Fertilization and Development

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!