Fertilization and Fertilization and Development Development
Jan 11, 2016
Fertilization and Fertilization and DevelopmentDevelopment
FertilizationFertilizationUntil this point, we have assumed
that the released egg was not fertilized by a sperm
However, if sperm are in the Fallopian tube, there is a chance that the egg will become fertilized
Sperm attaches by finding binding sites on egg
Layer containing sites then dissolves, removing possibility of polyspermy
Two haploid nuclei merge to form diploid zygote
Will begin to develop even before it reaches the uterus
Early DevelopmentEarly DevelopmentDivided into three stages:
◦Implantation◦Gastrulation◦Neurulation
NeurulationNeurulationAfter fertilization, zygote goes
through a series of mitotic divisions, with very little time between each
Reaches around 64 cells (solid ball), called a morula
Morula is not much larger than original single cell
Begins to hallow out, becomes blatocyst
Blastocyst will secrete digestive enzymes to burrow into uterine lining (recall it should be thickest now) about one week after fertilization
This is the implantationCan cause spotting for the
female
Blastocyst cells can begin to differentiate
Cluster of cells form inside blastocyst
This inner cell mass will become the embryo
Trophoblast surrounding this will become the placenta and umbilical cord
GastrulationGastrulationICM differentiates into three
layers◦Ecotoderm – skin and nervous
system◦Endoderm – lining of digestive track
and internal organs◦Mesoderm – inner tissue and organs
Mesoderm
Amniotic cavity
Primitive streak
Ectoderm
Endoderm
Primitive streak is where cell grow out of to form mesoderm
NeurulationNeurulationFormation of the CNSBlock of mesoderm differentiates
into notochord – eventually becoming the spine
Neural crest Neural fold
Notochord
• As the notochord develops, the neural groove changes shape, producing neural folds
Neural crest Neural tube
Ectoderm
Notochord
Gradually, these folds move together to create a neural tube from which the spinal cord and the nervous system develop
Extraembryonic Membranes
Trophoblast and mesoderm continue to develop, forming two membranes called amnion and the chorion
The amnion develops into a fluid-filled amniotic sac, which cushions and protects the developing embryo
Chorion forms finger-like projections, connecting mother to fetus
Uterus
Amnion
Fetus
Amniotic sac
Placenta
Umbilical cord
Fetal portion of placenta
Maternal portion of placenta
Maternal artery
Maternal vein
Umbilical vein
Umbilical arteries
Umbilical cord
Amnion
Chorionic villus
•The chorionic villi and uterine lining form the placenta
The PlacentaThe PlacentaServes as embryo’s organ for
respiration, nourishment, and excretion
Protects fetus from many, but not all, harmful substances
Viruses such as HIV and substances such as alcohol pass through
First TrimesterFirst TrimesterEmbryo becomes fetus at eight
weeksMost major organs and tissues
formedUmbilical chord develops,
connects baby to mother
Second TrimesterSecond TrimesterThe heart can be heard with a
stethoscopeBone replaces cartilage that
forms the early skeletonA layer of soft hair grows over
the fetus’s skinThe fetus grows and the mother
can feel it moving
Third TrimesterThird TrimesterThe fetus doubles in massIt can now regulate its body
temperatureThe central nervous system and
lungs completely developGestation
ChildbirthChildbirthMother’s pituitary gland secretes
oxytocinCauses rhythmic, involuntary
contractions of muscles around uterine wall
This is known as labourAmniotic sac will break (water breaking)Opening to cervix expands, convulsions
push baby out through vaginaThe Birth Video!