CHAPTER 5 NCM 107

Stages of Fetal (Prenatal) Development

Prof. Dymphna Casquejo

Fertilization - implantation
o when the outer cells of blastocyst
How far does a sperm have to swim? contact cells of the endometrium lining
- a sperm cell is about two thousandths of an the uterus, the blastocyst embeds in
inch long the endometrium
- can swim roughly 8 inches in an hour o occurs about a week after fertilization
- to reach an egg, they will ultimately they have
to swim around 192,000 times their own length Embryo Growth
- if fertilization occurs, the egg will have all the
"instructions" to grow into a new organism Growth and Development of the Embryo
Embryo
Cleavage and Implantation - what the blastocyst is called after implantation
- sperm are deposited in the vagina during Embryonic Stage
sexual intercourse - lasts through the 8th week following fertilization
- propel themselves through the uterus and - the embryo grows in size and becomes more
enter a fallopian tube complex
o where fertilization usually takes place o develops specialized cells and tissues
- when a sperm penetrates the egg, it triggers and starts to form most organs
the egg to complete meiosis
o sperms tail falls off, and Formation of Cell Layers
its nucleus fuses with the nucleus of During the second week after fertilization, cells in the
the egg embryo migrate to form three distinct cell layers:
o  resulting cell = zygote 1. Ectoderm (external layers)
 contains all o skin cells of epidermis
the chromosomes needed for o neuron of brain
a new human organism
o pigment cell
 half the chromosomes come
from the egg and half from the 2. Mesoderm (middle layer)
sperm o cardiac muscle
o skeletal muscle cells
Morula and Blastocysts Stages o tubule cell of the kidney
- uterus, where the zygote will stay o red blood cells
o when the zygote travels to the uterus it o smooth muscle (in gut)
will undergo mitosis
o it forms a ball of cells called morula
3. Endoderm (interior layer)
o lung cell (alveolar cell)
o cell divisions are called cleavage
o thyroid cell
- increase the number of cells but not the overall
size of the new organism o pancreatic cell
- blastocyst 4. Germ Cells
o a fluid filled cavity forms inside the ball o sperm
of cells o egg
o the cells of the blastocyst form an
inner cell mass and an outer cell Differentiation of Cells
layers Differentiation
o inner cell mass = embryoblast - occurs during the 3rd week after fertilization
 will soon develop into an - process by which unspecialized cells become
embryo specialized
o outer cell layer = trophoblast - occurs as certain genes are expressed
 develop into other structures (“switched on”) while other genes are switched
needed to support and nourish off
the embryo - cells develop unique structures and abilities
that suit them for their specialized functions
Implantation
- 4 to 5 days Organ Formation
o time for the blastocyst to reach the eight week of development – embryo about 30
uterus after fertilization millimeters in length
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[NCM 107] 5. Stages of Fetal (Prenatal) Development – Prof. Dymphna Casquejo

- may also have begun to move o consists of water and dissolved

substances
Embryonic Development (Week 4 – 8) o fluid allows the fetus to move freely
- heart begins to beat until it grows to fill most of the
- arm buds appear available space
Week 4 o cushions the fetus and helps protect it
- liver, pancreas, and gall bladder start to form from injury
- spleen appears
- eyes start to form Prenatal Development
- leg buds appear
Growth and Development of the Fetus
Week 5
Fetus
- hands appear as paddles
- end of the 8th week until birth
- blood begins to circulate
- facial features start to develop
Birth
Week 6
- occurs at about 38 weeks after fertilization, so
- lungs start to form
the fetal period generally lasts about 30 weeks
- fingers and toes form
Week 7
Fetal Development (Week 9 – 38)
- hair follicles start to form
Week 9 – 15
- elbows and toes are visible
- reproductive organs form
Week 8
- tooth buds appear
- face begins to look human
- eyelids form
- exteral ears starts to form
- fetus is very active
- brain activity can be detected
The Placenta
Week 16 – 26
Placenta and Related Structures - brain develops rapidly
Placenta - alveoli form in the lungs
- where nutrients and oxygen exchange with - internal parts of the eyes and ears form
waste products - eyebrows, eyelashes, and nails appear
- muscles develop
Placenta
- temporary organ that begins to form from the Week 27 – 38
trophoblast layer of cells shortly after - body fat increases rapidly
implantation - bones complete their development
- head hair gets coarser and thicker
Fully Developed Placenta - brain is continuously active
- made up of a large mass of blood vessels from
the mother and fetus 38-week
- fetus normally ranges from 36 to 51
Maternal and Fetal Vessels centimeters (14 – 20 inches) in length and
- close together but separated by tiny spaces weighs between 2.7 and 4.6 kilograms (about
- allows the mother’s and fetus’s blood to 6 – 10 pounds)
exchange substances across their capillary
walls without the blood actually mixing

Umbilical Cord
- connects the fetus to the placenta
- a tube that contains two arteries and a vein
- blood from the fetus enter the placenta through
the umbilical arteries, exchanges gases, and
other substances with the mother’s blood, and
travels back to the fetus through the umbilical
vein

Amniotic Sac and Fluid

Amniotic Sac
- attached to the placenta
- an enclosed membrane that surrounds and
protects the fetus
- contains amniotic fluid
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