Download Second Week of Development

Lecture 4
Development
Introductory Terminology
 Sexual reproduction - the process by which organisms produce offspring
by making sex cells called gametes
 Male gametes are called sperm (spermatozoa)
 Female gametes are called secondary oocytes
 The organs that produce gametes are called gonads, (testes in the male,
ovaries in the female)
 Pregnancy is a sequence of events that begins with fertilization, proceeds
to implantation, embryonic development, and fetal development, and
normally ends with birth about 38 weeks later
 Developmental biology is the study of the sequence of events from the
fertilization of a secondary oocyte to the formation of an adult organism.
 Embryology is the study of the developing embryo
 From fertilization through the eighth week of development, the developing human is
called an embryo and the stage is called the embryonic period
 The fetal period begins at week nine and continues until birth
 During this time the developing human is called a fetus
 Prenatal development includes both embryonic and fetal periods
 It is divided into three periods of three calendar months each called trimesters (first,
second, and third trimesters).
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The Path to Fertilization
 Fertilization: the genetic material from the
haploid spermatozoon and haploid
secondary oocyte merges into a single
nucleus creating a diploid cell called a
zygote
 The oocyte is viable for 12 to 24 hours
 Sperm is viable 24 t0 72 hours
 For fertilization to occur, coitus must
occur no more than:
 Three days before ovulation
 24 hours after ovulation
 Fertilization normally occurs in the
uterine (fallopian) tube about 12 to 24
hours after ovulation
 Sperm swim (due to the actions of their tails) from the vagina into the cervical
canal; the journey through the rest of the uterus and then into the uterine
tubes results mainly from contractions of the walls of these organs
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Fertilization
 Sperm undergo maturation in the epididymis,
they are not able to fertilize an oocyte until they
undergo capacitation in the female
reproductive tract
 During this process, the sperm’s tail beats
more vigorously and its plasma membrane’s
ability to fuse with the oocyte’s plasma
membrane is enhanced
 Binding of sperm cells to zona pellucida
receptor molecules triggers the acrosomal
reaction in which acrosomal enzymes are
released to help the sperm cells penetrate the
corona radiata and the zona pellucida
 Normally only one spermatozoon penetrates
and enters a secondary oocyte- this event is
called syngamy and it triggers events that
prevent polyspermy
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Fertilization
 When a spermatozoon enters a
secondary oocyte:
 The oocyte completes meiosis II
 The nucleus of the ovum develops into a
female pronucleus
 The nucleus of the sperm develops into a
male pronucleus
 The two pronuclei fuse to form a single
diploid nucleus that contains 46
chromosomes (23 from each
pronucleus)
 The fertilized ovum is called a zygote
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First Week of Development
 Cleavage: rapid mitotic cell divisions that occur
immediately after fertilization
 Cell division occurs without cell growth
producing progressively smaller cells called
Blastomeres
 A morula is a solid sphere of blastomeres
 From about 16 cells until a central cavity forms
 Remains about the same size as the original
zygote
 Still surrounded by the zona pellucida
 Blastocyst formation:
 After 4 or 5 days, the morula enters the uterine
cavity
 It becomes bathed by glycogen-rich uterine
milk secreted by the endometrium
 At about the 32-cell stage, this fluid enters the
morula to form a fluid-filled blastocyst cavity
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Fertilization
 The blastocyst hatches from the zona pellucida
 A blastocyst has the following components:
 An embryoblast or inner cell mass which develops into the
embryo
 An outer covering of cells called the trophoblast which ultimately
forms the outer chorionic sac about 5 days after fertilization
 Twins
 Dizygotic (fraternal) twins are produced from the independent
release of two secondary oocytes and the subsequent fertilization of
each by different spermatozoa
 Monozygotic (identical) twins develop from a single fertilized
ovum that splits at an early stage in development, usually within 8
days after fertilization, into two embryos
 Conjoined twins identical twins are joined together and share
some body structures from separations that occur later than 8 days
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Implantation
 Implantation is the process by which the blastocyst
attaches to and embeds itself within the endometrium;
 Occurs about 6 days after fertilization
 Following implantation, the endometrium is called the
decidua
