The brood pouch of seahorses can be divided into four sequential stages based on the characteristics of the altered tissue layers during gestation: the normal stage, the embryo-carrying stage, the embryo-release stage and the repair stage. The brood pouch is composed of a folded inner pseudostratified columnar epithelium and a smooth outer stratified cuboidal epithelium. Three tissue layers between the inner and the outer epithelia are an inner loose connective tissue layer, a middle smooth muscle layer and an outer dense irregular connective tissue layer. In the normal stage, the inner loose connective tissue layer is thick and vascularised with small blood vessels; the muscle layer consists of scattered unorganised muscle fibres. In the embryo-carrying stage, the inner epithelial and inner loose connective tissue layers become distended and highly vascularised with enlarged blood vessels. In the embryo-release stage, the inner loose connective tissue layer is extensively vascularised with very large blood vessels and the smooth muscle fibres invade the outer dense irregular connective tissue layer. Structures altered during gestation gradually resume their normal condition in the repair stage. Extensive vascularisation of the brood pouch during gestation suggests an intricate paternal–embryo relationship implying other significant roles besides protective function of the pouch.