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Chapter 3 Outline
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Overview of Embryology
Gametogenesis
Pre-embryonic Period
Embryonic Period
Fetal Period
Stages of Prenatal Development
Pre-embryonic period
First 2 weeks after fertilization of egg/formation
of zygote
Embryonic period
Third through eighth weeks when all major
organ systems begin to develop
Fetal period
Ninth through thirty-eighth weeks when growth
dominates; fetal period ends at birth
Human Development
• From fertilized egg through adult
maturation
– Fertilization to birth = embryogenesis
– After birth
• maturation of body and reproductive
organs
• production of sex cells (gametes),
eggs or sperm = gametogenesis
Life Cycle of Humans
Figure 3.1
Gametogenesis
• Gamete (sperm or egg) production
– gametes are haploid (contain 23 chromosomes)
– all other body cells are diploid (have 23 pairs of
chromosomes)
• in mitosis a diploid cell produces two
genetically identical diploid “daughter” cells
• Reproductive organs produce haploid cells
by meiosis
Meiosis
• Division of a diploid cell producing two
haploid “daughter” cells
– resulting cells are not identical to each other
– crossing over may occur allowing exchange
of genetic material between paired
homologous chromosomes
• producing gametes that contain a
combination of genes from both parents =
genetic diversity
Meiosis
• Occurs in diploid cells of testes and
ovaries
– each contain 46 chromosomes (23 from
each parent)
– results in 4 haploid cells
– involves two division cycles
• meiosis I and meiosis II
Meiosis I
Figure 3.2
Meiosis I: Prophase I
1. Nuclear envelope breaks down.
2. Homologous double-stranded
chromosomes pair up in a process called
synapsis to form a tetrad.
3. Crossing over ensures genetic diversity
in future generations.
Meiosis I: Metaphase I
1. Pairs of homologous tetrads form two
rows in the center of the cell.
2. Each row is a mix of tetrads from mother
and father.
3. Spindle fibers from centrioles attach to
the paired chromosomes.
Meiosis I: Anaphase I
1. Pairs of homologous (double-stranded)
chromosomes are pulled to opposite
ends of cell.
2. Daughter cells will receive a random
combination of maternal and paternal
sister chromatids.
3. This random separation of maternal and
paternal sister chromatids is called
reduction division.
Meiosis I: Telophase I
1. Chromosomes arrive at far ends of the
cell.
2. Nuclear membranes form around the two
sets of chromosomes.
3. A cleavage furrow forms and cytoplasm
is divided by cytokinesis into two
daughter cells.
4. Each daughter cell has 23 doublestranded chromosomes (each
chromosome has two sister chromatids).
Meiosis I
Figure 3.2
Meiosis II
Figure 3.2
Meiosis II: Prophase II
1. Resembles prophase of mitosis
2. Nuclear envelope breaks down in
daughter cells from meiosis I
3. Double-stranded chromosomes collect
near center of cell
4. Crossing over occurs in first meiotic
prophase only
Meiosis II: Metaphase II
1. Double-stranded chromosomes form a
single line at the equator of each
daughter cell.
2. Spindle fibers extend from the centrioles
and attach to the centromeres of the
double-stranded chromosomes.
Meiosis II: Anaphase II
1. Sister chromatids of each doublestranded chromosome are pulled apart at
the centromere.
2. Each chromatid is now a single-stranded
chromosome.
3. The single-stranded chromosomes
migrate to opposite poles of the cell.
Meiosis II: Telophase II
1. Nuclear envelopes form around each set
of single-stranded chromosomes at
opposite ends of the cell.
2. A cleavage furrow forms and the cell’s
cytoplasm divides by cytokinesis.
3. The daughter cells are now haploid
containing only 23 single-stranded
chromosomes.
Summary of Meiosis
1. Starts with one diploid cell
2. Meiosis I produces two diploid daughter
cells
3. Meiosis II turns two diploid cells into four
haploid cells
4. Crossing over only occurs in prophase I
Oogenesis
• Parent cells that produce haploid oocytes
(eggs) through meiosis are oogonia.
• Oogonia are located in the ovaries and
enter prophase I during fetal development.
• Oogenesis stops in females until puberty.
• The cells in prophase I are primary
oocytes.
Oogenesis─Continued
• Monthly, after puberty, a number of primary
oocytes begin to mature by resuming meiosis I.
• Meiosis I produces two daughter cells but
cytokinesis divides the cells unequally.
– The smaller cell is a polar body and will die.
– The larger cell is the secondary oocyte,
which stops developing at metaphase II and
will be ovulated.
Oogenesis─Concluded
• If fertilized, the secondary oocyte completes
meiosis II.
– Meiosis II produces two daughter cells with
uneven division of cytoplasm.
