Embryonic Development Two Cells: who cares? New-born baby: everybody cares!

Embryonic Development Fertilization Implantation The Extra-Embryonic membranes Placenta Back to the Tube Within a Tube Continued Fetal Growth

Fertilization—the spermatozoa Sperm capacitation –Become motile when mixed with secretions from seminal vesicles –Become capable of fertilization in female reproductive tract Few sperm reach egg –200 million introduced at ejaculation –10,000 reach Fallopian tube/oviduct –As few as 100 reach oocyte

Fertilization—the oocyte Gametes (egg and sperm)—with haploid (1x) number of chromosomes unite to form diploid (2x) zygote Oocyte much larger than sperm Oocyte is suspended at Metaphase of Meiosis II

Moment of fertilization Repeated attempts allow one sperm to penetrate zona pellucida Sperm and egg cell membranes fuse Oocyte activation –Egg cell membrane depolarizes—Na + ions rush in, Ca 2+ is released from endoplasmic reticulum –Cortical reaction releases enzymes that inactivate sperm receptors and harden zona pellucida—no more sperm allowed in –Meiosis II completes and second polar body forms –Cell metabolic rate increases markedly

Fertilization and Implantation Approximately one week to implantation No growth, just mitosis producing smaller cells Ectopic pregnancy— fertilized egg implants but not in uterine wall

ICM (Inner Cell Mass) forms actual embryo Implantation with ICM towards uterine wall Trophoblast forms in chorion, destined to become placenta –Cytotrophoblast (cells around ICM) –Synctiotrophoblast (synctium of cells that becomes placenta)

Placenta formation Lacunae form within synctiotrophoblast--maternal blood fills these spaces Villi form with embryonic capillaries down middle

Placenta formation Villi bathed in maternal blood in lacunae--exchange of nutrients, O2, CO2 After 13 weeks, full placenta is now pancake-shaped organ Umbilical cord forms from body stalk to connect [link]link

Other extraembryonic tissues Cellular trophoblast forms placenta. Embryonic germ layers are within trophoblast sphere Embryo itself is only part of what forms from ectoderm/endoderm Ectoderm forms amniotic sac Fetal cells shed into amniotic cavity which fills with fluid from fetal kidneys excreted through ureter, bladder, urethra Amniocentesis--sample of amniotic fluid taken with hypodermic needle Endoderm forms yolk sac and allantois Part of allantois “sucked” into embryo to form endodermic bladder

Yolk sac, allantois, amniotic sac and gastrulation From a blue and yellow line…. ….to a tube within a tube

Board Drawing Cross Section Chorion from trophoblast (sphere of cells around embryo)forms placenta Outer tube with notocord, neural tube Gut tube Yolk sac, allantois from endoderm Amniotic sac from ectoderm

Basic body plan laid down No organs formed Placenta develops Very little growth, fetus still tiny Early embryonic development--summary

From fetus to adult Fetal “germ layers” destined to become specific adult structures Overall adult body reflects early fetal form –Guts inside –Brain/spinal cord dorsal midline –Muscles, bones in flesh of outer part of body