Fertilization And Implantation Flashcards
Fertilization
Occurs in the fallopian tubes
Acrosome from many sperm burrow through the corona radiata (cells around the egg) and zona pellucida (gelatinous layer)
Once one sperm head enters the ovum, the membrane changes
No other sperm can enter
Sperm and ovum nuclei unite
Fertilized ovum (2n) – a single cell called a zygote
Morula
After cleavage, once zygote reaches 16 cells it is morula
Blastocyst
Once morula reaches uterus=Blastocyte (Day 5)
Outer layer (trophoblast) – develops into extraembryonic structures
Inner mass of cells (embryoblast) – develops into the embryo
Implantation
Occurs 5-7 days after fertilization
The blastocyst attaches to the endometrium
The trophoblast secretes enzymes which embed into endometrium
Implantation complete by day 10-14
human chorionic gonadotropin (hCG)
Implanted trophoblast (the chorion layer) secretes human chorionic gonadotropin (hCG)
hCG has the same effects as LH by Maintaining the corpus luteum so E & P stay high during pregnancy
Maintains the endometrium (prevents menstruation)
hCG secreted for first 3 months of pregnancy (then the placenta secretes E & P)
Extraembryonic Structures (chorion, amnion, yolk sac, allantois)
Chorion – secretes hCG & controls passage of nutrients, gases and wastes
•hCG needed to maintain corpus luteum (and E & P levels)
Amnion – inner layer, contains amniotic fluid (insulates, hydrates, protects)
Yolk Sac – site of early red blood cell formation and digestive tract development
Allantois – forms foundation of the umbilical cord
Placenta
Trophoblast & endometrium supplies developing embryo. As development occurs chorionic villi extend into the endometrium
Rich in blood vessels for metabolic exchange between baby and mother’s blood
Blood is not exchanged
By 3rd month of pregnancy, the placenta produces Estrogen and Progesterone
Umbilical cord
Forms by the 8th Week
•Yolk sac shrinks
•Amniotic Sac increases
•Umbilical Cord forms from the Allantois
Contains one vein and two arteries
Connects naval of fetus to placenta
Nutrient and gas transfer
Umbilical arteries carry deoxygenated blood away from the fetal heart to the placenta
Umbilical veins carry oxygenated blood from the placenta to the baby’s heart
Gases (O2, CO2), waste products and nutrients diffuse between mom’s blood and baby’s blood at the placenta
Morphogenesis
Development of an organism through:
•Differentiation (cell specialization) – cells begin to take shape and perform specific functions
•Growth (increase in size)
Gastrulation
The process of the inner mass of cells (embryoblast) flattening and forming distinct layers
Occurs through a process of folding and cell specialization
Embryo is now called a gastrula (in 2nd week)
Germ Layers Produced through Gastrulation
Endoderm (inner layer)
•Digestive system, lungs, bladder, liver, endocrine glands
Mesoderm (middle)
•Muscles, vessels, kidneys, reproductive organs, bones
Ectoderm (outer)
•Skin, hair, nervous system, eyes, ears, teeth
Neurulation
Formation of the neural tube (develops into the brain and spinal cord)
Folds in from ectoderm in the 3rd week
•A section of mesoderm cells form along the embryonic disc
•These form the notochord – will make up the skeleton •Nervous system develops from the ectoderm located above the notochord
Development
First trimester (fertilization to end of third month)
•50 mm at end of third month
•Very sensitive to the environment
Second trimester (third to sixth month)
•All organs formed, bones present
Third trimester (sixth month to term)
•Rapid growth
Embryonic Development
First 8 weeks (2 months) – Embryonic Stage
Stage of organ formation – germ layers separating and specializing
3rd week – heart beat
4th week – early eyes and limb buds
5th week – brain and spinal cord
8th week – hand and feet, testosterone released in males, no change in females
Embryonic development ends when it is recognizably human
