The development of a human embryo begins with the formation of three primary germ layers: the ectoderm, the endoderm, and the mesoderm. The ectoderm is the outermost layer, forming the nervous system and the epidermis, while the endoderm creates the lining of the digestive and respiratory tracts and associated glands. Sandwiched between these two layers, the mesoderm is the progenitor tissue for the body’s most widespread structural, circulatory, and connective tissues. This middle layer creates the framework, transport systems, and support structures that connect the inner and outer components of the body.
Formation and Early Subdivisions of the Mesoderm
The mesoderm first appears during gastrulation in the third week of embryonic development, transforming the two-layered embryonic disc into a three-layered structure. Cells from the epiblast layer migrate inward through the primitive streak. These migrating cells form the endoderm and then settle in the space between the endoderm and the remaining epiblast cells, which become the ectoderm.
Once established, the intraembryonic mesoderm organizes itself into distinct functional domains along the embryo’s central axis. Medially, running alongside the notochord, is the paraxial mesoderm. Lateral to this lies the intermediate mesoderm, a narrow band of tissue, and finally, the outermost region is the lateral plate mesoderm.
This early arrangement determines the final fate of the cells. The paraxial mesoderm segments into somites, establishing the body’s segmental pattern. The intermediate mesoderm forms the urogenital structures, while the lateral plate mesoderm is responsible for the lining of the body cavities and the circulatory system.
Building the Body Frame: Skeletal and Muscular Structures
The paraxial mesoderm is the primary source for the body’s skeletal and voluntary muscular system, beginning with the formation of somites. These somites are paired structures that appear sequentially along the neural tube, establishing the segmental organization of the trunk. Each somite differentiates into three components: the sclerotome, the myotome, and the dermatome.
The sclerotome is the precursor for the axial skeleton, forming the cartilage and bone of the vertebrae and ribs. Its cells migrate to surround the notochord and neural tube. A process called resegmentation occurs, where parts of adjacent sclerotomes merge to form a single definitive vertebra, ensuring proper passage for spinal nerves.
The myotome differentiates into all skeletal muscle of the body, including the muscles of the back, trunk, and limbs. The dermatome contributes to the dermis, the inner layer of the skin, particularly over the dorsal regions. Specialized connective tissues, such as tendons and ligaments, also arise from the paraxial mesoderm lineage.
Developing the Circulatory and Urogenital Systems
The body’s filtration and reproductive systems are constructed from the intermediate and lateral plate mesoderm regions. The intermediate mesoderm forms the urogenital system, including the successive stages of kidney development and the definitive adult kidney. This tissue also gives rise to the gonads (ovaries and testes) and their associated ducts. A derivative of this region is the adrenal cortex, the outer glandular layer of the adrenal gland, which produces steroid hormones.
The lateral plate mesoderm splits into two layers, creating the intraembryonic coelom, the precursor to the main body cavities. This mesoderm is the source of the entire cardiovascular system, including the myocardium, all blood vessels, and all circulating blood cells. This region also contributes significantly to the connective tissue, bone, and cartilage of the limbs and the sternum.
Internal Membranes, Glands, and Clinical Significance
The splitting of the lateral plate mesoderm results in two distinct layers that line the body cavities and cover the organs. The somatic (parietal) layer forms the serous membranes lining the interior of the pleural, pericardial, and peritoneal cavities. Conversely, the splanchnic (visceral) layer adheres to the organs, giving rise to the serous membranes that cover the lungs, heart, and abdominal organs.
The mesoderm also contributes the smooth muscle and connective tissue layers within the walls of the digestive and respiratory tracts, providing support and contractile function. The mesoderm is also the source of the spleen, a large lymphoid organ, and the lymphatic vessels integral to immune surveillance and fluid balance.
Because the mesoderm is responsible for many structural and fluid-handling systems, its faulty development is linked to a range of congenital conditions. Errors in the paraxial mesoderm can result in vertebral defects or skeletal muscle abnormalities. Malformation of the intermediate mesoderm can lead to renal agenesis, the absence of one or both kidneys. Defects in the lateral plate mesoderm often manifest as congenital heart defects, which are among the most common birth anomalies.