THE CELLULAR BASIS OF HYDROID MORPHOGENESIS

Abstract

Until quite recently, the general framework within which the maintenance of form in the face of continual cell renewal in hydroids has been viewed has been that elaborated by Paul BRIEN, modified with respect to the sites of proliferation by Richard CAMPBELL and the disposition of cells by Stanley SHOSTAK and his students. The growth of hydroids, according to this model, and to the generative scheme described by KUHN, was viewed as similar to the growth of meristematic plants. The form of the animal was believed to arise as a consequence of the location of pwliferative regions. Essential to this view of hydroid morphogenesis has been two tenets: (1) that, with the exception of cnidocytes, cells in hydroids move as coherent sheets, and (2) that cells of each of the two layers retain the integrity of their layer of origin-i.e. there is no crossing over the mesoglea. As early as 1930, however, KANAJEW described the movement of vitally stained cells from the epidermis of hydra to the gastrodermis. Subsequently considerable conflicting data has appeared indicating that growth in hydroids is not similar to the meristematic growth of plants, but rather that the sites of cell proliferation are removed in space from the sites of utilization, that cells migrate individually, actively as amoebocytes through the epidermis or passively as epitheliocytes, carried along in the hydrocoel, to their sites of utilization, and that considerable migration across the mesoglea occurs. A new model of hydroid morphogenesis and morphostasis can now be constructed based upon new information regarding the sites of cell proliferation and cell migration, and accounting for the form of the colony in terms of cellular proclivities such as amoebocytic or epitheliocytic tendencies and cell stickiness, and identifying the decision points of cellular differentiation as a consequence of which colony form is generated. The majority of the data for this model is derived from my studies of the colonial marine hydroid Podocoryne carnea. This model will take the form of a flow chart which accounts for the source, distribution and disposition of cellular elements and attempts to account for colony form as a consequence of cellular activities.