Embryonic Development

Living things that reproduce sexually begin their existence as a single egg cell. That egg cell gets fertilized by a sperm cell and then begins the process of developing into a new organism. Once fertilization has occurred, the fusion of sperm and egg causes several things to happen:

• Additional sperm are prevented from entering the egg cell.
• The pH (acidity) of the egg cytoplasm (watery interior) changes.
• Protein synthesis (making of proteins) in the egg begins.
• Cell divisions begin to change the one cell into multiple cells.

The developing organism at this stage is called a zygote. Almost everything the newly formed zygote needs comes from the mother. The egg is so much larger than the sperm that it has sufficient space to hold organelles (small cellular structures, such as mitochondria for energy), nutrients, and transcription factors (molecules that help make proteins). The sperm contributes its haploid nucleus to produce a complete set of chromosomes. It also adds centrioles. These structures help form the zygote's centrosomes, small structures near the edges of the cell. This structure is important for the attachment of spindle fibers when the cell undergoes mitosis. The spindle fibers help the cell divide.

As the zygote begins to divide, the egg cytoplasm rearranges itself to prepare for determination, the process by which each cell becomes a specific type of cell, such as a skin cell or a muscle cell. The place where the sperm enters the egg determines the polarity (axes) of the zygote, and signal molecules align according to their charges. This results in these molecules not being distributed evenly. The rearrangement of the cytoplasm creates two distinct regions of the zygote, the vegetal hemisphere and the animal hemisphere. The vegetal hemisphere contains most of the yolk from the egg, while the animal hemisphere contains relatively little.

Animals are all multicellular organisms, which means they are made up of many cells. To get to this stage, the original egg must undergo cleavage, the splitting or dividing of cells during development. Cleavage is the early cell divisions that change the diploid zygote into a larger collection of generic, undifferentiated cells. As these cell divisions occur, the cytoplasm and other materials within it maintain their uneven distribution. Cleavage begins with rapid DNA replication, like just the interphase of mitosis, but in this case, there is no cell growth. As a result, the developing embryo becomes a mass of smaller and smaller cells. Over time, a fluid-filled cavity called a blastocoel will form in this mass. When this happens, the mass of cells is called a blastula, and each cell is called a blastomere. A blastula is the stage of animal embryo development that consists of a hollow ball of cells surrounding a central cavity.

There are three main patterns of cleavage that influence the formation of the blastula.

• Complete cleavage: occurs in eggs that have very little yolk (nourishment). The egg is divided completely.
• Incomplete cleavage: happens in eggs with a large amount of yolk. Cleavage furrows do not penetrate all the way through the egg.
• Superficial cleavage: a variation of incomplete cleavage. Mitosis occurs without cell division, resulting in cells with multiple nuclei called a syncytium. These nuclei move toward the outsides of the cells, and the membrane moves inward. This creates a blastoderm, the layer of cells that surrounds the blastula.

These early stages of development and cleavage in mammals are quite different from those of other organisms. First, these stages occur over a much longer period of time. Mammalian cell division can take between 12 and 24 hours compared to the minutes the process takes in other types of organisms.

The cytoplasm of the egg cell is rearranged into a number of smaller cells during cleavage. Cells in different regions of the blastula have different nutrients. The blastocoel keeps these different areas from coming into contact with each other, but that changes during the next stage of development. The cells begin to move around and interact with each other during this blastula stage by sending instructions to each other. Blastomeres become determined; that is, they develop a particular purpose (cell type).

The next stage after the blastula stage is the change of the blastula, which is a fluid-filled ball of cells, into a gastrula, which is the structure formed from the blastula that has three distinct layers of cells. Gastrulation involves the formation of three distinct layers of tissues through the movement and rearrangement of cells. Each layer is called a germ layer. The endoderm is the innermost layer, which gives rise to the lining of the digestive tract, respiratory tract, pancreas, and liver. The mesoderm is the middle layer, which gives rise to organs such as the heart, blood vessels, muscles, and bones. The ectoderm, the outer cell layer, gives rise to the nervous system, skin, and structures derived from the skin (hair, nails, and feathers, for example).

The process of gastrulation is very complex, and it varies among different organisms. However, in all animals, gastrulation leads to organogenesis, the development of organs and organ systems during animal development. Important steps in gastrulation thus include the development of cells that will lead to the mouth, anus, and digestive tract. Gastrulation also involves the breaking of the developing organism's symmetry. That is, structures within the gastrula segregate to one side or the other. This helps the developing organism to later form the yolk, placenta, or other structure that nourishes the embryo.

The following discussion focuses on development in mammals. Once the gastrula has formed, the embryo now has the three cell layers it needs to start developing and differentiating into an organism. It does this through the process of organogenesis. Organs and organ systems develop at the same time after neurulation, the stage of vertebrate development when the nervous system starts to form. While there are slight differences, neurulation happens in the same manner in all classes of chordates.

The first tissue that forms during neurulation is the notochord, the structure that supports the nervous system. This comes from a rod of mesodermal cells that extend down the midline of the embryo. The notochord also gives structural support to the developing embryo. It will be replaced by the vertebral column in vertebrates as the organism grows. The outer ectoderm layer covering the notochord will flatten and thicken, forming an area called the neural plate. The edges of this plate run in an anterior–posterior direction and form additional folds and ridges, which will eventually join with the midline. The folds then fuse to form a cylinder of neural tissue called the neural tube. The neural tube forms bulges at its anterior end, which will eventually become the parts of the brain. The rest of the tube will develop into the spinal cord.

As the neural tube forms, mesodermal tissues align themselves along the sides of the notochord, and the cells aggregate to form what will become each of the body's segments. Arthropod development occurs in a similar manner, except that instead of forming the vertebrae, ribs, muscles, and appendages, these cells form a head and thorax. The specific genes that control these formations are called HOX genes, or homeobox genes. These genes control the development of body segmentation and are distributed in vertebrates according to where the particular segment needs to develop along the anterior–posterior axis. Other genes provide signals for the dorsal–ventral development of body structures.