Topic 20 Human Development. From
conception to Death
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Objectives
1. List the stages of development and
describes what happens during each.
2. Where is the egg fertilized and where does
it eventually end up?
3. What is organogenesis?
4. How many sperm are deposited in the
vagina? How many
sperm fertilize the egg.
5. What is implantation?
6. What role does the placenta play in development?
7. Describe an embryo at the end of eight weeks?
8. What happens during the 2nt trimester?
9. Describe the process of birth and lactation?
10. How does the maternal lifestyle affect development?
11. What happens to the body as it ages?
12. What are the causes of aging?
13. What happens to the body system
as it ages?
1. The birth of a child
1)
Fertilization begins when a sperm penetrates an egg and is
completed when the sperm nucleus fuses with the egg nucleus, resulting in
formation of the zygote.
2) Cleavage
(repeated mitotic divisions) convert the zygote to a ball of cells: cell
numbers increase but not individual cell size.
B.
Gastrulation: Primary
Tissues Form
1)
Endoderm is the inner layer: it gives rise to the lining of
the gut and lungs and organs derived from it.
2) Mesoderm
is the middle layer: muscle, organs of circulation, reproduction, excretion,
and skeleton are derived form it.
3) Ectoderm
is the outer layer: it gives rise to the nervous system and the outer layers of
the integument.
C.
Organogenesis,
Growth , and Tissue Specialization
1) Organogenesis begins as germ layers subdivide into populations of
cells destined to become organs and tissues that are unique in structure and
function.
2) During the first several weeks of development two key
processes are at work:
a)
In cell
differentiation, even though cells have the same number and kind of genes,
controls on the expression of those genes, makes possible their expression in
the form of structures, products, and functions found in some cells but not in
other cells.
3)
Morphogenesis is the organization of differentiated cells into tissues and
organs by means of localized cell division, movements of tissues, folding, and
the like.
3. Early events in Development:
A.
Fertilization
and cleavage
1)
If the 150 million to 350 million sperm deposited in
the vagina during coitus, only a few hundred ever reach the upper region of the
oviduct where fertilization occurs.
a)
On the
journey the acrosome of the sperm becomes
structurally unstable in a process called capacitation
b) Many sperm will bind to the zona
pellucida of the egg
2)
Only one sperm will successfully enter the cytoplasm of the
secondary oocyte because of changes in the egg’s
membrane that prevents additional sperm entry.
a)
The arrival of
that sperm inside stimulates the completion of meiosis II in the secondary oocyte, which yields a mature ovum.
b) The sperm nucleus fuses with the egg nucleus to
restore the diploid chromosome number of 46.
3)
Repeated mitotic
divisions of the zygote produce a ball of many cells that are not necessarily
larger but these cells differ in size, shape
and activity.
4) Identical twins are the result of a separation of the two cells produced by the first
cleavage: fraternal twins are not identical because the are the result of two separate fertilizations.
1)
During the first
few days after fertilization, the zygote undergoes repeated cleavages as it
travels down the oviduct.
2) By the time the zygote reaches the uterus, it is a
solid ball of cells (morula), which is
transformed into a blastocyst, consisting of a
surface layer of cells- the trophoblast- and an inner
cell mass.
a)
Before the first
week ends, the blastocyst contacts and adheres to the
uterine lining (endometrium); the enzymes form
the trophoblast cells digest the maternal tissue to
allow implantation.
b) The inner cell mass of the blastocyst
is transformed into an embryonic disk, which will develop into the embryo
proper within the next week: sometimes the implantation proceeds outside the
uterus, resulting in an ectopic pregnancy.
c) The implanted embryo releases HCG(human chorionic gonadotropin) which
prods the corpus luteum to secrete estrogen and
progesterone to maintain the uterine lining: the presence of HCG in the
mother’s urine is the basis for home pregnancy tests.
4. Extraembryonic Membranes:
1) Several days after implantation the Extraembryonic membranes begin to form
2) The membranes and their functions are:
a) The yolk sac gives rise to the digestive tube and is
a source of early blood cells
b) The amnion is a fluid filled sac that keeps
the embryo form drying out and acts as a shock absorber.
c) The allantois gives
rise to blood vessels that will become enclosed in the umbilical cord
d) The chorion is a
protective membrane around the embryo,
forms a portion of the placenta, and secretes HCG that maintains the uterine
lining after implantation.
1) The placenta is a combination of endometrial tissue
and embryonic chorion
a) The maternal tissue consists of tissues high in
arterioles and venules
b) The embryo’s chorion
extends into the maternal tissue as tiny chorionic villi
2)
Materials are
exchanged from blood capillaries of mother to fetus, and vice versa, by
diffusion: the maternal and fetal bloods do not mix!
