Animal Development Campbell’s Biology Ch 43 – 4 th ed./Ch 47 – 6 th ed

Fertilization Many steps have been studied in the sea urchin, an echinoderm Acrosome Reaction –Acrosome at head of a sperm releases hydrolytic enzymes that penetrate the jelly coat of the egg –Specific sperm molecules bind with receptors on the vitelline membrane (outside the plasma membrane) of the egg. This is a species specific reaction

Fertilization, cont’d –Membrane is dramatically depolarized by influx of sodium ions (fast block to polyspermy) –No other sperm can penetrate the membrane

Fertilization, cont’d Cortical reaction –Fusion of egg and sperm sets up a signal transduction pathway that leads to a release of large amounts of Ca 2+ from the ER into the cytoplasm –High Ca 2+ causes changes in cortical granules (vesicles located under the plasma membrane) –Vitelline membrane develops into a hard fertilization envelope that further resists entry of sperm (slow-block to polyspermy)

Fertilization, cont’d Activation of the Egg –Rise in Ca 2+ activates egg and development begins –Eggs can be activated artificially by electrical stimulation or by injection with Ca 2+ –Parthenogenesis = development of an unfertilized egg Ex: drone honeybees = haploid males developed naturally by parthenogenesis

Comparing & Contrasting Fertilization in Mammals & Sea Urchins Zona pellucida (ZP) in mammals replaces vitelline membrane of sea urchin ZP = extracellular matrix of egg & is composed of 3 filamentous glycoproteins ZP3 serves as species specific sperm receptor Depolarization of membrane = fast block to polyspermy same as in sea urchin

Comparing & Contrasting Fertilization in Mammals & Sea Urchins, cont’d Cortical granules/Hardening of zona pellucida = slow block to polyspermy similar to mechanism in sea urchin Sperm and egg nuclei do not fuse before cell division begins

Three Stages of Embryonic Development CleavageGastrulationOrganogenesis –Will compare these processes in three types of organisms with differing amounts of yolk: a sea urchin, a frog, and an egg

Cleavage Rapid mitotic cell division of zygote that occurs immediately after fertilization In protostomes, cleavage is spiral and determinant –The future of each cell is already decided by the four cell stage In deuterostomes (Echinoderms and Chordata), cleavage is radial and indeterminate –Each cell retains the capacity to develop into a complete and normal embryo

Cleavage, cont’d In both sea urchins and frogs first two cleavages are vertical. The third division is horizontal. The result is an eight-celled embryo with two tiers of four cells. A morula, ball of cells is eventually formed.

Continued cleavage produces the morula. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 47.8b

In birds the yolk is so plentiful that it restricts cleavage to the animal pole: meroblastic cleavage. In animals with less yolk there is complete division of the egg: holoblastic cleavage.

Cleavage, cont’d Cleavage produces a fluid filled ball called a blastula The hollow fluid-filled middle is referred to as a blastocoel The cells are referred to as blastomeres

Gastrulation A process that involves rearrangement of the blastula Observe changes in: –Cell motility –Cell shape –Cellular adhesion to other cells –Molecules of extracellular matrix

Gastrulation, cont’d Gastrulation begins with formation of the blastopore, an opening in the blastula Gastrulation begins with formation of the blastopore, an opening in the blastula –The blastopore becomes the mouth of protostomes –The blastopore becomes the anus of deuterostomes

Gastrulation, cont’d Cells migrate into the blastopore to form a new cavity called the archenteron or primitive gut Result is a three-layered embryo called a gastrula Three embryonic layers formed are the –Ectoderm –Mesoderm –Endoderm

Embryonic Germ Layers Ectoderm gives rise to skin and nervous system Endoderm will form the viscera including the lungs, liver, and digestive organs Mesoderm gives rise to muscle, blood, and bones Note: Sponges & cnidarians form a mesoglea, a noncellular layer instead of a mesoderm

Zygote → Morula → Blastula → Gastrula → Late Embryo Cleavage = rapid cell division with no cell growth in between divisions Cleavage encompasses zygote through blastula stage

Organogenesis Cells continue to differentiate to produce organs from the three embryonic germ layers Three morphogenic changes are first evidence of organ building –Folds –Splits –Condensation = dense clustering Once organs are developed, the embryo simply increases in size

The Frog Embryo: Fertilization One third of frog egg is yolk Yolky portion of egg = vegetal pole Top half of egg = animal pole with a pigmented cap Pigmented cap rotates to point of penetration by sperm Gray crescent appears opposite to point of entry by sperm/essential to normal development of growing embryo

The Frog Embryo: Cleavage & Gastrulation Cleavage is uneven because of yolk Blastopore forms at border of gray crescent and vegetal pole Involution

The Bird Embryo: Cleavage & Gastrulation Bird’s egg has so much yolk that development of the embryo occurs in a flat disc or blastodisc that sits on top of the yolk A primitive streak forms instead of a gray crescent Cells migrate over the primitive streak and flow inward to form the archenteron As cleavage and gastrulation occur, the yolk gets smaller

The Bird Embryo, cont’d Four extra-embryonic membranes outside the embryo support the growing embryo inside the shell Yolk Sac AmnionChorionAllantois

Extra-embryonic Membranes Yolk Sac –Encloses the yolk which provides food for the growing embryo Amnion –Encloses the embryo in protective amniotic fluid Chorion –Lies under the shell and allows for diffusion of respiratory gases between the outside and the growing embryo

Extra-embryonic Membranes Allantois –Analagous to the placenta in mammals –A conduit for respiratory gases between the environment and the embryo –Repository for uric acid, the nitrogenous waste from the embryo

Hox Genes Homeotic or homeobox genes are master genes that control expression of genes responsible for specific anatomical structures –Ex: the “place legs here” instructions in a developing embryo