Developmental Neurogenesis
Cnidaria develop from two layers of tissue, the ectoderm and the endoderm, and are thus termed diploblasts. The ectoderm and the endoderm are separated by an extra-cellular matrix layer called the mesoglea. Cnidaria begin to differentiate their nervous systems in the late gastrula. In hydrozoa and anthozoa, interstitial stem cells from the endoderm generate neuroblasts and nematoblasts which migrate to the ectoderm and provide for the formation of the nervous system along the anterior-posterior axis. Non-hydrozoa lack interstitial stem cells, and the neurons arise from epithelial cells, which are most likely differentiated from the ectoderm as occurs in vertebrates. Differentiation occurs near the aboral pore and this is where most neurons remain.
In cnidaria larvae, neurons are not distributed homogenously along the anterior-posterior axis; cnidaria demonstrate anatomical polarities during the differentiation of a nervous system. There are two main hypotheses that attempt to explain neuronal cell differentiation. The zootype hypothesis says that regulatory genes define an anterior-posterior axis and the urbilateria hypothesis says that genes specify a dorsal-ventral axis. Experiments suggest that developmental neurogenesis is controlled along the anterior-posterior axis. The mechanism by which this occurs is similar to that concerning the anterior to posterior patterning of the central nervous systems in bilaterians. The conservation of the development of neuronal tissue along the anterior-posterior axis provides insight into the evolutionary divergence of coelenterates and bilaterians.
Neurogenesis occurs in cnidaria not only during developmental stages, but also in adults. Hydra, a genus belonging to cnidaria, is used as a model organism to study nerve nets. In the body column of hydra, there is continuous division of epithelial cells occurring while the size of the hydra remains constant. The movement of individual neurons is coupled to the movement of epithelial cells. Experiments have provided evidence that once neurons are differentiated, epithelial cell division drives their insertion into the nerve net. As neurogenesis occurs, a density gradient of neuronal cells appears in the body. While each cnidarian species has a unique composition of its nerve net, distribution of neurons throughout the body occurs by a density gradient along the proximal-distal axis. The density gradient goes from high to low from the proximal to the distal end of the hydra. The highest concentration of neurons is in the basal disk and the lowest (if neurons are even present) is in the tentacles. During development of hydra, the amount of neurons gradually increases to a certain level, and this density is maintained for the duration of the organism’s life-span, even following an amputation event. After amputation, regeneration occurs and the neuron density gradient is reestablished along the hydra.
Read more about this topic: Nerve Net