1. Detail the events associated with fertilization in sea urchins. Include the acrosomal/cortical reactions and how polyspermy is prevented.
2. Describe the body plans of animals. Include types of symmetry, germinal tissues, body cavities, and protostome vs. deuterostome development.
Ans 1.) Four important events of fertilization in the sea urchin are: 1) the acrosome reaction of the sperm, 2) sperm-egg fusion, 3) the cortical reaction of the egg, and 4) the formation of the fertilization coat. The acrosome reaction is triggered by contact of the sperm with the jelly coat, a complex extracellular matrix surrounding the egg. This causes rapid fluxes of ions, fusion of the acrosome membrane with the plasma membrane, and extension of the acrosomal filament. The acrosome membrane inserted into thesperm plasma membrane covers the acrosomal filament and contacts the plasma membrane of the egg to initiate sperm-egg fusion. One consequence of sperm-egg fusion is insertion of the sperm plasma membrane into the egg plasma membrane, producing a mosaic patch. The sperm components inserted persist in development and can be identified by quantitative methods after gastrulation. Another consequence of sperm-egg fusion is the cortical reaction in which thousands of vesicles fuse with the egg surface, thereby adding their membranes to the egg plasma membrane and releasing their contents upon the egg surface. This results in an approximate doubling of the amount of membrane on the egg surface in a few seconds and produces a mosaic topography. The excess surface membrane is accommodated by elongation of egg microvilli. The cortical reaction causes a detachment of the egg glycocalyx or vitelline membrane, and this layer is elevated from the egg surface. Contents of the cortical granules combine with and alter the vitelline membrane by a hardening reaction to produce the fertilization coat. Hardening involves a peroxidase-mediated tyrosine crosslinking, requiring a burst of oxygen consumption by the egg to generate hydrogen peroxide and resulting in chemiluminescence. These events are followed by activation of metabolic processes in the egg and changes which protect the egg against polyspermy.
The term "body plan" refers to the general similarities in development and form and function among members of a particular phylum. Another name for these similarities is baüplan, which is the German word for "body plan."
A body plan is a group of structural and developmental characteristics that can be used to identify a group of animals, such as a phylum. All members of a particular group share the same body plan at some point during their development—in the embryonic, larval, or adult stage. Biologists have long observed that anatomy and embryology reflect shared underlying structural plans. These plans can be used to define taxonomic groups (usually phyla) and to construct hierarchical classifications within groups (organisms with similar body plans tend to be more closely related).
Symmetry= Axis formation in the embryo is responsible for determining patterns of symmetry and polarity. Organisms may be asymmetrical (no symmetry) or symmetrical (a single line, or plane, of symmetry). Symmetry may be spherical, radial, or bilateral. Animals with spherical symmetry, like sea urchins, have a hollow globe of cell layers organized around a central point. Animals with radial symmetry, like jellyfishes, have body parts that radiate from a central point, like the spokes of a wheel. Animals with bilateral symmetry , like earthworms, have bodies that if cut lengthwise, form right and left halves that are mirror images.
Most metazoan body plans can be described as a "tube-within-a-tube," with a body wall made up of layers of different tissue types surrounding a central cavity. In almost all metazoans, the body wall has three cell layers (ectoderm, mesoderm, and endoderm), although some, such as sponges (Porifera), have no organized cell layers, and others, such as jellyfishes (Cnidaria ) have only two layers in the adult. Multicellular metazoan ancestors had an inside-outside, two-layered organization with an endoderm and ectoderm. In triploblasts , such as flatworms, a middle layer of mesoderm also evolved.
The body wall surrounds a coelum (central cavity) between the digestive tract and body wall that is completely lined by mesoderm. The coelom allows the digestive system and body wall to move independently. Because of this, internal organs can be more complex. The coelom may also serve as a storage area for eggs and sperm, facilitating development of these gametes within the animal body. Coelomic fluid helps in respiration and circulation by diffusing nutrients and in excretion by accumulating wastes. This fluid has the same function as several organ systems in the higher animals. In addition, coelomic fluid protects internal organs and serves as a hydrostatic skeleton.