Embryonic and early post-hatch developmental morphology of the Thymus in Nigerian indigenous chickens (Gallus gallus domesticus)

The avian thymus is a primary lymphoid organ that produces T-lymphocytes that engage in active cell-mediated immune responses. The pre- and early post-hatch development of this organ in Nigerian indigenous chickens of the south-eastern region was understudied from embryonic incubation day (EID) 10 to day (D) 42 post-hatch to determine the pattern of growth and the timeline of development of the vital immunological structures using gross anatomical, histological, and morphometric techniques. The embryos were harvested from the gravid eggs for the pre-hatch studies. The thymus was collected through a ventral neck incision in both embryos and post-hatch chicks, observed for gross features, weighed and fixed in Bouin’s fluid. The fixed tissues were routinely processed and stained with hematoxylin and eosin (H&E) for histological studies. The thymus is composed of a bilateral chain of 7-9 irregular flat and pale-coloured lobes whose sizes vary with age. The absolute and relative weights of post-hatch thymus increased significantly between Day 0 (D 0), Day 14 (D 14) and Day 42 (D 42), and between D 0 and D 14, respectively. The lobation of thymic tissues was not quite significant until EID 18. Between D 0 and D 42 post-hatch, thymic lobes were outstanding. Histologically, the thymus at EID 10 appeared as tissue buds on both sides of the neck. By EID 14, each lobe was enveloped by a connective tissue capsule from which thread-like trabeculae penetrated the thymic substance. At EID 18, the parenchyma showed marked differentiation of tissues into cortical and medullary regions with remarkable cortical lymphocyte accumulation. At hatch (D 0), Hassall’s corpuscles were observed in the thymic medulla, and thymic cell density also significantly increased. Subsequent ages were marked by an increase in both thymic cell densities and thickness of the connective tissue capsule and trabeculae, together with the accompanying blood vessels. The early attainment of high relative weight and the high rate of proliferation of thymic lymphocytes, especially at late embryonic and early post-hatch ages, imply early establishment of cell-mediated immunity, which may offer immunological protection prior to vaccination in Nigerian indigenous chicken.