 As the blastocyst implants, usually on the posterior wall
of the fundus or body of the uterus, it is oriented so that
the inner cell mass faces the endometrium
 Different regions of the decidua are named based on
their positions relative to the site of the implanted
blastocyst
 Decidua basalis is the portion of the endometrium located
between the embryo and the stratum basalis; it becomes
the maternal part of the placenta
 Decidua capsularis is the portion of the endometrium
located between the embryo and the uterine cavity
 Decidua parietalis is the remaining modified
endometrium that lines the noninvolved areas of the rest of
the uterus
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Second Week of Development
 The trophoblast develops two
layers in the region of contact
between the blastocyst and
endometrium:
 Outer syncytiotrophoblast
which secretes enzymes that
enable the blastocyst to penetrate
into the endometrium
 Inner cytotrophoblast that is
composed of distinct cells
 the trophoblast produces human
chorionic gonadotropin (hCG)
 Bilaminar embryonic disc: a flat disc formed from the inner cell mass
that differentiates into the:
 Epiblast (primitive ectoderm)
 Hypoblast (primitive endoderm)
 A small cavity appears in the epiblast and eventually enlarges to form the
amniotic cavity
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Second Week of Development
 Amnion: a thin, protective membrane that
forms from the epiblast and initially
overlies the bilaminar embryonic disc
 Comes to completely surround the
embryo, creating an amniotic cavity
 Filled with amniotic fluid
 Serves as a shock absorber for the fetus
 Helps regulate fetal body temperature
 Prevents adhesions between the fetal skin
and surrounding tissues
 Embryonic cells sloughed off into amniotic
fluid may be examined via amniocentesis
 Usually ruptures just before birth and with
its fluid constitutes the “bag of waters”
 Yolk sac: a thin, exocoelomic membrane that forms from the hypoblast,
formerly called the blastocyst cavity
 In humans, is small, relatively empty, and progressively decreases in size
 Several important functions include supplying nutrients during the second and
third weeks of development, providing blood cells, differentiating into primitive
germ cells, forming part of the gut, acting as a shock absorber, and preventing
dehydration of the embryo
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Second Week of Development
 Sinusoids:
 Develop from small spaces within
the syncytiotrophoblast called
lacunae
 Lacunae fuse to form larger,
interconnected lacunar networks
 Endometrial capillaries expand to
form sinusoids
 Maternal blood and endometrial
secretions enter the lacunar
networks to provide embryonic
nutrition and to serve as a disposal
site for embryonic wastes
 Extraembryonic coelom:
 Extraembryonic mesoderm develops around the amnion and yolk sac
 Cavities develop in the extraembryonic mesoderm, which then form a single larger
cavity called the extraembryonic coelom
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Second Week of Development
 Chorion develops from the trophoblast and the extraembryonic mesoderm
 Surrounds the embryo and, later, the fetus
 Eventually becomes the major embryonic component of the placenta
 Protects the embryo and fetus from maternal immune responses and produces
human chorionic gonadotropin (hCG)
 Inner layer of the chorion fuses with the amnion and the extraembryonic
coelom is now called the chorionic cavity
 The bilaminar embryonic disc becomes connected to the trophoblast by the
connecting (body) stalk, the future umbilical cord
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Third Week of Development
 Gastrulation: the bilaminar embryonic disc
is transformed into a trilaminar embryonic
disc consisting of three primary germ
layers
 Gastrulation begins with formation of the
primitive streak, a faint groove on the dorsal
surface; at the head end, a rounded primitive
node develops
 Invagination results in formation of the three
primary germ layers: ectoderm, mesoderm and
endoderm
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Third Week of Development
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A hollow tube called the notochordal process develops; then later forms a
solid cylinder called the notochord, which plays a very important role in tissue
induction
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The oropharyngeal membrane develops, but subsequently degenerates to
connect the oral cavity to the throat and the remainder of the GI tract
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The cloacal membrane develops, but subsequently degenerates to form the
openings of the anus, urinary, and reproductive tracts
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The allantois develops from an outpouching of the yolk sac that functions in early
formation of blood and blood vessels and is associated with development of the urinary
