1. Human Development
Biology 12

2. Joke of the day:

4. Fertilization: Sperm enters egg
Resulting cell (egg + sperm) is called a zygote

5. Ovulation to implantation:

6. Cleavage:
Occurs immediately after fertilization and splits the zygote into many smaller cells
Repeated mitotic divisions of zygote
Unequal divisions establishes body plan
different cells receive different portions of egg cytoplasm & therefore different regulatory signals

7. Embryonic development
Day 1: 1st cleavage
1 cell becomes 2 (2-cell stage)
Day 2: 2nd cleavage
4-cell stage
Day 3: 6-12 cell stage
can test at this stage for genetic diseases if done by IVF
Day 4: cell stage
solid ball of cells = morula

8. Embryonic development
Day 5:
solid morula develops into hollow, fluid-filled blastula
embryo will develop from the inner cell mass, or embryonic disc
Day 6 -7:
blastocyst attaches to the endometrium and implantation occurs (now considered an embryo)
*blastocyst starts to secrete HCG = human chorionic gonadotropin which stimulates estrogen & progesterone to prevent menstrual flow
pregnancy test measures the amount of this hormone!

9. In pictures:
zygote
gastrulation
morula
blastula

10. gastrulation
Embryo cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells (blastula) into a multi-layered organism.
Many of the cells at or near the surface of the embryo move to a new, more interior location.
Days 7 to 10

11. Gastrulation: Establishes 3 cell layers Ectoderm Mesoderm Endoderm
outer body tissues
skin, nails, teeth
nerves, eyes, lining of mouth
Mesoderm
middle tissues
blood & lymph, bone & notochord, muscle
excretory & reproductive systems
Endoderm
inner lining
digestive system
lining of respiratory, excretory & reproductive systems
ectoderm
mesoderm
endoderm

12. Embryonic developmental
Days : Pregnancy becomes established
Amnion (inner membrane)develops
Fluid filled amniotic cavity starts to form
Yolk sac starts to form (make blood cells, germ cells)
Placenta (site of exchange of nutrients and waste b/w mother and fetus) starts to form

13. Neurulation Formation of notochord & neural tube Neural tube Notochord
develop into nervous system
Neural tube
develops into CNS (brain & spinal cord)
Notochord
develops into vertebral column

14. Organogenesis
is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism.
Internal organs start
development within
the 3rd to 8th week

15. Review: Embryonic Development
Cleavage
Gastrulation
Ectoderm
Mesoderm
Endoderm
Zygote
2-cell embryo
Gastrula (cross section)
Morula
Blastocyst (cross section)

16. Video: Showing gastrulation, etc.

17. 4 weeks: The organs begin to develop
The formation of the placenta, which is used for nutrition and waste removal
The formation of the amnion, which contains amniotic fluid for protection

18. 4 week embryo showing tail & limb buds

19. Week 5: beginning of the eye can be seen, as well the bulging heart & the umbilical cord

20. Lens of the eye can be seen forming, the mass of the heart bulging from the chest, & the beginnings of the finger rays

21. What’s next? Embryo continues to grow and divide
After 8 weeks referred to as a fetus
Cells continue to differentiate (develop into specialized cells)
Fetus grows and develops
Born at ~40 weeks

22. 8 weeks Measures approximately the size of a bean
The tail disappears, turns into coccyx (tailbone)
Formation of the appendages, hands and joints
Formation of the tubes for the respiratory system.
Nervous system begins to form
Ossification (bone formation) begins

24. 8 weeks: formation of the nose, eyelids, ear flap & well defined toes & fingers

25. 12 weeks Measures 5 cm More developed eyes and ears
Kidneys become functional
More nervous system connections formed
The fetus can suck its thumb

27. 16 weeks Measures 10 cm and weighs 100g
Circulatory and urinary systems become functional
The heart beats strongly
The eyes begin to move
Toenails begin to grow

29. 20 weeks
A waxy substance (vernix) coats the skin to provide protection from the amniotic fluid
The fetus is able to swallow
The production of meconium begins (digestive system)

31. 24 weeks Measures 30 cm and weighs 2 kg Chance of survival if born now
The fetus is thin and begins to accumulate fat
The skin is thin and translucent
The formation of taste buds
The lungs continue to develop and are able to inflate due to the production of a fluid (pulmonary surfactant

33. 24 weeks (6 months)
Fetus is covered with fine, downy hair called lanugo.
It’s skin is protected by a waxy substance called vernix

34. Getting crowded in there!!
Growing, growing, growing…….
32 weeks (8 months)
The fetus sleeps 90-95% of the day & sometimes experiences REM sleep, an indication of dreaming

36. 38 to 42 weeks….Birth!