Epigenesis is the belief that the form of an embryo gradually develops from a formless egg. This idea was originally proposed by the Greek philosopher Aristotle. Modern biologists have found that the organism's development is mostly determined by the zygote's genome and the organization of the egg's cytoplasm. As the zygote undergoes cleavage, the cytoplasm is compartmentalized causing the nuclei of the different cells to be exposed to different cytoplasmic environments. These different cytoplasmic environments result in the expression of different genes in different cells. Inherited traits then emerge, in an orderly fashion, in space and time by mechanisms controlling gene expression.
Embryonic development involves cell division, differentiation, and morphogenesis. Cell division results in an increase in the number of cells. All the cells result from mitotic divisions beginning with the zygote. Differentiation is the development of specialized cells that are organized into tissues and organs. This differentiation is controlled by the cell's expression of different genes brought on by its contact with the environment. Morphogenesis includes the physical processes that give shape to the animal's body and organs.
Fertilization: activates the egg and brings together the nuclei of the sperm and egg. Fertilization forms the diploid zygote and triggers the onset of embryonic development. The steps in the fertilization process are as follows:
This reaction is caused by a discharge of hydrolytic enzymes from the arcosome of the sperm cell. When the head of the sperm contacts the egg the enzymes are released from a vesicle. This allows the arcosomal process to elongate and penetrate the the jell coat of the egg. The protein coating the tip will attach to the exact receptors on the egg's vitelline layer to ensure it is the correct species of sperm. The enzymes continue to digest the membrane allowing the tip to reach the plasma membrane of the egg. The two membranes fuse, allowing the sperm's nucleus to enter the egg. A quick depolarization then occurs locking out all other sperm from the egg.
The fusion of the sperm and egg membranes stimulates a series of changes in the egg's cortex. The egg-sperm fusion causes a signal transduction pathway involving a G- protein to stimulate the release of Ca2+ from the egg's endoplasmic reticulum. The Calcium acts as a second messenger and results in a change in the egg's cortical granules. This increase in calcium causes the cortical granules to fuse with the plasma membrane and release their contents into the perivitlline space outside of the plasma membrane. Eventually the area will swell and become hard forming a fertilization membrane. This membrane will prevent additional sperm from entering the egg .
The sharp rise in cytoplasmic calcium also incites metabolic changes that activates the egg. Cellular respiration and protein synthesis increase; cytoplasmic pH becomes more basic due to a loss of H+ ; the sperm nucleus swells and merges with the egg nucleus to form the zygote and DNA replication begins with the first division occurring in about 90 minutes.
Cleavage: is a succession of rapid mitotic cell divisions following fertilization and produces a multi cellular embryo, the blastula. During cleavage the S and M stages of the cell cycle occur, while the G1 and G2 phase are skipped. Very little DNA transcription occurs causing very little growth in the embryo. A definite polarity develops in the egg caused by the concentration of materials such as mRNA, proteins, and yolk. The yolk is key in determining polarity and influencing cleavage in many animals. The egg's vegetal pole contains the highest concentration of yolk. The animal pole contains the lowest concentration and is the area where polar bodies bud off of the cell. The animal pole marks where the most anterior part of the animal will form. The animal hemisphere is gray due to the presence of the pigment melanin. The vegetal hemisphere is slightly yellow due to the yellow yolk.
Cleavage in the animal hemisphere is more rapid than in the vegetal hemisphere. If there is little yolk in the vegetal hemisphere cleavage will proceed equally. The first two cleavage divisions are vertical and divide the embryo into 4 cells. The third cleavage plane is horizontal and produces an 8 cell embryo with two levels. Deuterostome cleavage forms 2 tiers of cells one exactly above the other. Protostome cleavage the upper tier of cell align directly over the grooves of the lower tier. Continual cleavage produces a solid ball of cells called the morula. A fluid filled cavity, called the blastocoel develops within the morula forming a hollow ball of cells called the blastula.
Gastrulation rearranges the blastula to form a three-layered embryo with a primitive gut. The three layers produced by gastrulation are embryonic tissues called embryonic germ layers. These three germ layers will eventually develop into all parts of the adult animal. The ectoderm is the outer layer of the gastrula. The nervous system and the outer layer of the skin in adult animals develop from the ectoderm. The endoderm produces the lining of the digestive tract and associated organs (liver, pancreas). The mesoderm partially fills the space between the ecto and endoderm. The kidneys, heart, muscles, inner layer of the skin and most other organs develop from the mesoderm.
Gastrulation during frog development begins with a small crease on the blastula where the blastopore will eventually form. Invagination occurs due to a cluster of cells burrowing inward. This process produces a dorsal lip where the gray crescent was located on the zygote. Involution then occurs. This is the process where cells on the surface roll up and move into the embryo's interior away from the blastopore. The archenteron ( primitive gut ) forms within the endoderm.
Organogenesis: forms the organs of the animal body from the three embryonic layers. The first evidence of organogenesis is morphogentic changes (folds, splits, condensation of cells) that occur in the layered embryonic tissues. The neural tube and notochord are the first organs to develop in frogs and other chordates. The ectoderm above the beginning notochord thickens to form the neural plate that sinks below the the embryo's surface and rolls itself into a neural tube which will become the brain and spinal cord. The notochord stretches the embryo lengthwise and forms the core around witch the mesoderm cells will develop the muscles of the axial skeleton. As organogenesis continues, other organs and tissues develop from the embryonic germ layers: Ectoderm: epidermis, epidermal glands, inner ear, and eye lens, Mesoderm: notochord, coelomic lining, muscles, skeleton, gonads, kidneys, and most of the circulatory system, Endoderm: forms the digestive tract lining, liver, pancreas, and lungs.