What Are Squamous Cells?
Squamous cells are dead, flattened keratin-filled cells that form the outermost protective layer of the skin (stratum corneum), representing the final stage of keratinocyte differentiation in stratified squamous epithelium. 1
Cellular Origin and Location
Squamous cells originate from keratinocytes, which comprise 95% of epidermal cells and are the major cell type of the epidermis. 1 These cells undergo a specific maturation process through multiple layers of the skin:
- Stratum basale: Contains mitotically active stem cells (deepest layer)
- Stratum spinosum: Irregular polyhedral cells connected by desmosomes
- Stratum granulosum: Diamond-shaped cells with keratohyalin granules
- Stratum lucidum: Thin transparent layer (only in thick skin)
- Stratum corneum: The outermost layer consisting of keratin and horny scales—these are the squamous cells, which were formerly live cells but are now dead 1
Structural Characteristics
Squamous cells are characterized by their large size, large nucleus, and low nucleo-cytoplasmic ratio when examined in superficial epithelial layers. 2 The term "squamous" refers to their flattened, scale-like appearance—derived from the Latin word for "scale."
Tissue Distribution
Squamous epithelium is found throughout the body in areas requiring protective barriers:
- Skin epidermis (most extensive)
- Oral cavity, pharynx, and esophagus
- Cervix, vagina, and anogenital regions
- Corneal epithelium (superficial layers) 2
- Thymus (forms three-dimensional mesh) 3
Functional Role
The primary function of squamous cells is to provide a protective waterproof barrier that shields the body from:
- UV light damage
- Water loss
- Microbial invasion
- Chemical exposure
- Physical trauma 1
Clinical Significance
In Diagnostic Testing
Squamous cells serve as important quality indicators in clinical specimens:
- Urine samples: Elevated squamous epithelial cells (>21.35 cells/µL in women, >5.05 cells/µL in men) indicate probable contamination from skin or genital tract 4
- Thyroid FNA: Presence of squamous cells is abnormal and requires careful evaluation for malignancy, metastatic disease, or developmental cysts 5
In Pathology
The presence of squamous cells in unexpected locations may indicate:
- Squamous metaplasia: Transformation of normal epithelium to squamous type (usually benign but can predispose to malignancy)
- HPV infection: Specifically targets basal keratinocytes of stratified squamous epithelium 6
- Dysplasia or malignancy: Atypical squamous cells warrant further investigation 7
Common Pitfall
Do not confuse squamous cells with keratinocytes—keratinocytes are the living, metabolically active cells that produce structural proteins, while squamous cells are the dead, terminally differentiated end-product of keratinocyte maturation that have completed the cornification process. 1, 8