Mechanisms of Developmental Organization PDF

BIO119 DEVELOPMENTAL AND REPRODUCTIVE BIOLOGY Mechanisms of Developmental Organization Carl Jerwin J. Gonzaga The Cycle of Life The stages of development between fertilization and birth are called embryogenesis 1. Fertilization involves the fusion of gametes (male and female pronuclei), and give...

BIO119 DEVELOPMENTAL AND REPRODUCTIVE BIOLOGY Mechanisms of Developmental Organization Carl Jerwin J. Gonzaga The Cycle of Life The stages of development between fertilization and birth are called embryogenesis 1. Fertilization involves the fusion of gametes (male and female pronuclei), and gives the embryo its genome 2. Cleavage is a series of rapid mitotic divisions where the zygote cytoplasm is divided into blastomeres, then into a blastula 3. Gastrulation forms the three germ layers (endoderm, mesoderm, ectoderm) 4. Cells rearrange to form tissues and organs in a process called organogenesis 5. In species where the orgniasm born is not sexually mature (larva), it undergoes metamorphosis 6. A group of cells, the germ cells, is set aside for reproductive function (all other cells of the body are called somatic cells). The development of gametes is called gametogenesis A Frog’s Life Gametogenesis and Fertilization Life cycles are often controlled by environmental factors In most frog species, fertilization is external Some species lay their eggs in pond vegetation so the egg jelly adheres to the plants and anchors the eggs Sex is the recombination of genes; reproduction is the generation of a new individual The haploid pronuclei recombine to form the diploid zygote Fertilization activates the molecules necessary to begin cleavage Cleavage and Gastrulation During cleavage, the volume of the frog egg styas the same, but is divided into tens of thousands of cells called a blastomere, the cell is called a blastula Gastrulation begins 180° opposite the sperm entry with the formation of the blastopore Cells migrating through the blastopore into the embryo becomes the mesoderm and endoderm; cells remaining outside become the ectoderm Four common characteristics of vertebrate embryo Organogenesis Begins when cells on the most dorsal region of the mesoderm form the notochord (notochord is an embryonic organ) Cells of the notochord produce chemical signals that redirect the fate of the ectodermal cells above it (Ectodermal cells become the nervous system); the cell at this stage is called the neurula Neural precursor cells form the neural tube; future epidermal cells of the back cover the neural tube The mesodermal tissue adjacent to the neural tube become somites (precursors of back muscle; spinal vertebrae; and dermis) The embryo develops mouth and anus; neuromuscular connections; and gills The hatched tadpole will feed for itself after the yolk is exhausted Metamorphosis and Gametogenesis Tadpole Adult frog Hindlimbs Locomotion Tail and forelimbs Skull Cartilage Bone Developed Horny Feeding organ mouth; jaw; teeth and tongue Intestine Lengthy Shortened Respiration Gills Lungs Metamorphosis and Gametogenesis Metamorphosis is initiated by the tadpole’s thyroid gland Speed of metamorphosis depends on environmental conditions As metamorphosis ends, development of the germ cells begins To become mature, germ cells must be competent to complete meiosis Comparative Embryology Patterns of clevage In most species, cell division is facilitated by proteins and mRNAs in the oocyte, later by the organism’s own genome The zygote cytoplasm is divided into increasingly smaller cells (blastomeres) which forms the blastula Cleavage occurs rapidly in invertebrates to restore the nuclear- cytoplasmic ratio; invertebrates skip the G1 and G2 phase The pattern of embrypnic cleavage is determined by: 1) Amount and distribution of yolk protein in the cytoplasm 2) Factors in egg cytoplasm influencing the angles of mitotic spindles Comparative Embryology Patterns of clevage Cellular divisions occur faster in yolk-free poles (animal pole) than in yolk-rich poles (vegetal pole) Zygote nucleus is typically on the animal pole Cleavage Types Gastrulation The most important time of your life Cells of the blastula are given new positions The multilayered body plan is established (endoderm; mesoderm; and ectoderm) Cells that will form the endoderm and mesoderm are brought to the interior of the cell; the cells that become the skin and nervous system are spread over the exterior Types of cell movements during gastrulation Axes of a bilaterally symmetrical animal The primary germ layers The primary germ layers The germ layers do not form their organs autonomously Induction is the interaction with other tissues that determines the cells’ final form Complexity of differentiation takes different trajectories in different classes Pharyngeal arches become gill supports in fish; becomes jaws and ears in mammals Evolution of pharyngeal arch structures Adult Amphibia fish n Pharyngeal arches Mamma The four principles of Karl Erst von Baer He recognized that the three germ layers give rise to the same organs among vertebrates He discovered the notochord and the mammalian egg The four principles of Karl Ernst von Baer 1. The general features of a large group of animals appear earlier in development than do the specialized features of a smaller group 2.Less general characters develop from the more general, until finally the most specialized appear 3.The embryo of a given species, instead of passing through the adult stages of lower animals, departs more and more from them 4.Therefore, the early embryo of a higher animal is never like a lower animal, but only like its early embryo Fate Maps and Cell Lineages Embryonic cells do not remain in one place, nor do they keep the same shape (Larsen & McLaughlin, 1987) Two major types of cells in the embryo: 1) Epithelial cells - tightly connected 2) Mesenchymal cells - loosely or unconnected Variations in cell development: Direction and number of cell divisions Cell shape changes Cell migration Cell growth Cell death Changes in the composition of the cell membrane and secreted products Fate Maps Following individual cells allows us to see what those cells become Embryonic cells are labeled with dyes to see what they become in an adult organism Some organisms’ cytoplasms have different colored pigments Mucle-forming cells of Styela partita Dye marking Vital dyes can be applied to a region of interest They stain cells but do not kill them Setback: Vital dyes become diluted with each cell division Fluorescent dyes are intense and can still be detected many cell divisions later Dye marking Vital dye (amphibian embryo) Fluorescent dye (Zebrafish embryo) Genetic labeling Embryos that contain cells with different genetic constitutions Chimeric embryos (chimera) are embryos that are made of tissues from more than one genetic source Chick-quail embryos are a fine example There are species-specific proteins that can be used to find individual quail cells Showed how melanin from chicks originated from neural crest cells  Transgenic DNA chimeras A transgene is a gene that contains DNA from another species One version is to infect the embryonic cells with a virus whose genes have been altered with green fluorescent protein (GFP) Infected embryo When the infected cells are transplanted into another host, only the donor cells will express GFP Transplanted infected neural crest cells Evolutionary embryology Community of embryonic structure reveals community of descent (Charles Darwin, 1859) Darwin noted how organisms developed vestigial organs He also noted that differences among species within genera become greater as development persists  Embryonic homologies Homologous structures are derived from a common ancestor Analogous structures perform a similar function Homology Genetic malformations and syndromes Abnormalities caused by genetic events are called malformations A syndrome is a condition where two or more malformations are expressed together (e.g. Holt Oram syndrome) Disruptions and Teratogens Disruptions are developmental abnormalities caused by exogenous agents (teratogens) Thalidomide was used as a mild sedative for pregnant women in the 1960s, but caused a deficiency in the long bones of infants’ limbs known as phocomelia