Download 3.4 Prenatal Development

3.4 Prenatal Development
• A prenatal human is
considered an embryo for the
first eight weeks.
•Prenatal development after
the eighth week is the fetal
period.
•During this time, rudiments of
all body parts form.
•This is the time when
structures grow and
specialize.
•The embryo in the fist week
is considered to be in a
“preimplantation” stage
because it has not yet settled
into the uterine lining.
•From the start of the ninth
week until birth, the prenatal
human organism is a fetus.
Human Reproduction and Development
Three Trimesters of Development
 Human development takes around 266
days from fertilization to birth.
 The time span is divided into three
trimesters.
Human Reproduction and Development
The First Trimester
 All tissues, organs, and organ systems begin
to develop.
 At the end of eight weeks, the embryo is
called a fetus.
Human Reproduction and Development
The Second Trimester
 Period of growth
 The fetal heartbeat might be heard.
The Third Trimester
 The fetus continues to grow at a rapid
rate.
 Fat accumulates under the skin to
provide insulation for the fetus once it is
born.
Fertilization
• Formation of diploid zygote –
single cell from fusion of haploid
gametes.
• Gametes formed by cell division
known as meiosis which is a
process by which the chromosome
content is reduced to 23.
(Spermatogenesis or Oogenesis)
• Process allows for mixing of
maternal and paternal genes
Acknowledgement for pictures is
courtesy of Alpha (Scientist in
Reproductive Medicine
Permission confirmed by D Cole
24.5.05).
Human Reproduction and Development
Fertilization
 Process of a sperm
joining with an egg
 Sperm and eggs
each are
haploid, and
each normally
has 23
chromosomes.
 Fertilization restores the diploid number of 46
chromosomes.
Human Reproduction and Development
Human Development Before Birth
 The tip of each sperm cell is a specialized
lysosome called an acrosome that weakens
the plasma membrane surrounding the egg.
 Eventually the plasma membrane becomes weak
enough that one sperm can penetrate the egg.
 Immediately following this penetration, the egg
forms a barrier to prevent other sperm from
entering the now-fertilized egg.
Human Reproduction and Development
Early Development
 The fertilized egg
is called a zygote.
 Around 30 hours after
fertilization, the zygote
undergoes its first
mitosis and cell
division
 By the third day, the embryo, called a morula, leaves
the oviduct and enters the uterus.
Mitosis
• Mitotic cell division –
formation of identical
cells
• Cells have identical
genetic material
• Totipotent – clones
• Day 4: embryo 16-20
cells & is known as a
Morula
Acknowledgement for pictures is
courtesy of Alpha (Scientist in
Reproductive Medicine Permission
confirmed by D Cole 24.5.05).
Human Reproduction and Development
Human Development Before Birth
 By the fifth day, the
morula has developed
into a blastocyst.
 The blastocyst
attaches to the
endometrium around
the sixth day and is
fully implanted by Day
10.
Blastocyst
• Cells division becomes asymmetrical
• Cells polarize
• Lose their totipotency and begin to differentiate
• Outer cell mass becomes the trophoblast which
develops into the placenta
• Implants day 6
By day 14
• From implantation of the blastocyst ,the
inner cell mass is known as the embryo
• Amniotic & Chorionic cavities
Embryo 3-8 weeks
• Critical time for normal
development
• Particularly sensitive to
external factors, environmental
hazards, pharmacological
agents, drug misuse
• Organogenesis
• Trilaminar disc folds into C
shaped cylindrical embryo
• Co-ordinated by genes –
Homeobox
• Cell differentiation
• Tissue interaction &
communication
• Folding is due to different
rate of growth
Cells
• 350 different types of
human cells
• Different functions
• Cells process: division,
differentiation, induction,
migration & death
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
Extraembryonic Membranes
 Four
extraembryonic
membranes form.
 Amnion
 Chorion
 yolk sac
 allantois.
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
The Placenta
 Provides food and oxygen and removes
wastes
 The placenta has two surfaces
 A fetal side that forms from the chorion
and faces the fetus
 A maternal side that forms from uterine
tissue
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
A. As an embryo
develops, the
chorionic villi begin to
grow into the uterine
wall.
B. Nutrients, oxygen,
and wastes diffuse
across maternal and
fetal blood vessels,
and are carried to
and from the fetus
through the umbilical
cord.
C. The placenta
contains tissue from
both mother and
fetus.
Chapter
36
Human Reproduction and Development
Gastrulation – Week 3
• Primitive streak in the midline
• 2mm long
• Bilaminar disc is converted into
Trilaminar disc
• Notochord forms
• Primitive heart
• 3 Germ layers (gastrula)
:Ectoderm, Mesoderm &
Endoderm
• Mother first missed menstrual
period
Trilaminar disc
• Ectoderm will form the epidermis & central
nervous system
• Mesoderm will form the bones, muscles and
heart, blood vessels, kidneys and reproductive
organs
• Endoderm will form digestive tract, respiratory
tract, glands & mucous membranes
Stages of Development
Table 3.2
Formation of the Neural tube - Neurulation
• Starts at 22- 23 days
• Folding starts in the middle in
both the cranial and caudal
direction.
