Assembly of contractile and cytoskeletal elements in developing smooth muscle cells
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R G Chou1, M H Stromer, R M Robson
1Department of Animal Science, Iowa State University, Ames 50011.
Insights
This study details smooth muscle development in chick gizzards, identifying key stages from embryonic day 6 to posthatch day 7. It reveals the sequential assembly of contractile and cytoskeletal elements, crucial for muscle formation.
Area of Science:
- Developmental Biology
- Cell Biology
- Muscle Physiology
Background:
- Smooth muscle development involves the intricate assembly of contractile and cytoskeletal proteins.
- Understanding the precise timeline and sequence of these events is crucial for comprehending muscle formation and function.
Purpose of the Study:
- To characterize the specific developmental changes in smooth muscle during chick embryo and posthatch development.
- To identify and define distinct developmental stages based on the appearance and growth of contractile and cytoskeletal elements.
Main Methods:
- Comparative analysis of chick gizzard smooth muscle from embryonic day 6 to posthatch day 7.
- High-resolution electron microscopy to observe ultrastructural changes.
- Immunogold labeling with anti-desmin to identify intermediate filament composition.
Main Results:
- Myoblast proliferation occurs early, with cytoplasmic dense bodies (CDBs) and membrane-associated dense bodies (MADBs) appearing by embryonic day 8.
- Thick filaments emerge by embryonic day 12, and smooth muscle cells retain the capacity for division until embryonic day 20.
- The axial intermediate filament bundle (IFB), containing desmin, becomes prominent in posthatch cells, with myofilaments and dense bodies assembling before the IFB.
Conclusions:
- Smooth muscle development in chick gizzards follows a defined sequence, with contractile elements preceding major cytoskeletal structures like the IFB.
- Seven distinct developmental stages were identified, characterized by the progressive organization of myofilaments and cytoskeletal components.
- The findings provide insights into the in vivo assembly mechanisms of smooth muscle.
Abstract:
Specific developmental changes in smooth muscle were studied in gizzards obtained from 6-, 8-, 10-, 12-, 14-, 16-, 18-, and 20-day chick embryos and from 1- and 7-day posthatch chicks. Myoblasts were actively replicating in tissue from 6-day embryos. Cytoplasmic dense bodies (CDBs) first appeared at Embryonic Day 8 (E8) and were recognized as patches of increased electron density that consisted of actin filaments (AFs), intermediate filaments (IFs), and cross-connecting filaments (CCFs). Although the assembly of CDBs was not synchronized within a cell, the number, size, and electron density of CDBs increased as age increased. Membrane-associated dense bodies (MADBs) also could be recognized at E8. The number and size of MADBs increased as age increased, especially after E16. Filaments with the diameter of thick filaments first appeared at E12. Smooth muscle cells were able to divide as late as E20. The axial intermediate filament bundle (IFB) could first be identified in 1-day posthatch cells and became larger and more prominent in 7-day posthatch cells. Immunogold labeling of 1- and 7-day posthatch cells with anti-desmin showed that the IFB contained desmin IFs. The developmental events during this 23-day period were classified into seven stages, based primarily on the appearance and the growth of contractile and cytoskeletal elements. These stages are myoblast proliferation, dense body appearance, thick filament appearance, dense body growth, muscle cell replication, IFB appearance, and appearance of adult type cells. Smooth muscle cells in each stage express similar developmental characteristics. The mechanism of assembly of myofilaments and cytoskeletal elements in smooth muscle in vivo indicates that myofilaments (AFs and thick filaments) and filament attachment sites (CDBs and MADBs) are assembled before the axial IFB, a major cytoskeletal element.