bladder
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Neurulation and Development of Somites
 The embryonic process by which
the neural plate, neural folds and
neural tube form is called
neurulation
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The head end of the neural tube
develops into the brain
The mesoderm segments into
cube-shaped structures called
somites
Each somite differentiates into a:
 Myotome, which develops
into skeletal muscles of the
neck, trunk and limbs
 Dermatome, which forms
connective tissue, including
the dermis
 Sclerotome, which
develops into vertebrae and
ribs
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Development of the Intraembryonic Coelom
 Small spaces appear in the lateral plate mesoderm and merge into a
larger cavity called the intraembryonic coelom
 Consequently, the lateral plate mesoderm is split into the splanchnic
mesoderm and the somatic mesoderm, each of which develops into
specific structures
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Development of the Cardiovascular System
Copyright © 2004 Pearson Education, Inc.
 Angiogenesis, the formation of blood vessels, begins when
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Mesodermal cells differentiate into hemangioblasts
Which then develop into angioblasts
Which then aggregate to form blood islands;
Which develop into blood vessels
 Blood cells arise from pluripotent stem cells
 The heart develops when:
 The cardiogenic area forms a pair of endocardial tubes
 Which form a single primitive heart tube
 The latter becomes S-shaped, begins to beat, and then joins with blood
vessels
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Placentation
 The process of forming the placenta, the
site of exchange of nutrients and wastes
between the mother and fetus
 Shaped like a pancake, the placenta
consists of a fetal portion formed by
chorionic villi and a maternal portion formed
by the decidua basalis
 As embryonic tissue invades the uterine wall,
maternal uterine vessels are eroded and
maternal blood fills spaces called lacunae
 Fingerlike projections of the chorion called
chorionic villi grow and blood capillaries
develop within them
 Maternal and fetal blood do not normally
mix
 Exchange of substances occurs between the
maternal blood in the intervillous spaces and
fetal blood flowing through capillaries within
the chorionic villi
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Placenta
 The placenta is a protective barrier, most microorganisms cannot pass
through it
 Certain viruses such as those that cause AIDS, German measles,
chickenpox, measles, encephalitis, and poliomyelitis can cross the placenta
 Certain drugs and alcohol can cause birth defects as they pass freely
through the placenta
 The umbilical cord forms the actual connection between the placenta
and embryo, which develops from the connecting stalk; it consists of:
 Two umbilical arteries carry deoxygenated fetal blood and wastes from the
fetus to the placenta
 One umbilical vein carries oxygenated blood and nutrients from the
placenta into the fetus
 Supporting mucous connective tissue is called Wharton’s jelly
 After birth, the placenta detaches from the uterus and is called the
afterbirth
 The area where the umbilical cord was attached to the infant becomes scar
tissue called the umbilicus (navel or belly button)
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Fourth Week of Development
 Organogenesis, the
development of body
organs and systems,
begins during the fourth
through eighth weeks of
development
 Embryonic folding
produces formation of:
 A head fold, tail fold,
and two lateral folds
 A primitive gut,
 Future pharyngeal
region (throat)
 Future ears and eyes
 Upper limb buds and
lower limb buds
 A visible heart
prominence
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Fifth Through Eighth Weeks of Development
 Rapid development of the
brain and head
 Development of distinct limb
regions
 Further development of the
eyes and ears
 The tail disappears
 The external genitals begin
to differentiate
 By the end of the eighth
week, the embryo has
clearly human
characteristics
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Fetal Development
 The remaining months of development are the fetal period, during which
time the fetus continues to develop and grows at a remarkable rate
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The fetus takes on a human appearance
Organs grow rapidly
By the end of the third month, the placenta is functioning
Third month
Second trimester
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A time of growth
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Bone formation occurs
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Hair and body are
covered with fine hair
called lanugo
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By the end of the 6th
month, the fetus is 30
