Microorganization of ovaries and oogenesis of Haplotaxis sp. (Clitellata: Haplotaxidae)

Ovaries of Haplotaxis sp. were studied in active and nonactive states, that is, in a sexually mature specimen and in specimens outside of the reproductive period. Two pairs of ovaries were found in segments XI and XII. Especially in the nonactive state, they were in close contact with copulatory glands. Each ovary was composed of germ cells interconnected with syncytial cysts, which were enveloped by a layer of somatic cells. Within cysts each germ cell had one ring canal connecting it to the common anuclear cytoplasmic mass called a cytophore. During oogenesis clustering germ cells differentiated into nurse cells and oocytes; thus, the oogenesis was recognized as meroistic. Vitellogenic oocytes were detached from the ovaries and continued yolk absorption within the body cavity. Because recent studies have shown the variety of ovaries and germ line cyst organization in clitellates and suggest their evolutionary conservatism at the family or subfamily level, the data presented here can be valid in understanding the phylogenetic relationships among Clitellata. In this context, ovaries found in Haplotaxis sp. resembled those of the “Tubifex” type. “Tubifex” ovaries are characteristic for numerous microdrile taxa (tubificines, limnodriloidines, propappids, lumbriculids, and leech‐like branchiobdellids) and can be regarded as the primary character for these Clitellata in which germ‐line cysts are formed during early oogenesis. As the family Haplotaxidae is currently considered to be paraphyletic and the species studied here belongs to Haplotaxidae sensu stricto, our results support the close relationship of Haplotaxidae sensu stricto to the clade consisting of Lumbriculidae, Branchiobdellida, and Hirudinida, in which lumbriculids are sister to the latter two.     

Experimentally verified mathematical approach for the prediction of force developed by motor units at variable frequency stimulation patterns

During normal daily activity, muscle motor units (MUs) develop unfused tetanic contractions evoked by trains of motoneuronal firings at variable interpulse intervals (IPIs). The mechanical responses of a MU to successive impulses are not identical. The aim of this study was to develop a mathematical approach for the prediction of each response within the tetanus as well as the tetanic force itself. Experimental unfused tetani of fast and slow rat MUs, evoked by trains of stimuli at variable IPIs, were decomposed into series of twitch-shaped responses to successive stimuli using a previously described algorithm. The relationships between the parameters of the modeled twitches and the tetanic force level at which the next response begins were examined and regression equations were derived. Using these equations, profiles of force for the same and different stimulation patterns were mathematically predicted by summating modeled twitches. For comparison, force predictions were made by the summation of twitches equal to the first one. The recorded and the predicted tetanic forces were compared. The results revealed that it is possible to predict tetanic force with high accuracy by using regression equations. The force predicted in this way was much closer to the experimental record than the force obtained by the summation of equal twitches, especially for slow MUs. These findings are likely to have an impact on the development of realistic muscle models composed of MUs, and will assist our understanding of the significance of the neuronal code in motor control and the role of biophysical processes during MU contractions.