Mesoglea represents a unique biological structure, a distinctive non-living, gelatinous layer found in certain simple multicellular animals. Despite its seemingly simple appearance, it serves various significant purposes within these organisms. This layer contributes to the overall form and function of the animals that possess it.
Defining Mesoglea
Mesoglea is a jelly-like substance that acts as an extracellular matrix. It is found between the two primary cell layers, the outer ectoderm and the inner endoderm (also called gastrodermis), in animals classified as diploblastic. While largely acellular, it is not primarily composed of living cells, but can contain scattered cells, such as wandering amoebocytes.
This layer is not considered a true tissue in the same way that muscle or nerve tissues are. Instead, it serves as a form of connective tissue, providing a scaffold for the organism’s structure. Unlike the mesoderm in triploblastic animals, mesoglea does not form organs.
Where Mesoglea Resides
Mesoglea is characteristic of specific animal groups, primarily the phyla Cnidaria and Ctenophora. Cnidarians include jellyfish, sea anemones, corals, and hydras, while Ctenophores are known as comb jellies.
The prominence of the mesoglea varies considerably among these groups. In large jellyfish, for instance, the mesoglea can constitute the bulk of the animal’s body, giving them their characteristic bell shape and gelatinous consistency. Conversely, in smaller cnidarians like hydras, this layer may be relatively thin, sometimes thinner than the surrounding cell layers. The presence of mesoglea in these organisms is linked to their evolutionary simplicity, as it provides a basic yet effective structural solution without requiring complex tissue development.
The Makeup of Mesoglea
The mesoglea is predominantly composed of water, often making up 95% to 99% of its mass. Beyond water, the layer contains a network of structural proteins.
These proteins include collagen-like fibers and heparan sulfate proteoglycans, which provide structural integrity and elasticity. Although it is largely acellular, the mesoglea can contain migratory cells, such as amoebocytes, which are involved in processes like phagocytosing debris and bacteria, and producing antibacterial chemicals. These cells also contribute to the formation of mesogleal fibers in some species.
Roles of Mesoglea
The mesoglea performs several functions for cnidarians and ctenophores. It acts as a hydrostatic skeleton, providing structural support and helping to maintain the organism’s body shape, especially in the aquatic environment where buoyancy is a factor. For example, it gives jellyfish their distinct bell shape.
This gelatinous layer also provides elasticity and flexibility, allowing for movements such as the pulsating contractions observed in jellyfish, which aid in their locomotion. The elastic properties help the body return to its original shape after muscle contractions. Furthermore, the mesoglea facilitates internal transport, allowing for the diffusion of nutrients, gases, and waste products between the outer and inner cell layers, sometimes serving as a primitive circulatory system.
The mesoglea can also play a role in defense by contributing to the organism’s overall toughness or forming a protective barrier. In some species, it may even store lipids or other energy reserves, although this is not a universal function.