Fetal Development

UWEC BIOL 314 A&P II

Fertilization of egg duration of time

egg fertilized within 12 to 24 hours of ovulation (usually occurs in the ampulla)

Dangers to sperm in the uterus

destroyed by vaginal acid, drained out of the vagina, destroyed by WBCs, sperm go up the wrong tube

Process of sperm fertilization

when sperm encounter the egg they release digestive enzymes to penetrate the egg

Egg prevention of polyspermy

fast block and slow block

Fast block

binding of the sperm to the egg opens Na+ channels in egg membrane (electrically repel sperm)

Slow block

involves secretory granules, cortical granules, just below membrane (impenetrable fertilization membrane blocking the egg)

Major events of fertilized egg to embryo

1st, 2nd, 3rd trimesters

1st trimester events

from fertilization to 12 weeks; half of all embryos die; most vulnerable stage to stressors/chemicals

2nd trimester events

weeks 13-24; organs complete development; fetus looks human; chance of survival outside womb becomes viable

3rd trimester events

weeks 25 to birth; fetus grows rapidly and organs become fully functional

Preembryonic stage

first 16 days after fertilization resulting in an embryo

Preembryonic stage major processes

Cleavage, Implantation, Embryogenesis

Cleavage

mitotic divisions that occur in the first 3 days while the egg migrates down uterine tube; produces smaller and smaller blastomeres

Morula stage

divides into 100 cells causing the zona pellucida to disintegrate and release the blastocyst

Blastocyst

a hollow sphere which implants into the uterine wall (trophoblast and embryoblast)

Throphoblast

outer layer which gives rise to placenta

Embryoblast

inner layer which gives rise to the embryo

Implantation

process of attachment to uterine wall (begins when blostcyst adheres to uterus)

Embryogenesis

arrangement which makes up the primary germ layers of the embryo (ectoderm, mesoderm, endoderm)

Ectoderm

outermost germ layer; forms skin, hair, nails, lens of eyes, and nervous system

Mesoderm

middle germ layer; forms the musculoskeletal system (bones, cartilage, muscles, connective tissue) and circulatory system

Endoderm

innermost germ layer; forms digestive tract, lungs, liver, pancreas, thyroid gland, linings of organs

Fetal circulatory system

O2 is provided to fetus through the placenta; lungs are filled with amnionic fluid

Major organs developed during fetal stage (end of 8 weeks)

bones calcify; skeletal muscles spontaneously contract; heart starts beating at 4 weeks circulating blood; heart and liver are large; head is nearly half the body length