The mesoderm is one of the three major layers in a developing embryo, referred to as germ cell layers. Other germ cell layers are the endoderm and ectoderm. When the embryo develops, different organs and body parts develop from these germ cell layers. Developing from the mesoderm are muscles, bones, some skin tissue, connective tissues, the reproductive and urinary systems and other internal organs. Embryos of all complex animals, including humans, contain mesodermal layers in their early development.
How the mesoderm functions in the development of an embryo is representative of the complex process by which humans and other vertebrates develop in the womb. Each of the various layers in the early embryo begin as relatively simple structures, then change and grow to form the numerous interacting parts necessary to create a functioning living creature. Each layer becomes organs and tissue that are dependent on tissues and organs created in other layers, so that the development of each layer must occur properly to develop into a viable fetus.
Within the mesoderm itself are additional layers that differentiate into different organs and parts of the body during gestation. Each of these subdivisions develops into a different set of organs and other body tissues. The other two layers, the ectoderm and endoderm, eventually become external tissue such as skin and teeth (ectoderm) and internal tissue that lines most of the internal organs (endoderm). This process of development is similar among most vertebrates.
One of the layers of the mesoderm is the lateral plate mesoderm. This portion of the embryonic tissue divides into layers that develop into the body wall as well as the wall of the intestines and the circulatory system. Another layer, the intermediate mesoderm, becomes the urinary and reproductive systems. Paraxial mesoderm becomes facial muscles, vertebrae, skeletal muscles and the dermis layer of the skin. Chordamesoderm, or axial mesoderm, develops into various parts of the spine.
Another benefit to this means of development is that a coelom, or body cavity, forms early in the process. Within this cavity, there is room for the various internal organs to develop. When the animal is mature, the body cavity, located in the abdomen, houses most of the vital organs, providing protection for such delicate and important body parts as the heart, lungs and intestines. The lining of the abdominal cavity, called the peritoneum, also is derived from the mesoderm during early development.