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The role of thecal glands in the ovary of birds remains controversial. Using transmission electron microscopy and immunohistochemistry, immunohistochemical localisation of cyclooxygenase I and II (COX-1 and COX-2), oestrogen receptor α and β (ER-α and ER-β), androgen receptor (AR) and progesterone receptor (PR), a detailed analysis of the thecal glands was performed. Our ultrastructural studies revealed that the thecal glands of the quail ovary consist of 2 cell types, steroid-producing cells (SPCs) and enclosing cells (ENCs). The SPCs are large, light cells containing a varying number of lipid droplets. Their cytoplasm is characterised by a large amount of smooth endoplasmic reticulum. The ENCs are always located at the periphery of the gland. Some ENCs contain an abundant number of microfilaments, but lipid droplets and dense bodies were rare. Within 1 gland, SPCs with distinct COX-2 immunostaining were interspersed between usually larger numbers of moderately COX-2-positive cells. A completely different staining pattern was observed for COX-1, where the cytoplasm of the ENCs was distinctly immunopositive. The SPCs stained only weakly with antibodies to COX-1. The thecal glands showed distinct reactions for ER-β but only a weak to negative one for ER-α, PR, and AR. Our immunohistochemical and ultrastructural data support our hypothesis that the thecal glands of the quail are involved in steroid hormone and prostaglandin synthesis. The prostaglandins secreted by the thecal glands probably contribute to the ovulation of the follicle first in the hierarchy.