Embryonic Development Essay, Research Paper
The theory of embryonic development we follow today was proposed 2000 years ago by Aristotle. This idea, called epigenesis, the form of an animal arises gradually from a generally formless egg. The organisms development is largely determined by the genome of the zygote and the organization of the cytoplasm of the egg cell. The first part of the development of a new animal is fertilization.
The gametes, the sperm and the egg, are both highly specialized haploid cells that will combine chromosomes to produce the diploid zygote. The union of the sperm with the egg activates the egg, triggering the onset of embryonic development. The mammalian egg is cloaked by follicle cells that were released with the egg during ovulation. The sperm must migrate through this before it reaches the zona pellucida, the extracellular matrix of the egg. To get there the acrosome of the sperm (tip) releases hydrolytic enzymes to the zona pellucida enabling sperm to enter egg. The zona pellucida is made of 3 different glycoproteins. The protein ZP3 functions as a sperm receptor. In non mammals the acrosomal reaction releases hydrolytic enzymes that enable an elongating acrosomal process to penetrate the jelly coat of the egg and to bind to the vitelline layer.
When the sperm meets with an egg, the sperm undergoes a acrosomal reaction; the reaction releases hydrolytic enzymes that enable the sperm or sperm nucleus to enter the egg. The tip of the acrosomal process is coated with a protein that binds to a specific receptor molecules just outside the plasma membrane of the egg. This “lock and key” enables only gametes of the same species to fertilize. The acrosomal reaction leads to the fusion of the sperm and the egg.
The fusion of the egg and the sperm causes ion channels to open allowing sodium to enter the cell, changing the membrane potential. This blocks other sperm cells from entering the egg and is called the fast block to polyspermy. Along with the fast block to polyspermy the calcium “activates” the cell, substantially increasing metabolism.
Another effect of the fusion of gametes is the cortical reaction, where calcium is released into the cytosol. Cortical granules, lying under the plasma membrane, sense the calcium increase and release their contents to form the fertilization membrane, which resists the entry of other sperm. This functions as the slow block to polyspermy to serve after the fast block has ended.
Fingerlike extensions of the mammalian egg cell, microvilli, take the entire sperm into the egg. The basal body of the sperm s flagellum divides and forms two coatrooms in the zygote. These will generate the mitotic spindle for cell division. The nuclei of the sperm and egg do not immediately fuse as they do in non-mammals, instead the envelopes of both nuclei disperse and the chromosomes from the two share a common spindle apparatus during the first mitotic cell division; thusly forming a diploid cell.
Following fertilization a process called cleavage creates a multicultural embryo. During cleavage the cells undergo DNA synthesis and mitosis phases of the cell cycle but usually skip the G1 and G2 phases. The embryo does not enlarge during cleavage, it simply divides the single large zygote into many smaller cells called blastomeres each with its own nucleus.
With the exception of mammals, most eggs of animals have a distinct polarity divided into a vegetal pole (having most amount of yolk) and animal (having least amount of yolk). Yolk is the nutrients stored in the egg. The polarity effects the speed of the division. Continued cleavage forms produces a solid ball known as morula. A fluid filled cavity called the blastocoel forms within the morula, creating a hollow ball stage of development called the blastula.
Once cleavage is finished gastrulation begins. Gastrulation is a morphogenetic process is a dramatic rearrangement of cells. Gastrulation differs from one animal group to another but follows the same fundamental principals. They are changes in motility, shape, adhesion to other cells and molecules in extracellular matrix. The essential result of gastrulation is that some of the cells near the surface of the blastula move to new, more interior location. This transforms the blastula into a three layered embryo called the gastula.
The three layers produced by gastrulation are the ectoderm, endoderm, and mesoderm (known collectively as the embryonic germ layers). The ectoderm forms the outer layer of the gastrula, the endoderm lines the embryonic digestive tract, and the mesoderm fills the space between the other two. The outer layer of skin and the nervous system arise from the ectoderm, the innermost lining of our digestive tract and the associated organs come from the endoderm, and most other organs and tissues (examples: heart, muscles, skeleton) arise from the mesoderm.
Various regions of the three germ layers develop into rudimentary organs during organgenesis. Folds, splits, and condensation are the first evidence of organ building. The first organs to be built in chordates are the neural tube and the notochord, the skeletal rod characteristic of all chordate embryos. The neural tube forms the central nervous system, the brain and spinal cord. The notochord is formed by condensation of the mesoderm and is later condensed even more into separate blocks called somites. Somite form the backbone vertebrates and muscles associated with axial skeleton. As organgenesis continues the organ become more and more refined. The neural crest, unique to vertebrate embryos, pinches off ectoderm and migrates to other parts of the cell; forming pigment cells of skin, teeth, and other peripheral components.
All vertebrates require an aqueous environment to develop and terrestrial animal evolved to suit reproduction in dry environments. Either within a shell or a uterus, birds, reptiles and mammals are surrounded by a fluid filled pouch called the amnion.
