# Integrated Sensory and Immune Cell Organization in the Lip Skin of the Goldfish (Carassius auratus, Linnaeus, 1758)

## TL;DR

This study explores the specialized lip skin of goldfish, revealing a unique combination of sensory and immune cells that work closely together.

## Contribution

The study is the first to simultaneously identify multiple immune and sensory cell types in goldfish lip skin using histological, ultrastructural, and immunohistochemical methods.

## Key findings

- Goldfish lip skin contains a high density of sensory and immune cells, including neuromasts, Merkel cells, and macrophages.
- Telocytes and synapse-like structures were observed, suggesting roles in cellular communication and sensory processing.
- Immunohistochemical markers confirmed the presence of specific cell types and their spatial organization.

## Abstract

Fish lips are specialized sensory regions involved in foraging, substrate exploration, and environmental detection. In goldfish, the lip skin contains a high concentration of sensory and immune cells, yet its detailed structure has not been fully described. Histological and ultrastructural observations revealed diverse cell types, including Merkel cells, rodlet cells, eosinophilic granular cells, and intraepidermal macrophages, as well as specialized sensory organs such as neuromasts and tuberous-like sensory units. Immunohistochemical labeling patterns for CK20, S100, CD68, CD64, CD117, and E-cadherin were consistent with the corresponding histological and ultrastructural observations. Telocytes were also identified in the dermis, indicating roles in cellular communication. Our results highlight the structural specialization of the lips as a region where sensory and immune-related elements are closely associated.

This study examines the histological, ultrastructural, and immunohistochemical features of the lip skin of the goldfish (Carassius auratus, Linnaeus, 1758), a sensory-rich region that plays an essential role in feeding and environmental perception. Our findings highlight the coexistence and close association of immune, epithelial, and sensory cells within the epidermis and dermis. For the first time in goldfish, intraepidermal macrophages, eosinophilic granular cells, rodlet cells, Merkel cells, and several specialized sensory structures—neuromasts, taste buds, and tuberous-like sensory units—were simultaneously identified within the same integumentary field. Quantitative morphometry demonstrated a high density of eosinophilic granular cells, rodlet cells, and neuromasts per unit epithelial area, reinforcing the functional specialization of the goldfish lip as a sensory–immune interface. Immunohistochemical markers (CK20, S100, CD68, CD64, CD117, and E-cadherin) were applied as complementary tools to describe phenotypic labeling patterns. These findings are interpreted cautiously as supportive evidence consistent with epithelial, neural-associated, stromal, and immune cell distributions observed morphologically. Transmission electron microscopy further uncovered fine structural details such as synapse-like contacts in taste buds and Merkel cells, dense-core granules in eosinophilic granular cells, and telocyte–nerve fiber associations in the dermis. By integrating cellular, structural, and immunohistochemical perspectives, this study provides a novel descriptive reference for the goldfish lip skin as a region characterized by the close spatial association of sensory and immune-related elements, underscoring its value as a model for vertebrate cutaneous biology and neuroimmunology.