Preformation • How does an egg become an animal? • Until the end of the 18th century, the prevailing theory was that the embryo was a miniature infant. • This idea of preformation also included the thought that each embryo contained all of the descendents as smaller embryos.
Another version of preformation included the idea of a homunculus- the sperm contains a preformed infant which grows.
Epigenesis • Another theory proposed by Aristotle 2,000 years earlier was that of epigenesis. • The form of an animal emerges gradually from a relatively formless egg. • Microscopy allowed scientists to witness the progressive development of embryos- thereby validating Aristotle’s theory.
Fertilization • Fertilization in vertebrates is, of course, the union of two haploid gametes to reconstitute a diploid cell - a cell with the potential to become a new individual. • Fertilization is a not a single event. Rather, it is a series of steps that might be said to begin when egg and sperm first come into contact and end with the intermingling of haploid genomes.
Events of Fertilization • Contact & recognition between sperm and egg • Regulation of sperm entry into the egg. Only one can enter - others inhibited from entering • Fusion of genetic material • Activation of egg metabolism to start development
Acrosomal Reaction • Binding of sperm to the zona pellucida (egg membrane) is the easy part of fertilization. • The sperm then faces the daunting task of penetrating the zona pellucida to get to the oocyte. • Evolution's response to this challenge is the acrosome - a huge modified lysosome that is packed with zona-digesting enzymes and located around the anterior part of the sperm's head - just where it is needed.
The acrosome reaction provides the sperm with an enzymatic drill to get throught the zona pellucida.
As the acrosome reaction progresses and the sperm passes through the zona pellucida, more and more of the plasma membrane and acrosomal contents are lost. • By the time the sperm traverses the zona pellucida, the entire anterior surface of its head, down to the inner acrosomal membrane, is denuded.
The constant propulsive force from the sperm's flagellating tail, in combination with acrosomal enzymes, allow the sperm to create a tract through the zona pellucida. • Once a sperm penetrates the zona pellucida, it binds to and fuses with the plasma membrane of the oocyte.
Egg Activation • Prior to fertilization, the egg is in a quiescent state, arrested in metaphase of the second meiotic division. • Upon binding of a sperm, the egg rapidly undergoes a number of metabolic and physical changes that collectively are called egg activation. • Prominent effects include a rise in the intracellular concentration of calcium, completion of the second meiotic division and the so-called cortical reaction.
Zona Reaction • The critical importance of the zona reaction is that it represents the major block to polyspermy in most mammals. • This effect is the result of two measurable changes induced in the zona pellucida: • The zona pellucida hardens. • Sperm receptors in the zona pellucida are destroyed
Stages of Development • In animals, one can usually distinguish 4 stages of embryonic development. • Cleavage • Patterning • Differentiation • Growth
Cleavage • Mitosis and cytokinesis of the zygote, an unusually large cell, produces an increasing number of smaller cells, each with an exact copy of the genome present in the zygote. • However, the genes of the zygote are not expressed at first. The activities of cleavage are controlled by the mother's genome; that is, by mRNAs and proteins she deposited in the unfertilized egg. • Cleavage ends with the formation of a blastula.
Patterning • During this phase, the cells produced by cleavage organize themselves in layers and masses, a process called gastrulation. The pattern of the future animal appears: • front and rear (the anterior-posterior axis) • back side and belly side (its dorsal-ventral axis) • left and right sides.
Gastrulation forms three major "germ layers": ectoderm, mesoderm, and endoderm. • By gastrulation, the genes of the zygote genome are being expressed.
Differentiation • In time, the cells of the embryo differentiate to form the specialized structures and functions that they will have in the adult. This is called differentiation. • They form neurons, blood cells, skin cells, muscle cells, etc., etc. • These are organized into tissues, the tissues into organs, the organs into systems – This is called organogenesis
Growth • After all the systems are formed, most animals go through a period of growth. Growth occurs by the formation of new cells and more extracellular matrix.
Germ Layers • Each of these will have special roles to play in building the complete animal. • Some are listed in the table on the next slide.
Eggs and Zygotes have Animal and Vegetal Poles • Egg cells are very large: • Sea urchin: ~70 to 150 microns • Human ~100 microns • Frogs & fishes, some insect eggs: 1000 to 2000 microns(1-2 mm) • Birds & reptiles: millions of microns (many cm) • Eggs store materials needed for development of the embryo • Yolk: lipids, carbohydrates and proteins organized into granules
Yolk settles to bottom of egg, producing a gradient of stored material • Top of egg, with little yolk, is called the animal pole • Bottom of egg, rich in yolk, is called the vegetal pole • Polar axis goes from animal to vegetal pole
Eggs have different amounts of yolk • Large animals developing outside mothers body (birds, reptiles) have large eggs with lots of yolk • Large animals developing within mother's body (mammals) have small eggs with very little yolk; they get their food from the mother through the placenta • Animals which develop into small feeding larvae (sea urchins, sea stars) also have small, simple eggs • Frogs and fish are intermediate in egg size and yolk content • Almost all of the zygote volume comes from the egg, giving the zygote an animal & vegetal pole
Embryological Development • To go from a single-cell to an organism the embryo must repeatedly divide by mitosis • The early set of rapid cell divisions is called cleavage
In a Series of Mitotic Divisions the Zygote Becomes a Hollow Ball (Blastula) • The first set of cleavage divisions are synchronized and there is no cell growth between divisions • The size of the embryo does not change, but the egg material is partitioned into more and more cells • DNA synthesis does occur between divisions since each new cell needs a nucleus. • The cells arrange themselves into a ball (blastula; called blastocyst in mammals) with the cell layer surrounding the fluid-filled interior (blastocoel)
The Archenteron • After the blastula is finished the wall folds inward at one point • Forms a tube, the archenteron or primitive gut • The opening to the archenteron is called the blastopore • Cells at the animal pole grow and spread over outer surface, forcing other cells inward through the blastopore • In bird & mammal embryos there is a long furrow, the primitive streak instead of a blastopore
Organogenesis • In chordates the first organ to form is the notochord and the neural tube (later becomes the central nervous system – brain and spinal cord) • The neural tube forms from ectoderm • The notochord forms from the mesoderm • Embryonic Induction –The ability of a group of embryonic cells to influence the development of another group of embryonic cells.
Cytoplasmic Determinants • Certain materials are distributed unequally within the egg. These substances are not the same in each of the cells that are formed and determine the fate of the cells and the cell’s pattern of gene expression. • Hans Spemann did an experiment with the gray crescent of an embryo to show the importance of the cytoplasm in development. • Dissected an embryo in the two ball stage in different ways and only the one with the gray crescent developed normally.
The striped area is the Grey Crescent shows that if it is cut in a way where it does not get some of the grey crescent the embryo does not develop properly.
Induction • Discovered by Spemann • Proved that the dorsal lip of the bastopore normally initiates inductions that forms the neural tube. • Grafted a piece of dorsal lip from one embryo onto a second embryo and cause the development of two neural tubes. • The grafted tissue caused the abdomen tissue to become neural tissue. • Spemann called the dorsal lip the primary organizer.
Homeotic, Homeobox, or Hox Genes • Master genes that control the expression for anatomical features. It is like a paint by number – Place legs here, arms here, head here, etc. They direct the placement of structures in the correct place.