Germ Layer Definition
A germ layer is a group of cells in an embryo that interact with each other during the development of the embryo 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.
Some animals, such as cnidarians, produce two germ layers (the ectoderm and the endoderm), which makes them diploblastic. Other animals, such as chordata, produce a third layer (the mesoderm) between these two layers, making them triploblastic. All tissues and organs of an animal are formed through the germ layers through the process of organogenesis.
Caspar Friedrich Wolff observed the organization of the early embryo in leaf-like layers. In 1817, while examining chicken embryos, Heinz Christian Pander discovered three original germ layers.
Between 1850 and 1855 Robert Remak refined the cotyledon concept further and found that the outer, inner and middle layers form the epidermis, the intestine and the muscles in between and the vascular system. The term “mesoderm” was introduced into English by Huxley in 1871 and “ectoderm” and “endoderm” by Lankester in 1873.
Among the animals, sponges show the simplest organization with a single germ layer. Although they have differentiated cells (e.g., collar cells), they lack real tissue coordination.
Diploblastic animals, Cnidaria and Ctenophora, show an increase in complexity with two germ layers, the endoderm and the ectoderm. Diploblastic animals are organized into recognizable tissues.
All higher animals (from flatworms to humans) are triploblastic and have a mesoderm in addition to the germ layers found in diploblasts. Triploblastic animals develop recognizable organs.
Fertilization leads to the formation of a zygote. During the next stage, cleavage and mitotic cell division transform the zygote into a hollow cell ball called a blastula. This early embryonic form becomes gastrulated and forms a gastrula with either two or three layers (the germ layers). In all vertebrates, these progenitor cells differentiate into all adult tissues and organs.
In the human embryo, after about three days, the zygote forms a solid cell mass called a morula through mitotic division. This then changes to a blastocyst, which is made up of an outer layer called a trophoblast and an inner cell mass called an embryoblast.
The blastocyst, filled with uterine fluid, breaks out of the zona pellucida and is implanted. The inner cell mass initially consists of two layers: the hypoblast and the epiblast. At the end of the second week a primitive streak appears.
The epiblast in this region moves towards the primitive streak, dipping into it and forming a new layer, the endoderm, which pushes the hypoblast out of the way (this forms the amnion). The epiblast moves on and forms a second layer, the mesoderm. The top layer is now called the ectoderm.
What are the Three Germ layers?
1. Endoderm Germ Layer
The endoderm is one of the germ layers that are formed during embryonic development in animals. Cells that migrate inward together with the archenteron form the inner layer of the gastrula, which develops into the endoderm.
The endoderm initially consists of flattened cells that then become columnar. It forms the epithelial lining of the entire digestive tract except for part of the mouth and pharynx and the terminal part of the rectum (which are lined by involutions of the ectoderm).
It also forms the lining cells of all of the glands that open into the digestive tract, including those of the liver and pancreas; the epithelium of the auditory tube and the tympanic cavity; the trachea, bronchi, and alveoli of the lungs; the bladder and part of the urethra; and the follicular lining of the thyroid and thymus.
The endoderm forms: the pharynx, esophagus, stomach, small intestine, large intestine, liver, pancreas, bladder, epithelial parts of the trachea and bronchi, lungs, thyroid, and parathyroid.
2. Mesoderm Germ Layer
The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the cells that migrate inward contribute to the mesoderm, an extra layer between the endoderm and ectoderm. The formation of a mesoderm leads to the development of a coelom.
Organs formed in a coelom can move, grow, and develop freely regardless of the body wall, while fluid cushions protect them from impact. The mesoderm is made up of several components that develop into tissues: intermesoderm, paraxial mesoderm, lateral plate mesoderm, and chordamesoderm.
The chordamesoderm develops into the notochord. The intermediate mesoderm develops into the kidneys and gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis. The lateral plate mesoderm develops into the circulatory system (including the heart and spleen), the intestinal wall, and the wall of the human body.
The mesodermal cells begin the differentiation process through cell signal cascades and interactions with the ectodermal and endodermal cells.
The mesoderm forms: muscles (smooth and striped), bones, cartilage, connective tissue, adipose tissue, circulatory system, lymphatic system, dermis, dentine of the teeth, urogenital system, serous membranes, spleen and notochord.
3. Ectoderm Germ Layer
The ectoderm creates the outer layer of the embryo and is formed from the embryo’s epiblasts. The ectoderm develops into the surface ectoderm, the neural crest, and the neural tube.
The surface ectoderm develops into the epidermis, hair, nails, lens of the eye, sebum glands, cornea, tooth enamel, epithelium of the mouth and nose.
The nerve ridge of the ectoderm develops into the peripheral nervous system, adrenal medulla, melanocytes, and facial cartilage.
The neural tube of the ectoderm develops into the brain, spinal cord, posterior pituitary gland, motor neurons, and retina.
Note: The anterior pituitary develops from the ectodermal tissue of Rathke’s pouch.