5. A closer look at the early embryo
1)
Gastrulation results in
the formation of the three germ layers and the primitive streak, which will
give rise to the nervous system.
2) By week three, sometimes- the beginnings of
connective tissues, bones and muscles form: the heart begins to beat.
B.
Neurulation is the
first stage in the development of the nervous system
1) Ectodermal cells at the midline of the embryo elongate to form
a neural plate
2) Cells of the neural plate fold over and meet at the
midline to form a neural tube which will eventually form the spinal cord and
brain: an incomplete closure results in spina
bifida.
6. Emergence of distinctly human
features
3) Gonad development begins by the second half of the
first trimester
4) An embryo with both an X and a Y chromosome will
produce from the embryonic testes male
hormones that will influence further sex differentiation
5) An XX embryo will become a female because of the absence
of testosterone
6) At the end of eight weeks of development, the embryo
is designated a fetus.
1) The spontaneous expulsion of the uterine contents
occurs in about 20 percent of all conceptions, usually during the first
trimester.
2) More than half of all spontaneous abortions occur
because of genetic disorders in the embryo/fetus
7. Fetal Development
1) The suckling reflex is evident: the fetus is about
4-5 inches long.
7) Fuzzy hair (lanugo)
and a cheesy coating (vernix caseosa) covers the body.
1)
The third
trimester extends form month seven until birth: the earliest delivery in which
survival on its own is possible is the middle of this trimester
2) Babies born before seven months’ gestation often
suffer form respiratory distress syndrome.
C.
Fetal
Circulation
1)
Deoxygenated
blood is carried from the fetus to the placenta in two umbilical arteries:
Oxygenated blood is returned to the fetus via the umbilical vein.
2) The lungs are partially bypassed due to the foramen
ovale that allows blood to pass directly form the right atrium to the left
atrium and the ductus arteriosus
which shunts blood to the aorta.
3) The ductus venosus allows blood to proceed
directly form the placenta to the heart, bypassing the liver.
8. From Birth Onward
1)
Birth (parturition)
usually takes place about 39 weeks after fertilization by a process of “labor “
involving stimuli from oxytocin and prostaglandin’s.
2) Three stage of labor are recognized.
a)
Birth begins
with contractions of the uterine muscles: the cervical canal dilates, and the
amniotic sac ruptures
b) The fetus is forcefully expelled accompanied by fluid
and blood
c) The umbilical cord is severed: the afterbirth is
expelled.
B.
Lactation
1)
The mammary
glands first produce a special fluid (colostrum)
for the newborn: then under the influence of prolactin,
they produce milk.
2) Suckling by the baby stimulates the pituitary to
release oxytocin, which in turn forces milk into the
ducts of the breast tissue.
3) For a nutrient breakdown of human mile see http://classes.aces.uiuc.edu/AnSci308/HumanLact.html
9. How
maternal Lifestyle Affects Early Development
1.
maternal diet,
especially vitamins and minerals, is important to the proper development of the
fetal tissues.
2. Severe restrictions of the maternal diet can result
in underweight babies.
B.
Risk of
infections
1)
Risk of
infections in the fetus can be minimized by antibodies that cross over into the
fetal blood.
2) However, viral diseases in the mother (such as German
measles) can cause fetal malformations.
C.
Prescription
drugs, Illegal Drugs, and Alcohol
1)
Thalidomide can cause limb deformities: retinoic acid
increases the risk of facial and cranial deformities: tetracycline
causes yellowed teeth, and streptomycin causes hearing problems.
2) Cocaine disrupts
the function of the nervous system of both fetus and mother
3) Alcohol
can cross the placenta and cause many effects collectively known as fetal
alcohol syndrome
D.
Effects of
cigarette smoke
1)
Cigarette
smoking can cause miscarriage, stillbirth, and premature delivery.
2) The critical period appears to be the last half of
pregnancy.
10. Summary of Developmental Stages
1.
The stages of
postnatal development are: neonate (first two weeks)>> infant
(two weeks to 15 months)>> child (to 12 years)>> pubescent
(individual at puberty)>> adolescent (form puberty to 3-4 years
later)>> adult>>>old age
2. Certain of these stages are characterized by more
noticeable changes such as the growth spurts and reproductive changes of
puberty.
B.
Adult Life
1.
Aging (senescence) is the progressive cellular and bodily
deterioration built into the life cycle of all living things.
a)
Beginning around
30 there is a gradual decline in bone and muscle mass, increased shin
wrinkling, and more fat deposition.
b) Metabolic rates decline, reflexes become slower, and
reduced collagen contents make tissues all over the body less elastic.
2.
