Is the metrial gland really a gland?

An intriguing history surrounds the ‘metrial gland’. It is a term coined by Selye and McKeown (1935) to describe a pregnancy-specific modification of the mesometrial uterus juxtaposed to the developing chorioallantoic placenta of the rat. Selye and McKeown were justifiably fascinated with the prominent secretory granules present within cells of the structure and were aware of the earlier speculations about the endocrine activity of the tissue (Ancel and Bouin, 1911; Weill, 1919; Gerard, 1927). The term, ‘metrial gland’, has been used to describe similar anatomical regions in other species, including the mouse. Experimental approaches employed to study the ‘metrial gland’ have generally had some bias. The metrial gland has been viewed as a panacea for the adventurous endocrinologist, immunologist, and reproductive biologist. In recent years, there has been considerable discussion about the suitability of the term ‘metrial gland’ (Croy, 1999; Stewart, 1999, 2001; Pijnenborg, 2000). In her provocative commentary Croy questioned the use of the term ‘metrial gland’ to describe the uterine mesometrial compartment (Croy, 1999). She stated: “The ‘metrial gland’ is not epithelial in origin, is dissimilar in histological structure to other glands and has not been found to have endocrine or exocrine functions.” We take exception with this assessment, as is addressed below. Croy further recommended the use of the term ‘mesometrial lymphoid aggregate of pregnancy’ instead of ‘metrial gland’. Pijnenborg and Stewart concurred with the limitations of the term ‘metrial gland’. They aptly discussed aspects of the complex cellular composition of the structure, its dynamic nature, and its species-specific characteristics; and likewise offered their own preferred terminology (Stewart, 1999, 2001; Pijnenborg, 2000). We have been drawn to the ‘metrial gland’ of the rat from yet a different vantage point. A population of cells within the ‘metrial gland’ is a target of prolactin (PRL)-like protein-A (PLP-A). PLP-A is a member of the PRL family of hormones/cytokines produced by trophoblast cells (Soares and Linzer, 2001). PLP-A is present in maternal circulation and