Kaj Holmberg

The eyes in three genera of hagfish (Eptatretus, paramyxine andMyxine)—A case of degenerative evolution

Abstract The eyes in three genera of hagfish (Eptatretus, Paramyxine andMyxine) have been investigated with the light microscope. A comparison of the eyes, their location and retinal organization shows that among hagfishes an evolutionary trend towards reduction of the visual capability and a degeneration of the eyes is evident.Eptatretus represents the plesiomorphic andMyxine the apomorphic condition, whereasParamyxine holds an intermediate position. The different degenerative stages of the eyes are concomitant with various other characters interpreted as evolutionary adaptations in the ecological differentiation of the hagfishes to niches at different depths.

Fine structure of retinal synaptic organelles in lamprey and hagfish photoreceptors.

Abstract Synaptic ribbons comparable with those in retinalphotoreceptorsof gnathostome chordates occur in the lamprey (Lampetra fluviatilis)retina. The hagfishesEptatretus burgeri, E. stouti andParamyxine atami have instead spherical synaptic bodies. These organelles are absent inMyxine glutinosa. The results show that synapticribbons do not only occur in the gnathostome chordates but also in one of the agnath subdivisions, the lampreys, while the other agnath subdivision, the hagfishes, instead have synapticbodies.

The hagfish retina: electron microscopic study comparing receptor and epithelial cells in the Pacific hagfish, Polistotrema stouti, with those in the Atlantic hagfish, Myxine glutinosa.

SummaryThe fine structure of receptor and epithelial cells in the retina of the pacific hagfish, Polistotrema stouti, has been investigated and compared with previous observations made in the atlantic hagfish, Myxine glutinosa. The receptor cells in Polistotrema have cylindrical outer segments which consist of numerous discs arranged perpendicularly to the long axis of the cell. Characteristic synaptic bodies (synaptic lamellae) occur at the receptor base. Membranous inclusions in the epithelial cells suggest phagocytosis of outer segments. In Myxine, the outer segments are whorl-like, and synaptic bodies are absent at the receptor base. There are no signs of phagocytosis in the epithelial cells. The results are discussed from a functional and phylogenetical point of view.

ERG-recordings from the retina of the river lamprey (Lampetra fluviatilis).

Abstract Light- and dark-adapted isolated eyes of adult animals were stimulated with “white” light within the range of 400–850 nm with stimuli varying between 0.5 and 25 sec. The occurrence of a c-wave and a cornea-positive off-response (d-wave) indicates that the retina of the river lamprey is functionally similar to a mixed rod-cone retina in spite of the rod-like fine structure of both the long and the short photoreceptor cells.

Fine structure of the brain and brain nerves ofOikopleura dioica (Urochordata, Appendicularia)

SummaryEach second brain nerve consists of only one single fibre terminating at two different types of touch receptors in the oral region. The two nerves are the dendrites of two perikarya in the forebrain and are the master neurons for ciliary reversal in the stigmata, which is a two-neuron reflex. By axoaxonal synapses they control one motor neuron in the midbrain, i.e. the command neuron for ciliary reversal in both rings. This cell sends one axon branch in each third nerve to the cilia cells. In the left nerve this fibre is closely associated with a coarsely granulated accessory fibre, which apparently regulates the ciliary beat. The third nerves also contain one fibre each from another motor neuron in the hindbrain. These fibres make synaptic contacts at some specialized epidermal cells in the lateral trunk behind the ciliary rings. A few previously unknown nerves in the dorsal forebrain innervate epidermal cells. It is likely that the complicated epidermal motor innervation regulates the secretory activity of the oikoplasts or of the epidermal cells in constructing a new house, including the necessary complicated filters and food trapping mechanisms.

Ultrastructure and response to background illumination of the melanophores of the Atlantic hagfish, Myxine glutinosa, L

Abstract The melanophores of the Atlantic hagfish, Myxine glutinosa, L. have been investigated by light and electron microscopical methods. Animals have also been illuminated against black or white backgrounds in an attempt to evoke color change. 1. 1. The melanophores have basically the same structure as other investigated melanophores in poikilothermic vertebrates, but differ in two respects: 1.1. (a) they have cytoplasmic tubules, 400–700 A in diameter, 1.2. (b) they lack an inner membrane as described in Lebistes reticulatus. 2. 2. No color change was observed with the naked eye, nor did microscopy reveal any migration, fading, or other change in the melanophore due to the influence of light. 3. 3. The mortality observed in animals illuminated in white aquaria is discussed.

Light- and electron-microscopic investigation of the optic nerve fiber layer in the river lamprey (Lampetra fluviatilis)

Abstract The optic nerve fibers are organized into discreet fascicles which course from the periphery of the retina to the optic disc along the vitreal margin of the inner nuclear layer. Thus, in contrast to gnathostome chordates. the optic fiber layer in lampreys is located sclerally to the inner synaptic layer. A hypothesis is put forward which suggests that the fascicle organization is a device which enables cell processes from the inner nuclear layer to extend vitreally beyond the optic fiber layer and thus facilitate the development of the inner synaptic layer without a splitting process of the inner neuroblastic layer.

A transmission electron microscopic investigation of the sensory vesicle in the brain of Oikopleura dioica (Appendicularia)

SummaryThe structure of three cell types in the sensory vesicle is described: (1) The statocyte, with its intracellular statolith, is attached to the medial wall of the vesicle via delicate shaft cells. (2) Cells along the dorsal, ventral and lateral walls which contact the surface of the statocyte with long, slender cilia. These cells are presumed to be primary sensory cells. (3) Presumed secretory cells, along the rostral and dorsal walls, may have a dual function: (a) secretion of the vesicle fluid, and (b) stabilization of the wall by turgor created in characteristic intercellular cavities. The sensory vesicle in Oikopleura contains undoubtedly typical “statocyst components” adequate for a free-swimming animal, whereas the ascidian system is suggested to be a device that responds to gravitational stimuli and, together with temporary photoreceptors, aids the larva in finding optimal settling conditions.

Critical flicker fusion frequency of the river lamprey (Lampetra fluviatilis).

Abstract Enucleated eyes of the river lamprey ( Lampetra fluviatilis ) were investigated by flicker stimulation. The critical fusion frequencies (CFFs) of the ERG were determined at various intensities of the stimulating light. The relation between CFF and light intensity shows two phases. On the average, the shift between these two phases occurs at 9.8 Hz and −3.0 log units relative intensity (170 lux). The scotopic fusion frequencies are low compared to those of most other vertebrates hitherto investigated and may reflect an extraordinarily slow decay of the photoreceptor potentials in the river lamprey. The intensity at which the CFF phase-shift occurs indicates that the river lamprey retina takes an intermediate position between rod- and cone-dominated retinas with regard to the cone/rod ratio. The results agree with previous morphological findings and suggest a duplex organization of the river lamprey retina.

The neural connection between the Langerhans receptor cells and the central nervous system in Oikopleura dioica (Appendicularia)

SummaryTransmission electron microscopic analysis of serial sections showed that the receptor cells are innervated by only one neuron and not two as previously believed. The neurons two dendrites constitute the afferent sensory nerves to the caudal ganglion where the neurons cell body is located. Its neurite was traced a few micrometers, but the synaptic terminals were not identified. This sensory system in Oikopleura is compared with a similar caudal sensory system in the tadpole larva of Diplosoma macdonaldi investigated by Torrence and Cloney. ‘Wiring diagrams’ are proposed for the two systems. The ganglia, which receive the afferent sensory neurons, are discussed in terms of models for further research on simple integration systems.