TDK conference 2024

Introduction: In birds, the gut-associated lymphoid tissue (GALT) contains a large number of lymphatic organs, which are scattered along the cranio-caudal axis of the intestinal tract. These include the esophageal-, pyloric-, and coecal tonsils, Peyer's patches, or the bursa Fabricii, a peculiar organ responsible for the avian B cell development. In addition to the larger lymphatic organs, many smaller lymphatic tissues with scattered germinal centers (bursa Fabricii peripheral fold, dorsal diffuse infiltrated area (DIA)) also belong to the GALT. The specific anatomical position of the lymphatic tissues associated with the cloaca raises the chance that one of the possible routes of external antigens and infectious agents entering the body leads through the cloaca.

Objective: The main objective of my student research work is the morphological (histology, immunocytochemistry, confocal, and electron microscopy) characterization and ontogenesis of the cloacal lymphatic tissues. According to our preliminary observation, using the serial paraffin sections of the rectum-cloacal-bursa Fabricii region of the adult chicken, in addition to the DIA and the peripheral fold, the ventral part of the bursa duct, specifically in the mucosal fold facing to the colorectum, is rich in lymphoid tissue, which has not been described before.

Methods and Results: Using immunocytochemistry, we confirmed that there is extensive lymphatic tissue in the ventral fold of the bursa duct opening to the cloaca. This lymphatic tissue is located ventral to the DIA. According to CD45 immunoreactivity, numerous lymphomyeloid cells are concentrated in the lamina propria. With double immunofluorescence, we demonstrated that the lymphoid tissue is connected to the surface cloacal epithelium and is organized into B- and T-dependent regions. B cells cluster in germinal centers (GCs), which contain CSF1R+/74.3+ dendritic cells and TIM4+ macrophages, respectively, while the interfollicular lymphoid tissue, located between the GCs, is rich in T cells. Lymphatic tissue of the bursa duct consists of individual tonsillar units separated in the lamina propria by collagen-rich connective tissue septum.

Summary: Using histological and immunocytochemical methods, we describe a novel lymphoepithelial organ associated to the duct of the bursa of Fabricius. Detailed immunocytochemical characterization confirms that this lymphoid organ is consistently found in the cloacal of the chicken and consists of tonsillar units identical to the pyloric and coecal tonsils. We named this lymphoid tissue as the cloacal tonsil.