The stomochord is a flexible, hollow tube found in hemichordates. Stomochords arise in

embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...

as an outpocketing from the roof of the embryonic gut anterior to the pharynx. In adults, they extend dorsally from the pharynx into the proboscis, and it communicates with the oral cavity. Their walls are composed primarily of

epithelial cells Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

, including ciliated and glandular cells.

Relationship to chordate anatomy

Stomochords were initially considered a variant of a primitive

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

, a defining feature of

chordate A chordate ( ) is a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

s. However, they are now recognized to not share histological composition to that of the notochord found in chordates, lacking the fibrous sheath characteristic of a notochord. In addition,

gene expression Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...

studies have failed to provide any evidence for homology between the notochord and the stomochord, or between the notochord and any hemichordate structure. The

Brachyury T-box transcription factor T, also known as Brachyury protein, is encoded for in humans and other apes by the ''TBXT'' gene. Brachyury functions as a transcription factor within the T-box, T-box family of genes. Brachyury Sequence homology, homo ...

(T) gene, which is expressed in the ascidian and other chordates notochord, is not expressed in the stomochord, and

collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

is absent.

References

{{reflist Endoderm Histology