Background

The medial upper eyelid fat compartment has long been recognized clinically as being firmer, whiter, and more fibrous than the central preaponeurotic fat. Despite its surgical relevance, systematic quantitative histological characterization remains limited.
Objectives

The objective of this study was to quantitatively compare the histological and immunohistochemical characteristics of medial and central upper eyelid fat compartments, with particular focus on connective tissue composition, vascularization, and stromal nuclear density.
Methods

In a paired-sample design, medial and central upper eyelid fat compartments were harvested from 10 patients undergoing upper blepharoplasty. Each compartment was analyzed using hematoxylin and eosin (H&E) staining, acid fuchsin + orange G staining, and CD31 immunohistochemistry. Connective tissue proportion, capillary density, and stromal nuclear density were assessed.
Results

The medial compartment demonstrated a significantly higher proportion of connective tissue compared with the central compartment (67.74 ± 5.63% vs 56.63 ± 5.56%, P < .001). In contrast, capillary density was significantly reduced in the medial compartment (1.60 ± 0.54% vs 3.04 ± 1.14%, P = .002). Stromal nuclear density was also lower in medial fat compared with central fat (0.85 ± 0.66% vs 1.76 ± 1.06%, P = .004). Across all parameters, the central compartment consistently showed higher stromal nuclear density and vascularization, whereas the medial fat exhibited increased fibrous density.
Conclusions

The medial upper eyelid fat compartment differs histologically from the central compartment by an increased connective tissue content and reduced vascular and stromal cell density. These findings provide a structural basis for its clinically observed firmness and potentially altered regenerative behavior.