Biology 322 Human Anatomy Human Embryology

One of the best ways to understand the structure of the adult human body is to understand how it developed. Similarly:

Human Development: 3 Periods Pre-embryonic: Embryonic: Fetal:

Fertilization: Resulting cell =

As Fluid-filled

Pre-embryonic period now over, Embryonic period begins. Fertilization – 2 wks = Pre-embryo 2 wks – 8 wks = Embryonic period 8 wks – birth = Fetal period

First: View From Top, Showing Ectoderm View From Edge (Cross-section) Showing All Three Layers

As embryo develops:

The first system to begin developing

~ Day 22 – 23, By day 28:

~ Day 28: begin developing. Length ~ 4mm (width of pencil)

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

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

Placenta well developed:

In placenta:

Ninth week: (Organs and systems continue development) Fetus = Tenth week: (Organs and systems continue development) Fetus =

After tenth week: (Organs and systems continue to develop)

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.

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

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 X