Animal Development

Dr. Chambers

I. The Stages of Early Development

A.Epigenesis

-The idea that an animal emerges gradually from a relatively formless egg-Proposed by Aristotle 2,000 years before the idea of preformation embryos within embryos

B. FERTILIZATION

Activates egg and brings together the nucleic of

sperm and egg

Acrosomal reaction1. Sperm+Egg2. Hydrolytic enzymes digest through material substance of cell

Cortical Reaction-Lets only one sperm in and then blocks off

Mammalian fertilization- Cortical Reaction hardens a zona as a block to

polyspermy

C. Cleavage-A period of rapid cell division without growth-Result=blastomeres=large number of cells-Holoblastic

Complete division of eggYolk-rich eggs

-MeroblasticIncomplete division of eggYolk-rich eggs

-Creates blastula, a multicellular ball, which contains the blastocoel

D. G

astr

ulati

on-Rearranges the blastula to form a three-layered embryo with a primitive gut-Transforms blastula into a gastrula-The gastrula contains the archenteron and three embryonic germ layers: the ectoderm, the endoderm, and the mesoderm

-Organs form from the three embryonic layers

-Includes:-Formation of the notochord by

condensation of dorsal mesoderm-Development of the neural tube

from folding of the ectodermal neural plate

-Formation of the coelom from splitting of lateral mesoderm

E. Organogenesis

F. Amniote Embryos

-Develop in a fluid-filled sac within a shell or uterus-Meroblastic cleavage in the eggs of birds and reptiles is restricted to a small disc of cytoplasm at the animal pole-The blastodisc forms and begins gastrulation with the formation of the primitive streak.-Three germ layers give rise to the four extraembryonic membranes:the yolk sac, amnion, chorion, and allantois.-Eggs of placenta mammals=small, store little food, and exhibit holoblastic cleavage w/no obvious polarity-Gastrulation and organogenesis resemble the processes in birds and reptiles

F. Amniote Embryos cont.

-After fertilization, and early cleavage in the oviduct, the blastocyst implants in the uterus-Trophoblast begins formation of fetal portion of the placenta-Embryo proper develops from a single layer of cell, the epiblast, within the blastocyst.-Membranes homologous to birds and reptiles function in intrauterine development

II. The Cellular And Molecular Basis of Morphogenesis and Differentiation

In AnimalsA. Morphogenesis

Involves specific changes in cell shape, position, and adhesion

Cytoskeletal rearrangements are responsible for changes in both shape and position of cells

Cell adhesion molecules on cell surfaces are also important for cell migration and for holding cells together in tissues

The extracellular matrix provides anchorage for cells and also helps guide migrating cells toward their destinations

B. Fate Mapping-Can reveal cell genealogies in chordate embryos

-Maps of embryos have shown that specific regions of the zygote or blastula develop into specific parts of older embryos

C. Eggs of Vertebrates

-Have cytoplasmic determinants that help establish the body axes and differences among cells of the early embryo

-Cytoplasmic determinants serve as the basis for setting up differences among parts of the egg

-Cells that receive different cytoplasmic determinants undergo different fates

-Drive differentiation and pattern formation in vertebrates-In a developing embryo, cells interpret positional info that varies w/ location-Info=form of signal molecules-Special “organizer regions Dorsal lip of the blastopore Apical ectodermal ridge of the vertebrate limb bud-Signal molecules influence gene expression in the cells that take them

Leading to differentiation and the development of particular structures

D. Inductive Signals