ANIMAL DEVELOPMENT Alyssa & Karenn INTRODUCTION: Humans begin life as a zygote, a fertilized egg. At 6-8 weeks, development occurs which is determined by cytoplasmic determinants. Some Key Concepts: Cell differentiation- the specialization of cells in structure and function. Morphogenesis- the process where an animal starts taking shape or form. Model Organisms- representatives of a larger group that are studied (i.e. frogs, chicks). EMBRYONIC DEVELOPMENT: 3 STAGES 1) Cleavage: cell division creates a hollow ball of cells called blastula. 2) Gastrulation: cells are rearranged into a three-layered gastrula made up of an ectoderm, mesoderm, and endoderm. 3) Organogenesis: three germ layers that interact and move to give rise to new organs. Frogs are often studied for this stage. CLEAVAGE The cleavage stage results in rapid mitosis and no mass change or growth. Afterwards, fertilization follows which is the combining of sperm and egg to create a zygote. Then, a blastula is formed made of fluid-filled cavities called blastocoel. TWO TYPES OF CLEAVAGE: Holoblastic Cleavage- complete division of egg; occurs in species with little yolk (i.e. frogs). Meroblastic Cleavage- incomplete dominance of egg; occurs in species with lots of yolk (i.e. birds). Eggs found in animals, except mammals, have polarity that is defined by the distribution of yolk (stored nutrients). Vegetal Pole: more yolk Animal Pole: less yolk GASTRULATION Stage where cells are rearranged into a three-layered gastrula. THREE LAYERS: Ectoderm: outside layer; i.e. the epidermis of skin, tooth enamel, nervous system, and cornea and lens of eyes. Mesoderm: middle layer; i.e. the notochord, skeletal system, muscular system, reproductive system, and excretory system. Endoderm: inside layer; i.e. the liver, pancreas, epithelial lining of the digestive tract, and thymus. This stage occurs in frogs, sea urchins, and chicks. FERTILIZATION: SPERM + EGG = ZYGOTE Sperm and egg contact initiates metabolic reactions that trigger development; acrosomal and cortical reactions. These reactions activate the egg. ACROSOMAL REACTION: the acrosome tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg. CORTICAL REACTION: induces a rise in Ca2+ that stimulates cortical granules to release the contents outside the egg. Forms fertilization envelope as well. AMNIOTES Embryos of birds, reptiles, and mammals that develop in a fluidfilled sac in a shell or uterus. Develop extra-embryonic membranes: 1) 2) 3) 4) Chorion (outermost layer): functions in gas exchange Amnion: encloses amniotic fluid Yolk Sac: encloses yolk Allantois: disposes nitrogenous waste products and contributes to gas exchange MAMMALIAN DEVELOPMENT: 1) Inner cell mass develops into an embryo and forms extraembryonic membranes. 2) Trophoblast (outer epithelium of a blastocyst) initiates implantation in the uterus. 3) Once implantation is completed, gastrulation begins. 4) Epiblast cells invaginate the forming mesoderm and endoderm. 5) Placenta formed from trophoblast. 6) By the end of gastrulation, embryonic germ layers have formed. MORPHOGENESIS Changes in cell shape, position, adhesion Convergent Extension: cells of a tissue become narrower and longer; cell migration is involved. Cell shape and movement causes tissue invagination. Cell Adhesion Molecules: located on cell surfaces; contribute to cell migration and stable tissue structure. i.e. Cadherins- important in the formation of frog blastula. DIFFERENTIATION Differences in cell type = differential gene expression PRINCIPALS: 1. During early cleavage divisions, embryonic cells must become different from one another. If the egg’s cytoplasm is heterogeneous, dividing cells vary in the cytoplasmic determinants they contain. 2. After cell asymmetries set up, interactions among embryonic cells influence fate, causing changes in gene expression. This mechanism is called induction and is mediated by diffusible chemicals/cell to cell interactions . Fate Maps: general diagrams of embryonic development. Dorsal Lip: Organizer Discovered by Hans Spemann and Hilde Mangold A blastopore’s dorsal lip is the organizer of embryos (initiates inductions). PATTERN FORMATION Development of spatial organization. Is controlled by its positional information (axes) and determines how the cell and its descendants respond to future molecular signals. i.e. chicks THREE AXES: 1. 2. 3. Proximal- distal axis Anterior- posterior axis Dorsal- ventral axis QUIZ 1. What is fertilization? 2. What are the two types of cleavage and what is the difference between each? 3. What are amniotes? 4. List the three stages of embryonic development in order. 5. What are the two principals of differentiation?