1. Stages of Human Development
Fertilization
Embryo
Cleavage
Gastrulation
Neurulation
Fetus
Childhood
Puberty
Adulthood (Johnson does not discuss development past the fetus, so I will assign some additional reading.)

2. Fertilization:
Fertilization involves fusion of sperm and egg plasma membranes.
Entry of sperm into egg triggers blocks to polyspermy,
and in mammals, signals the egg to complete meiosis and begin development.
See next figure.

3. figure jpg
Figure 43.4
Figure 43.4

4. Fertilization: Sperm and egg contribute differentially to the zygote.
The sperm contributes a haploid nucleus and, in some species, a centriole.
The egg contributes a haploid nucleus, cytoplasm including nutrients, ribosomes, mitochondria, and informational molecules that control early stages of development.

5. Fertilization:
The cytoplasmic contents of the egg are not distributed evenly, and are used to set up the major axes of the future embryo.
See next figure.

6. Fertilized Egg
Figure 43.5
Figure 43.5

7. Fertilization: The animal pole will become the future embryo.
The vegetal pole will become yolk or yolk cells (food for the developing embryo, like the yolk of a chicken egg).

9. Stages of Animal Development
After fertilization comes: Embryogenesis
Three subphases:
Cleavage
Gastrulation
Neurulation

10. Cleavage: Repackaging the Cytoplasm
Cleavage follows fertilization.
During cleavage, the cytoplasm of the zygote is repackaged into smaller and smaller cells.

11. Cleavage:
Cleavage is a period of rapid cell division without cell growth or gene expression.
The result of cleavage is a mass of ~1000 cells called a blastula that is ~size as the egg.

12. Early Development: the Embryo
Cleavage
Zygote to Blastula
Increase in cell number, not size
Blastula
Hollow sphere of dividing cells
Blastocyst—in mammals

13. Embryonic Development
Blastocyst (the mammalian blastula)
One week for egg to travel to uterus
About 100 cells in human
Trophoblast (gray cells)
Is an outer layer of blastocyst cells
Becomes the placenta
Inner cell mass (red cells)
Clustered inside the trophoblast
Becomes the embryo

14. Implantation Into endometrium Placenta develops Amnion Yolk sac
Inner lining of uterus
Placenta develops
2 Network of blood vessels
Exchange nutrients and waste
Amnion
Sac surrounding embryo
Yolk sac
Produces blood cells
Normally a source for food, but not in mammals

15. Embryo phase: Gastrulation
After cleavage, the blastula develops three layers and is called the gastrula
Different layers give rise to various tissues
Endoderm
Mesoderm
Ectoderm
Three layers move relative to one another

16. table jpg
Table 43.1
Table 43.1
Do not memorize!

17. Embryo phase: Gastrulation
Gastrulation is when most embryonic genes are first expressed.
Prior, most proteins came from the mother.
Many Gastrulas die due to errors or mutations in the embryonic genome.
Gastrula cells migrate (move from one place to another).

18. Embryo phase: Neurulation
The Neurula follows the Gastrula.
Cells migrate over the blastopore and induce the overlying ectoderm to fold in on itself to form a neural tube.
The nervous system develops from the neural tube.
The next Figure shows neurulation views from the top and inside.

20. Fetal Development Follows neurulation Development of organs
1st trimester in humans
Includes embryo & early fetal development
Vulnerable to disruption
Most miscarriages
Susceptible to the environment (smoking, alcohol)
2nd and 3rd trimesters
Further development of existing organs & growth

21. Childbirth Estrogen levels increase Oxytocin Cervix opens
Produced by fetus and mother’s pituitary gland
Stimulates uterine muscle contraction
Positive feedback
Contractions cause release of more oxytocin
Cervix opens
Fetus is pushed out --birth
Placenta follows --afterbirth

22. Birth:
First stage: opening of the cervix.
Second stage: expulsion of the baby from uterine contractions.
Third stage: expulsion of the placenta.

23. Gene Expression: Development
Embryo development depends on gene expression
Timing of expression is complex, yet vital
Cell and timing specific expression
Controlled by cascades of gene expression that coordinate development of specific structures

24. Control of Development
Genes are turned on or off during development
Gene activation during development
Transcription of genes is regulated as is the processing of transcribed RNA
Changes gene expression resulting in development
Chemicals and hormones
Switch genes on or off, controlling or affecting development
Environment
Can influence gene regulation in many animals
Why an embryo/fetus must be protected fr/ env. insults (alcohol, drugs, pollution, etc.)

25. Changes in Development
Apoptosis
Programmed cell death
Remove cells from further development
Webbing between digits
Remove unstimulated neurons

26. Childhood Dev does not end w/ birth Growth after birth
Continuation of fetal growth
Humans—one quarter of life reaching full size
Many tissues/organs developing
Immune system
Brain

27. Puberty Continuation of fetal growth
Many tissues/organs still developing
Brain till early 20s
Sexual development
Secondary sex characteristics
Reproductive organs develop

28. Adulthood Cessation of Growth Sexual maturation Cells still divide
Development only ends at what time?

29. Post-Reproductive Years
Females
Reproduce less effectively after ~40 years
Eggs decline in quality
Menopause
End of reproductive life
Males
Decline in sperm
Quality and quantity
No distinct end of reproductive life

30. Regeneration of Cells Limited life span of cells
Must regenerate and be replaced
Example: skin cells
Two methods for regeneration
Simple cell division
Stem cells

31. Stem Cells Unspecialized cells Can develop into mature cells
Unspecialized to various degrees
Depending on tissue and stage of life
Embryonic stem cells
Cells haven’t undergone differentiation
Greatest potential for developing into any tissue

32. Concept Quiz
Meiosis is the process of producing gametes in both spermatogenesis and oogenesis. Which of the two developments results in four gamete cells during human reproductive cell production?
Oogenesis
Spermatogenesis
The correct answer is B.
Answer A is incorrect because meiosis in the female results in one egg and three polar bodies instead of four equally functional ova.

33. Concept Quiz
Stem cells are considered to be a great potential source for regenerating damaged or nonfunctional tissue. Which of the following stem cells provide the greatest potential for regenerating different types of tissues?
Embryonic stem cells
Bone marrow stem cells
Skin stem cells
Answer A is correct because embryonic stem cells have undergone the least amount of differentiation. Therefore, they possess the greatest ability to become any tissue of the body.
Bone marrow stem cells (hematopoietic stem cells) can become any of the blood cells, but they have already undergone a series of differentiation steps.
Skin stem cells likewise have undergone a series of differentiation and, although they can develop into a number of different types of skin cells, their “plasticity” or degree of differentiation is limited.

34. Concept Quiz Fertilization of the human egg occurs in which location?
Ovary
Uterus
Oviduct
The correct answer is C.
The ovary is where the egg is produced. During ovulation it is released and swept into the oviduct. Most successful fertilization events take place in the oviduct, after which the egg completes meiotic division and spends the next week rapidly dividing and moving downward into the uterus where it is implanted.
Fertilization usually occurs long before the egg reaches the uterus. Several changes must take place in the uterus prior to its acceptance of and implantation of the fertilized egg.

35. How Identical Twins Form: