Mesoderm and Its Derivatives

The mesoderm is the middle of the three primary germ layers and gives rise to most of the body's structural and circulatory tissues — muscle, bone, connective tissue, the heart and blood vessels, blood, kidneys, and gonads. After gastrulation it organizes into longitudinal columns on either side of the midline, each of which produces a characteristic set of derivatives.

Definition

The mesoderm is the middle germ layer formed during gastrulation; it gives rise to the axial and appendicular skeleton, skeletal, cardiac, and smooth muscle, connective tissue, the cardiovascular and lymphatic systems and blood, the urogenital system, and the serous membranes.

Scope

This topic covers the formation of the intraembryonic mesoderm, its subdivision into paraxial, intermediate, and lateral plate columns, the segmentation of paraxial mesoderm into somites, and the principal tissues each mesodermal region forms. It is a reference account of germ-layer fate and does not provide clinical guidance.

Core questions

• How does the intraembryonic mesoderm form during gastrulation?
• How is it subdivided into paraxial, intermediate, and lateral plate mesoderm?
• How does paraxial mesoderm segment into somites?
• Which tissues derive from each mesodermal region?

Key concepts

• Paraxial mesoderm and somites
• Intermediate mesoderm
• Lateral plate mesoderm (somatic and splanchnic layers)
• Axial mesoderm (notochord)
• Sclerotome, myotome, and dermatome
• Intraembryonic coelom
• Cardiovascular and urogenital origins

Key theories

Dorsal-ventral mesoderm patterning by BMP and its antagonists

A gradient of BMP activity, opposed by organizer-derived antagonists such as noggin, dorsalizes the mesoderm; high BMP yields ventral (e.g. blood-forming) mesoderm and BMP inhibition yields dorsal (e.g. axial and somitic) mesoderm.

Mechanisms

Mesoderm arises when epiblast cells ingress through the primitive streak and spread between the ectoderm and endoderm. Mediolaterally, it organizes into three columns: paraxial mesoderm flanking the midline, intermediate mesoderm, and lateral plate mesoderm. Paraxial mesoderm segments rhythmically into paired somites, which differentiate into sclerotome (vertebrae and ribs), myotome (skeletal muscle), and dermatome (dermis of the back). Intermediate mesoderm forms the kidneys and gonads with their ducts. Lateral plate mesoderm splits into a somatic (parietal) layer, contributing to the body wall and limb connective tissue, and a splanchnic (visceral) layer, contributing to the heart, smooth muscle and connective tissue of the gut, and blood vessels; the cavity between them becomes the intraembryonic coelom. Dorsoventral identity within the mesoderm is set by a BMP gradient opposed by organizer antagonists such as noggin and chordin.

Clinical relevance

The shared mesodermal origin of the skeleton, muscle, heart, kidneys, and gonads underlies why some malformation patterns involve several of these systems together, and the segmental (somite-derived) organization of the vertebrae and body-wall muscles is reflected in clinical dermatomes and myotomes. This entry describes developmental origins for reference and is not a basis for diagnosis or treatment.

Evidence & guidelines

The account here is based on experimental and molecular developmental biology in vertebrate models and standard human embryology texts, synthesized in reviews of gastrulation and mesoderm patterning rather than clinical guidelines.

History

Classical embryology recognized the middle germ layer and its columnar organization, and fate-mapping studies in amniotes detailed how streak-derived cells populate the paraxial, intermediate, and lateral plate domains. Molecular work in the 1990s, including the cloning of organizer antagonists such as noggin, clarified how BMP signalling and its inhibitors pattern the mesoderm along the dorsoventral axis.

Key figures

• Patrick Tam
• Lilianna Solnica-Krezel
• Richard Harland
• Olivier Pourquié

Seminal works

• smith-1992-noggin
• tam-behringer-1997
• solnica-krezel-2012

Frequently asked questions

What tissues come from the mesoderm?

Skeletal, cardiac, and smooth muscle; bone, cartilage, and other connective tissue; the heart, blood vessels, blood, and lymphatics; the kidneys and gonads; and the serous membranes lining the body cavities.

What are somites?

Somites are the paired, segmental blocks that form by segmentation of the paraxial mesoderm; they differentiate into sclerotome (vertebrae and ribs), myotome (skeletal muscle), and dermatome (dermis of the back).