• The larger cell is the ovum, containing 23
chromosomes that will combine with the 23
provided by the sperm that fertilized it.
• The smaller cell is a polar body that dies.
• If the secondary oocyte is not fertilized, it
degenerates in about 24 hours.
Ovulation
•
The ovum is expelled from the ovary with
two surrounding structures:
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–
•
the corona radiata─several layers of
cuboidal cells
the zona pellucida─a clear layer of
proteins on the ovum under the corona
radiata
Sperm must penetrate both structures in
order to fertilize the ovum
Spermatogenesis
• Parent cells that produce haploid sperm
through meiosis are spermatogonia
– only live in the testes of the male
– each spermatogonium divides by mitosis to
produce two genetically identical cells called
primary spermatocytes
Spermatogenesis
• Each primary spermatocyte undergoes
meiosis producing four haploid
spermatids containing 23 chromosomes.
– Spermatids must undergo further
changes called spermiogenesis to
become mature sperm.
Structure of Mature Sperm
Figure 3.3
Mature Sperm
• Sperm deposited in the female reproductive tract
are unable to fertilize a secondary oocyte.
– They must undergo capacitation or
conditioning in the vagina to change the
membrane of the acrosome, a membranous
cap at the head of the sperm.
– The acrosome contains digestive enzymes
that will be released upon contact with the
cells of the corona radiata and facilitate the
penetration of the sperm’s nucleus into the
cytoplasm of the egg.
1. Pre-embryonic Period
• Fusion of sperm and secondary oocyte is
fertilization
– usually occurs in upper 1/3 of uterine tube
– nucleus of ovum fuses with nucleus of sperm
(properly called pronuclei prior to fusion)
– resulting single diploid cell is the zygote
• on rare occasions, two or more sperm may
penetrate the egg’s cytoplasm, a condition
called polyspermy that is immediately fatal
Figure 3.3
Week 1 (Early)
• After the zygote is formed, it undergoes a series
of mitotic divisions called cleavage.
– The number of cells increase, but total size remains
the same.
– This process, called compaction, results in increased
contact between the cells.
• A 16-cell stage organism is called a morula.
– The morula arrives in the uterine cavity about day 3 or
4.
Week 1 (Late)
• One to 2 days after the morula enters the
uterine cavity, it develops a fluid-filled
cavity in its center.
• This cavity is the blastocyst cavity and
the organism is now a blastocyst.
Figure 3.4
Week 1 (Late)
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Shortly after blastocyst formation,
differentiation forms two regions:
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trophoblast─outer ring of cells that will
develop into the chorion
embryoblast (inner cell mass)─ cluster
of tightly packed cells inside one portion of
the trophoblast
• cells of the inner cell mass are
pluripotent (able to differentiate into
any cell type found in the human body)
Week 1 (Late)
•
At the end of the first week after
fertilization, the zona pellucida has
degraded.
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The trophoblast can make direct contact
with cells that line the inside of the uterus.
• The cells that line the inside of the
uterus form a layer called the
endometrium.
Week 1 (Late)
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The endometrium consists of two layers:
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deep basal layer and superficial functional layer
•
blastocyst invades the functional layer
•
its trophoblast turns into two layers:
– inner cellular layer─cytotrophoblast
– outer thick layer─ syncytiotrophoblast,
which continues to invade the
endometrium and pulls the blastocyst
deeper into the endometrium
– by end of week 2, the blastocyst has
disappeared from the surface of the
endometrium
Figure 3.6
Week 2 (Early)
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By day 8, the cells of the embryoblast
differentiate into two distinct types:
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hypoblast─layer of small cuboidal cells
facing the blastocyst cavity
epiblast─layer of columnar cells deep to
the hypoblast
Together, these two layers form a flat
disc called the bilaminar germinal disc
Week 2 (Early)
•
The bilaminar germinal disc and
trophoblast produce three
extraembryonic membranes:
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yolk sac
amnion
chorion
Week 2 (Early)
• The yolk sac is formed from and is
continuous with the hypoblast layer.
• It does not store yolk in humans but does
serve as a site for early blood cell and
vessel formation.
Week 2 (Early)
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The amnion is a thin layer of cells that forms
above and is derived from the epiblast.
A fluid-filled amniotic cavity appears between
the amnion and epiblast layer.
– The fluid is produced by the cells of the
amnion and will protect the embryo from
“drying out.”
Figure 3.6
Week 2 (Early)
• The chorion is the outermost membrane and is
formed by the rapidly expanding
syncytiotrophoblast and cytotrophoblast.
• A major function of the chorion is the formation
of the placenta.
Figure 3.7
Week 2 (Late)
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The placenta is a highly vascularized organ
that serves as a physical and biochemical
interface between embryo and mother.
