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