o Basic stages
o Similarity to vertebrate development
o Larval nervous system
Aplysia has been studied in many areas of developmental biology. For example, the process of gastropodal torsion and spiralian development has been examined in some detail (1). For a general overview of the life cycle of Aplysia, as well as general information, see the GeoChemBio Aplysia page. This page will focus on the development of the nervous system.
Aplysia develops in four basic stages. The embryonic stage (10-12 days) lasts from fertilization to hatching. This includes the early cleavage stage, blastula, gastrula, segmentation cavity (blastocoel), trochophore (celomic cavity development) and first veliger stage. The animal hatches as a Stage 1 veliger which is free-swimming. This develops through six stages (32-34 days) before metamorphosing (2-3days) into the crawling benthic juvenile stage. After further development (80-90 days), the Aplysia is a mature adult which can reproduce. Full development takes about 120 days (2).
The development of the nervous system begins in the embryonic stage and continues into the juvenile stage(2). First, the order of ganglia development was determined. The cerebral and pedal ganglia develop when the Aplysia hatches, about 10 days post-fertilization. The others develop subsequently in the following order: abdominal, pleural, osphradial (3 weeks), buccal (4 weeks) (3).
However, before the ganglia develop, the nervous system appears as putative neurons in the larval stage. There are three initial sites of neuron development. The apical sensory organ (ASO) is located in the anterior and consists initially of vase-shaped cells which express catecholamines and neuropeptides. At the posterior, neurons marked by containing FMRF-amide-related peptides develop. Vase-shaped catecholaminergic cells also develop on the ventral and lateral sides of the mouth. This release dopamine and norepinephrine which inhibit ciliary beating and feeding(4).
Adult ganglia begin development in the middle of the larval stage, separating from proliferative zones in the body wall. These zones continue to make post-mitotic neurons which migrate inwards towards the gangia, but their organization is often lost to observers as they migrate and fuse. The entire larval body is also twisting and untwisting at this stage, complicating further studies(4). A study conducted in 1983 determined that the pattern of neurogenesis resembles that of vertebrates, even more so than many other invertebrates.
Schematic of Neurogenesis in Aplysia
Proliferative zones of the body wall are the source of ganglion cells of the CNS. The sequence of cell proliferation in the proliferative zones and the migration to the nearby ganglia are illustrated schematically in magnified views of the body wall as it changes over time during neurogenesis. (1-6) and in cross sections of two developmental stages of the whole animal (a, gastrula to segmentation cavity embryo, 1 week following labeling with [3H] thymidine; b, trochophore stage embryo, 1 week following labeling. The top of each figure is dorsal, and the bottom is ventral). Proliferative zones (Pz) are established in regions of the body wall which are adjacent to underlying mesodermal cells (M) (see a and b for orientation in cross-section of whole animal). The proliferative zones contain ectodermal cells which have elongated from a flat or cuboidal state to a columnar shape (1-2). Multilayered placodes (Pl) form in a central portion of each proliferative zone (2-4). Mitosis of the columnar and placodal cells of the proliferative zones in the presence of [3H] thymidine leads to the production of heavily labeled cells ([3H]thymidine-labeled cell nucleus as compared to unlabeled cell nucleus). The regions has a pesudostratified appearance with nuclei seeming to migrate to the basal area of the cell during the cell cycle. Some labeled cells leave the surface and migrate inward along the paths indicated by the arrows to join the nearby forming ganglia (5-6, a and b). Each gangion (C, cerebral, Pe, pedal indicated here) is formed by cells generated in a unique adjacent proliferative zone. Other internal structures indicated include the esophagus (E) and the otocysts (Ot).
However, this comparison is based mainly on qualitative data and relationship between vertebrate and Aplysia neuronal development remains unclear from this study.
More recently, scientists have examined pre-ganglionic neuronal system development. The embryonic stage Aplysia contains an extensive, previously unrecorded nervous system at the trochophore stage. The existence of this nervous system is based on immunocytochemical assays. These showed the presence of two pairs of cells in the trochophore stage which can be labeled with antibodies to FMRFamide (see product page) and EFLRIamide (both neuropeptides). In other species, a single FMRF-amide-like immunoreactive (LIR) cell has been observed. This may have been missed by the cytochemical assay due to brief prescense or low reactivity to the antibodies. In contrast to these early-developing neuronal cells, the ganglia of the adult CNS do not develop until late embryonic stages. The anterior ends of these cells terminate under the apical tuft, a region which develops into the apical sensory organ or cerebral commissure or both.
However, this immunocytochemical study did not examine later stages of the embryos, to the fates of these larval neuronal cells remains unclear. They are present in the same regions where adult ganglia develop in later veliger stage, but the study did not demonstrate a relation between these early cells and the developing adult CNS ganglia. They may be incorporated into or guide the development of the adult ganglia.
In addition, a pair of more lateral apical seroton-like immunoreactive (LIR) cells were found which extend to the velar lobes, foot, abdomen, and visceral regions of the veliger. The apical organ likely controls larval behavior such as swimming, feeding and crawling because it innervates the vellum. This has been observed in other mollusks too.
Just before hatching, catecholamine-containing cells appear around the mouth and in the foot.
Overall, this study found the presence of posterior neurons and fibers before torsion of the embryo, when the visceral loop makes a figure-eight pattern. Together, the posterior cells, apical organ and catecholaminergic cells form the larval nervous system(5). Microarray analysis of the transcriptome supports the immunocytochemical evidence for the role of FMRF in embryonic behavior. Additionally, FMRF may have a role in patterning the adult nervous system(6). Further study is needed to determine the fate and mechanism of development of this system(5). Further evidence has shown that the larval and adult nervous systems are probably separate structures, raising interesting questions about how two nervous systems co-coordinate during the Aplysia development(6).
Resources: Several of these were given in the main text, but have been repeated here for convenient access.
Life cycle figure with hyperlinked to videos
Aplysia_cerebral_ganglia at the NeuronBank wiki
Aplysia_californica_pedal_ganglia at the NeuronBank wiki
1. Wollesen, T., Wanninger, A., and Klussmann-Kolb, A. (2008) Myogenesis in Aplysia californica (Cooper, 1863) (Mollusca, Gastropoda, Opisthobranchia) with special focus on muscular remodeling during metamorphosis, J. Morphol. 269, 776-789.
2. Jacob, M. (1984) Neurogenesis in Aplysia californica resembles nervous system formation in vertebrates, The Journal of Neuroscience 4, 1225-1239.
3. Kriegstein, A. R. (1977) Development of the nervous system of Aplysia californica, Proc. Natl. Acad. Sci. U. S. A. 74, 375-378.
4. Croll, R. P. (2009) Developing Nervous Systems in Molluscs: Navigating the Twists and Turns of a Complex Life Cycle, Brain, Behavior & Evolution 74, 164-176.
5. Dickinson, A., Croll, R., and Voronezhskaya, E. (2000) Development of embryonic cells containing serotonin, catecholamines, and FMRFamide-related peptides in Aplysia californica, Biol Bull 199, 305-315.
6. Heyland, A., Vue, Z., Voolstra, C. R., Medina, M., and Moroz, L. L. (2011) Developmental transcriptome of Aplysia californica’, Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 316B, 113-134.