 An organism’s development is planned by a genetic program involving the genome of the zygote and the molecules placed in the egg by the mother › These molecules are called cytoplasmic determinants › As the zygote divides, differences occur between early embryonic cells due to the uneven distribution of cytoplasmic determinants and to signals from neighboring cells › The process of cell specialization in structure and function is called cell differentiation › Development involves morphogenesis, the process by which an organism takes shape and the differentiated cells occupy their appropriate locations

 The main function of fertilization is the combining of haploid sets of chromosomes from two individuals into a single diploid cell › Contact of the sperm with the egg’s surface starts metabolic reactions within the egg and “activating” it  The acrosomol reaction begins when a specialized vesicle at the tip of the sperm, called the acrosome, discharges hydrolytic enzymes  Egg activation is the outcome of the sharp rise of Ca 2+ concentration in the egg’s cytosol, as well as the binding and fusion of sperm  Unfertilized eggs of many species can be artificially activated by the injection of Ca 2+  Fertilization in mammals is mostly internal, which causes an increase in sperm motility

 During cleavage, cells carry out the S and M phases of the cell cycle. Cleavage divides the cytoplasm of a zygote into smaller cells called blastomeres › Eggs and zygotes have a definite polarity which have a specific pattern that needs to be followed › The polarity is defined by the uneven distribution of substances in the cytoplasm, including cytoplasmic determinants and yolk (stored nutrients)  The distribution of yolk is a key factor influencing the pattern of cleavage. It’s often concentrated toward one pole of the egg, called the vegetal pole ; the yolk concentration decreases toward the opposite pole, the animal pole  Establishment of the three body axes occurs early in development. The animal and vegetal hemispheres of the zygote can be distinguished by color (animal is gray and vegetal is yellow)  Cortical rotation is when the plasma membrane and associated cortex rotate with respect to the inner cytoplasm

 Is the morphongenetic process in which cells take up new locations that will allow the formation of tissues and organs. The result is that some cells at or near the surface of the blastula move to a new interior location, and three cell layers are established  UkTaGYw UkTaGYw

 During organogenesis, various regions of the three embryonic germ layers develop into the rudiments of organs › Involves localized shape changes in both tissues and individual cells

 Is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion (using structures such as flagella, cilia, and lamellipodia ), and plays important roles in both intracellular transport and cellular division › Cell adhesion molecules (CAMs) are a key group of proteins that contribute to cell migration and stable migration and stable tissue structure

 Fate maps of embryos have shown that specific regions of the zygote or blastula develop into specific parts of older embryos

 In nonamniotes, unevenly distributed cytoplasmic determinants in the egg are important in establishing the body axes and in setting up differences between the blastomeres resulting from cleavage of the zygote  Cells that receive different cytoplasmic determinants result in different outcomes

 Cells in a developing embryo receive and respond to positional information that differs with location › This information is in the form of signaling molecules hidden by cells in special “organizer” regions of the embryo  The signaling molecules influence gene expression in the cells that receive them