Week #1: Fertilization………

MORULA MORULA

to Implantation

Mitosis : chromosomes double 1x, the nucleus (cell) divides 1x

Meiosis : chromosomes double 1x, the nucleus (cell) divides 2x

– Each is haploid

• contains 23 chromosomes

being begins with the fusion of an ovulated oocyte and a sperm

called germ cells

A. Fertilization (1)

• The development of a human

fertilization

– The oocyte and sperm are

following meiosis

• this is the amount of DNA needed by somatic (body) cells

• This produces a zygote

single cell

diploid • it contains 46

A. Fertilization (2)

zygote –this is a

–it means “united”

–it is

chromosomes

• like all somatic cells

B. Cleavage

• This results in an

–increase the number of cells

• The embryo now rapidly undergoes a series of mitotic divisions called cleavage

C. Cell Differentiation • By day 4, the embryo

forms a morula –a ball containing 16- cells

–the cells have now differentitated into

• an inner cell mass – gives rise to the morula embryo proper

(namely YOU) (means “mulberry”)

•an outer cell mass – gives rise to the

embryonic contribution

to the placenta

outer cell mass

cavity

“blastocyst”

and the outer cell mass

cavity

“cavitation”

D. Cavitationand the Blastocyst

• Next a fluid-filled inner cell mass forms

between the inner cell mass

• This is called

• This gives rise to the blastocyst

“blastocyst”= “germ-bag”

E. Implantation • On about day 7,

the blastocyst implants itself into the wall of the mother’s uterus

Week #2: BilaminarDisk Formation (1)

inner cell mass now differentiates into 2 distinct cell layers

bilaminar embryonic disk

epiblast

hypoblast

•The

–the epiblast

–the hypoblast

• This is called the

amniotic cavity

• A cavity forms next to the epiblast –It is called the amniotic

cavity

amniotic fluid

Week #2: BilaminarDisk Formation (2a)

–it becomes filled with

–this will eventually surround and suspend the embryo and provide

• a shock-free environment

• a constant temperature

–and allow free movement

amniotic cavity

amniocentesis

Week #2: BilaminarDisk Formation (2b)

• The cells lining the amniotic cavity are shed on occasion

• These cells, along with proteins, carbohydrates, fat, enzymes, hormones and embryonic excretions can be extracted and examined

• This procedure is called an

amniotic cavity

yolk sac

respiratory and digestive system structures

Week #2: BilaminarDisk Formation (3)

• A cavity also forms next to the hypoblast and is called the yolk sac

–it will contribute to

outer cell mass embryo’s

A few words about the outer cell mass (1)

•The –is the

contribution to the placenta

–becomes intimately associated with the inner lining of the uterus in which the embryo is embedded

blood vessels

–they exchange nutrients and wastes via diffusion

A few words about the outer cell mass (2) • Where the 2 surfaces join

–many from the embryo and from the mom come close together

–no actual blood flows between the embryonic and maternal circulation

• With further cell differentiation, a 3rdcell layer forms between the epiblastand hypoblast

• The embryo is now a trilaminardisk

• This process is called gastrulation

Week #3: Gastrulation (1)…

called the ectoderm

called the endoderm

Week #3: Gastrulation(2)…

• The epiblastis now

• The hypoblast is now

• The middle layer, composed of cells that migrated from the epiblast, becomes the mesoderm

• These 3 cell layers give rise to all the cells of your body!

ectoderm –organs that maintain contact with the outside

world

• epidermis (epithelial lining of the skin)

• nervous system

Week #3: Gastrulation(3a)… •The will give rise to

•The endoderm will give rise to –the epithelial lining of the digestive and

respiratory tracts

• liver • pancreas

Week #3: Gastrulation(3b)…

–plus outgrowths of the digestive tract that give rise to the:

•The mesoderm will give rise to –connective tissues

• connective tissue proper That is, basically everything between the inner and outer linings of your body!

• bone • cartilage • blood

–muscle

Week #3: Gastrulation(3c)…

Week #3: …and Notochord formation

• It is called the notochord –it extends the length

to tail) –it will contribute to the axial skeleton

notochord

cross section

• The formerly round, 3-layer disk elongates

• A rod-like structure forms in the mesoderm

of the embryo (from head

Notochord at 5 weeks

• This is to give you a better idea of the notochord extending from head to tail

• It forms the basis for the axial skeleton

This is our next lecture! The Axial Skeleton gives rise to 3 Components:

• the skull

• the vertebral column

• the thoracic cage:

–the sternum

–the ribs

Week #4: Neurulation

–invaginates and

into –differentiates the neural tube

nervous system

• brain

• spinal cord

….

• Ectoderm overlying the notochord now

–this becomes the

Neural Tube Formation

• Here is a view of the neural tube as if forms superficial to the notochord

Here are the brain and spinal cord in you!

– “C”-shaped… • (from side to side) • (from head to toe)

tube

Week #4:and Formation of the Fetus (1)

• The shape of the embryo changes drastically – from an elongated disk it rolls up into a

–an outside • lined by ectoderm

–an inside • lined by endoderm

• from mesoderm

endoderm ectoderm

Week #4:and Formation of the Fetus(2)

• The embryo now has –and everything in between

–and is now called a fetus • all the major organ systems have now been established

Week #4:and Formation of the Fetus(3)

• And the fetus now bulges into the amniotic cavity

the umbilical cord

Week #4:and Formation of the Fetus(4)

• It remains connected to the placenta via

• It contains very important blood vessels!

Placental Blood Supply

• Oxygenated blood for the fetus is supplied by the Mom

(high O2

from mom)

Summary • Week #1: – Fertilization →zygote formation

– Cleavage →morulaformation

– Beginning of cell differentiation and cavitation→blastocyst formation

– Implantation

• Week #2: – Differentiation continues →bilaminardisk formation

– Formation of amniotic cavity and yolk sac

• Week #3: – Gastrulation→trilaminardisk

– Notochord formation

• Week #4: –Neurulation

– Major shape changes in embryo

– Formation of the fetus

upper and lower limb buds have

the 4thweek of

Four Weeks

• By the end of

gestation the

formed • (The fetus is at

very high risk of malformations at this age)

Figure 29–7a, b

the 8thweek the fingers and toes have

Figure 29–7c, d

• The face

Eight Weeks

• By the end of

formed now begins to look human-like

• (The fetus remains at very high risk of malformations through the 8th

week)

Figure 29–7c, d

Twelve Weeks

• This is the period of functional maturation of all organ systems

• (By this age, the fetus is at lower risk of malformations)

Fertilization is defined as the

A. increase in cell number

B. joining of the sperm and oocyte

C. formation of an inner and outer cell mass

D. development of a cavity

E. formation of a trilaminardisk

The ectoderm gives rise to

A. muscle

B. bones

C. lining of the digestive system

D. liver

E. skin and nervous system

The neural tube gives rise to

A. muscle

B. connective tissue

C. skin

D. liver

E. brain and spinal cord

At what developmental stage does the embryo implant into the uterus?

A. fertilized oocyte

B. morula

C. blastocyst

D. trilaminardisk

E. fetus

At what developmental stage does the trilaminardisk form?

A. fertilized oocyte

B. morula

C. blastocyst

D. gastrulation

E. fetus