The recognition that various adult tissues arose from a common embryonic anlage emerged from embryological studies in the early 19th century. In 1817, Christian Pander studied the early development of chicken embryos, and noted the embryonic blastoderm was organized into three distinct layers. He called the middle layer the ‘vessel-layer’ on account of its later association with the vascular system. This middle layer later was termed “mesoderm”, meaning middle-skin, by T.H. Huxley. The other two layers that Pander originally called the serous and mucous layers, came to be called the ectoderm and endoderm.
Carl Ernst von Baer examined a variety of other vertebrate embryos and in 1837 asserted that germ layers were present throughout the group. Rathke identified germ layers in the invertebrate Astacus in 1825, and germ layers were noted in many invertebrates by Kowalevski. However, T.H. Huxley noted that during cnidarian development, embryos were subdivided into only two layers that he called “foundation layers”. These were later renamed ectoderm and endoderm. This established the idea that mesoderm, as a distinct germ layer, arose in early animal evolution. However, the precise definition of the mesodermal germ layer was somewhat contested: von Baer asserted it was divided into separate layers (splanchnic and somatic layers, currently recognized as subsets of the lateral plate mesoderm), while Remak saw the mesoderm as a ‘secondary’ germ layer, which formed as a subset of one ‘primary’ germ layer, the endoderm.
Mesoderm is a crucial developmental lineage characterized by a distinct location between ectoderm and endoderm, and which possesses a distinct set of morphological derivatives necessary for locomotion, respiration, and associated fluid movement. Mesoderm probably had a monophyletic origin from a mesodermal precursor prior to the separation of protostome and deuterostome ancestors, and evolution in the surviving daughter lineages of that organism has led to variability in each aspect of mesodermal development within extant bilaterian animals. These aspects are discussed further below.
In all bilaterians, mesoderm is described as arising and moving to the middle layer of the animal, between the ectoderm and endoderm. This is the most consistent aspect of mesoderm, and while there is variability in how cells migrate into the interior (whether by enterocoely, as described below, or by schizocoelic opening of coeloms from within mesodermal bands), there is no variation in the position of early mesoderm relative to ectoderm and endoderm. Mesoderm is always in the middle at embryonic stages, and so its name is appropriate.
Mesoderm is characterized by a set of distinct fates. A homologous suite of mesodermally derived tissues has been described broadly in bilaterians and include skeletal muscle, smooth muscle, nephridia, mesothelia (coelomic lining), white blood cells, and pigment cells. However, novel mesodermal tissues have evolved independently in various lineages. For example, echinoderms have developed a water vascular system, while vertebrates developed a phosphatic endoskeleton.
The presence of shared use of genetic controls, similar topological position, and similar derivative tissues suggests that mesoderm across the protostomes and deuterostomes is definitely homologous, and shared from a common ancestor that was probably prebilaterian. Homologs of some genes required in bilaterian mesoderm specification have been found to be expressed within the endoderm of the anthozoan Cnidarian Nematostella vectensis. This suggests that mesoderm might have arisen by partitioning mesodermal and endodermal precursors.