The mesoderm is a primary germ layer that gives rise to connective tissue, muscle, bone, blood vessels, and mesothelium in the embryo. A mesothelium is a simple squamous epithelial sheet of cells that lines all coelomic cavities and organs, providing a non-adhesive surface for which organs can freely move within the body cavity. Both heart and intestinal mesothelia share conserved characteristics that include: initially both organs lack a mesothelium, then mesothelia cover the organs, and cells from the mesothelium undergo epithelial-to-mesenchymal (EMT) transition, and contribute cells to the vasculature of the organ.
Development of the Lateral Plate Mesoderm and Coelomic Cavities
Proper specification and differentiation of the lateral plate mesoderm (LPM) is required for the formation of the coelomic organs in the developing vertebrate embryo. During avian gastrulation, epiblast cells converge and ingress (migrating inward and then moving laterally) along the primitive streak to form the mesodermal layer just below the epiblast (future ectoderm) and above the hypoblast (future endoderm). Once gastrulation is complete, the mesodermal layer is specified as the chordamesoderm at the midline, and moving laterally from the midline: the paraxial or somitic mesoderm, the intermediate mesoderm, and the LPM. Studies have demonstrated that regulation of bone morphogenic protein (BMP) signaling is involved in specification of each region in the mesoderm. At this stage, the avian embryo is flat and all three germ layers are extending and developing concurrently.
The LPM will eventually give rise to the body wall, coelomic cavities, and organs within the cavities. The LPM divides into two layers, the somatic mesoderm and the splanchnic mesoderm. The somatic mesoderm is located dorsally and associates with the ectoderm to form the somatopluere. The splanchnic mesoderm is ventral and unites with the endoderm, now referred to as the splanchnopleure. In the mouse, hepatocyte nuclear factor-2/forkhead homologue-8 (HFH-8/Foxf1) is involved in formation of mesodermally-derived tissues, including the lung and intestine, specifically during vasculogenesis and development of extraembryonic membranes. Next, the LPM splits to generate an open cavity, known as a coelom, between the somatopluere and splanchnopleure. The epithelial histology of the mesodermal layers facing the lumen of the coelom reflects one another in an apical-apical manner (personal observations). The split of the LPM and formation of a coelom initiates the process of organogenesis in the embryo.
Organogenesis of the viscera begins essentially as the formation of a tube. This process is defined by the embryo folding ventrally toward its midline, which fuses and forms the primitive gut tube in the anterior region of the embryo. The gut tube is composed of endoderm, splanchnic mesoderm, and a thin space between containing an endothelial plexus. Ventral to the gut tube is the forming primitive heart tube. The heart tube does not contain embryonic endoderm, but instead two endocardial primordia fuse to form the endocardium in the ventral-most region of the embryo. The heart tissue is now comprised of two layers, the endocardium and myocardium, and is attached to both the dorsal and ventral body wall. Eventually, the heart tube detaches from the ventral aspect. After the organ tubes have formed, the left and right somatopleure adhere at the midline, and the left and right coelomic cavities fuse to form the pericardial cavity (the space around the heart), pleural cavity (the space around the lungs), and the peritoneal cavity (the space around the gut). The coelom provides a fluid filled cavity for organs, a space in which organs can separate from the body wall to become free-form entities, which creates space for other organs to expand and develop. Coelom formation is intimately involved with LPM specification and separation, all fundamental to organogenesis. The next section will focus on the development of the small intestine.