1. Biology 212 Anatomy & Physiology
Human Development

2. One of the best ways to understand the structure of the adult human body is to understand how it developed.
Similarly: abnormalities during development produce
abnormalities in adult structure.

3. Assuming:
You paired up with an appropriate partner
You elicited normal sexual responses from each other
A few milliliters of semen,
containing 200- to 600-million
sperm, were deposited high
in the vagina, just below
the cervix of the uterus

4. Scenario I:
The woman is not within a few days before or after ovulation
Her cervix will be plugged with thick mucous, so few if any sperm will enter the uterus and subsequently the Fallopian tubes.
The few sperm which might make it into the Fallopian tubes are not pushed distally (toward the ampulla)
The sperm do not meet an oocyte and die within 48 hours
No fertilization occurs

5. Scenario 2:
The woman has just ovulated, or she will ovulate within days.
The mucous plug in her cervix will be thin and watery; sperm can easily penetrate it
Most sperm will die in her vagina, but a few million (1 - 2%) are pulled through the cervix, into the uterus, and into the Fallopian tubes where they are propelled toward the ampulla (+100,000 make it this far)
Sperm meet the oocyte, and fertilization can occur

6. At ovulation, oocyte is surrounded by a thick membrane called the zona pellucida, and by follicular cells called the corona radiata
Fimbria surrounding the opening of the Fallopian tube sweep this structure into the infundibulum

7. Oocyte is viable for 12-24 hours, so fertilization must occur within this time.
Fertilization occurs within ampulla of Fallopian tube
If fertilization does not occur, oocyte will degenerate and die

8. A few thousand sperm reach the ampulla of the Fallopian tube within 30 to 60 minutes after ejaculation
But
They are not capable of fertilizing oocyte
Must undergo processes called Capacitation
and Acrosome Reaction
which will take 8 to 10 hours after ejaculation

9. Capacitation:
Prior to ejaculation,
membrane around head of sperm is strong
Semen contains chemicals to prevent weakening
Tail is moderately active
After ejaculation, Chemicals in semen diluted / washed away.
Secretions from uterus & oviducts cause membrane to weaken.
Movement of tail increases.

10. Acrosome Reaction:
Occurs as sperm make contact with corona radiata and zona pellucida
Requires 15 to 60 minutes
Acrosome swells, becomes leaky, releases digestive enzymes

11. (Acrosome swells, becomes leaky,
releases digestive enzymes)
Enzymes digest path through corona radiata and zona pellucida
Hundreds of sperm usually involved in this digestion

12. Head of one sperm makes contact with outside of oocyte, binds onto it, and is quickly taken into it.
Two systems prevent entry of more than one sperm:
Fast Block: Plasma membrane of oocyte changes its electrical charge, which prevents other sperm from binding onto it.
Slow Block: Zona pellucida swells, pushing other sperm away from the oocyte.

13. Once head of sperm enters oocyte, its nucleus is released into the cytoplasm.
Nucleus of sperm (23 chromosomes) and nucleus of oocyte (23 chromosomes) meet and fuse together.
Cell now called Zygote with 46 chromosomes

14. Human Development: 3 Periods
Pre-embryonic: Fertilization weeks
Embryonic: weeks weeks
Fetal: weeks weeks (birth)

15. Fertilization: Lateral part of oviduct (Fallopian tube)
One to two days after ovulation
Resulting cell = Zygote

16. Zygote undergoes repeated divisions (mitosis), called cleavage, as it is transported through oviduct toward uterus
Eventually forms solid mass of 20 – 30 cells called morula, same size as the zygote. This reaches uterus 3 to 4 days after fertilization

17. Morula does not embed in uterus immediately.
Stays loosely attached for 4 to 5 days, nourished by secretions of endometrium.
Continues dividing into +100 cells, forming hollow sphere called blastocyst.
Fluid-filled cavity in center = blastocyst cavity.

18. Blastocyst develops mass of cells at one end, called embryoblast or inner cell mass, which will develop into the embryo.
Cells surrounding cavity, called trophoblast, will develop into the placenta.

19. Cells of trophoblast secrete digestive enzymes, forming finger-like projections into endometrium of uterus.
Blastocyst burrows into endometrium, which will heal over. This process is called implantation.

