Chapter 47 Animal Development Introduction Embryonic development involves common stages set in a common order Fertilization Cleavage Gastrulation and Organogenesis 47 1 Fertilization and Cleavage Initiate Embryonic Development FERTILIZATION Sperm penetrates or dissolves the protective layer surrounding the egg to reach the plasma membrane Molecules on the sperm surface bind to receptors on the egg surface to ensure same species fertilization POLYSPERMY multiple sperm nuclei enter egg is prevented by changes at the surface of the egg The Acrosomal Reaction As soon as a sperm head contacts the jelly coat of the egg molecules in the jelly trigger the discharge of hydrolytic enzymes from the ACROSOME specialized vesicle at tip of sperm The enzymes then digest the jelly coat enabling the sperm structure called the acrosomal process to elongate and penetrate the coat which allows for the binding to specific receptors This binding serves as a very important lock key model which is important in external fertilization This recognition triggers for the sperm and egg to fuse their plasma membranes Depolarization inflow of sodium ions leads to the fast block to polyspermy The Cortical Reaction The slow block to polyspermy serves as longer lasting change to prevent polyspermy This is done when vesicles called cortical granules fuse with the egg plasma membrane The secreted contents clop off sperm binding receptors and cause the fertilization envelope to form Egg Activation Fertilization initiates and speeds up the metabolic reactions that trigger the onset of embryonic development Fertilization initiates this by indicating that a rise in Ca 2 concentration that causes the cortical reaction also causes egg activation Fertilization in Mammals Mammals mostly fertilize eggs internally Support cells of the developing follicle surrounding the mammalian egg before and after ovulation A sperm must travel through a layer of follicle cells before it reaches the ZONA PELLUCIDA which is the EC matric of the egg There the binding of sperm to a sperm receptor induces acrosomal reaction which facilitates sperm entry Mammals don t have a fast block to polyspermy and fertilization is a much slower process CLEAVAGE After fertilization is complete zygotes undergo rapid cell divisions called Cleavage The S DNA synthesis phase and M mitosis phase are the primary stages of the cell cycle Growth is skipped and thus cells are divided into smaller cells called blastomeres A hollow ball of cells called the BLASTULA surrounds the fluid filled cavity called the BLASTOCOEL The pattern of cleavage divisions differs among species Cleavage Pattern in Frogs In frogs and many other animals cleavage is asymmetric reflecting the asymmetric division of the yolk Yolk is often concentrated toward one pole VEGETAL POLE and away from the opposite ANIMAL POLE When an animal cell divides a cleavage furrow forms as cytokinesis divides the cell in half The first 2 cleavage furrows in eh frog form parallel to the line connecting the 2 poles and the yolk slows completion of cytokinesis However the cells keep on dividing the yolky cytoplasm and this result in uneven distribution The third division is effected from the yolk and results in an equatorial division that produces an 8 celled embryo The blastocoel forms entirely in the animal hemisphere due to the unequal division Cleavage Patterns in Other Animals HOLOBLASTIC cleavage is when the cleavage furrow still passes entirely through the egg This is seen in echinoderms mammals and annelids These organisms have little amount of yolk MEROBLASTIC cleavage is incomplete cleavage of a yolk rich egg and is found in chickens and other birds due to the high volume of yolk preventing complete division Regulation of Cleavage RNA and proteins deposited in the egg during oogenesis help with initial development since there is too little DNA to produce mRNA to make proteins During the blastomere phase the cells can make enough RNA to program the cell s metabolism and development 47 2 MORPHOGENEIS IN ANIMALS INVOLVES SPECIFIC CHANGES IN CELL SHAPE POSITION AND SURVIVAL GASTRULATION establishment of cell layers and ORGANOGENESIS the formation of organs are responsible for MORPHOGENESIS the process by which the animal body takes shape GASTRULATION Gastrulation is a dramatic reorganization of the hollow blastula into a three layered embryo called a GASTRULA The cell layers produced are called GERM LAYERS and they are the Endoderm Mesoderm and Ectoderm The Endoderm forms the embryonic digestive compartment and lining The Ectoderm forms the outer layer while the mesoderm forms between the Ectoderm and Endoderm and is found in vertebrates with bilateral symmetry Gastrulation in Sea Urchins Involves cell migration as well as INVAGINATION in folding of a sheet of cells into the embryo The infoldings form a tube called the ARCHENTERON and the open end becomes the anus and is called the BLASTOPORE The forms the second opening This trait is shared by all chordates vertebrates and Deuterostomes Gastrulation in Frogs Frogs and other bilaterally symmetrical animals have a dorsal top side and a ventral bottom side left right and posterior and anterior sides The cell movements that begin gastrulation begin on the dorsal side of the blastula Gastrulation in Chicks Starting point for gastrulation in chick is an embryo consisting of upper and lower layers epibast and hypoblast which lie atop the yolk All the cells that will form the embryo come from the epiblast Cells from the epiblast move toward the midline of the blastoderm and pileup producing a PRIMATIVE STREAK The most downward cells form the endoderm and push aside the hypoblast cells while other migrate laterally to form the mesoderm The cells left behind on the surface will become the ectoderm Formation of the Primitive Streak is central to human embryo gastrulation Gastrulation in Humans Embryo has 100 cells arranged around cavity and is called a blastocyst mammalian version of blastula Inner Cell Mass are clustered at one end of the blastocyst cavity which will develop into the embryo proper Embryo implantation is initiated by the TROPHOBLAST the outer epithelium of the blastocyst Enzymes secreted by the Trophoblast break down the endometrium and allow invasion of the blastocyst Finger like extensions cause blood to spill out from capillaries and be captured by trophoblast tissue Around implantation inner cell mass of the blastocyst undergoes implantation