The mesohyl, formerly known as mesenchyme, is the gelatinous matrix within a sponge. It fills the space between the external pinacoderm

Pinacoderm

The pinacoderm is the outer most layer of cells in the phylum Porifera , equivalent to the epidermis in other organisms....

 and the internal choanoderm

Choanoderm

The choanoderm is composed of flagellated collar cells, or choanocytes. The sponge body is mostly a connective tissue, the mesohyl, over which are applied epithelioid monolayers of cells, the outer pinacoderm and the inner choanoderm...

. The mesohyl resembles a type of connective tissue

Connective tissue

"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 and contains several amoeboid cells

Amoeba

Amoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...

 such as amebocyte

Amebocyte

An amebocyte or amoebocyte is a mobile cell in the body of invertebrates such as echinoderms, mollusks or sponges. They move by pseudopodia. Similarly to some of the white blood cells of vertebrates, in many species amebocytes are found in the blood or body fluid and play a role in the defense of...

s, as well as fibrils and skeletal elements.
For a long time, it has been largely accepted that sponges lack true tissue

Tissue (biology)

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

, but it is currently debated as to whether mesohyl and pinacoderm

Pinacoderm

The pinacoderm is the outer most layer of cells in the phylum Porifera , equivalent to the epidermis in other organisms....

 layers are in fact tissues.

The mesohyl is composed of the following main elements: collagen

Collagen

Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

, fibronectin-like molecules, galectin

Galectin

In molecular biology, galectins are a family of lectins which bind beta-galactoside. The family is defined by having at least one characteristic carbohydrate recognition domain with an affinity for beta-galactosides and sharing certain sequence elements...

, and a minor component, dermatopontin. These polypeptides form the extracellular matrix

Extracellular matrix

In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...

 which provides the platform for specific cell

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 adhesion as well as for signal transduction

Transduction

Transduction is a mechanism whereby genetic material may be transferred from the genes of a bacterium to another bacterium. This may include the actual covalent-bonding of new genetic markers...

 and cellular growth.