Fertilization Animation

Fertilization Several sperm penetrate the corona radiata Several sperm attempt to penetrate zona pellucida ONE sperm enters the egg and their nuclei fuse

Embryonic Development 3 Stages Cleavage - Development of Cells Gastrulation – Development of Tissues Organogenesis – Development of Organs

Cleavage - Cell Development Rapid mitotic cell division of the zygote that occurs immediately after fertilization Protostome Cleavage Spiral & Determinate Deuterostome Cleavage Radial & Indeterminate

Radial

Cleavage - Cell Development Produces a fluid filled ball of cells call a BLASTULA. BLASTOMERES – cells of the blastula BLASTOCOEL – the fluid filled center of the blastula

Gastrulation- Tissue Development Gastrulation is the process that involves rearrangement of the blastula & begins with the formation of the BLASTOPORE (an opening in the blastula) Protostome – Blastopore becomes mouth Deuterostome – Blastopore becomes anus

Gastrulation- Tissue Development Some of the cells on the surface of the embryo migrate into the blastopore to form a new cavity called the ARCHENTERON (primitive gut) As a result of this movement, a three-layered embryo called a GASTRULA is formed. Animation

Gastrulation- Tissue Development The gastrula consists of three differentiated layers called EMBRYONIC GERM LAYERS Ectoderm Endoderm Mesoderm

Organogenesis –Development of Organs In chordates, the first organs to form are the NOTOCHORD & the NEURAL TUBE. Notochord – formed from the dorsal mesoderm Neural Tube – formed from the dorsal ectoderm

Development of Neural Tube and Coelom in a Frog Embryo Nervous system Neural folds develop on either side of neural groove Neural grove becomes the neural tube Nervous system Develops from midline ectoderm located just above the notochord Thickening of neural plate is seen along dorsal surface of the embryo

Factors that Influence Embryonic Development Development requires Growth Cellular Differentiation Morphogenesis

Cellular Differentiation Occurs when cells specialize in structure & function. Certain Genes (and not others) are turned on in differentiated cells. Cytoplasmic Segregation Induction

Cellular Differentiation Each body cell contains full set of chromosomes; so differentiation is not due to parceled out genes. Cells in adult body are TOTIPOTENT, they contain all instructions to form any specialized cell. Only red blood cells produce hemoglobin, etc.; therefore there is differential gene expression.

Cytoplasmic Segregation Differentiation begins before we recognize specialized cell types. Eggs contain substances called maternal determinants that influence course of development. Cytoplasmic segregation parcels out maternal determinants as mitosis occurs and determines how various cells of morula develop Cytoplasmic Segregation

Early experiments showed cytoplasm of a frog egg is not uniform in content.After first cleavage of a frog embryo, only a daughter cell that receives a portion of gray crescent develops into a complete embryo Hans Spemann (Nobel Prize in 1935) found particular chemical signals within gray crescent turn on genes that control development. This experiment allows us to speculate that the gray crescent must contain particular chemical signals that are needed for development to proceed normally

Induction The ability of one embryonic tissue to influence the development of another tissue Developmental path of cells is influenced by neighboring cells Work of Hans Spemann Animation

Control of Nervous System Development

Hans Spemann & Hilde Mangold worked on dorsal side of embryo where notochord & nervous system develop. Presumptive notochord tissue induces formation of the nervous system Warren Lewis found developing lens induces optic vesicle to form optic cup in frog embryo. The process of induction is thought to go on continuously with cells always influencing each other. Cells closest to the primary organizer become endoderm; those farthest away become ectoderm. A molecular concentration gradient acts as a signal to induce germ layer differentiation. Cell migration occurs during gastrulation; one set of cells can influence migratory path of another set. Spemann showed dorsal lip of a blastopore (primary organizer) was necessary for development Induction is the ability of one tissue to influence development of another tissue. As development proceeds, differentiation involves signals from neighboring cells. Induction

Development of C. elegans, a Nematode Fate map – shows that as cells arise by cell division, they are destined to become particular structures A 1mm long worm with only 959 cells

Morphogenesis Morphogenesis produces a change in shape and form of a body part; includes cell movement and pattern formation. HOMEOTIC, HOMEOBOX or HOX GENES are Master Genes that control the expression of genes responsible for specific anatomical structures. They play a critical role in normal embryonic development. A Homeotic Gene might give the instructions “place legs here” in the developing embryo.

Extraembryonic Membranes Tissues outside the embryo forms four extraembryonic membranes nessary to support the growing embryo inside the shell Yolk sac Amnion Chorion allantois

Allantois collects nitrogenous wastes. Yolk sac surrounds remaining yolk that provides nourishment. Amnion contains protective amniotic fluid that bathes developing embryo. Chorion lies next to shell and carries on gas exchange. Allantois collects nitrogenous wastes.

Yolk sac is first site of blood cell formation. Amnion surrounds embryo and cushions it with amniotic fluid. Allantoic blood vessels become umbilical blood vessels. Chorion develops into fetal half of placenta.

Human Development Before Implantation

Human Embryonic & Fetal Development Human gestation time - time from conception to birth - approximately nine months Embryonic Development - Months 1-2 Fetal Development - Months 3-9 Extraembryonic Membranes Chorion Amnion Allantois Yolk Sac

Embryonic Development First Week Morula transformed into blastocyst Gives rise to chorion Second Week Implanting begins Gastrulation occurs Inner cell mass flattened into embryonic disk Ectoderm and Endoderm differentiate

Embryonic Development Third Week Nervous system and circulatory system appear Fourth and Fifth Weeks Umbilical cord is fully formed Limb buds appear Head enlarges Sense organs more apparent