Myoepithelial cells

Myoepithelial cells are specialized contractile cells found in various exocrine glands, including salivary, mammary, sweat, and lacrimal glands. They play a critical role in supporting glandular architecture and facilitating secretion. Their unique structural and functional properties make them important both in normal physiology and in pathological conditions.

Introduction

Myoepithelial cells are located between the basal lamina and the secretory epithelial cells of exocrine glands. They exhibit both epithelial and smooth muscle characteristics, allowing them to contribute to the contractile activity of glands while maintaining structural support. Their presence is essential for efficient secretion and maintenance of glandular integrity.

Histological and Morphological Features

Cell Shape and Structure

Myoepithelial cells are typically stellate, spindle-shaped, or pyramidal, depending on the gland and species. They extend cytoplasmic processes that interdigitate with adjacent epithelial cells, providing both mechanical support and contractile function.

Location in Glands

These cells are positioned between the basal lamina and secretory cells in exocrine glands, forming a continuous layer around acini and ducts. They are prominently found in:

• Salivary glands
• Mammary glands
• Sweat glands
• Lacrimal glands

Ultrastructural Characteristics

Under electron microscopy, myoepithelial cells exhibit abundant cytoplasmic filaments composed of actin and myosin, which enable contractility. They form desmosomal and hemidesmosomal junctions, anchoring themselves to epithelial cells and the basal lamina, ensuring both stability and functional coordination.

Immunohistochemical Profile

Marker Expression

Myoepithelial cells express a characteristic set of proteins that allow their identification in tissue samples. Common markers include:

• Smooth Muscle Actin (SMA): Confirms contractile capacity
• Calponin: Associated with actin filament stabilization
• p63: Nuclear marker often used in diagnostic pathology
• Cytokeratins: Indicative of epithelial origin
• S100 Protein: Present in certain myoepithelial populations

Diagnostic Utility

Immunohistochemical staining of myoepithelial markers is crucial for identifying these cells in biopsies. This is especially important in distinguishing benign lesions from malignant tumors and in recognizing tumors with myoepithelial differentiation.

Development and Differentiation

Embryological Origin

Myoepithelial cells are derived from epithelial progenitor cells during gland development. They arise alongside secretory cells and acquire both epithelial and contractile properties through differentiation.

Maturation and Functional Differentiation

During maturation, myoepithelial cells develop cytoplasmic actin and myosin filaments, desmosomal attachments, and the ability to contract. These changes enable them to support secretory function, maintain glandular architecture, and participate in paracrine signaling within the gland.

Functions of Myoepithelial Cells

Contractile Function

Myoepithelial cells facilitate the expulsion of secretions from glandular acini and ducts. Their contractile activity, mediated by actin and myosin filaments, ensures efficient movement of saliva, milk, sweat, or tears into the ductal system.

Structural Support

By surrounding secretory cells and anchoring to the basal lamina, myoepithelial cells maintain glandular architecture. They provide mechanical stability to acini and ducts, preventing collapse and preserving tissue integrity.

Paracrine and Regulatory Roles

Myoepithelial cells secrete signaling molecules that influence the behavior of neighboring epithelial and stromal cells. They contribute to tissue homeostasis, regulate epithelial differentiation, and modulate glandular function through paracrine interactions.

Barrier Function

Myoepithelial cells form a physical and functional barrier that limits the invasive potential of epithelial cells. They help restrict the spread of neoplastic cells and maintain the compartmentalization of glandular tissue.

Myoepithelial Cells in Disease

Benign Lesions

Myoepithelial cells are prominent in several benign tumors, including:

• Pleomorphic adenoma of the salivary gland
• Syringoma of sweat glands
• Other myoepithelial-rich adenomas

These cells contribute to the histological diversity and stromal interactions observed in benign lesions.

Malignant Transformation

Myoepithelial carcinomas and adenoid cystic carcinomas arise from or involve myoepithelial cells. These malignancies can exhibit aggressive behavior, perineural invasion, and potential for metastasis, highlighting the importance of recognizing myoepithelial components in diagnosis.

Role in Tumor Suppression

In normal glands, myoepithelial cells exhibit anti-invasive and anti-angiogenic properties. They produce extracellular matrix components and inhibitory factors that limit tumor progression and invasion of surrounding tissues.

Pathophysiological Alterations

Changes in myoepithelial cell function or density can occur in autoimmune disorders, fibrosis, and glandular dysfunction, contributing to altered secretion, structural compromise, and predisposition to disease.

Techniques for Studying Myoepithelial Cells

Histology and Microscopy

Conventional light microscopy allows visualization of myoepithelial cells in tissue sections using hematoxylin and eosin staining. Electron microscopy provides detailed ultrastructural information, revealing cytoplasmic filaments, desmosomes, and interactions with secretory cells.

Immunohistochemistry and Immunofluorescence

These techniques utilize antibodies against myoepithelial markers such as SMA, calponin, and p63 to specifically identify myoepithelial cells. Fluorescent labeling allows localization and quantification within tissue sections and aids in distinguishing them from neoplastic epithelial cells.

Cell Culture and Molecular Studies

Primary culture and in vitro studies of myoepithelial cells allow investigation of contractility, signaling pathways, gene expression, and interaction with epithelial or stromal cells. Molecular techniques such as PCR, Western blotting, and RNA sequencing provide insights into the functional biology of these cells.

Clinical and Diagnostic Significance

Histopathological Diagnosis

Recognition of myoepithelial cells in biopsy or surgical specimens is critical for diagnosing glandular lesions. Their presence helps differentiate benign from malignant tumors, and their pattern can inform on tumor architecture and behavior.

Prognostic Value

The integrity and abundance of myoepithelial cells are associated with tumor suppression. Loss or disruption of myoepithelial layers in carcinomas often correlates with increased invasiveness, metastatic potential, and poorer patient outcomes.

Therapeutic Implications

Targeting pathways involved in myoepithelial cell function or harnessing their tumor-suppressive properties represents a potential avenue for cancer therapy. Preservation or restoration of myoepithelial integrity may improve outcomes in glandular malignancies.

References

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