Chapter 3
Fetal Development
Copyright © 2012, 2008 by Saunders, an imprint of Elsevier Inc.
1
Genetics and Fertilization
Copyright © 2012, 2008 by Saunders, an imprint of Elsevier Inc.
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Objectives
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Define key terms listed.
Define a chromosome and give the number in
each human body cell.
Compare a gene and a chromosome.
Explain how the sex of an individual is
determined.
Describe human fertilization and implantation.
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Genetics
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Our genes are programmed to use
deoxyribonucleic acid (DNA)
DNA is the master protein controlling
development and functioning of all cells
Spiral shaped strands found in nucleus of all
human cells
Genetic codes are programmed into the new
individual’s cells by DNA
Single mistake or variation in sequence can
effect cell(s) development
Can lead to mutations/disease
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Chromosomes
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Threadlike, spiral structure
Occur in pairs; made up of long chains of
DNA
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Each cell contains 46 chromosomes
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One from mother, one from father
There are 22 arranged pairs known as autosomes
The remaining pair are gametes, or sex cells,
which determine the sex of the developing fetus
Each chromosome is composed of genes;
defined as segments of DNA controlling
heredity (gene single bead; chromosome
strand of beads)
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Cell Division and Gametogenesis
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Division of a cell begins in its nucleus
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Contains gene-bearing chromosomes
Two types of cell division
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Mitosis=body cell division
Meiosis=reproductive cell division
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Mitosis
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Continuous process
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Each “daughter cell” contains same number
of chromosomes as parent cell
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Body grows, develops, and dead cells are
replaced
46 chromosomes in a body cell are called diploid
number
Process of mitosis
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In sperm—spermatogenesis
In ovum—oogenesis
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Meiosis
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Reproductive cells undergo two sequential
divisions
Number of chromosomes is reduced by half
(23)
Each cell contains only one sex chromosome,
called haploid
Process completed in sperm before traveling
toward fallopian tubes; in ovum occurs after
ovulation, if fertilization occurs
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Fertilization
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When sperm and ovum unite
New cell contains 23 chromosomes from
sperm and same amount from ovum
Chromosome number is returned to 46
(diploid)
Traits from mother and father are inherited
Formation of gametes by this type of cell
division is gametogenesis
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Genes
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Segment of DNA chain
Coded for inheritance
Carry instruction for dominant and recessive
traits
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Dominant Genes
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Dominant overpower recessive traits and are
passed on to the offspring
If only one parent has dominant trait, 50% of
offspring will have it too
If both parents carry a recessive trait, there is
a chance that one of the offspring will display
that trait
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Recessive Genes
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If each parent carries a recessive trait, there
is a chance that one offspring will display that
trait
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Sex Determination
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Determined at time of fertilization
Spermatozoon (sperm cell) has either an X or
Y chromosome
Female ovum only contains an X
chromosome
XX—female
XY—male
The male determines the sex of the fetus
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Beginning of Embryonic
Development
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Fertilization
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Conception: union of egg and sperm; also
known as fertilization
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Normally occurs in outer third of fallopian tube
After a single sperm enters ovum, a membrane
forms to prevent the entry of more sperm
Ovum can be fertilized from 6 to 24 hours
after its release from the ovary
Sperm can survive up to 5 days after
ejaculation
Once fertilized, the nucleus of the sperm and
the ovum unite to form a zygote
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Implantation
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Zygote starts a
series of rapid cell
divisions as it
travels from the
fallopian tube to
the uterus
Over several days
various stages of
development
continue to occur
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Embryonic Period
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Trophoblastic cells aid in the implantation
process and provide nutrients to the
developing embryo
Embryonic development is from the 2nd to
8th week after conception
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At this point teratogenic agents may exert
profound and damaging effects on the developing
embryo
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Fetal Period of Development
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Lasts from the 8th week until birth
Care must still be taken to prevent harm to
the developing fetus
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Audience Response System
Question 1
The placental transfer of gases, nutrients, waste
materials and other substances can be
increased by maternal:
A. Hyperglycemia
B. Hypoglycemia
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Fetal Development
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Objectives
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Discuss fetal development.
Explain the development and function of the
placenta.
Review the functions of amniotic fluid and the
umbilical cord.
Diagram fetal circulation to circulation after
birth.
Discuss multifetal pregnancy, and compare
two types of twins.
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Embryonic Cell Differentiation
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Occurs during 2nd and 3rd week after
conception
All tissues and organs of the embryo or fetus
come from three cell layers
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Ectoderm
 Mesoderm
 Endoderm
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Fetal Membranes and
Amniotic Sac
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Amnion—protects developing embryo
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Forms a cavity in which the embryo or fetus floats
Expands to accommodate growing fetus
Chorion is outer membrane that encloses
growing amnion
Both fuse together as fetus continues to grow,
becoming amniotic sac (“bag of waters”)
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Amniotic Fluid
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Clear and slightly straw-colored
Consists of 98% water
Traces of protein, glucose, fetal lanugo (hair),
fetal urine, and vernix caseosa
Most derived from maternal blood with fetal
urine contributing to the amount of fluid in
later development
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Amniotic Fluid (cont.)
