Animal Development

The Mystery of Development The main problem of embryology is this: How, in the course of development, does a cell of one type change into other types of cells? How can differentiation be made to conform to the fact of mitotic cell division, a process that seems to give daughter cells identical with each other and with the parent cell?

The Mystery of Development Preformation~ until 18th century; miniature infant in sperm or egg embryo must contain all descendants Eve stored all of humanity within her Epigenesis ~ Aristotle Form of animal emerges from relatively formless egg No progressive development steps

So what determines development? Genome of zygote Exposure of nuclei of different cells to varying cytoplasmic environments Inherited traits emerge as mechanisms that control gene expression lead to differentiation of cells Part I: Embryonic Development Part II: Cellular and Molecular Mechanisms

Embryonic development/fertilization (Sea Urchin) AT CONCEPTION Acrosomal reaction ~ hydrolytic enzyme action on egg jelly coat…. Fast block to polyspermy ~ membrane depolarization prevents multiple fertilizations…. Cortical reaction ~ release of calcium causes hardening of egg outer layer and creates a... Slow block to polyspermy and... Egg activation ~ increases metabolic activity; protein synthesis WHY?

SEA URCHIN FERTILIZATION

Fertilization in Mammals

Generally internal fertilization Secretions in female rep. tract= decrease sperm motility Layer of follicle cells Zona pellucida- extra cellular matrix of egg Basal body of sperm’s flagellum divides and forms centrosomes- generate mitotic spindle for cell division

Fertilization in Mammals Fallopian Tubes ? 1. Sperm binds to receptor molecules on zona pellucida 2. Acrosomal reaction 3. Membrane proteins on sperm bind to membrane proteins on egg 4. Plasma membranes fuse 5.Enzymes releases=cortical reaction

NEXT… StageResults in… 1 CleavageMulticellular embryo 2 GastrulationThree layered embryo 3 OrganogenesisRudimentary organs

1.Cleavage Partitions cytoplasm of one large cell into many smaller cells- Blastomeres

The Fertilized Egg - Amphibians Polarity- defined by distribution of substances in cytoplasm Yolk ~ nutrients stored in the egg Vegetal pole ~ side of egg with high yolk concentration Animal pole ~ side of egg with low yolk concentration

Cleavage Vocabulary Blastomeres~ resultant cells of cleavage/mitosis Morula~ solid ball of cells Blastocoel ~fluid-filled cavity in morula Blastula ~hollow ball stage of development

2.Gastrulation Gastrula ~ 2 layered, cup- shaped embryonic stage Different in all organisms, General trends include: Changes in cell motility Changes in cell shape Changes in cellular adhesion THREE CELLS LAYERS ESTABLISHED SEA URCHIN

2.Gastrulation 3 Embryonic germ layers: Ectoderm~ outer layer; epidermis; nervous system, etc. Endoderm ~ inner layer; digestive tract and associated organs; respiratory, etc. Mesoderm ~skeletal; muscular; excretory, etc.

2.Gastrulation Vocabulary Invagination ~ gastrula buckling process to create the... Archenteron ~ primitive gut Blastopore ~ open end of archenteron (becomes anus in whom?)

3.Organogenesis: Vocabulary Notochord ~ from dorsal mesoderm Neural tube ~ beginning of spinal cord Blastodisc ~ cap of cells on top of yolk Primitive streak ~ invagination of blastodisc Somites~ vertebrae and skeletal muscles Neural crest ~ bones and muscles of skull

3. Gastrulation: Human cells around central cavity- blastocyst reaches uterus 2. trophoblast initiates implantation- secretes enzymes, epiblast and hypoblast 3. extraembryonic membranes develop-chorion, amnion, placenta 4. inward movement of cells, 3 layered embryo, four extraembryonic membranes

Amniote embryos Extraembryonic membranes : yolk sac (support; circulatory function) amnion (fluid-filled sac; protection) chorion (placenta formation) allantois (nitrogenous waste)