• Cranial opening closes day 25 ,
caudal opening closes on day
27days
• Folic acid is involved in DNA
synthesis
• Most women at this stage do no
know they are pregnant
http://embryology.med.unsw.edu.au/wwwhuma
n/Stages/Images/CSt13.gif
Development of the skeletal vertebral column
• Commences at week 4
• Week 6 cartilaginous stage
• Week 8 Ossification begins
Week 4
• Heart begins to beat
approximately 85 beats
/minute
• Outline of eyes
• Upper limb buds
• Lungs begin to form
• Parts of gastrointestinal
tract can be identified.
http://brillbaby.com/img/Pregnancy-embryo-fetus-week-6.jpg
Week 8
• Heart has 4 chambers
• Upper limbs longer bent at the
elbows
• Fingers distinct but webbed
• External genitalia still in
sexless state but have begun
to differentiate
• By end of week 8 all body
systems & organs are formed.
FETUS
http://brillbaby.com/img/Pregnancy-embryo-fetus-week-6.jpg
8-12 Weeks
• Eye lids fuse
• Fetal circulation functioning
• Moves freely
• Kidney’s function fetus passes urine ~10 weeks
• Abdominal gut needs to be withdrawn into cavity by
week 10
• Ossification of bones begins 8 weeks
By 20 Weeks
• Most organs capable of functioning
• Neurons formed between 10-18 weeks
• Skin covered with vernix and lanugo
• Brown fat deposited
• Limbs are at mature proportions
• Meconium present
in gut
24 Weeks
• Skin – thin, wrinkled, translucent &
dark red
• Lungs terminal sac phase (surfactant
started to be produced 22weeks,
increases significantly after
30weeks)
• Sensory organs develop, fetus
responds to noise
• Length 32 cm
• Weight 700g
• Periods of sleep & activity
28 Weeks
• Survival possible
• Eyelids open
• Length 37cm
• Weight 1200g
• Head circumference 26cm
32 Weeks
• Lanugo disappears from
face
• Ear cartilage soft
• Lengths 43cm
• Weight 2000g
• Accumulation of fat
36 Weeks
•
Head circumference > abdominal
circumference
• Breast tissue nodule present 12 mm
•
Plantar creases visible
• Skin pale
•
Head hair lengthens
•
Nails reach the tips of fingers
•
Lanugo vanishes from shoulder
• Length 49cm
• Head circumference 33cm
• Weight 2900g: Ready for birth
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
Diagnosis in the Fetus
 Ultrasound
 Procedure in which sound waves are
bounced off the fetus
 Determines if the fetus is growing properly
 Determines the position of the fetus in
the uterus
 Determines the gender of the fetus
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
Amniocentesis
 Amniocentesis is
performed in the
second trimester.
 Fluid from the
amniotic sac is
removed and
analyzed.
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth
Chorionic Villus Sampling
 Chorionic villus
sampling is performed
during the first
trimester.
 Cells from the chorion
are removed and
analyzed by
karyotyping.
Chapter
36
Human Reproduction and Development
36.3 Birth, Growth, and Aging
Birth
 Birth occurs in three stages: dilation,
expulsion, and the placental stage.
 The beginning of the birthing process is
called labor.
Chapter
36
Human Reproduction and Development
36.3 Birth, Growth, and Aging
Dilation
 Another sign the
baby is going to be
born is the dilation
of the cervix.
Chapter
36
Human Reproduction and Development
36.3 Birth, Growth, and Aging
Expulsion Stage
 The mother
consciously will
contract her
abdominal muscles
to help push the
baby, usually head
first, through the
vagina in the
expulsion stage.
Chapter
36
Human Reproduction and Development
36.3 Birth, Growth, and Aging
Placental Stage
 The placenta
detaches from the
uterus and leaves
the mother’s body
along with
extraembryonic
membranes in the
placental stage.
Chapter
36
Human Reproduction and Development
36.3 Birth, Growth, and Aging
Infancy
 The first two years of life
Childhood and Adolescence
 Childhood is the period of growth and
development that extends from
infancy to adolescence.
Chapter
36
Human Reproduction and Development
36.3 Birth, Growth, and Aging
 Puberty marks the beginning of
adolescence.
 Begins between ages 8 to 13 in girls and
ages 10 to 15 in boys.
Chapter
36
Human Reproduction and Development
36.3 Birth, Growth, and Aging
Adulthood
 At the end of adolescence, physical growth is
complete, marking the beginning of adulthood.
 Physical changes perhaps are the most
noticeable signs of aging.
 Other changes include a decrease in muscle
mass, a slowing of overall metabolism, and a
decreased pumping ability of the heart.
Chapter
36
Human Reproduction and Development
36.2 Human Development Before Birth