# Differential expression of myosin heavy chain genes in fast and slow muscles of the developing rat hindlimb in vivo and in vitro

#### Abstract

Adult mammalian muscles are composed of different types of muscle fibers with distinct contractile characteristics. How these patterns of muscle fiber diversity are established in adult muscles from a seemingly homogeneous population of fibers in the fetal animal is a fundamental question in muscle development. This study examined the stability of the differentiated phenotype of secondary generation myoblasts and the plasticity of this phenotype in response to thyroid hormone (T$\sb3$), a known modulator of gene expression, in vitro. The study used the isoforms of myosin heavy chain (MHC) as developmental and fiber-type specific markers to determine the patterns of MHC gene expression in rat hindlimb muscles during late fetal development and in primary cultures of 19-day fetal hindlimb muscles. The patterns of MHC gene expression were characterized by steady-state messenger RNA (mRNA) levels, determined by ribonuclease protection assays. This study found that all skeletal MHC genes were expressed in vitro, but the pattern of expression did not recapitulate the developmental sequence observed in vivo--there was no sequential replacement of immature isoforms by adult isoforms. Cultures of slow soleus muscles and fast extensor digitorum longus (EDL) muscles exhibited differential patterns of MHC gene expression consistent with their phenotypes in vivo. Soleus cultures had higher levels of slow ($\beta$ cardiac) MHC messages than EDL cultures; conversely, EDL cultures had higher levels of 2A and 2B messages than soleus cultures. All cultures responded to hyperthyroid conditions in vitro with increased 2A and 2B MHC gene expression, and decreased slow MHC gene expression. Neither 2A nor 2B MHC transcripts were detected in vivo until birth. Denervation of fetal muscles from day 14 through day 21 did not allow 2A or 2B genes to be expressed, and did not affect embryonic, neonatal, or slow MHC expression. Hyperthyroid conditions in utero accelerated the appearance of 2A and 2B transcripts from birth to 21-days fetal. The results indicated that differentiated phenotypes are established early in muscle development, these phenotypes are stable in vitro, and they respond to T$\sb3$ in a tissue-specific manner.

#### Subject Area

Anatomy & physiology|Molecular biology|Biology

#### Recommended Citation

Hoffman, Rebecca K, "Differential expression of myosin heavy chain genes in fast and slow muscles of the developing rat hindlimb in vivo and in vitro" (1990). Dissertations available from ProQuest. AAI9026579.
https://repository.upenn.edu/dissertations/AAI9026579

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