GESTATION

Honors Biology

Introduction

• Gestation is when sperm fertilizes an egg

• Results in a ZYGOTE

• How many chromosomes are in a zygote?

• In 9 months: zygote trillions of cells

FERTILIZATION

• Sperm: – Vagina cervix uterus fallopian tube– 48 – 72 hours may encounter an egg from ovulation– Head of sperm digests protective layer of egg

• Head of sperm and egg fuse together• Egg membrane changes so no more can enter• Haploid sperm (23 chromosomes) + haploid egg

(23 chromosomes) = diploid zygote (46 chromosomes)

Cleavage and Implantation

• In tube the zygote begins MITOSIS (cleavage)

• Makes a ball of cells same size as zygote (MORULA)

• Makes a fluid filled BLASTOCYST

• Reaches uterus and implants in lining (thick); IMPLANTATION

• PREGNANT (6 days after fertilization)

PREGNANCY

• First trimester– Ball of cells 3 layers; Differentiation– Amnion forms makes amniotic sac– Placenta; nutrients, gases, drugs pass forom

mother to fetus; umbilical cord– Blood never mixes; diffusion– HCG secreted to retain uterine lining (also

progesterone and estrogen)

PREGNANCY

• First Trimester– Brain, spinal cord and nervous system by 3rd

week– Heart beats at 21 days– Body plan developed– 8 weeks birth; fetus

PREGNANCY

• Second Trimester– Abdomen swells– Uterus enlarges– Hear heartbeat– Wake and sleeps– Movement– Swallow, hiccup, suck thumb– Eyes open

PREGNANCY

• Third Trimester– Fetus grows and gets ready to survive on the

outside– See light and dark– React to sound– Develops fat to

insulate body

CHILDBIRTH

• 270 days after fertilization• Hormones stimulate childbirth• Smooth muscles in uterus contract• Amniotic sac breaks• Muscles in vagina and cervix relax• Contractions push fetus out of vagina• Birth to afterbirth• Soon respiratory and circulatory becomes own!