J. L. S. Cobb

Song characteristics are age dependent in the willow warbler, Phylloscopus trochilus

Theory predicts that sexually selected characters should be condition dependent if they are to be used as honest signals of male quality. One consequence of this dependence is that young and older males will differ in the expression of these characters. Many bird song characteristics, such as repertoire size or song length, are considered to have evolved through sexual selection. Consistent with this, several studies have shown that repertoire size increases with age. However, many of these studies used cross-sectional methods, comparing song characteristics in different age classes. This approach has the disadvantage that differential survival related to song can confound the data, emphasizing or hiding individual changes. To avoid this we combined a cross-sectional with a longitudinal analysis, where the same cohort is compared at different ages, to study how song characteristics change with age in the willow warbler. The longitudinal analysis showed that song repertoire size (the set of different song elements) and element rate (number of elements produced divided by song duration) increased between the first and second year. The cross-sectional analysis gave similar results, but there were no changes in element rate. From these results we can infer that the development of several song characteristics is costly in this species. The different results provided by the two methods for element rate suggest a negative correlation between element rate and the probability of survival. The discrepancy between the methods implies that cross-sectional methods can provide erroneous conclusions in the study of changes with age in bird song.

The fine structure of the heart of the teleost, Pleuronectes platessa L.

SummaryTissue from the four chambers of the heart of the plaice (Pleuronectes platessa, L.) has been examined in the electron microscope in order to describe the morphology of the heart at a fine structural level.The sinus venosus is a thin walled chamber between 60–90 μ thick consisting of a connective tissue matrix in which are situated the plexus of the parasympathetic cardiac ganglion and localised bundles of myocardial cells. The myocardial cells do not form a continuous layer but are associated in particular with the region of the cardiac ganglion and are innervated by it.The sino-auricular junction has hitherto been described as a pacemaker region but the myocardial cells in this region are identical in morphology to myocardial cells in other parts of the heart. There is a large complex of nerves, derived from the cardiac plexus, that runs around the junction before branching to innervate the auricle.The myocardial tissues consist of an outer layer of myocardium forming the wall of the heart and a profusion of trabeculae. The endocardium invaginates into the endocardium to divide up the cells into populations of approximately 25 cells in profile. There is no well-defined coronary blood supply although capillaries are occasionally seen. The myocardial cells themselves are small in diameter (3.5–5.5 μ) and show some primitive features which are: a short sarcomere (1.4–2.0 μ), the absence of any sarcoplasmic reticulum, and very scarce fasciae occludentes. In the atrium in particular, there are many groups of 1500 Å membrane-bound, dense-cored vesicles in the myocardial cells. Ventricular cells contain more myofilaments and mitochondria than do atrial cells and have many vesicles of 0.1–0.3μ diameter whose function and contents are unknown.Connective tissue is very evident in the plaice heart, being an integral part of the sinus venosus and the auriculo-ventricular junction and being the sole constituent of the auriculoventricular valve and the bulbus arteriosus.

The significance of the radial nerve cords in asteroids and echinoids.

SummaryThe radial nerve cords of a number of different species of echinoids and asteroids have been examined with the electron microscope. They have been shown to consist of tracts of small naked axons with discrete regions of neuropile. Synapses within these areas of neuropile have been described and their significance discussed. A careful examination has failed to reveal any connections between the ectoneural nerve and the mesodermal nerves and muscles. Synapses across the connective tissue sheet separating these regions have been proposed to account for known functional connection. A hypothetical model of the nervous system has been proposed to account for known structure and function.

An ultrastructural study of mitotic division in differentiated gastric smooth muscle cells

SummaryAn ultrastructural examination of tissue from the gizzards of chicks just before and just after hatching showed numerous mitotic divisions in the well differentiated and functional smooth muscle. The nuclei in the very elongate, dividing cells were located centrally. The cytoplasm immediately adjacent to the nuclei contained the normal fully differentiated complement of myofilaments. During the active stages of division, after the breakdown of the nuclear membrane, myofilaments were shown to lie between the individual chromosomes. The process of division only occupied a small portion of the long muscle cells; the ultrastructural changes seen appeared similar to those described in other cell types.