The main functions of the placenta are
– exchange of nutrients, waste products,
and blood gases between embryo and
mother.
– transmission of maternal antibodies to the
embryo.
– production of many hormones,
predominantly estrogen and progesterone.
Week 2 (Late)
• The placenta is comprised of tissues from
both embryo and mother.
– The embryonic portion of the placenta is the
chorion.
– The maternal portion is from the functional
layer of the endometrium.
Week 2 (Late)
• The early embryo is attached to the
placenta by a structure called the
connecting stalk.
• Eventually, the connecting stalk will
develop into the umbilical cord through
which the umbilical arteries and veins will
be transmitted.
Week 2 (Late)
• Fingerlike projections called chorionic
villi appear at the leading edge of the
chorion.
– The villi project into the functional layer of the
endometrium.
– Inside the villi are branches from umbilical
blood vessels (embryonic source).
– Outside the villi is maternal blood.
– Metabolic exchange in the placenta occurs
across the wall of the villi.
Figure 3.7
2. Embryonic Period
• Weeks 3–8
• One of the earliest events to occur during
week 3 is the establishment of three
primary germ layers from which all adult
human structures are derived (except the
embryonic part of the placenta)
• By the end of week 8, the main organ
systems have developed
Gastrulation
• The process by which cells from the
epiblast migrate to form all three primary
germ layers
– starts during week 3 with formation of
the primitive streak
• Once all three germ layers are present,
the trilaminar structure can be called an
embryo
Primitive Streak
Figure 3.9
Primitive Streak
• A thin depression on the surface of the
epiblast
– the cephalic end of the streak is raised and
thickened forming the primitive node
– a depression in the node is the primitive pit
• Cells from the epiblast layer move through
the primitive streak to locate themselves
between the epiblast and hypoblast layers
Primitive Streak
Figure 3.8
Primary Germ Layers
• The cells between the epiblast and
hypoblast layers become the primary germ
layer known as mesoderm.
• Other migrating cells displace the
hypoblast cells and become endoderm.
• Cells remaining in the epiblast will become
ectoderm.
• All three germ layers are derived from the
epiblast.
Folding of the Embryonic Disc
• Early in week 3, the embryo is a flattened
disc-shaped structure.
• During late week 3, the embryo begins
growing faster than the space in which it
resides.
• In order to continue growing, the embryo
must begin a series of folds.
Folding of the Embryonic Disc
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Three folds of the embryo occur during
weeks 3−4:
– cephalocaudal (cephalic = head;
caudal = tail) fold
– transverse (lateral) fold
Folding of the Embryonic Disc
Figure 3.10
Ectoderm
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Ectoderm is located on the external
surface of the embryo
Ectoderm cells will eventually develop
into the following structures:
– epidermis of the skin
– derivatives of epidermis, including
hair and nails
– nervous system
Neurulation
• The formation of the neural tube from overlying
ectoderm cells is called neurulation.
– The neural tube will develop into the nervous
system.
• The formation of the neural tube begins with the
appearance of the notochord, which is derived
from mesoderm.
– The notochord is a rod-shaped structure
internal and parallel to the primitive streak.
Notochord
Insert Figure 3.11a
Figure 3.11
Neurulation
• The notochord induces the overlying ectoderm
to begin the formation of the neural tube.
– Thickening of the overlying ectoderm forms a neural
plate.
– The lateral edges of the neural plate form neural
folds.
– The depression between the folds is the neural
groove.
– The neural folds approach midline and fuse to form
the neural tube.
Figure 3.11
Mesoderm
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The middle germ layer forms five regions:
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notochord─tightly packed midline cells
paraxial─beside notochord, develops into units
called somites that form axial skeleton, muscle,
dermis of the skin, and most connective tissues
intermediate─lateral to paraxial, develops into
most of the urinary and reproductive systems
lateral plate─lateral to intermediate, forms most
components of cardiovascular system, lining of all
body cavities, and connective tissue of the limbs
head mesenchyme─forms the connective tissue
and musculature of the face
Figure 3.11
Endoderm
• Will develop into many internal structures
following the foldings of the embryo
– linings of the digestive, respiratory, and
urinary systems
Derivatives of the Germ Layers
Figure 3.12
Organogenesis
• The process constructing the organs of the body
– rudimentary forms of most organ systems are
complete by the end of the embryonic period
(week 8)
– during this period normal development of
organs can be interfered with by agents called
teratogens
• a teratogen is any agent that can cause
congenital malformations (birth defects)
3. Fetal Period
• Begins at week 9 and ends at birth
(usually week 38)
• Characterized by maturation and growth of
tissues and organs
Fetal Period