Human heel fat pad development before birth.

The prenatal structural organization of the human heel fat pad (HHFP) remains poorly understood, despite its relevance for foot development and related disorders. It is composed of neural, vascular, fibrous, and elastic components arranged into a system of honeycomb-like architecture of microchambers and macrochambers embedded within adipose tissue. This study aimed to characterize the histological and morphometric development of the HHFP, with particular emphasis on compartmentalization and layer-specific maturation, identifying developmental weeks potentially relevant to congenital abnormalities. Embryonic and fetal specimens (8-34 gestational weeks) well preserved anatomical integrity and without histological evidence of malformation or tissue degradation of the feet were included from the Department of Anatomy and Embryology of the Complutense University of Madrid were analyzed. Early stages were characterized by undifferentiated mesenchymal tissue with prominent vascularization and absence of organized adipose structures. Based on structural organization, HHFP development was classified into three stages: Stage A, defined by early organization without a clearly identifiable horizontal fibrous band (HFB); Stage B, representing the progressive formation of the HFB and the onset of compartmentalization into superficial subcutaneous microchamber (SSM) and deep subcutaneous macrochamber (DSM) layers; and Stage C, characterized by a well-defined HFB, established compartmentalization, and progressive adipocyte maturation. Morphometric analysis demonstrated a progressive increase in HHFP thickness, with consistently greater thickness and adipocyte diameter in the DSM compared to the SSM. These findings indicate that HHFP development followed a rapid growth between weeks 14 and 17, with region-specific pattern in which compartmentalization precedes adipocyte maturation and occurs in an asynchronous, layer-dependent manner. This study provides the first structural framework for understanding prenatal HHFP organization, extending beyond classical models of adipose tissue development.