1. Fertilization
Animation

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

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

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

5. Radial

7. 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

9. 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

11. 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

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

15. 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

16. 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

18. Factors that Influence Embryonic Development
Development requires
Growth
Cellular Differentiation
Morphogenesis

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

20. 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.

21. 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

23. 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

24. 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

25. Control of Nervous System Development

26. 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

27. 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

28. 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.

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

30. 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.

31. 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.

32. Human Development Before Implantation

33. 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

34. 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

35. 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