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1) Chapter Fertilization i) Fertilization involves four events (1) The sperm contacts the egg and recognition occurs. (2) The sperm or sperm nucleus enters the egg. (3) The egg becomes activated and developmental changes begin. (4) The sperm and egg nuclei fuse. b) The first step in fertilization involves contact and recognition. i) Eggs are active in fertilization. ii) Coverings facilitate fertilization as well as no fertilization by sperm of the wrong species. iii) Outside the plasma membrane of the sperm, there is a thin vitelline envelope, and a thick glycoprotein jelly coat. iv) The egg secretes chemical substances the draw sperm to it. v) When a sperm contacts an egg, it undergoes an acrosome reaction, in which the acrosome membrane fuses and enlarges its pores. vi) If the species of sperm and egg are the same, bindin, a species-specific protein adheres the egg + sperm. vii) Before a sperm partakes in fertilization it must first undergo capacitation, a maturation process in the female reproductive tract. viii) This allows sperm to have their acrosome reaction and makes them more motile. c) Sperm entry is regulated i) In the FAST BLOCK to polyspermy, the egg membrane depolarizes, and no more sperm can bind to it. ii) In the SLOW BLOCK to polyspermy, which can take 7 minutes, the cortical reaction bring calcium ions into the cell to enlarge the space between the two membranes and make it impossible for more sperm to get in. iii) The vitelline envelop ^ becomes elevated from the plasma membrane. d) Fertilization activates the egg i) Ca ions trigger the ACTIVATION PROGRAM, a series of metabolic changes within the egg. e) Sperm and Egg pronuclei fuse restoring the diploid state. i) Microtubules bring the pronuclei together. ii) They fuse to form a diploid zygote 2) Cleavage i) The zygote is totipotent, and gives rise to all the cell types in a new individual. ii) The bulk of the cytoplasm comes from the ovum. iii) Shortly after fertilization, the zygote undergoes cleavage, a series of rapid mitotic divisions with no growth. iv) Repeated divisions generate the blastomeres, or cells that make up the embryo. v) At the 32 cell stage, the embryo is a solid ball of cells called a morula. vi) After that stage it is called a blastula. vii) The fluid filled cavity that forms is called the blastocoel.
b) The pattern of cleavage is affected by yolk. i) Many eggs contain yolk, which is a mixture of proteins, phospholipids, and fats that serves as food for the embryo. ii) Most invertebrates and chordates have isolecithal eggs with relatively SMALL amounts of yolk. iii) This type of egg divides completely with holoblastic cleavage. iv) Cleavage is radial or spiral. v) In Radial Cleavage, the first division is vertical, and the rest are at right angles, and then horizontal. vi) In Spiral Cleavage, the plane of division becomes diagonal to the polar axis. Each cell is above and between those underlying it. vii) Many vertebrate eggs are Telolecithal, and have large amounts of yolk concentrated at one end of the egg, the vegetal pole. The viii) other end where the cells are concentrated is called the animal pole. ix) DIVISION IS SLOWED BY YOLK. x) The eggs of reptiles and birds have their cleavage restricted to the blastodisc, the small disc of cytoplasm at the animal pole. xi) This cleavage is termed meroblastic. c) Cleavage may distribute developmental determinates. i) The pattern of cleavage depends on factors other than yolk distribution. ii) The mosaic development of cells is through to be due to the unequal distribution of materials in the cytoplasm. iii) Some embryos have the exact opposite scheme of development. iv) In the frog embryo, fertilization causes some of the cytoplasmic determinants to migrate to a specific point on the cell. v) This grey crescent is though to hold many determinants necessary in the frog's development. vi) The grey crescent develops into the dorsal half of the embryo. d) Cleavage provides building blocks for development. i) Cells begin to arrange themselves during cleavage, using the proteins on the outside as locators. 3) Gastrulation i) The process by which an embryo becomes a three layered gastrula is gastrulation. ii) The stages of development: (1) Zygote (2) Early Cleavage Stages (3) Morula (4) Blastula (5) Gastula iii) Germ layers: (1) Ectoderm, the outer layer (2) Mesoderm, middle layer (3) Endoderm, the inner layers b) The pattern of gastrulation is affected by yolk
i) Gastrulation begins when a group of cells at the vegetal pole cause part of the blastula's wall to flatten and then to invaginate. ii) This eventually meets the opposite wall creating the digestive tract. iii) The newly formed internal wall lines the Archenteron, the newly forming gut. iv) The opening of the archenteron to the exterior, the blastopore, becomes the anus in deuterostomes. v) In frog embryos, the cells move up the exterior until they reach the grey crescent and then move to the inside. vi) This is called the dorsal lip of the blastopore. vii) The yolk filled cells then fill in the blastopore and form the yolk plug. viii) In birds, cells also migrate up the outside or epiblast, only they form the primitive streak, which elongates as the embryo develops. ix) The center of this is called the primitive groove. x) The bird embryo doesn't have a cavity like the archenteron. xi) The anterior of the primitive streak forms the notochord. xii) They concentrate in a thickened knot known as Hensen's Node. 4) Organogenesis i) In development, organogenesis is the formation of organs. ii) The development of the notochord leads to the formation of the neural plate, the precursor of the CNS. iii) This is an example of induction, where cells stimulate their neighbors to differentiate. iv) The neural plate dips down eventually to form the neural groove, with the cells flanking it becoming the neural folds. v) These fold come closer together until they eventually form the neural tube. vi) This develops into the brain and spinal chord. vii) The Neural Crest grows off of these structured and allows for the neurons to reach all over the body.