1. Biology 322 Human Anatomy
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.
Today: Discuss basic embryology
Future lectures: Discuss development of specific
system we are discussing

3. Assuming:
The woman has recently ovulated, releasing an oocyte (egg) from the surface of an ovary. This cell is haploid, with 23 chromosomes, and is surrounded by a membrane called
the zona pellucida and a layer of cells called the corona radiata.
After ovulation, it is pickup up by the open end of the oviduct (Fallopian tube) and transported into a region of this organ called its ampulla.
This is where fertilization should normally occur.

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

5. A few thousand sperm reach the
ampulla of the oviduct within
30 to 60 minutes after ejaculation
but
They are not yet capable of
fertilizing the oocyte (egg)
Sperm must undergo processes called capacitation and acrosome reaction
which will take 8 to 10 hours after ejaculation

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

7. 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

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

9. 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 (“polyspermy”):
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.

10. 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

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

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

13. 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

14. 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.

15. 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.

16. 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.

17. 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)

18. 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 weeks = Pre-embryo
2 weeks – 8 weeks = Embryonic period
8 weeks – birth = Fetal period

19. First: Cells of embryonic disk separate into 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

20. 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

21. 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

22. ~ 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.

23. ~ 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)

24. 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

25. 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

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

27. 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

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

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

30. 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.

32. 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

33. 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

34. 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 X

35. As body cools, brain stimulates heat production.
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 foramen ovale and
ductus arteriosus close (days).