Nerve cells in hydra: monoclonal antibodies identify two lineages with distinct mechanisms for their incorporation into head tissue.

The relationship between populations of nerve cells defined by two monoclonal antibodies was investigated in Hydra oligactis. A population of sensory nerve cells localized in the head (hypostome and tentacles) is identified by the binding of antibody JD1. A second antibody, RC9, binds ganglion cells throughout the animal. When the nerve cell precursors, the interstitial cells, are depleted by treatment with hydroxyurea or nitrogen mustard, the JD1+ nerve cells are lost as epithelial tissue is sloughed at the extremities. In contrast, RC9+ nerve cells remain present in all regions of the animal following treatment with either drug. When such hydra are decapitated to initiate head regeneration, the new head tissue formed is again free of JD1+ sensory cells but does contain RC9+ ganglion cells. Our studies indicate that (1) nerve cells are passively displaced with the epithelial tissue in hydra, (2) JD1+ sensory cells do not arise by the conversion of body column nerve cells that are displaced into the head, whereas RC9+ head nerve cells can originate in the body column, (3) formation of new JD1+ sensory cells requires interstitial cell differentiation. We conclude from these results that the two populations defined by these antibodies are incorporated into the h ad via different developmental pathways and, therefore, constitute distinct nerve cell lineages.

Maintenance of proliferative capacity during avian myogenesis is independent of fusion-permissive calcium ion concentration.

Skeletal muscle differentiation is normally accompanied by the permanent withdrawal of myogenic nuclei from the proliferative cycle. However, embryonic Japanese quail (Coturnix coturnix japonica) myoblasts which have been prevented from fusing in vitro by the addition of EGTA to the culture medium retain the capacity to re-enter the cell cycle following accumulation of muscle-specific myosin. We have therefore investigated the roles of Ca2+ and fusion in the withdrawal of myogenic cells from the cell cycle. Using three defined media which differ in Ca2+ and in the ability to promote fusion, we examined the ability of differentiation-competent myoblasts to resume proliferation with increased time in G1. Under these conditions, there is a periodic variation in the ability of the myoblasts to respond to mitogenic stimulation, irrespective of the medium employed. These results indicate that loss of proliferative capacity during myogenesis is independent of Ca2+.

Regulation of interstitial cell differentiation in Hydra attenuata. III. Effects of I-cell and nerve cell densities.

The interstitial cell (i-cell) of hydra, a multipotent stem cell, produces two classes of differentiated cell types, nerve cells and nematocytes, throughout asexual growth. Using a new assay, the regulation of i-cell commitment to either nerve cell or nematocyte differentiation was investigated. This assay was used to determine the fractions of i-cells differentiating into nerve cells and nematocyte precursors in a variety of in vivo cellular milieus produced by hydroxyurea treatment, differential feeding, and reaggregation of dissociated cells. Nematocyte commitment was found to be positively correlated with the size of the i-cell population and independent of the axial position of the i-cells along the body column. This indicates that i-cell commitment to nematocyte differentiation may be regulated by feedback from the i-cell population. Nerve cell commitment was found to be correlated with regions of high nerve cell density. This suggests that nerve cell commitment is regulated by feedback from the nerve cell population or is dependent on axial position. Implications of such mechanisms for the regulation of i-cell population size and distribution are discussed.

Regulation of interstitial cell differentiation in Hydra attenuata. IV. Nerve cell commitment in head regeneration is position-dependent.

In hydra, nerve cells are a differentiation product of the interstitial cell, a multipotent stem cell. Nerve cell commitment was examined during head regeneration in Hydra attenuata. Within 3 h of head removal there is a 10- to 20-fold increase in nerve cell commitment in the tissue which subsequently forms the new head. Nerve cell commitment is unaltered in the remainder of the gastric region. This local increase in nerve cell commitment is responsible for about one half the new nerve cells formed during head regeneration, while one half differentiate from interstitial cells that migrate into the regenerating tip.

Regulation of interstitial cell differentiation in Hydra attenuata. V. Inability of regenerating head to support nematocyte differentiation.

Nematocyte differentiation was examined during head regeneration in Hydra attenuata. Nematocyte precursors were found to decrease in head-regenerating tissue. This decrease could not be attributed to decreased stem cell commitment or to altered cellular kinetics. The nematocyte precursors could be 'rescued' by regrafting a head onto the initially regenerating tissue only prior to the time at which head determination occurred. These results suggest that concurrent with head determination an irreversible change occurs in the tissue environment, resulting in decreased survival of cells committed to nematocyte differentiation.