During organogenesis, the three germ layers of the embryo differentiate and further specialize to form the various organs of the body.
Describe the process of organogenesis in vertebrates
- Cells in the ectoderm are signaled by molecules called growth factors to form the neural plate, which rolls up to form a structure called the neural tube; the neural tube will eventually develop into the brain and spinal cord.
- The differing expression of various genes controls the differentiation of the mesoderm into connective tissue, as well as the ribs, spine, skeletal muscle, and lungs.
- The endoderm forms the lining of the digestive tract, as well as the linings of all the glands that will empty into the digestive tract; it also forms a wide variety of internal organs.
- organogenesis: the formation and development of the organs of an organism from embryonic cells
- ectoderm: outermost of the three tissue layers in the embryo of a metazoan animal, which will produce the epidermis (skin) and nervous system of the adult
- mesoderm: one of the three tissue layers in the embryo of a metazoan animal, which will produce many internal organs of the adult such as the muscles, spine and circulatory system
- endoderm: one of the three tissue layers in the embryo of a metazoan animal, which will produce the digestive system and other internal organs of the adult
- neural plate: a thick, flat bundle of ectoderm formed in vertebrate embryos after induction by the notochord
Organogenesis is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism. Organs form from the germ layers through the differentiation: the process by which a less-specialized cell becomes a more-specialized cell type. This must occur many times as a zygote becomes a fully-developed organism. During differentiation, the embryonic stem cells express specific sets of genes which will determine their ultimate cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. Therefore, the process of differentiation is regulated by cellular signaling cascades.
In vertebrates, one of the primary steps during organogenesis is the formation of the neural system. The ectoderm forms epithelial cells and tissues, as well as neuronal tissues. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were disrupted, then the entire ectoderm would differentiate into neural tissue. The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube. In further development, the neural tube will give rise to the brain and the spinal cord.
The mesoderm that lies on either side of the vertebrate neural tube will develop into the various connective tissues of the animal body. A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites, with spaces between them. The somites will further develop into the ribs, lungs, and segmental (spine) muscle. The mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.
The endoderm consists, at first, of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube (except part of the mouth and pharynx) and the terminal part of the rectum (which is lined by involutions of the ectoderm). It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and air cells of the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus. Additionally, the endoderm forms internal organs including the stomach, the colon, the liver, the pancreas, the urinary bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.
Vertebrate Axis Formation
Through the expression patterns of different genes, the three axes of the body are established, aiding in tissue and organ development.
Describe the formation of body axes in vertebrates
- As an animal develops, it must organize its internal and external structures such that the anterior/posterior (forward/backward), dorsal / ventral (back/belly), and lateral/medial (side/middle) axes are correctly determined.
- Proteins that are part of the Wnt signaling pathway help determine the anterior/posterior axis by guiding the axons of the spinal cord in an anterior/posterior direction.
- Together with the sonic hedgehog (Shh) protein, Wnt determines the dorsal/ventral axis; Wnt levels are highest in the dorsal region and lessen toward the ventral region, while Shh levels are highest in the ventral region and lessen toward the dorsal region.
- dorsal: with respect to, or concerning the side in which the backbone is located, or the analogous side of an invertebrate
- ventral: on the front side of the human body, or the corresponding surface of an animal, usually the lower surface
- notochord: a flexible rodlike structure that forms the main support of the body in the lowest chordates; a primitive spine
- Wnt signaling pathway: a group of signal transduction pathways made of proteins that pass signals from outside of a cell through cell surface receptors to the inside of the cell
Vertebrate Axis Formation
Even as the germ layers form, the ball of cells still retains its spherical shape. However, animal bodies have lateral-medial (toward the side-toward the midline), dorsal-ventral (toward the back-toward the belly), and anterior-posterior (toward the front-toward the back) axes. As the body forms, it must develop in such a way that cells, tissues, and organs are organized correctly along these axes.
How are these established? In one of the most seminal experiments ever to be carried out in developmental biology, Spemann and Mangold took dorsal cells from one embryo and transplanted them into the belly region of another embryo. They found that the transplanted embryo now had two notochords: one at the dorsal site from the original cells and another at the transplanted site. This suggested that the dorsal cells were genetically programmed to form the notochord and define the dorsal-ventral axis. Since then, researchers have identified many genes that are responsible for axis formation. Mutations in these genes leads to the loss of symmetry required for organism development. Many of these genes are involved in the Wnt signaling pathway.
In early embryonic development, the formation of the primary body axes is a crucial step in establishing the overall body plan of each particular organism. Wnt signaling can be implicated in the formation of the anteroposterior and dorsoventral axes. Wnt signaling activity in anterior-posterior development can be seen in several organisms including mammals, fish, and frogs. Wnt signaling is also involved in the axis formation of specific body parts and organ systems that are a part of later development. In vertebrates, sonic hedgehog (Shh) and Wnt morphogenetic signaling gradients establish the dorsoventral axis of the central nervous system during neural tube axial patterning. High Wnt signaling establishes the dorsal region while high Shh signaling indicates in the ventral region. Wnt is also involved in the dorsal-ventral formation of the central nervous system through its involvement in axon guidance. Wnt proteins guide the axons of the spinal cord in an anterior-posterior direction. Wnt is also involved in the formation of the limb dorsal-ventral axis. Specifically, Wnt7a helps produce the dorsal patterning of the developing limb.