Congenital fetal rhabdomyoma in a foal.

This research article chronicles the birth of an Appaloosa filly (a young female horse) with a rare, benign tumor known as a “fetal form” rhabdomyoma and discusses the breakthrough it represents in veterinary science, as this is the first such reported case.

Article Overview

The article begins by recounting the birth of a filly born with a palpable ventral midline subcutaneous mass, a type of tumor located on the neck of the horse. This mass measured roughly 8 x 12 x 15 cm.

• The mass was non-encapsulated, meaning it was not isolated or contained within a partition in the body.
• The tumor was composed of intertwined or randomly assembled bundles of spindle cells – cells that become long and thin while the filly was forming in the womb.
• These cells existed on a moderate to abundant loose myxomatous stroma – a supportive or connective tissue full of mucoid substance.

Cell analysis

A deeper analysis of the cell population in the mass showed:

• A portion of these cells illustrated cross striations, small alternating light and dark bands, with minor nuclear variation, indicating small differences in cell nuclei.
• The proliferatory ability of these cells, indicated by mitotic rate, was low, a characteristic typical of benign tumors.

Immunohistochemical Staining

Later, the article describes the results of immunohistochemical staining, a technique used to identify certain antigens (proteins) in cells of a tissue section.

• The tumor cells show positive responses to myoglobin, desmin, actin, vimentin, and S-100, muscles and neural-related antigens, indicating that they may be derived from muscle or nervous tissue.
• They were negative for glial fibrillar antigen and keratin. The former is mainly located in the nervous system, and the latter is a type of protein that protects epithelial cells from damage or stress. Their absence suggests that the tumor doesn’t originate from these tissues.

Rhabdomyomas

The article then delves specifically into the classification and characteristics of rhabdomyomas, benign tumors of the striated muscle, concluding that:

• Rhabdomyomas are a rare form of benign tumor.
• Previously, the rhabdomyomas described in veterinary literature mimicked the “adult form” analogous to human rhabdomyomas. They are typically associated with adult animals and humans.
• The tumor found in the filly is the first reported unique case in a) clinically and histologically paralleling the “fetal form” in human rhabdomyomas, and b) being a congenital extracardiac rhabdomyoma, a tumor that arises outside the heart and is present from birth.