Sea urchins (Strongylocentrotus purpuratus) are small, spiny animals which make up the class Echinoidea in the Echinoderm phylum. These organisms are found in all oceans across the world, and generally inhabit the shallows and tide pools of ocean environments. Sea urchins have been used as model organisms in biology since the 1800s after the invention of the microscope.
Figure 1: Sea Urchin.
The sea urchin embryo has long been used as a model organism to address many questions in developmental biology. There are a number of important features that make the sea urchin an ideal system. The straightforward artificial spawning, fertilization and rearing, and embryo optical transparency make this organism a great resource. Also, the simple organization makes the sea urchin embryo a great organism in which to study morphogenetic movements and effects (Hardin 1995). Though in the Echinoderm phylum, sea urchins have a genome which encodes for many vertebrate immune system-related genes. The recent sequencing of the sea urchin genome has made this model organism even more important in the study of development (Rast et al., 2006).
Because the early development of the sea urchin has been studied in such depth, the steps of its fertilization are known in much more detail than many other organisms.
Male and Female sea urchins release their gametes directly into the marine environment they inhabit. The sheer number of gametes released by these organisms is staggering- sea urchins of various species are known to produce between 8 and 20 million eggs.
Figure 2: Sea urchin sperm, after locating an egg that has already been fertilized, are unable to fertilize the egg themselves.
The sea urchin sperm follows a typical sperm structure. It has a long flagella, which allows it to move quickly through the water it is released into. The flagella is connected to a middle piece which contains the mitochondrion necessary for powering the flagella as the sperm moves towards the egg. The middle section is connected to the sperm’s thick head, which contains the nucleus carrying genetic information and the acrosome, which contains enzymes used for breaking down the jelly-like layers surrounding the egg.
The egg is similarly simple in structure. It is a stationary structure with the three parts: the outermost part is made of a jelly-like substance which protects the egg, the second structure is the vitelline layer, which is composed primarily of glycoproteins, and the innermost layer surrounding the genetic information is the plasma membrane, common to all egg cells.
Process of Fertilization
Because the sperm must travel such comparatively vast distances to locate a compatible egg, it is directed to the egg through chemotaxis. Upon reaching an egg, protein receptors located on the surface of the head of the sperm interact with the jelly-like coating on the egg. This initiates a response in the acrosome of the sperm, resulting in the release of acrosomal enzymes. These enzymes break down the jelly coat.
Figure 3: Steps of the acrosomal reaction. Specialized components of the sperm initiate the breakdown of the jelly layer of the egg, allowing the sperm to reach the egg’s plasma membrane.
The sperm is then allowed to continue on to the plasma membrane of the egg. Finally, the plasma membranes of the egg and the sperm fuse, allowing the egg to be fertilized with the genetic material of the sperm.
Adversity in Fertilization
The first issue that an egg and a sperm of any organism type face in successfully producing an embryo is possibility of polyspermy, the fertilization of an egg by multiple sperm. This can be disastrous to the developing embryo, and sea urchins have developed ways of preventing this from occurring. To read more about these pathways in greater detail, visit Sea Urchins: Avoiding Polyspermy.
If you are interested in seeing viewing the process of fertilization, please visit the Fertilization Video.
Sea urchins undergo radial, reductive cleavage. The first two cleavages are meridional, the third one is equatorial, and the fourth division is unequal. After the fourth cell division, a protein called Vasa selectively accumulates in the small micromeres of the embryo. Vasa is in the ATP-dependent DEAD-box helicase family, which unwind nucleic acids. This protein is found in many animal phyla, and is almost completely conserved across the Echinoderm phylum. The vasa protein is thought to play a major role in germ cell marker. To read more about the role of Vasa accumulation, please visit “The role of role of Vasa protein accumulation in sea urchin embryogenesis.”
Eventually, a blastula is formed. At this point, a hatching enzyme is released, which degrades the fertilization envelope. The vegetal pole starts to thicken. This will give rise to primary mesenchyme cells and the archenteron during gastrulation. But before this happens, the endoderm and mesoderm layers must segregate. This process involves a complex succession of regulatory interactions between cWnt and Notch signaling. To read more about this mechanism please visit “Regulation of Endomesoderm Segregation.” After the endoderm and mesoderm have differentiated, gastrulation can then occur. Sea urchins’ transparency makes gastrulation processes easy to study.
Here is an image of of the cell movements that take place during gastrulation.
The first major movement is the ingression of primary mesenchyme cells. The complex movement of these cells depend on environmental and internal cues. To read more about how these cells migrate to make the skeleton continue onto, “Primary and Secondary Mesenchyme“. Then the vegetal plate invaginates to create the archenteron. There are three possible mechanisms for how this happens.
1. Apical Constriction
2. Apical Tractoring
3. Swelling of Proteoglycan
Then a second invagination takes place when the archenteron elongates across the blastocoel towards the animal pole. This happens at the same time of the onset of seconday mesenchyme cell’s filopodia. If you want to read more about how the filopodia find their target destination, read more at “Primary and Secondary Mesenchyme“.
Below is a graph of cell fates.
Rast, JP; Smith, LC; Loza-Coll, M; Hibino, T; Litman, GW (2006). “Genomic Insights into the Immune System of the Sea Urchin”. Science 314 (5801): 952–6.