Embryology

Embryology: The scientific study of the development of an embryo from the point of the ovum's fertilization to the fetus stage in its growth.

There are many steps in the development of the embryo. The acrosomal reaction is the first step of fertilization. In this reaction, the sperm must fuse with and then penetrate the egg in order to fertilize it. Fusing the egg is generally very easy for the sperm, however breaking through the egg's protective, hard outer shell can pose a problem for the sperm. This is why the sperm go through the acrosome reaction. The acrosome is a cap-like structure on the head of the sperm. It is made up of surface antigens and numerous enzymes that are responsible for breaking down the egg's outer shell. When the sperm approach the zona pellucida of the egg, the egg's hard outer membrane, they undergo acrosomal reaction. During the acrosomal reaction, the membrane surrounding the acrosome fuses with the plasma membrane of the sperm and exposes the contents of the acrosome. Because its contents break down the tough exterior of the egg, the acrosome allows fertilization to occur.

Once the sperm has broken through the egg's plasma membrane, preventing other sperm from attaching to and entering the egg, the cortical reaction occurs. When the sperm contacts the egg's plasma membrane, it triggers calcium to be released from storage sites within the egg. In response to this, cortical granule membranes fuse with the plasma membrane of the egg, freeing the contents of the granules to the extracellular space. As the release of calcium travels across the egg, a wave of cortical granule fusion results. The contents of the granules differ between species, and are not entirely understood. Take the sea urchin, for example. In this organism, the granule contents alter a protein coat on the outside of the plasma membrane, known as the vitelline layer, so that it is released from the membrane. The freeing of the protein is known as "elevation of the fertilization envelope". As this occurs, non-fertilizing sperm are lifted away from the egg’s membrane and thus prevented from entering the egg, which prohibits polyspermic fertilization.

Once the egg has been fertilized, the organism begins to develop. Cleavage is the first step in the development of all multicellular organisms. It is the division of the cells in the early stages of the growth of the embryo. The single-celled zygote expands into a multi-celled embryo through cleavage. The blastula is produced by the mitosis of the zygote. Also called a blastosphere, the blastula consists of a spherical layer of cells surrounding the blastocoel, a fluid-filled cavity. Due to their rapid division, the size of the cells decreases. However, this increases their surface to volume ratio, which allows a more efficient oxygen exchange to take place between the cells and their environment. During this step of fertilization, RNA is dispersed throughout the blastula. These differentiations in molecular development are the basis for the next stages in development.

Gastrulation is the next step in embryology. It involves a series of cellular changes to positions where they form the three main cell layers. The ectoderm forms the tissues associated with the outer layer of our body such as the skin, hair and sweat glands. Also, the brain and nervous system develop from the ectoderm. The next layer is the mesoderm. It forms structures such as muscles, cartilage, bone, blood and all other connective tissue that support body movement. The mesoderm is also responsible for forming the reproductive systems organs and the kidneys. The inner layer is called the endoderm. It forms tissues and organs associated with the digestive and respiratory systems, as well as many endocrine structures.

Organogenesis is the series of organized, integrated processes that transform an unstructured mass of cells into a complete organ in the developing embryo. Basically, this is the process that creates the definite characteristics of the organs. Internal organs initiate development in humans within the third to eighth weeks in utero. The germ layers differ by three processes: folds, splits and condensation.

So how does this all happen in a sea urchin?

Sea urchin eggs are produced by the female and shed into the water. Sperm from the male is then spread over the eggs. Because sea urchin eggs free-float in the sea and the fertilization process is external, they must protect them. To prevent the egg and sperm from being swept away (or eaten), sea urchins have evolved methods to bring the gametes together, like magnets. When a sperm cell encounters an egg of the same species, it binds itself to the plasma membrane of the egg, triggering the release of calcium that facilitates fertilization. Both the sperm and egg have specific receptors for the other that must transmit a signal sequentially for fertilization to occur. However, all these forces that attract to one another can work too well and bring many sperm to the egg. The fertilization envelope protects the egg from polyspermic fertilization, which would produce an inviable zygote.

Following fertilization, the egg begins the process called cleavage. These early cell divisions occur rapidly. The egg synthesizes proteins using mRNA provided by the mother sea urchin. There are 4 main divisions in the cleavage of the sea urchin embryo. The first 3 cell divisions bisect the embryo equally. The first 2 cleavage planes from the top, known as the animal pole, to the bottom, called the vegetal pole, while the third runs across the middle and separates the embryo into “animal” and “vegetal” halves. The fourth cleavage divides the bottom half unequally, creating large cells called macromeres and small cells called micromeres.

Shortly after hatching, the offspring of the micromeres at the vegetal end detach and move into the blastocoel. These are known as primary mesenchyme cells and they form the calcium carbonate spicules of the larval skeleton.

The descendants of the macromeres thicken to form the vegetal plate, which invaginates to form the primordial gut. This process is known as gastrulation and, in addition to forming the gut, it results in a multilayered body plan.