Embryonic DevelopmentTwo 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, Ca2+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]
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