Today’s Objective: 2.1 The student will list the germ layers and their derivatives The student will be able to sequence the stages of animal development

Embryology Study of development from fertilized egg until all major organ systems are formed.

Sperm must penetrate gel coat of an egg. Fertilization Sperm must penetrate gel coat of an egg.

Sperm Cells on Egg Surface

Types of Eggs Macrolecithal Mesolecithal Microlecithal

Fusion of acrosomal membrane and egg membrane Egg Activation Fusion of acrosomal membrane and egg membrane

Acrosome Reaction

Stages of Egg Activation Egg membrane wraps microvilli around sperm head Sperm is drawn into egg cytoplasm Formation of fertilization membrane Vitelline layer lifts off of the egg Vitelline layer strengthens and thickens Cortical changes take place (gray crescent in amphibians)

Fast Electrical Block to Multiple Fertilizations

Other changes with fertilization Formation of animal pole Formation of vegetal pole

Holoblastic Meroblastic Cleavage Holoblastic Meroblastic

Primary Germ Layers

Ectoderm Nervous tissue Skin epidermis Sensory organs

Endoderm Gut lining Digestive glands Lining of respiratory tract

Mesoderm Connective tissues Excretory structures Reproductive structures Muscles

Echinoderm Embryology Echinoderm – “spiny skin” Sea stars, sea urchins, etc.

Initial Cleavage Eggs are microlecithal Cleavage is holoblastic Cleavage (2 cell, 4 cell, etc.) initially forms the morula

Blastula Formation Interior of morula is pulled away Forms blastocoel Hollow ball = BLASTULA (layer is one cell thick)

Gastrula formation - morphogenesis Invagination begins at vegetal pole (blastopore – becomes the anus) Archenteron is formed – embryonic gut Embryo lengthens – forms anterioposterior axis Coelom devlops from outpockets of archenteron Mouth formed when gut breaks through anterior wall

Vertebrate Embryology

All Chordates Possess Dorsal hollow nerve tube Notochord Pharyngeal gill slits Postanal tail

Amphibian Embryology

Initial Cleavage Eggs fertilized externally Animal pole (gray) Vegetal pole (white) – lots of yolk Cleavage forms a morula that rests ON TOP of the vegetal pole

Gastrulation

Amphibian Gastrulation Groove forms between gray crescent and vegetal pole Involution occurs Cells spread from blastopore toward animal pole Forms a “yolk plug.”

Yolk Plug

Results of Gastrulation Cells spread from animal pole toward the dorsal lip of the blastopore – layers thin Archenteron formation Epiboly – ectoderm spreads over entire embryo

Mesoderm Roll over the dorsal lip of blastopore Detach from endoderm and forms CHORDOMESODERM Spreads anteriorly Thickens along the sides of the embryo – form SOMITES Forms body cavidy - COELOM

Somites

Neural Tube Formation Neurulation Nerual plate on dorsal ectoderm begins to roll inward

Neural Tube Formation

Closing the Neural Fold

Avian Embryology

Structure of Avian Egg

Initial Cleavage Meroblastic Formation of BLASTODERM Blastoderm begins to move and sorts cells into two layers: Epiblast Hypoblast

Initial Cleavage

Gastrulation Forms a primitive streak Henson’s node Eventually forms the notochord and neural tube

Avian Egg Membranes Yolk sac – holds nutients for embryo Amnion – shock absorber Chorion – gas exchange Allantois – grows off gut, holds waste products and gas exchange