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At term can be 800 to 1000 mL
Hydramnios (excessive fluid) if greater than
2000 mL
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Associated with malformations of CNS
Oligohydramnios (not enough fluid) if less
than 300 mL
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Typically associated with renal abnormalities
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Functions of Amniotic Fluid
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Allows embryo or fetus to move about freely
Prevents amnion from adhering
Cushions against injury from external sources
Maintains a constant temperature
surrounding the embryo or fetus
Provides fluid homeostasis
Prevents umbilical cord compression
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Placenta
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Two sections
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Maternal
Fetal
Permits exchange of materials carried in the
bloodstream between mother and fetus
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Placental Circulation
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Placenta Formation
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Begins at the outer layer of the blastocyst
Trophoblastic cells multiply and develop the
chorionic villi
Villi penetrate the lining of the uterus
(decidua)
Uteroplacental circulation begins around 17
days postconception
Allows for the absorption of nutrients and
oxygen and the excretion of wastes
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Uteroplacental Blood Flow
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50 mL/min at 10 weeks gestation
500 to 600 mL/min at term
Maternal and fetal circulatory systems are
separate
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Placental Functions
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Protection
Nutrition
Respiration
Excretion
Hormone production
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Placental Transfer
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Movement of gases, nutrients, waste
materials, drugs, and other substances
across the placenta
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Can be modified by maternal nutritional status,
exercise, and disease
 Can be increased by maternal hyperglycemia
 Can be decreased by reduced uteroplacental
blood flow
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Immunologic Functions
of the Placenta
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Protects fetus from pathogens
Prevents rejection by the mother
Allows most viruses, some bacteria,
pollutants, and drugs to cross the membrane
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Placental Blood Flow
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Can be reduced if uterine artery is constricted
(e.g., maternal hypertension)
Harm can come from substances that cross the
placental barrier (e.g., alcohol, nicotine, carbon
monoxide, some prescription and illicit drugs)
Contractions of the uterus can decrease flow;
therefore, important to monitor for uterine
relaxation between contractions
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Placental Hormones
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Progesterone
Estrogen
Human chorionic gonadotropin (hCG)
Human placental lactogen (hPL)
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Progesterone
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First produced by corpus luteum, then by
placenta
Functions during pregnancy
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Maintain uterine lining for implantation of zygote
Reduce uterine contractions to prevent spontaneous
abortion
Prepare gland of breasts for lactation
Stimulate testes to produce testosterone, which aids
the male fetus in developing the reproductive tract
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Estrogen
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During pregnancy
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Stimulates uterine growth
Increases blood flow to uterine vessels
Stimulates development of breast ducts
Effects not directly related to pregnancy
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Increased skin pigmentation (mask of pregnancy)
Vascular changes in skin and mucous membranes
Increased saliva production
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Umbilical Cord
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Lifeline between fetus and the placenta
Contains
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Two umbilical arteries
One umbilical vein
Embedded in Wharton’s jelly (prevents
kinking of cord and interference with
circulation)
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Umbilical Blood Vessels
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Before birth
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Blood flows through the umbilical arteries to
the placenta
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Umbilical arteries carry deoxygenated blood
Umbilical veins carry oxygenated blood
Carbon dioxide and other waste products are then
released
Umbilical vein carries oxygen and nutrients
from placenta to fetus
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Circulation Before
and After Birth
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Embryonic Circulation
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Fertilized ovum derives nutrition first from its
own cytoplasmic mass
Then from the decidua by activity of
trophoblastic cells
By 4th week after conception, embryo gains
circulation and nourishment from yolk sac
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Fetal Circulation
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After 4th week of gestation
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Blood circulation through placenta to fetus is wellestablished
Fetus does not breathe, but does have breathing
movements
Liver does not process most waste products
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Fetal Circulatory Shunts
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Ductus venosus: diverts some blood away
from liver as it returns from placenta
Foramen ovale: diverts most blood from right
atrium directly to left atrium
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Bypasses lungs
Ductus arteriosus: diverts most blood from
pulmonary artery into aorta
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Circulation Before Birth
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Oxygenated blood enters fetal body through
umbilical vein
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Remainder enters inferior vena cava (IVC)
through ductus venosus
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Half goes to liver through portal sinus
Blood in IVC enters right atrium, passes directly
into left atrium through foramen ovale
Rest of blood from right ventricle joins blood from
left ventricle through ductus arteriosus
Waste products are returned to placenta
through umbilical arteries
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Circulation After Birth
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Fetal shunts not needed after birth
Foramen ovale closes when pressure in right
side of heart falls as lungs become more fully
inflated
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Little resistance to blood flow
Blood oxygen level rises, causes ductus
arteriosus to constrict
Ductus venosus closes when flow from
umbilical cord stops
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Closure of Fetal
Circulatory Shunts
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Foramen ovale closes functionally within 2
hours after birth
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Ductus arteriosus closes functionally within
15 hours
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Permanently by age 3 months
Permanently in about 3 weeks
Ductus venosus closes functionally when
cord is cut
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Permanently in about 1 week
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Fetal Development
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Three basic developmental stages include
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Zygote: conception to 2nd week
Embryo: 2nd to 8th week
Fetus: 9th week to birth
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Fetal Development (cont.)
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Factors that may affect development
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Quality of ovum and sperm
Inherited characteristics
Intrauterine environment
Injury by exposure to teratogenic chemicals
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Fetal Development (cont.)
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Most critical time in fetal development is the
first 8 weeks
This period is called the organogenesis
period
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Fetal Development (cont.)
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14th week: fetal movement
20th week: fetus may survive outside uterus
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Age of viability
28th week: surfactant is minimal and is
needed to help keep alveoli in lungs open
40th week: full-term and ready to survive
outside of uterus
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Multifetal Pregnancy
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More than one fetus is developing at the
same time
Monozygotic: identical twins
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From one egg and one sperm
Dizygotic: fraternal twins (nonidentical)
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From two ova and two sperm
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Audience Response System
Question 2
Which fetal circulatory shunt closes functionally
within 15 hours of delivery and permanently in
about 3 weeks?
A. Ductus arteriosus
B. Foramen ovale
C. Ductus venosus
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Review Key Points
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