BIOSCI 0815 1st Edition Lecture 10Outline of Last Lecture I. GametesII. MeiosisIII. Two partso Meiosis Io Meiosis IIo DifferencesOutline of Current Lecture I. Development Stepsa. Cleavageb. Gastrulationc. OrganogenesisII. Egg and SpermIII. Inside the EggIV. In Vitro FertilizationV. CleavageVI. Differences between mitosis and meiosisCurrent Lecture- Fertilization and Development- Development Stepso FertilizationThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Cleavage Repackaging the sytoplasm Cell divisiono Gastrulation Cells move around  Producing the body plan Types of cells develop This is particularly in animalso Organogenesis- Egg & Spermo Have a lifespan of 12 hours unless they combine.o Formed by meosis which resulted in haploid cells.o Egg: Rich in cytoplasm and protein Mitochondria (offspring’s mitochondria are from mother) Lots of nourishment The egg releases specific proteins so that the sperm can attract it. This is called a chemoattractant. The process is Chemotaxis. This is not specific to sperm, other cells moving in the body have similar trajectories. Five layers surround the egg so that it doesn’t dry out,- Layers 1 & 2: cells are tightly packed. The sperm has two enzymes on its membrane that loosen these tight associations- Layer 3 the sperm attaches to receptor which causes the release of enzymes from sperm head. These enzymes dissolve that layer - Layer 4: it gets to the outer membrane and there is another receptor. The sperm attaches to it and releases its haploid nucleus into the egg.  the coating of the egg is called the zona pellucida.o Sperm: Lots of mitochondria (to power movement, are not passed on) Designed to deliver its haploid cargo- Flagellum (tail) propels it towards the egg- It contains enzymes to dissolve coating on egg. Follows chemoattraction, “docking” recognizes the egg with a receptor, enzymes are released. Hyperactivity to get through layers and penetrationof egg coat. Contact with membrane of egg. The region that these enzymes are released from is called an acrosome.o How can we ensure that only one sperm fertilizes the egg? The egg is negatively charged overall, and the sperm is positively charged.Part of the attraction is that opposite charges attract. When the first sperm contacts the egg, sodium ions in the area around the egg move into the egg. They come through channels. This causes the egg to become positively charged which repels all the other positively charged sperm.  Also, when the first sperm binds, the egg releases chemicals that form a hard barrier to the other sperm. This is called a fertilization envelope. All the sperms on the egg surface are chopped off, and other receptors onthe surface of the cell go away. - Inside the egg:o The sperm releases its 1N nucleus. This is called a pronucleus. o Joins with egg’s nucleus. Forms diploid nucleus.o Now is when the egg completes the meiosis II.o This cannot happen until fertilization occurs. It gets rid of a polar body.o The DNA of the sperm and egg become de-condensed. Each one duplicates its DNA (this is the S phase). It needs to form sister chromatids in anticipation of the first mitotic division.o During mitosis: we have microtubules that attach to the centromere. The sperm brings one set of centrioles into the egg, but the egg has no centrioles to begin with. When the chromosomes have duplicated, so do the centrioles. This allows the cell to divide in its first mitosis. o What if two sperm get in? both replicate their centrioles, and you have a total of 54 chromosomes. The cell eventually disintegrates, because double fertilization doesn’t work and it cannot divide properly.- Science! IVFo Because we know the steps and the environment needed for fertilization, scientists are able to control fertilization outside of the bodyo This is called in Vitro Fertilization (IVF). This is a union of egg and sperm in a petri dish. In Vitro translates to “in glass”. o Why is IVF helpful? Some people are infertile. In order for it to be successful, women are put on a hormone regimen promotes maturation of several eggs from follicles. These eggs are harvested and stored. The male’s sperm is collectedand the active sperm are selected.o In the petri Dish The follicle cells are removed and they are exposed to sperm. After 18 hours they search for pronuclii.  If there is a low sperm count, direct injection of sperm into egg can be done. o The process is very expensive.- Cleavage:o After the egg and sperm have combined and mitosis is initiated, there is a series of rapid cell division.o The diploid cell is called a zygote. When it divides into a blastula. There is no growth between divisions. The blastula is the same size as the original egg. This isto repackage the DNA into many small packages. o The zygote moves down the fallopian tubes to the uterus. o It is now called a morula. The cells are undifferentiated at this point.Aao Blastocyst is when the cells orient themselves within and form a cavity. Around day 9 it implants to the uterine wall.o It must implant! The human egg has limited nutrition. Implantation allows the egg to obtain fuel and oxygen from the uterine wall. o Cell division creates different cells. As it divides it appears that the cells are the same but this is not the case. This is because initially the egg is not a container filled with homogeneous proteins. On the first division, one cell is exposed to a certain collection of chemicals while the other is not. The daughter cells will continue down “paths of differentiation” where they will go on to develop different types of cells.- Gastrulationo The creation of new cells begins to slow in comparison to cleavage. The division continues, and cells move to form major tissue layers. They ultimately form germ layers which represent basic embryonic tissue divisions. Three main layers: Endoderm (innermost), - Linings of digestive tract, makes liver and pancreas.  ectoderm (outer)- gives rise to epithelial, skin and nervous system mesoderm (middle layer).- Muscles and connective tissueo By the end of gastrulation, there are three germ layers. The process of creating organs develops. First it develops the nervous system in early organogenesis. Thisis the notochord- Organogenesis:o Formation of brain and spinal chord. Below