cm (1 foot) long
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Development is
essentially complete
 Except for lungs
and brain
Developing human is
now called a fetus
It carries out primitive
reflexes like sucking
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The developmental biology of the various body systems will be described in
their respective textbook chapters
Fetal Development & Birth
 After birth, the umbilical cord is tied off
and severed
 The short remnant of the cord withers
and falls away usually within 12-15
days after birth
Third trimester
 The area where the cord was
attached develops a scar called the
umbilicus (navel)
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Pace of growth accelerates
 Premature infant - “Preemie”
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Weight of fetus more than
doubles
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Nutrients are still provided
by mother’s blood via the
placenta
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Most major nerve tracts are
formed in the brain
 A baby weighing less than 2500 g at
birth;
 The body of such a baby is not yet
ready to sustain some critical
functions
 Thus survival is uncertain without
medical assistance
After Birth
Apgar Score
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At 1-5 minutes after birth, the
infant’s physical status is
assessed based on five signs:
heart rate, respiration, color,
muscle tone, and reflexes
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Each observation is given a score
of 0 to 2
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Apgar score = the total score of
the above assessments
 8-10 indicates a healthy baby
 Lower scores reveal problems
Post Natal Development
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Babies typically double birth weight within a few months
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Allometric growth: Different body parts grow at different rates
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Nerve cells produced at an average rate of > 250,000 per minute
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At 6 months, neuron production ceases permanently
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Labor
 Obstetrics is the medical specialty that deals with the management of
pregnancy, labor, and the neonatal period (the first 28 days after birth)
 Labor is the process by which the fetus is expelled from the uterus
through the vagina to the outside of the body
 parturition also means giving birth.
 True labor begins when uterine contractions (spreading as waves from
the top of the uterus and moving downward) occur at regular intervals
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The contractions usually produce pain
As the interval between contractions shortens, the contractions intensify
There may be localization of pain in the back, which is intensified by walking
Evidence of “show”, a discharge of blood-containing mucus that accumulates
in the cervical canal during labor
 Dilation of the cervix
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True Labor - Three Stages
 Stage of dilation is the time (6-12 hours) from the
onset of labor to the complete dilation (to 10 cm) of
the cervix There are regular contractions of the uterus
 The amniotic sac usually ruptures spontaneously
 Stage of expulsion is the time (10 minutes to
several hours) from complete cervical dilation to
delivery of the baby
 Placental stage is the time (5-30 minutes or more)
after delivery until the placenta or “afterbirth” is
expelled by uterine contractions
 These contractions constrict blood vessels that were
torn during delivery which decreases the likelihood of
hemorrhage
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Maternal Changes During Pregnancy
Anatomical Changes
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Chadwick’s sign – the vagina develops a purplish hue
Breasts enlarge and their areolae darken
The uterus expands, occupying most of the abdominal cavity and pushes against
abdominal organs
Lordosis is common due to the change of the body’s center of gravity
Relaxin causes pelvic ligaments and the pubic symphysis to relax
Typical weight gain is about 29 pounds
Specific changes in the following systems: cardiovascular, respiratory, digestive,
urinary, integumentary, and reproductive
Structure of Lactating Mammary Glands
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Modified sweat glands consisting of 15-25 lobes that radiate around and open
at the nipple
Breast size is determined by the amount of adipose tissue, not the number of
lobes and alveolar glands
Areola – pigmented skin surrounding the nipple
Suspensory ligaments attach the breast to underlying muscle fascia
Lobes contain glandular alveoli that produce milk in lactating women
Compound alveolar glands pass milk to lactiferous ducts, which open to the
outside