In avian (birds) development, a bird egg in cleavage forms a cap of divided cells called the blastodisc over the undivided yolk. The blast mere then sorts into upper and lower layers; the epiblast and hypoblast. The cavity between the layers in the avian version of the blastocoel, and this stage is equivalent of the blastula. During gastrulation, the inward movement of cells at the blastodisc s midline produces a groove called the primitive streak. All the cells that will form the embryo will come from the epiblast. Some epiblast cells move into the blastocoel, producing the mesoderm. other epiblast cells will migrate to the hypoblast and produce the ectoderm. Four extra embryonic membranes are produced and support embryonic development in egg. They are the yolk sac, amnion, chorion, and allantois.
In mammalian species, fertilization occurs in the oviduct and most development takes place on its way to the uterus. Unlike other terrestrial animals the eggs of mammals the yolks are relatively small. Also mammalian eggs lack polarity, however they follow gastrulation and early organgenesis similar to reptiles and birds. Cleavage in mammals is random and blastomeres equal in size. Compaction occurs at the eight cell stage, it adhere the cells closer together rather than a loose configuration. Around 7 days after fertilization the embryo has over 100 cells arranged round a central cavity. This stage is called the blastocyst. The inner cell mass protrudes into one end of the blastocyst cavity, which will later turn into the embryo proper and some extraembryotic membranes. The outer epithelium surrounding the cavity is the trophoblast which will form the fetal portion of the placenta.
The embryo reaches the uterus in the blastocyst stage and begins to implant soon then after. The trophoblast secretes enzymes that enable the blastocyst to penetrate the uterine lining. About the time the blastocyst implants the inner cell mass forms a flat disc with an upper layer of cells, the epiblast and the hypoblast, a lower layer. These are homologous to the ones of birds.
The four extraembryonic membranes are homologous to that of birds also. The chorion develops from the trophoblast and surrounds the embryo. The amnion begins as a dome above the proliferating epiblast and encloses the embryo in a fluid filled cavity. The yolk sac membrane of mammals does not contain yolk but is the site of blood cell synthesis. The allantois is incorporated into the umbilical cord where it forms blood vessels to transport oxygen and nutrients to the embryo and waste removal.
Organgenesis begins with the formation of the neural tube, the notochord, and somites. By the end of the first trimester of human development rudiments of are the major organs are in place.
Morphogenesis is important in both plants and animals but only in animals does it involve movement. Changes in the shape of a cell usually involve the reorganization of the cytoskeleton. The cytoskeleton is also used by cells to crawl from one place to another in developing animals. Cell crawling is also involved in convergent extension. In convergent extension the tissue layer becomes narrower and longer. Cell adhesion molecules (CAMs) are glycoproteins on the surface of a cell involved in cell migration and lend to a stable tissue structure. CAMs vary in amount and chemical identity from one type of cell to another and these difference help regulate morphogenic movements and tissue building.
Cadherins are an important cell-to-cell adhesion molecules. They received their names because they require the presence of calcium ions to properly functions. There are many types of cadherins and each is expressed at a certain time in embryonic development.
The basic body plan is the first step in morphogenesis and is a prerequisite to the development of tissues and organs. In mammals, the basic polarities are not known until after cleavage. But in most species the fundamental instructions are set down earlier.
In many species only the zygote is totipotent, can be separated and have equal developmental potential. The first cleavage divides the cytoplasm so that only certain parts will give rise certain parts of the embryo. In contrast mammalian cells remain totipotent until they become arranged in a trophoblast. In the eight cell stage, each cell can be divided and bring about an individual.
As embryonic cell division creates cells with different developmental potential, one group of cells can influence the neighboring cells, called induction. Inductive signals play a major part in pattern formation, the arrangement of organs and tissues in their characteristic place. The molecular cues that control pattern formation, called positional formation, tell a cell where it is with respect to the animals body and help determine how the cell and its decedents respond to future molecular signals.
The development of limbs is one type of pattern formation. A limb bud consists of a core of mesoderm tissue covered by a layer of ectoderm. There are organizer regions present in all vertebrate limbs. One type is the apical ectodermal ridge (AER), a thickened area of ectoderm at the tip of the bud. The AER is required for the outgrowth of the limb along the proximal-distal axis and for patterns along its axis. The cells of AER produce fibroblast growth factor, which appears to be the growth signal that promotes limb bud outgrowth. The second major limb-bud organizer is the zone of polarizing activity (ZPA). The ZPA is located where the posterior side of the bud is attached to the body. The ZPA is necessary for proper pattern formation along the anterior-posterior axis of the limb. Cells nearest to the ZPA give rise to the posterior structure and those farthest away form the anterior structure. ZPA secretes morphogens, the most important being a protein growth factor called Sonic Hedgehog. Production of this protein in the wrong part of the body sometimes results in an extra limb.