Doctoral Dissertations

Steven W. Gay

Date of Award

6-1982

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Zoology

Major Professor

Jeffery A. MacCabe

Committee Members

Jim Caponetti, M. L. Pan, John Fallon, Robert Kosher

Abstract

The purpose of these experiments was to determine levels of commitment to differentiation and pattern formation in the developing chicle limb bud. Two series of experiments were performed. In the first experiments, commitment to differentiation was assayed by testing the ability of limb bud tissue explants to form myotubes in culture. Strips of apical mesoderm, with or without the attached apical ectodermal ridge, and strips immediately proximal to these, were removed from stage 17 to 23 wing buds and placed in culture with a medium conducive to myogenesis. Relatively few cultures of apical explants contained myotubes when the explants were removed from stage 17 to 19 wing buds. No significant myogenesis was observed in cultures of tissue taken from wing buds of later stages. This result was not dependent on whether the apical ectodermal ridge was present or not. Cultures of more proximal explants generally contained myotubes by the second or third day of incubation. These results suggest that there is a special zone of subridge mesoderm that extends about 0.10-0.20 mm into the mesoderm. Cells in this zone are not able to differentiate when cultured.

In the second series of experiments, portions of stage 21 or 23 leg buds were grafted onto portions of stage 21 or 23 wing buds. Distal to proximal transplants created presumptive gaps in the proximodistal axis of the graft limb; proximal to distal transplants created presumptive extra segments. Regulation (correction of abnormalities in the presumptive proximodistal axis) was observed in distal to proximal graft limbs but not in limbs resulting from proximal to distal grafts. The ability of host wing tissue to participate in regulation decreased at more proximal levels within the limb bud and with the age of the limb bud. These results, and those of previous investigations, suggest that proximodistal pattern is initially specified in subridge mesoderm and that the mesoderm becomes increasingly committed to this pattern specification at more proximal levels.

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