Investigation of the Morphology of Adrenal Glands in Hens Kept in Two Different Housing Systems—A Pilot Study

SIMPLE SUMMARY: One of the most important aspects in keeping farm animals is the housing condition, which has to fulfill the behavioral needs of the housed animals and must be economically efficient at the same time. In non-European countries, laying hens are often kept in enriched cages, while in European countries single cages are only used for breeding to record the laying performances. In contrast, floor housing pens are predominant in Germany. We assume that laying hens in single cages suffer more from chronic stress than laying hens in floor pens. To test this hypothesis, we studied the effect of housing conditions on the adrenal gland, which consists mainly of adrenal cells and interrenal cells. While the adrenal cells secrete catecholamines such as adrenalin and noradrenalin along with other hormones, the interrenal cells release steroid hormones. These also include corticosterone, which also plays an important role in chronic stress. In our study, we histologically investigated whether there is a difference in the amount of these cell types between laying hens kept in single cages and floor pens. We have found a slight tendency of a higher adrenal–interrenal ratio in floor-housed hens which is expressed in a relatively lower total area of interrenal cells in floor-housed hens. In addition, the animals in the floor housing were significantly heavier than those in the cage housing during the entire test period. ABSTRACT: It is difficult to objectively assess the chronic effects of housing systems on livestock and particularly on laying hens. However, this seems to be important in the context of animal welfare. Therefore, we conducted the present study in order to compare the effect of two different housing conditions, single cage (SC) and floor pen (FP), on the morphology of the adrenal gland. A higher amount of interrenal cells, which secrete stress hormones, can lead to a difference in the relation of adrenal and interrenal cells, which could be interpreted as an indication of chronic stress. For this purpose, adrenal glands were extracted, prepared, stained and examined by microscopy, and total area of the cut, total area of interrenal cells and total area of adrenal cells were measured. As a result, all laying hens had a higher percentage of interrenal cells than adrenal cells (FP: interrenal cells/adrenal cells = 78.37%/21.63%; SC: 80.00%/20.00%). The median of adrenal–interrenal ratio did not differ significantly (FP = 0.2503, SC = 0.2499), while the variation of the ratio between laying hens in FP and SC showed a slight tendency of a higher ratio in adrenal glands of FP (p < 0.0870). Body weight and adrenal–interrenal ratio were significantly negatively correlated in laying hens in FP (r(S) = −0.943, p < 0.0048) but not in SC (r(S) = −0.162, p = 0.7283). There was no significant correlation between body weight and total cell area for interrenal cells or adrenal cells. Body weight was significantly lower for laying hens kept in SC than for laying hens kept in FP (p < 0.0001). Due to the present results, it can be concluded that keeping laying hens in single cages can have a negative effect on body weight.