mesohyl

The mesohyl is the gelatinous, middle layer of most sponges (phylum Porifera) that lies between the outer pinacoderm (epidermis) and the inner choanoderm. It is often described as a connective-tissue–like matrix and contains a diverse mix of cells and skeletal elements embedded in an extracellular matrix rich in collagen and other glycoproteins and glycosaminoglycans.

Cells and skeletal elements resident in the mesohyl include archaeocytes (amoeboid cells specialized for nutrient transport, digestion, and differentiation), sclerocytes (which secrete skeletal elements such as silica or calcium carbonate spicules), and spongocytes (which produce spongin fibers). The mesohyl may also contain collencytes or lophocytes that secrete collagen fibers. In many sponges, it houses skeletal components, including spicules and spongin, and can host symbiotic microbes in some species.

Functions of the mesohyl involve providing structural support for the aquiferous canal system, enabling transport of nutrients to choanocytes and other cells, storing food and pigments, and participating in wound healing and regeneration. Its composition and cellular makeup vary among sponge groups; demosponges typically have a mesohyl rich in spongin and silica spicules, while calcareous sponges rely more on calcium carbonate spicules.

Relation to other tissues: the mesohyl forms the connective-tissue–like core of the sponge body wall, contributing to the integrity of the body plan and the arrangement of the canal system. Together with the pinacoderm and choanoderm, it constitutes the basic tissue organization of sponges.