derived from the primitive somatosensory system of the earliest craniates.

Lamellated receptors in the skin of the hagfish, Myxine glutinosa

The adult hagfish, Myxine glutinosa, does not exhibit a lateral line system. The hypodermal layer of the dorsal head and body skin contains a prominent receptor system--lamellated corpuscles--arranged in a segmental pattern close to the body fascia. The topography and the structural organization of the lamellated receptors are described at the light- and electronmicroscopical levels. Spinal nerves supply the lamellated receptor organs. A mechanoreceptive function and evolutionary aspects are discussed.

Somatotopic organization of the primary sensory trigeminal neurons in the hagfish, Eptatretus burgeri

Primary sensory trigeminal projections were investigated in the hagfish following application of horseradish peroxidase (HRP) to the sensory branches. In our control preparations we were able to distinguish five sensory ganglia and their respective nerves. HRP application confirmed the almost exclusive relation of each of these nerves to their respective ganglia, with very little overlap. In normal frontal sections of the medulla oblongata, five columns of fibers surrounded by neuronal cell bodies could be clearly distinguished, but the number is probably fortuitous, for there was no one-on-one relationship with the five trigeminal ganglia. From their peripheral connections, we surmised that columns 1 and 3 handle general cutaneous sensation, columns 2, 4, and 5 handle taste sensation, and column 5 handles general mucous cutaneous sensation conveyed by utricular ganglion cells. Dorsally located columns received projections from nerves with dorsal peripheral connections, and more ventrally located columns received projections from nerves with ventral peripheral connections. This relation is the reverse of that seen in other vertebrates.

Evolution of myelin sheaths: both lamprey and hagfish lack myelin

Modern views of agnathan phylogeny consider Petromyzoniformes and Myxiniformes to belong to distinct classes that diverged from a common ancestor at a remote period, perhaps in the lower Cambrian, greater than 600 million years ago. Both are more primitive than elasmobranchs, holocephalans and bony fishes. Myelin is well developed in elasmobranchs and other fishes but was reported to be lacking in the spinal cord of lampreys. In order to search further for possible early myelin in some part of the nervous system of one of the agnathan stems, or for further evidence that it first appeared in chondrichthians, we extended the sampling to many parts of the brain and cord of hagfish. Transmission electron microscopy was used as a nearly ideal criterion. We find no trace or forerunner of the spiral, multilaminate glial wrapping. Many axons are embedded within one or more glial cells, like unmyelinated fibers in other vertebrates, or lie contiguously in bundles without an obviously complete glial investment. True myelin must be presumed to have been invented within the vertebrates, in ancestors of the living cartilaginous fishes after the agnathans branched from the vertebrate stem.