An Overview of Animal Germ Layers
The Role of Endoderm, Mesoderm, and Ectoderm in Embryogenesis
Aug 7, 2008
Germ layers, as they are identified in human embryos, are present throughout the animal kingdom, though they are particularly differentiated in vertebrates. Germ layers arise after fertilization has occurred, and the diploid zygote has been formed. Following fertilization, the zygote undergoes a series of mitotic cell divisions resulting in a ball of cells called the blastula. This undifferentiated embryonic structure then undergoes a phase of major cell migration called gastrulation, producing a gastrula with three separate germ layers: the endoderm, mesoderm, and ectoderm.
Endoderm
The endoderm is formed from cells that migrate toward the center of the developing gastrula. They are initially found as flattened cells, but eventually begin to pile on top of each other in a columnar arrangement. The function of the endoderm is to provide an epithelial lining for the body’s two major tubes: the digestive and respiratory tube.
The digestive tube, which extends from one end of the body to the other, pinches off at certain points to from the accessory gastrointestinal organs—namely the pancreas, liver, and gallbladder. Though the digestive system of an adult includes the mouth, pharynx, and terminal part of the rectum, the endoderm does not line these particular regions due to certain alignments of tissue during embryogenesis.
Here is a list of the specific structures the endoderm produces lining for:
- Digestive tube (except for mouth, pharynx, end of rectum)
- Accessory digestive organs (pancreas, liver, gallbladder)
- Auditory structures, such as the auditory tube and tympanic cavity
- Respiratory structures (trachea, bronchi, alveoli)
- Urinary bladder and urethra
- Thyroid gland and thymus
- The anterior pituitary gland
Mesoderm
The mesoderm is formed during gastrulation from cells that migrate inward and stop between the inner layer (endoderm) and the outer layer (ectoderm). The evolutionary significance of the mesoderm is that it led to the development of the coelom. Coelom is another term for body cavity, and refers to the region between an animal’s outer covering, and the outer covering of the gut cavity. The coelom is the region where the body’s internal organs are found.
The mesoderm gives rise to many of the “middle tissues,” but is dynamic in its ability to differentiate in atypical directions. That is to say, cells originally found in the mesoderm might end up migrating and differentiate into an endoderm tissue. Cells that stay in the mesoderm most notably give rise to the following structures:
- All muscle, bone, and connective tissue
- Entire vascular and lymphatic system, including blood
- Urogenital organs (kidneys, gonads, and reproductive ducts)
- The dermis (middle layer of the skin)
Ectoderm
Since it is the outermost layer found during gastrulation, the ectoderm is said to emerge first. The ectoderm is actually comprised of three separate layers: the external ectoderm, the neural crest, and the neural tube. The neural crest and neural tubes are important precursors to the nervous system. Located between the external ectoderm and the neural tube, the neural crest contains tissue that will become neurons and glial cells of the autonomic nervous system. The neural tube is the precursor to the central nervous system, which includes the brain and spinal cord.
As a whole, the ectoderm is noted for giving rise to the following structures:
- The entire nervous system
- Posterior pituitary
- Adrenal medulla
- Cornea and lens
- Epidermis of skin and its derivatives (i.e. hair, nails, sweat glands, sensory receptors)
- Nasal, oral, and anal epithelium
Reference:
Evers, Christine A., Lisa Starr. Biology: Concepts and Applications. 6th ed. United States:Thomson, 2006.