The giant neurone system in ophiuroids

SummaryThe radial nerve cords of members of the class Ophiuroidea consist of two parts, the ectoneural and the hyponeural tissues, which are separated by an acellular basal lamina. The hyponeural tissue is composed entirely of motor fibres. The cell bodies of the hyponeural neurones are arranged in ganglia, one to each segment of the arm, and each containing approximately one hundred cell bodies. Synaptic contact between the two tissues occurs across the basal lamina. Ultrastructural evidence shows that the majority of these synapses operate in the ectoneural to hyponeural direction. Three pairs of nerve bundles, each containing approximately thirty five large motor fibres arise from each ganglion and innervate the intervertebral muscles. The large motor fibres divide into a number of pre-terminal axons in the region in which the motor fibre enters the muscle block. The terminal axons run at right-angles across the muscle fibres and neuromuscular junctions are found at the points of contact between the two; each terminal axon makes contact with a large number of muscle fibres. The hyponeural axons also pass through the juxtaligamental tissue before they reach the muscle blocks and there is some evidence of synaptic contact with the juxtaligamental cells. The juxtaligamental tissue is thought to be associated with changes in the structural properties of the collagenous ligaments of the arm during arm autotomy (Wilkie 1979). Degeneration studies confirmed the layout of the hyponeural motor axons.

A fluorescence histochemical study of the degeneration and regeneration of noradrenergic nerves in the chick following treatment with 6-hydroxydopamine

SummaryThe fluorescence histochemical method of Falck and Hillarp for the cellular localisation of biogenic amines was used to follow the disappearance and reappearance of peripheral noradrenergic nerves in the chick after treatment with 6-hydroxydopamine. With certain exceptions, it seemed that the disappearance and reappearance of the nerves was due to their degeneration and subsequent regeneration. However, there was not a uniform destruction of the noradrenergic nerves associated with different effector tissues. The extent of degeneration was greater in older than in younger chicks following equivalent doses of the drug, possibly indicating a greater uptake efficiency of the older nerves. It appeared that the noradrenergic nerves in the younger chicks regenerated more rapidly than those of older chicks, but in none of the animals studied were there any obvious morphological modifications of the regenerating nerves.

A comparative study on the innervation and the vascularization of the bulbus arteriosus in teleost fish

SummaryThe structure of the bulbus arteriosus of a wide range of teleost fish is described with particular reference to the vascularization and innervation. The adventitia of the organ consists of blood vessels and large nerve bundles in a collagen matrix. The nerve bundles contain monoamines, and fluorescence studies show small terminal bundles penetrating the muscular media; this is confirmed by electron microscopy. The media consists of an extensive elastic tissue matrix with a spiral arrangement of smooth muscle cells joined end to end by desmosomes and presumed electrotonic junctions. The muscle cells are innervated only at the adventitia/media boundary and the significance of this innervation is discussed. It is proposed that there is a correlation between the degree of vascularization and innervation and the activity of a particular species offish.

Gap junctions in the heart of teleost fish

SummaryAn examination of cell contacts was made in the atrial and ventricular muscle of Teleost fish. The intercalated discs consist of two types of junction resembling fascia and macula adherens. Small focal areas of gap junctions were shown to be numerous away from the regions of intercalated disc between the myocardial cells.

Synaptic structure in the visceral ganglion of the lamellibranch mollusc, Spisula solida

SummaryAn examination using the electron microscope was carried out on the visceral ganglion of the marine bivalve mollusc Spisula solida. A range of fixation, block staining and section staining technique was used to study the structure of chemical synapses. Phosphotungstic acid employed as a block stain specifically stained pre- and post-synaptic structures associated with the membrane at synapses as well as one class of granular vesicle. The specialised contacts were however shown to be rare and in many parts completely absent. Many axons, containing several types of vesicle, were shown to be varicose and it is proposed that they may function in a similar way to the unspecialised varicose terminals of vertebrate autonomic neurons. The role of membrane specialisations in intercellular adhesion is discussed. This study concludes that many synapses may be morphologically unidentified using present criteria.

An ultrastructural study of the dermal papulae of the starfish, Asterias rubens, with special reference to innervation of the muscles

SummaryThe ultrastructure of the dermal papulae of a starfish (Asterias rubens) is consistent with a respiratory function. The present study has shown no regions specialized for excretory mechanisms. The papulae consist of an outer ectodermal epithelium of sensory, support and gland cells and a small basiepithelial nerve plexus. A true basement membrane lies underneath the epithelium and regularly arranged longitudinal muscle bundles lie within the connective tissue. The coelomic cavity of the papulae is lined with ciliated endothelial cells, which overlie an irregular layer of circular muscles. A system of canals that are not lined by cells occurs at the base of the papulae with the circular muscles. The longitudinal and circular muscles show a different gross morphological arrangement and innervation. This paper proposes that there are ‘skeletal’ and ‘visceral’ types of smooth muscle in echinoderms and discusses this proposal at length.