A germ layer is a group of cells in an embryo that interact with each other as the embryo develops and contribute to the formation of all organs and tissues. All animals, except perhaps sponges, form two or three germ layers. The germ layers develop early in embryonic life, through the process of gastrulation. During gastrulation , a hollow cluster of cells called a blastula reorganizes into two primary germ layers : an inner layer, called endoderm , and an outer layer, called ectoderm.
Diploblastic organisms have only the two primary germ layers ; these organisms characteristically have multiple symmetrical body axes radial symmetry , as is true of jellyfish, sea anemones, and the rest of the phylum Cnidaria.
All other animals are triploblastic, as endoderm and ectoderm interact to produce a third germ layer, called mesoderm.
Together, the three germ layers will give rise to every organ in the body, from skin and hair to the digestive tract. Gastrulation differs across species, but the general process is the same: the hollow ball of cells that forms the blastula differentiates into layers. The first phase of gastrulation produces a two-layered organism comprised of ectoderm and endoderm.
The ectoderm will form the outer components of the body, such as skin, hair, and mammary glands, as well as part of the nervous system. Following gastrulation , a section of the ectoderm folds inward, creating a groove that closes and forms an isolated tube down the dorsal midsection of the embryo. This process of neurulation forms the neural tube , which gives rise to the central nervous system.
During neurulation , ectoderm also forms a type of tissue called the neural crest , which helps to form structures of the face and brain. The endoderm produced during gastrulation will form the lining of the digestive tract, as well as that of the lungs and thyroid.
For animals with three germ layers , after the endoderm and ectoderm have formed, interactions between the two germ layers induce the development of mesoderm. The mesoderm forms skeletal muscle, bone, connective tissue, the heart, and the urogenital system.
Due to the evolution of the mesoderm , triploblastic animals develop visceral organs such as stomachs and intestines, rather than retaining the open digestive cavity characteristic of diploblastic animals. In the publications derived from his dissertation, Pander described how two layers of cells, which he called serous and mucous, gave rise to an intermediate layer, which he called vascular.
Pander wrote of the interdependence of these three layers as well as the necessity of their interaction to form organs. In , eight years after Pander's initial descriptions, Martin Rathke, a physician and embryologist from Prussia now Poland , discovered layers of cells in a developing invertebrate crayfish , Astacus astacus , that corresponded to those Pander had described in chicks. Rathke showed that the embryonic layers Pander described existed in animals outside of the vertebrate clade.
Observations and Reflections. Discussion of the germ layers dwindled over the next twenty one years, but they resurfaced when Thomas Henry Huxley , a natural historian in England, published "On the Anatomy and Affinities of the Family of the Medusae.
Huxley realized that a correlation existed between the body architecture of the adult jellyfish and the vertebrate embryo. After fertilization, the zygote undergoes cleavage to form the blastula. The blastula, which in some species is a hollow ball of cells, undergoes a process called gastrulation, during which the three germ layers form. The ectoderm gives rise to the nervous system and the epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to the digestive system and other internal organs.
Organogenesis is the formation of organs from the germ layers. Each germ layer gives rise to specific tissue types. Learning Objectives By the end of this section, you will be able to: Explain how the embryo forms from the zygote Discuss the role of cleavage and gastrulation in animal development Describe organogenesis.
Concept in Action Visit the Virtual Human Embryo project at the Endowment for Human Development site to click through an interactive of the stages of embryo development, including micrographs and rotating 3-D images.
Answers D A If multiple sperm fused with one egg, a zygote with a multiple ploidy level multiple copies of the chromosomes would form, and then would die. Glossary blastocyst: the structure formed when cells in the mammalian blastula separate into an inner and outer layer.
Previous: Next: The neural crest cells of the embryo form structures or cells related to many systems including endocrine system, Schwann cells of the nervous system, odontoblasts and cemetoblasts of teeth, and Merkel cells of the integumentary system. Neuroblasts or neurons and Giloblasts of the nervous systems are differentiations of the neural tube cells of the embryo.
However, all these types of cells, organs, and systems are formed by differentiating the basic germ cells of an ectodermal origin. Thus, the ectoderm of the early embryo could be regarded as one of the most important germ cell layers, which accounts for the skin colour, strength of teeth, nervous system including the brain, and many other features of a particular individual. Endoderm is the innermost layer of primary germ cells forming in the early embryos.
Endoderm starts with flattened cells but later the shapes are changed into columnar cells, and form the epithelial linings of many organs and systems of the body. The endoderm lines mainly the digestive tract, and it covers a vast majority of the gastrointestinal tract excluding the mouth, pharynx, and the anus. The ectoderm gives rise to the skin and the nervous system.
The mesoderm specifies the development of several cell types such as bone, muscle, and connective tissue. Cells in the endoderm layer become the linings of the digestive and respiratory system, and form organs such as the liver and pancreas. Contact the news team Vicky Hatch Communications Officer vhatch ebi. Subscribe to the email newsletter Subscribe to our publications. Tweets by emblebi.
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