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.

In Embryology....The first step of fertilization is The acrosomal reaction.  This is when the sperm must fuse and penetrate the egg in order to fertilize it. This is generally very easy for the sperm but it can be hard to break through the egg's outer shell. It is very hard and used for protection.  This is the reason for why the sperm go through the acrosome reaction. This acrosome is a cap-like structure on the head of the sperm. It is composed of  surface antigens and numerous enzymes. Its job is to break down the egg's outer shell. Once the sperm approach the zona pellucida of the egg, they undergo this acrosomal reaction. During the reaction the membrane around the acrosome fuses with the plasma membrane of the sperm and exposes the contents of the acrosome. This allows for fertilization to occur. 

The next step is for the cortical reaction to occur. Once the sperm reaches the egg's plasma membrane it triggers calcium to be released from the egg. From this, the cortical granule membranes mix with the plasma membrane of the egg. This frees the contents of the granules to the extracellular space. As this calcium circles the egg a wave of cortical granule fusion occurs. It is important to note that the contents of the granules are different among species of humans and other creatures.


Sea Urchin example:

The granule contents alter a protein coat on the vitelline layer, so that it is released from the membrane. This process of freeing the protein is referred to as "elevation of the fertilization envelope". Due to this, non-fertilizing sperm are lifted away from the egg’s membrane and are prevented from entering the egg. This prohibits polyspermic fertilization (this is when an egg is fertilized by more than one sperm). 




Once the egg is fertilized it is now time for the organism to develop. Cleavage is the first step in this process of developing all of the multicellular organisms. This is the division of the cells in the early stages of the growth of the embryo. The zygote (single-cell) expands into what is called a multi-celled embryo. Due to the process mitosis, the blastula is produced. The blastula is made up from a spherical layer of cells that surround the blastocoel. This blastocoel is like a fluid filled cavity. The size of the cells decrease because of the rapid division of the cells that takes place. This increases the cell's surface to volume ratio. Because of this a more efficient oxygen exchange takes place among the cells and the environment they are in. Throughout this stage of fertilization, RNA is spread through the blastula. 



The next step in embryology is known as Gastrulation. This is a series of cellular changes to positions where they form the three main cellsuch layers. The ectoderm forms the tissues associated with the outer layer of our body (skin, hair, sweat glands). The next layer is the mesoderm. This forms the muscles, cartilage, bone, blood along with other connective tissue that are responsible for supporting body movement. This layer also forms the reproductive systems organs as well as the kidneys. The last layer is called the endoderm; this is the inner layer. It is responsible for forming tissues and organs that are associated with the digestive system, the respiratory systems, and other endocrine systems too.

Organogenesis is the series of organized, integrated processes that transform an unstructured mass of cells into a complete organ in the developing embryo. This creates the characteristics of the organs. The internal organs begin this development in humans within the third to eighth weeks in utero (state of the embryo or fetus). The germ layers differ by three seperate processes. This consists of folds, condensation and splits.

CONNECTION WITH SEA URCHIN

Most sea urchins have their eggs free floating in the sea. Some keep them on their spines for protection. In order to prevent the sperm and egg from being washed away, these urchins have developed mechanisms that bring the gametes together. Once a sperm cell encounters an egg of the same species, components of the jelly coat bind and connect to specific egg receptors in the plasma membrane. This then triggers the release of calcium that facilitates fertilization. 

In  sea urchins, early cell divisions are rapid. The proteins that are synthesized during the cleavage process utilize mRNA that is found within the cytoplasm. The first three cell divisions bisect the embryo equally while the fourth cleavage divides the cells in the top half equally. The ones in the bottom half are divided unequally. At this point, the cells continue to divide until they form the blastula.There are ten cycles of cell division for sea urchin embryos in order to make a single epithelial layer. The embryo then begins the grastrulation process (a multipart process which rearranges and invaginizes cells to produce three germ layers).  

This fertilized egg then develops into a free-swimming blastula embryo in twelve hours. The blastula transforms into a echinopluteus larva. This larva has has elongated arms, nutrients, and a cilia  which is used to capture particles of food. It takes up to several months for a larva to be completely developed. The larva proceeds to sinking to the bottom of the sea once the development is complete and begins its process of turning into an adult. Shockingly enough this may only take an hour.