Embryonic Development Essay Research Paper The theory

Embryonic Development Essay, Research Paper

The theory of embryonic development we follow today was proposed 2000 years ago by Aristotle. This idea, called epigenesis, the form of an animal arises gradually from a generally formless egg. The organisms development is largely determined by the genome of the zygote and the organization of the cytoplasm of the egg cell. The first part of the development of a new animal is fertilization.

The gametes, the sperm and the egg, are both highly specialized haploid cells that will combine chromosomes to produce the diploid zygote. The union of the sperm with the egg activates the egg, triggering the onset of embryonic development. The mammalian egg is cloaked by follicle cells that were released with the egg during ovulation. The sperm must migrate through this before it reaches the zona pellucida, the extracellular matrix of the egg. To get there the acrosome of the sperm (tip) releases hydrolytic enzymes to the zona pellucida enabling sperm to enter egg. The zona pellucida is made of 3 different glycoproteins. The protein ZP3 functions as a sperm receptor. In non mammals the acrosomal reaction releases hydrolytic enzymes that enable an elongating acrosomal process to penetrate the jelly coat of the egg and to bind to the vitelline layer.

When the sperm meets with an egg, the sperm undergoes a acrosomal reaction; the reaction releases hydrolytic enzymes that enable the sperm or sperm nucleus to enter the egg. The tip of the acrosomal process is coated with a protein that binds to a specific receptor molecules just outside the plasma membrane of the egg. This “lock and key” enables only gametes of the same species to fertilize. The acrosomal reaction leads to the fusion of the sperm and the egg.

The fusion of the egg and the sperm causes ion channels to open allowing sodium to enter the cell, changing the membrane potential. This blocks other sperm cells from entering the egg and is called the fast block to polyspermy. Along with the fast block to polyspermy the calcium “activates” the cell, substantially increasing metabolism.

Another effect of the fusion of gametes is the cortical reaction, where calcium is released into the cytosol. Cortical granules, lying under the plasma membrane, sense the calcium increase and release their contents to form the fertilization membrane, which resists the entry of other sperm. This functions as the slow block to polyspermy to serve after the fast block has ended.

Fingerlike extensions of the mammalian egg cell, microvilli, take the entire sperm into the egg. The basal body of the sperm s flagellum divides and forms two coatrooms in the zygote. These will generate the mitotic spindle for cell division. The nuclei of the sperm and egg do not immediately fuse as they do in non-mammals, instead the envelopes of both nuclei disperse and the chromosomes from the two share a common spindle apparatus during the first mitotic cell division; thusly forming a diploid cell.

Following fertilization a process called cleavage creates a multicultural embryo. During cleavage the cells undergo DNA synthesis and mitosis phases of the cell cycle but usually skip the G1 and G2 phases. The embryo does not enlarge during cleavage, it simply divides the single large zygote into many smaller cells called blastomeres each with its own nucleus.

With the exception of mammals, most eggs of animals have a distinct polarity divided into a vegetal pole (having most amount of yolk) and animal (having least amount of yolk). Yolk is the nutrients stored in the egg. The polarity effects the speed of the division. Continued cleavage forms produces a solid ball known as morula. A fluid filled cavity called the blastocoel forms within the morula, creating a hollow ball stage of development called the blastula.

Once cleavage is finished gastrulation begins. Gastrulation is a morphogenetic process is a dramatic rearrangement of cells. Gastrulation differs from one animal group to another but follows the same fundamental principals. They are changes in motility, shape, adhesion to other cells and molecules in extracellular matrix. The essential result of gastrulation is that some of the cells near the surface of the blastula move to new, more interior location. This transforms the blastula into a three layered embryo called the gastula.

The three layers produced by gastrulation are the ectoderm, endoderm, and mesoderm (known collectively as the embryonic germ layers). The ectoderm forms the outer layer of the gastrula, the endoderm lines the embryonic digestive tract, and the mesoderm fills the space between the other two. The outer layer of skin and the nervous system arise from the ectoderm, the innermost lining of our digestive tract and the associated organs come from the endoderm, and most other organs and tissues (examples: heart, muscles, skeleton) arise from the mesoderm.

Various regions of the three germ layers develop into rudimentary organs during organgenesis. Folds, splits, and condensation are the first evidence of organ building. The first organs to be built in chordates are the neural tube and the notochord, the skeletal rod characteristic of all chordate embryos. The neural tube forms the central nervous system, the brain and spinal cord. The notochord is formed by condensation of the mesoderm and is later condensed even more into sepa