The present study aims to investigate the histological, histochemical and electron microscopic changes
of the caecal proximal part of Japanese quail during both pre- and post-hatching periods starting from
the 2nd embryonic day (ED) until four weeks post-hatching. On the 2nd and 3rd ED, the primordia of caeca
appeared as bilateral swelling on the wall of the hindgut. On the 7th ED, the lamina propria/submucosa
contained the primordia of glands. On the 8th ED, rodlet cells could be observed amongst the epithelial
cells. On the 9th ED, the caeca began to divide into three parts with more developed layers. With age,
the height and number of villi increased. On the 13th ED, immature microfold cells (M-cells) could be
identified between the surface epithelium of the villi. The caecal tonsils (CTs) appeared in the form of
aggregations of lymphocytes, macrophages, dendritic cells and different types of leukocytes. Telocytes
and crypts of Lieberkuhn were observed at this age. On hatching day, the crypts of Lieberkuhn were
well-defined and formed of low columnar epithelium, goblet cells, and enteroendocrine cells. Posthatching,
the lumen was filled with villi that exhibited two forms: (1) tongue-shaped villi with tonsils
and (2) finger-shaped ones without tonsils. The villi lining epithelium contained simple columnar
cells with microvilli that were dispersed with many goblet cells, in addition to the presence of a high
number of intra-epithelial lymphocytes and basophils. Moreover, the submucosa was infiltrated by
numerous immune cells. CD3 immunomarker was expressed in intraepithelial lymphocytes, while CD20
immunomarker showed focal positivity in CTs. In conclusion, the caecal immune structures of quails at
post-hatching were more developed than those in pre-hatching life. The high frequency of immune cells
suggests that this proximal part may be a site for immunological surveillance in the quail caecum. The
cellular organisation of the caecum and its relation to the immunity was discussed.