B I O LO GY 1 2

HUMAN DEVELOPMENT

JOKE OF THE DAY:

FERTILIZATION:• Sperm enters egg• Resulting cell (egg + sperm) is called a zygote

OVULATION TO IMPLANTATION:

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

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: 16-32 cell stage • solid ball of cells = morula

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!

IN PICTURES:

zygote

blastulamorula

gastrulation

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

• Establishes 3 cell layers• Ectoderm

• 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

GASTRULATION:

ectoderm

mesoderm

endoderm

EMBRYONIC DEVELOPMENTAL

• 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

Days 10 - 14: Pregnancy becomes established

NEURULATION• Formation of notochord & neural tube • develop into nervous system

Notochord

Neural tube

develops intovertebral column

develops into CNS (brain & spinal cord)

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

Cleavage Gastrulation

Zygote

2-cell embry

oMorula Blastocys

t (cross section)

Ectoderm

Mesoderm

Endoderm

Gastrula (cross

section)

REVIEW: EMBRYONIC DEVELOPMENT

VIDEO:

• Showing gastrulation, etc.• http://www.youtube.com/watch?v=UgT5rUQ9EmQ

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

• 4 week embryo showing tail & limb buds

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

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

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

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

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

12 WEEKS

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

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

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)

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

24 WEEKS (6 MONTHS)

• Fetus is covered with fine, downy hair called lanugo. • It’s skin is

protected by a waxy substance called vernix

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

38 TO 42 WEEKS….BIRTH!