Hormonal control of ventral diaphragm myogenesis during metamorphosis of the moth, Manduca sexta.

Both the proliferation and differentiation of ventral diaphragm myoblasts are controlled by ecdysteroid during metamorphosis of the moth, Manduca sexta, but the responses have different hormonal requirements. Tonic exposure to moderate levels of ecdysteroid are required to stimulate myoblast proliferation. This is due to the presence of an ecdysteroid-dependent control point in the G(2) phase of the cell cycle. As a result, proliferation can be repeatedly turned on or off simply by adjusting the concentration of ecdysteroid to be above or below a critical threshold concentration. In contrast, high levels of ecdysteroid trigger irreversible proliferative arrest and differentiation of myofibers. Myoblast proliferation and differentiation also differ in their response to the juvenile hormone mimic, methoprene. Ecdysteroid-dependent proliferative arrest and differentiation are blocked by coculture with methoprene but methoprene has no effect on ecdysteroid-dependent proliferation. In the animal, premature exposure to high levels of ecdysteroid in the absence of juvenile hormone triggers precocious differentiation of the myoblasts, resulting in the formation of several thin bands of muscle rather than a complete diaphragm. Thus, ecdysteroid and juvenile hormone collaborate to determine the size and shape of the adult musculature.

Neuromuscular metamorphosis in the moth Manduca sexta: hormonal regulation of synapses loss and remodeling.

The motor system of the moth Manduca sexta is completely remodeled during the pupal-adult transformation (PAT). It is stable until the formation of the pupal stage (0% PAT), but larval motor end plates become disrupted by 5% PAT and are lost by 10% PAT, at the time that the muscle has begun to degenerate. Most of the axonal arbor is retracted by 15% with the first signs of adult sprouts appearing by 20% PAT, coinciding with proliferative activity in the remains of the larval muscle. Extensive growth of the axonal arbor begins after 30% PAT, with an initial phase of rapid longitudinal growth (35-50% PAT) and then the production of short transverse branches that then form sprays of end plates (50-70% PAT). Growth and maturation of the end plates occupies the remainder of metamorphosis. Neuromuscular metamorphosis was interfered with by systemic or local treatment with a mimic of the insect juvenile hormone. The results of these treatments suggest that some aspects of the removal of larval axonal branches requires cues from the target. For the sprouting response, the rapid longitudinal growth over the muscle appears to be due to ecdysteroids acting directly on the cell body of the motoneuron. By contrast, the subsequent production and maintenance of transverse sprouts and the corresponding end plates may be an indirect response to stimulation of muscle growth and differentiation by ecdysteroids.

Hormonal regulation of the shape of identified motoneurons in the moth Manduca sexta.

Motoneurons MN-1 and MN-3 in the moth Manduca sexta go through 3 distinct phases during postembryonic life. During larval life their neuritic fields show allometric growth in concert with larval growth. Larval neurites are then lost during the larval-pupal transition, which is followed by the outgrowth of adult-specific neurites during adult development. In MN-1, the adult-specific arbor typically results from the outgrowth of neurites ipsilateral to the cell body. However, in a small percentage of cells, ipsilateral branches are not present and contralateral branches extend across the midline to fill the vacant space. This altered form of MN-1 is thought to result from the early failure of ipsilateral neurite extension. The steroid hormones, the ecdysteroids, are responsible for the outgrowth of adult neurites. The onset of the latter is correlated with the ecdysteroid increase that promotes adult differentiation and does not occur under conditions such as diapause, in which the normal steroid rise is absent. Artificial replacement of ecdysteroids, however, induces the adult-specific growth. This action of ecdysteroids to cause a change in neuronal form requires the absence of juvenile hormone (JH). Application of JH mimics prior to the onset of the program of adult outgrowth blocks this outgrowth. MN-1 and MN-3 show different times of JH sensitivity, which appear to be correlated with different times of neurite outgrowth. It is concluded that the role of JH is to maintain the status quo of central neurons and prevent changes in form in response to ecdysteroids. In the absence of JH, the ecdysteroids can then exert morphogenetic changes, but the nature of these actions, neurite outgrowth or regression, is likely a function of the developmental history of the cell.

Occupational health survey: an evaluation of potential health hazards in the workplace.

Critical reviews of production jobs, potential exposure to chemical and physical agents, and job placement policies are necessary to provide a safe and healthful working environment for employees. To achieve this objective, a five-phase survey has been developed at BASF Wyandotte Corporation. Phase one assembles a data bank which cross references chemical substances with job classifications. Phase two provides toxicological evaluation for each chemical, physical, and ergonomic stress. Phase three evaluates the magnitude of each stress for a particular job classification. Phase four identifies present members of the work force or applicants who may have a heightened susceptibility to any stress. Phase five provides an integrated health manual with useful data and materials disseminated to various staff groups, manufacturing personnel, and plant employees.

Dendritic reorganization of an identified motoneuron during metamorphosis of the tobacco hornworm moth.

In the tobacco hornworm, many larval motoneurons become respecified and supply new muscles in the adult. Changes in the morphology of one such neuron were examined through metamorphosis. The dendritic pattern of the adult comes about both by outgrowth from the primary and secondary branches of the larval neuron and by the development of new branches that are unique to the adult.