Development and Inheritance
I. The gradual modification of physical and physiological
characteristics during the period from conception to physical
maturity is called development.
A. An overview of topics in development
1. Development involves 2 things- the
division and differentiation of cells and the
changes that produce and modify
2. Development begins with fertilization or
3. Development is divided into periods
characterized by specific anatomical
4. Embryological development considers
the events that occur in the first 2 months
5. The study of these events in the
developing organisms or embryo is called
6. After 2 months of development the
embryo becomes a fetus.
7. Fetal development begins at the start of
the ninth week and continues to birth.
8. Together, embryological and fetal
development is referred to as prenatal
9. Postnatal development begins at
birth and continues to maturity, when
the aging process begins.
10. Inheritance refers to the transfer
of genetically determined
characteristics from generation to
11. Genetics is the study of the
mechanisms responsible for
1. Fertilization involves the fusion of
two haploid gametes to produce a
zygote with 46 chromosomes.
C. An overview of prenatal development
1. The time spent in prenatal development
is known as the period of gestation, or
2. Gestation occurs within the uterus over
a period of 36-40 weeks.
3. Prenatal development is divided
into 3 trimesters.
4. The first trimester: all the basic
components of each organ system
5. The second trimesters: the
development of organs and organ
6. Third trimester is fetal growth.
D. The first trimester
1. By the end of the first trimester the
fetus is about 3 in long and weighs about
2. Four general events occur in the 1st
trimester: cleavage, implantation,
placentation and embryogenesis.
3. Cleavage is a series of cell divisions that
subdivided the cytoplasm of the zygote.
4. The first cleavage division produces two
identical cells called blastomeres.
5. The first division is completed in about
30 hours after fertilization.
6. After 3 days of cleavage, the
embryo is a solid ball of cells, this
stage is called the morula.
7. Over the next 2 days, the
blastomeres form a blastocyst, a
hollow ball with an inner cavity
known as the blastocoele.
8. See table 20.1 the fates of the
primary germ layers.
E. At fertilization, the zygote is still 4 days
away from the uterus.
1. It arrives in the uterine cavity as a
morula, and over the next 2-3 days,
blastocyst formation occurs.
2. Implantation begins as the surface of
the blastocyst closest to the inner cell
touches and adheres to the uterine lining.
3. If fertilization occurs anywhere
else in the body it is called an ectopic
F. Formation of the extra embryonic
1. The first extra embryonic membrane to
appear is the yolk sac.
2. The yolk sac is present 10 days after
fertilization. The yolk sac becomes an
important site of blood cell formation.
3. The amnion is composed of
ectoderm and mesoderm.
4. It encloses fluid that surrounds
and cushions the developing embryo
5. The chorion is created .
1. The placenta is a temporary
structure in the uterine wall that
provides a site for diffusion between
the fetal and maternal circulatory
2. Placentation or placenta formation
takes place when blood vessels form in the
3. The fetus remains connected to the
placenta by the elongate umbilical cord,
which contains the allantois, umbilical
blood vessels and the yolk sac.
H. Placental Hormones
1. The placenta acts as an endocrine
2. The placental hormones produced include
human chorionic gonadotropin (hCG),
progesterone, estrogen, human placental
lactogen, placental prolactin and relaxin.
3. hCG appears in the maternal
bloodstream soon after implantation
4. The presence of hCG in blood or
urine samples is a reliable indication
5. hCG maintains the uterine lining and as
a result menstruation does not occur.
6. If there is no hCG, a woman would get
7. Human placental lactogen (hPL) and
placental prolactin helps prepare the
mammary glands for milk production.
8. Relaxin is a hormone secreted by the
9. Relaxin (1) increases the flexibility of
the pubic bones allowing the pelvis to
expand during delivery. (2) causes the
dilation of the cervix, making it easier for
the fetus to enter the vaginal canal. (3)
suppresses the release of oxytoxin by the
hypothalamus and delays the onset of
1. The process of forming an embryo
is called embryogenesis.
2. It begins as folding and
3. The first trimester is a critical
period for development, because
events during the first 12 weeks
establish the basis for organogenesis,
the process of organ formation.
SEE PAGE 624-625!!!!!!
II. The second and third
A. By the end of the first trimesters,
the rudiments of all the major organ
systems have formed.
1. Over the next 3 months, the fetus
will grow to a weight of about 1.4 lbs.
2. During the
the fetus, encircled
by the amnion,
grows faster than
3. During the third
trimester, the basic
components of all
the organ systems
appear, and most
become ready to
fulfill their normal
4. The rate of growth starts to
decrease, but the third trimester is
when the baby gains the most
5. In the last 3 months of gestation,
the baby gains about 5.7lbs, for a full
term weight about 7lbs.
B. Pregnancy and maternal systems
1. The developing fetus is totally
dependent on the maternal organ system
for nourishment, respiration and waste
2. These functions must be performed by
maternal systems in addition to their
3. The mother must absorb enough
oxygen, nutrients and vitamins for
herself and her fetus and she must
eliminate all of the generated wastes.
4. The demands grow as the baby
C. Major changes that take place in
maternal systems include the
1. The maternal respiratory rate goes
up and the tidal volume increases.