The causes of
aging are not well understood.
a)
While the
lifespan in the
i)
diabetes
ii)
heart
disease (at a cost of 330 billion in
2000)
iii)
parkinsons
b)
Many of the
current drugs that are being researched have something to do with these
problems.
c)
The telomers of chromosomes become shorter.
d) Some research has shown that knockouts in this gene
will increase the lifespan of worms.
11. The body as it ages.
1)
There is a
gradual loss of vitality.
2) The skin begins to noticeably wrinkle and sag: body fat
accumulates: injuries are more frequent
3) In the connective tissues more crosslinks
form in the collagen making it less pliable.
1) Mitochondrial disfunction
2) Free radicals of oxygen could cause damage to
proteins and mitochondrial DNA
3) There may be a decline in the ability of cells to
repair DNA
4) There may be gradual loss in the ability of cells to
divide and reproduce.
5) For some reason the telomers
of the chromosomes become shorter
1) is has been shown in studies of mice for over 70
years that restricting calories will increase the lifespan significantly
iv)
12. Aging of skin, muscle, the skeleton,
and internal transport systems
1)
Changes in the
skin include: slower replacement of epidermis, elastic fibers replaced with
more rigid collagen, fewer oil and sweat glands produce drier skin, and hair
loses its pigment.
2) Changes in muscle include: loss of mass and strength:
muscle replacement by fat
3) Changes in the skeleton include: bones become weaker,
more porous and brittle: loss of calcium deterioration of cartilage at joints.
B.
Aging in the
Cardiovascular and Respiratory Systems
1)
Changes in the
respiratory system are mainly in the breakdown of the alveoli resulting in less
respiratory surface.
2) Changes in the cardiovascular system include: hear
pumping capacity diminishes: blood vessels become stiffer: plaque deposits form
in the vessels.
13. Age-Related changes in selected
other body Systems.
1.
Neurons are not
replaced when they die.
2. Neurofibrillary tangles of microtubules form in the neurons, and
beta amyloid protein form between neurons: these are
present in people with Alzheimer’s disease
B.
Reproductive
systems and Sexuality
1)
Falling
secretions of estrogen and progesterone trigger menopause in women whereas
declining testosterone in men causes reduced fertility.
2) Because the effects of declining hormones may be more
troublesome in women, hormone replacement therapy may be recommended.
C.
Immunity,
Nutrition, and the urinary System
1)
B and T cells in
the immune system decline.
2)
Reduction in
size of the thymus gland
3) Fewer digestive enzymes are produced in the
intestines
4) Basal metabolic rate falls, resulting in weigh gain.
5) Urinary incontinence may occur.
14. Studies on
ways to prevent aging?
Possible
Questions:
1. What is death? How is death officially defined by doctors
and coroners? Would their definition be different form that of a developmental
biologist?
2. The placenta supplements or completely replaces the
activity of three organ systems in the fetuses? What are they?
Practice
multiple choice questions
1)
Fertilization
is the result
a) of the union of two eggs and a sperm resulting in twins
b) of the union of an egg and a sperm that produces two identical cells
c) of the union of an egg and sperm that produces one diploid cell that will further divide to produce the zygote
d) Is the division of a diploid cell
into a gamete
e) Occurs at the point that semen is
deposited in the vagina
2)
Which of
the following would be considered cell differentiation?
a) the fertilization of an egg by the
sperm
b) the division of the fertilized egg
into 8 cells
c) when cells divide into populations
of cells that will become organs and tissues
d) when a cell moves to a new area in
the body
e) when a cell divides and becomes a new cell capable of a specific function
3)
The most
critical step that follows fertilization is
a) sloughing of the endometrium
in the uterus
b) implantation in the oviduct
c) implantation in the uterine lining
d) the development of arms and legs
e) the binding of the egg to the
vaginal wall
4)
during
the fetal development which of the following events occur during the second
trimester
a) suckling reflex is evident
b) the fetus is 4 to 5 inches long
c) the fetus is covered by a fuzzy hair
d) the fetus could survive on its own without the mother
e) the lungs are fully functional and
begin taking in air
5)
drugs may
often have an effect on a fetus, which of the following are matched
correctly. The drug to the condition
a) Thalidomide – missing limbs
b) Tetracycline – yellowed teeth
c) Streptomycin – hearing problems
d) Alcohol – fetal alcohol syndrome
e) Cigarette smoking – stillbirth and premature delivery
6) After birth a person starts to age. After middle age, the deleterious effects of
aging are more pronounced than the positive effects. Which of the statements below correctly
describes some of the negative aspects of aging? (review)
A) No effect occurs on the nervous system,
because neurons a continuously replaced when they die.
B) The levels of estrogen and progesterone fall triggering