20. As trophoblast is causing implantation and developing into placenta,
Embryoblast is enlarging and developing into embryo
Fluid-filled amniotic cavity develops between embryoblast and trophoblast
Embryoblast = flat disk
between amniotic cavity
and blastocyst cavity.
Now called embryonic disk
or simply embryo.
(12 – 14 days after fertilization)

21. Pre-embryonic period now over,
Embryonic period begins.
During next six weeks of embryonic period, cells of embryoblast will differentiate into beginnings of all organs.
Fertilization – 2 wks = Pre-embryo
2 wks – 8 wks = Embryonic period
8 wks – birth = Fetal period

22. Endoderm - nearest the blastocyst cavity (changes name to yolk sac)
First: Cells of embryonic disk separate into two, and then three germ layers:
Endoderm - nearest the blastocyst cavity
(changes name to yolk sac)
Mesoderm - in middle
Ectoderm - nearest amniotic cavity
View From Top, Showing Ectoderm
View From Edge (Cross-section) Showing All Three Layers

23. As embryo develops:
Endoderm will form epithelial linings of digestive and
respiratory systems.
Mesoderm will form skeletal, muscular, urinary,
reproductive, circulatory systems,
as well as muscle and connective
tissue layers of digestive and
respiratory systems .
Ectoderm will form skin and nervous system

24. The first system to begin developing is the nervous system
The first system to begin developing is the nervous system. This appears as a groove along the ectoderm, which closes over to form a neural tube.
The neural tube enlarges at one end to form brain;
remains cylindrical at the other end
to form spinal cord

25. ~ Day 22 – 23, sides of flat embryonic disk begin to fold
toward yolk sac.
By day 28: Embryo has “normal” body shape, amniotic
cavity pulled around to surround it.
Placenta continues to develop from trophoblast; many blood vessels develop within it and form villi which project into pools of the mother’s blood in her endometrium.
Embryo floating in amniotic fluid; stays attached to placenta by umbilical cord containing umbilical vessels which carry blood between placenta and embryo.

26. ~ Day 28: Arm buds and leg buds begin developing.
Bones, muscles, and blood vessels develop from mesoderm as limbs get bigger, nerves grow out from spinal cord.
Other organs continue developing internally.
Length ~ 4mm (width of pencil)

27. Late embryonic period (4 – 8 weeks):
Nervous system continues to develop;
brain rapidly enlarges and folds
Eyes and ears begin to develop
Limbs continue developing, fingers and toes separate
Digestive system forms
Lungs bud off from digestive system and grow
Heart folds, divides into chambers, begins contractions
Kidneys, bladder, gonads develop
Face develops as two halves on side of head; move to front and fuse

28. End of embryonic period (8 weeks): Beginning of fetal period
Body has human shape
Almost all organs have begun developing
Head growth still most pronounced
Limbs complete
Length ~ 3 centimeters
Mass ~ 1 – 2 grams
Mass of quarter: 5.7 grams

29. Placenta well developed:
Fetal heart pumps blood from embryo to placenta through umbilical arteries:
Low in oxygen
High in carbon dioxide & other wastes

30. In placenta:
Carbon dioxide & wastes diffuse out of fetal blood into maternal blood
Oxygen & nutrients diffuse from maternal blood into fetal blood
Blood flows from placenta back to embryo through umbilical veins:
High in oxygen & nutrients
Low in carbon dioxide & other wastes

31. Ninth week:
(Organs and systems continue development)
Fetus = 5 cm long, grams
External genitalia develop, but male and female still very similar: penis / clitoris scrotum / labia
Tenth week:
(Organs and systems continue development)
Fetus = 7 cm long, grams
External genitalia easily determined to be male or female

32. After tenth week:
(Organs and systems continue to develop)
Fetus increases rapidly in size

33. After 17 weeks (four months)
Skin formed, but thin. Blood vessels easily seen through it
Hair starting to develop. Fingernails and toenails forming.
Eyelids still fused shut.
Ovaries form primordial follicles.
May suck thumb.
Moves arms, legs, head
Mother can feel movement.

35. After 26 weeks (six months):
Hair present over entire body. Will become longer on scalp
Eyebrows and eyelashes forming
Eyelids almost open
Lungs producing surfactant
Testes begin descent through inguinal canal
Swallowing amniotic fluid
Movement of hands, feet, face

36. After 34 weeks (eight months):
Skin pink, smooth
Longer hair on head
Eyes open, respond to light
Fingernails and toenails have reached ends of fingers and toes
Testes have reached scrotum
Fingers can grasp objects, fine movement of face and eyes

37. After 34 weeks (eight months):
Skin pink, smooth
Longer hair on head
Eyes open, respond to light
Fingernails and toenails have reached ends of fingers and toes
Testes have reached scrotum
Fingers can grasp objects, fine movement of face and eyes
Nine months: Ready for birth

38. Fetus: Surrounded by amniotic fluid ~ 1 liter/quart
All organs mature, but:
Lungs not inflated, but fetus inhaling amniotic fluid
No food in digestive system; Fetus swallowing amniotic fluid
Temperature regulation not active
Blood shunted away from lungs through two openings

39. Adjustments after Birth:
As body cools, brain stimulates heat production.
As infant nurses, digestive system begins to produce and secrete enzymes for digestion.
Starts breathing, lungs
inflate (many minutes).
Blood starts flowing to lungs as shunts close (days).