Mother’s lungs obtain extra oxygen
and are required to remove the
excess carbon dioxide by the baby.
2. The maternal blood volume
increases. This increase is because
(1) blood flowing into the placenta
reduces the volume in the rest of her
body. (2) baby changes the chemical
make up of the blood. BY the end of
gestation, the maternal blood volume
has increased by almost 50%!
3. The maternal requirements for
nutrients and vitamins climb 10-30%.
4. The maternal glomerular filtration
rate increases by 50%. (urinating)
5. The uterus undergoes an increase
6. The mammary
glands increase in
size and secretory
activity begins. BY
the end of the 6th
are fully developed.
D. Structural and functional changes in the
1. At the end of gestation, a typical uterus
will have grown from 3 inches in length
and 2 oz in weight to 12 inches in length
and 2.4 lbs.
2. It may then contain almost 5 liters of
fluid so the organ with all contents has a
total weight of about 22lbs! (without baby)
3. The uterus is muscle and stretches as it
4. In the early stages of pregnancy, the
contractions are weak, painless and brief
as the uterus grows.
5. Progesterone released by the placenta
prevents stronger contractions.
6. Three major factors oppose the
calming action of progesterone.
-Rising estrogen levels:Estrogen, also produced
by the placenta makes contractions more likely.
- Rising oxytocin levels: Stimulates an increase
force and frequency of uterine contractions.
-Prostaglandin production: Also stimulates uterine
7. After 9 months of gestation (36-40
weeks) multiple factors interact to
produce labor contractions.
8. Once begun, positive feedback
ensures that contractions continue
until delivery has been completed.
III. Labor and Delivery-forcible expulsion of
the fetus is called parturition.
A. The stages of labor
1. The dilation stage: begins with the
onset of labor, the cervix dilates and the
baby begins to slide down the cervical
canal. This is very variable in length but
takes ABOUT 8 or more hours. At the start
of labor contractions are every 10-30
minutes but their frequency and intensity
increases. Late in the process the amniotic
sac breaks, or “water breaks.
2. The expulsion stage: begins with the
cervix pushed open by the baby, dilates
completely. Expulsion continues until the
baby is out. Lasts usually less than 2
hours. The birth of the baby is called
delivery. If the vagina is too small an
episiotomy may be performed. If the baby
cannot be delivered vaginally a C-section
will be performed.
3. Placental stage: Muscle tension
builds in the wall of the empty uterus and
the organ decreases in size. Usually within
an hour after delivery, the placental stage
ends with the ejection of the placenta, or
after birth. There could be as much as
500-600ml of blood lost, but the increase
of maternal blood through pregnancy helps
See page 629.
B. Premature labor
1. Premature labor occurs when labor
contractions begin before the baby has
completed normal development.
2. Most babies are born between 25-27
weeks have a high risk of developmental
3. A premature delivery usually
refers to babies born 26-36 weeks.
But these babies have a good chance
of normal development.
C. Multiple births
1. Multiple births can occur for several reasons.
2. 1 out of 89 births are multiples.
3. About 70% of twins are fraternal or dizygotic.
4. Identical twins or monozygotic twins are less
IV. Postnatal development
A. In postnatal development, each
individual passes through a number
of life stages: neonatal, infancy,
childhood, adolescence, and maturity.
1. The neonatal, infancy and
2. The neonatal period extends from
the moment of birth to 1 month
3. Infancy then continues to 2 years
4. Childhood lasts until puberty
5. Two major events are under way during
these developmental stages.
a. The major organ systems, other than
associated with reproduction, become fully
operational and gradually acquire the
functional characteristics of adult
b. The individual grows rapidly, and body
proportions change significantly.
6. Pediatrics is medical specialty
focusing on postnatal development
from infancy through adolescence.
C. The neonatal period
1. Physiological and anatomical changes
occur as the fetus completes the transition
to the status of a newborn infant or
2. Typical heart rate of 120-140 beats per
minute and respiratory rates of 30 breaths
per minutes is normal for them.
3. The transition from fetus to
neonate can be summarized as
a. The lungs at birth are collapsed and filled
b. When the lungs expand, the pattern of
cardiovascular circulation changes due to
alternations in blood pressure and flow rates.
c. Before birth, the digestive system
remains relatively inactive, although it does
accumulate a mixture of bile secretions,
mucus and epithelial cells.
d. As waste produces build up in the
arterial blood, they are excreted out
e. The neonate has little ability to
control body temperature, particularly
in the first few days after delivery.
4. By the end of the 6th month of
pregnancy the mammary glands are fully
5. The first few days after delivery the
glands will secrete colostrum, this contains
more proteins and less fat than breast
milk. Many of the proteins are antibodies.
6. As colostrum production declines,
the mammary glands will start to
7. The most rapid growth occurs
during prenatal development, and the
rate of growth declines after delivery.
8. Postnatal growth during infancy
and childhood occurs under the
direction of hormones.
9. Adolescence begins at puberty,
the period of sexual maturation and
ends when growth is complete.
a. The hypothalamus increases its
production of gonadotropin releasing
b. Because of GnRH, levels of FSH and LH
c. This causes the formation of gametes,
production of sex hormones, and an
increase of growth rate.