Observations on the Secretory Processes in the Corpus Cardiacum of the Stick Insect, Carausius morosus

The organization of the corpora cardiaca and of the nerve tracts linking these with the brain of the stick insect has been examined in the electron microscope. Particular attention has been paid to the appearance of neurosecretory products within the axons of the nervi corporis cardiaci, and within the cell processes of the intrinsic secretory neurons of the corpus cardiacum; these observations are considered to be an essential preliminary to a consideration of the mechanism of release of secreted products from the organ into the ambient haemolymph.

The corpus cardiacum contains cells of three main categories: axons of neurons located in the brain, intrinsic secretory neurons, and glial cells. Within the organ the cellular components form a loosely knit tissue incorporating ramifying extracellular spaces confluent with the haemolymph via the peripheral basement-membrane sheath of the organ. The present electron-microscopic account relates particularly to the mode of release of neurosecretory products into the extracellular milieu. This evidence indicates that the corpus cardiacum is the site of release of both extrinsic and intrinsic secretions, but does not support the suggestion that intrinsic neurosecretory materials synthesized within the corpus cardiacum are discharged via axon terminals at sites outside the corpus cardiacum. It is suggested that release of both extrinsic and intrinsic material takes place by fusion of the membrane surrounding the neurosecretory droplet with the plasma membrane of the nerve cell in regions where the glial sheath is absent.

The evidence on the mechanism of extrusion of materials initially secreted and sequestered within nerve cells and their processes is described and discussed in terms of present concepts of the passage of neural and other materials across plasma membrane boundaries.

It is stressed that, in this functionally versatile organ, the electron microscope may reveal the mechanism of release of active neurohumoral materials, but is at present unable to differentiate between the rich variety of these pharmacologically active products which affect numerous target organs throughout the body.

The myofilament arrangement in the femoral muscle of the cockroach, Leucophaea maderae fabricius

The structure of the femoral muscle of the cockroach, Leucophaea maderae, was investigated by light and electron microscopy. The several hundred fibers of either the extensor or flexor muscle are 20 to 40 micro in diameter in transverse sections and are subdivided into closely packed myofibrils. In glutaraldehyde-fixed and epoxy resin-embedded material of stretched fibers, the A band is about 4.5 micro long, the thin filaments are about 2.3 micro in length, the H zone and I band vary with the amount of stretch, and the M band is absent. The transverse sections of the filaments reveal in the area of a single overlap of thick and thin filaments an array of 10 to 12 thin filaments encircling each thick filament; whereas, in the area of double overlap in which the thin filaments interdigitate from opposite ends of the A band, the thin filaments show a twofold increase in number. The thick filament is approximately 205 to 185 A in diameter along most of its length, but at about 0.2 micro from the end it tapers to a point. Furthermore, some well oriented, very thin transverse sections show these filaments to have electron-transparent cores. The diameter of the thin filament is about 70 A. Transverse sections exhibit the sarcolemma invaginating clearly at regular intervals into the lateral regions of the A band. Three distinct types of mitochondria are associated with the muscle: an oval, an elongate, and a type with three processes. It is evident, in this muscle, that the sliding filament hypothesis is valid, and that perhaps the function of the extra thin filaments is to increase the tensile strength of the fiber and to create additional reactive sites between the thick and thin filaments. These sites are probably required for the functioning of the long sarcomeres.

The organization of flight muscle fibers in the Odonata

The cytological organization of flight muscle fibers of Odonata has been investigated. These fibers, in representatives of the Zygoptera and Anisoptera, have been compared and found to be similar, except that, in the former, pairs of lamellar fibrils, rather than single fibrils, alternate with the mitochondria. In each instance, in these synchronous muscles, the actin filaments of the myofibrils are found to lie opposite to and midway between pairs of myosin filaments-a configuration previously reported in asynchronous flight muscle fibers. The disposition of the T system and sarcoplasmic reticulum membranes in glutaraldehyde-fixed anisopteran muscle is described in detail: the T system tubules are shown to be radially continuous across the fiber, and are derived as openmouthed invaginations from the surface cell-membrane. The detailed organization of the dyad junctions between these tubules and the adjoining cisternae of the sarcoplasmic reticulum is described. The accessibility of the T system interior to diffusion exchange with the general extracellular milieu has been investigated by studies on the penetration of ferritin into the fiber: molecules of this marker have been found to diffuse solely along the T system tubules, and their presence in the tubule extremities adjoining the centrally placed nuclei confirms the morphological evidence suggesting that these tubules provide open diffusion channels extending across the radius of the fiber. The possible physiological role of these membrane components and their distribution in synchronous muscles of insects and vertebrates and in asynchronous insect flight muscle are discussed.

The Physiology of Excretion in the Cotton Stainer, Dysdercus Fasciatus Signoret

1. The intestine of Dysdercus is discontinuous in the larval instars, and urine from the Malpighian tubules is therefore uncontaminated by gut contents.

2. Excretion has been studied during the fifth instar, which lasts 8 days. Feeding only occurs in the first 4 days, during which there is a marked fall in water content. In the last part of the instar, when the animal only drinks, the water content returns to its original level.

3. The urine of Dysdercus is always liquid. There is a rapid rate of excretion during the first half of the instar, but when feeding ceases there is no further micturition and urine is retained in the rectum. Therefore there are two phases of excretion, designated excretory and post-excretory phases respectively.

4. The rectal epithelium is incapable of reabsorbing water against an osmotic gradient.

5. The urine which is retained in the rectum during the post-excretory phase acts as a water store; evaporative water loss is balanced by withdrawing water from this reservoir. During the excretory phase, a large volume of liquid is lost via the excretory system, but the loss is made good by drinking.