Human Development Biology 12

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 gastrulation morula blastula

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

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

Embryonic developmental Days 10 - 14: 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

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

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

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

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!