Abstract
Background:
To ensure food security, it is crucial to examine the immune system of Ross broiler chickens, which are widely used locally.

Aim:
Though a major lymphoid organ, the thymus also exhibits secondary lymphoid traits. This study analysed the post-hatching thymus and B-lymphocytes of this breed of chicken to establish baseline, age-related developmental, and immunological data for the thymus and B-lymphocyte ((Bu-1+)) population in unvaccinated Ross broiler chickens. And Its provides crucial, normal histological and morphometric parameters.

Methods:
H&E staining and a mouse anti-chicken monoclonal anti-Bu-1 antibody were used to analyse Ross grill chicken thymus from day 1 to day 58 post-hatching. The study examined B-lymphocytes, Hassall's corpuscles, and their developmental alterations. The cortex and medulla thickness and thymic lobule size were also measured during the investigation.

Results:
A study found that the thymus was mostly made of lymphocytes, with additional cell types appearing as early as day 1 post-hatching. These cells proliferated with age. Myoid cells (MCs) and Hassall's corpuscles (HCs) were found in the thymus cortex and medulla. MCs were classed as Kind 1, 2, and 3, while HCs were Type I, II, and III based on morphology. By day 7, MCs and HCs were more common, and by day 58, their numbers increased. Thymic lobules grew dramatically with age. By day 28, the cortex and medulla were significantly thicker than before. The medulla of the thymic lobules had Bu-1-positive B-lymphocytes from day 14 post-hatching.

Conclusion:
Thymic maturation in unvaccinated Ross broiler chickens is age-depentent with no mortaltily, indicating a functional immune system that may support future vaccination strategies.

Key words: Chicken; Histomormphmetric; Immunohistochemistry; Thymus.