Abstract

Multiple states of differentiation developed within the same undivided egg cytoplasm of ascidian zygotes cleavage-arrested with cytochalasin B. Complex ultrastructural traits of up to four quite diverse cell lineage components were observed in regions of the common cytoplasm in such multinucleate homokaryons of Ciona intestinalis: epidermal, muscle, notochordal, and neural. Almost all specimens among those selected as showing differentiation contained two such features, half of them had at least three, and a few expressed all four. The histospecific morphological characteristics noted were the extracellular test material of epidermal cell origin, muscle myofilaments and myofibrils, sheath components (leaflets and filaments) associated with notochordal cells, and the particular localized combinations of microtubules, filamentous structures, and cilia indicative of ascidian neural tissues. Cleavage-arrested one-celled embryos of Ascidia ceratodes served to demonstrate that those which were found cytochemically to contain muscle acetylcholinesterase always had myofibrils and myofilaments. Other arrested zygotes of Ascidia (unstained specimens) also had quite fully formed test material as well as myofilaments and myofibrils. The occurrence within the same cell of so many specific markers of diverse pathways of development is consistent with a theory about a primary level of regulation based on autonomous gene activation factors already present in the fertilized egg. If further investigation substantiates a real cytoplasmic continuity within these cleavage-arrested embryos, other theories that invoke cell interactions, temporal sequences of metabolically distinct microenvironments, and gradients of substances as causes of determinative change seem inadequate to account for the coexisting expressions of differentiation described here.

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.