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HUMAN
DEVELOPMENT
Dr. HARI RAM A/L RAMAYYA
OBSTETRICIAN & GYNAECOLOGIST,
HOSPITAL TUANKU FAUZIAH,
KANGAR, PERLIS.
Outline
FERTILIZATION
EMBRYOGENESIS
ORGANOGENESIS
Embryology
The study of the developmental events
that occur during the prenatal period
 Begins with a single fertilized cell that
divides to produce all of the cells in the
body.

The Prenatal Period
The first 38 weeks of human development,
which occurs between fertilization and birth.
 The pre-embryonic period is the first 2 weeks
of development when the zygote becomes a
spherical, multicellular structure.
 The embryonic period includes the third
through eighth weeks of development during
which all major organ systems appear.

The Fetal Period
Includes the remaining weeks of
development prior to birth
 The fetus continues to grow
 Its organs increase in complexity

The Stages of Embryogenesis
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Cleavage The zygote divides by mitosis to
form a multicellular structure called a
blastocyst.
Gastrulation The blastocyst cells form three
primary germ layers, which are the basic
cellular structures from which all body tissues
develop.
Organogenesis  The three primary germ
layers arrange themselves in ways that give
rise to all the organs within the body.
Cell Division by Mitosis
Mitosis is a type of cell division that produces
genetically identical cells.
•During mitosis DNA replicates in the parent
cell, which divides into two new cells, each
containing an exact copy of the DNA in the
parent cell.
•The only source of genetic variation in the
cells is via mutations.
Gametogenesis
Following birth, an individual undergoes
maturation.
  the body grows and develops
  the sex organs become mature
  the sex organs then begin to produce
gametes

Chromosomes
Human somatic cells contain 23 pairs of
chromosomes for a total of 46.
 22 pairs of autosomes and one pair of sex
chromosomes.
 Autosomes contain genetic information for most
human characteristics.
 A pair of similar autosomes are called
homologous chromosomes.

DNA and Chromosomes
•The DNA molecule in a single human cell is
99 cm long, so is 10 000 times longer than the
cell in which it resides (< 100mm).
•Since an adult human has about 1014 cells, all
the DNA is one human would stretch about 1014
m, which is a thousand times the distance
between the Earth and the Sun.
• Different species have different
number of chromosomes, but all
members of the same species have
the same number.
• Humans have 46, chickens have 78,
goldfish have 94, fruit flies have 8,
potatoes have 48, and so on.
• The number of chromosomes does
not appear to be related to the
number of genes or amount of DNA.


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The chromosomes are numbered from largest to
smallest.
Chromosomes come in pairs, called
homologous pairs ("same shaped"). So there are
two chromosome number 1s, two chromosome
number 2s, etc, and humans really have 23
pairs of chromosomes.
Homologous chromosomes are a result of
sexual reproduction, and the homologous pairs
are the maternal (inherited from the mother) and
paternal (inherited from the father) versions of
the same chromosome, so they have the same
sequence of genes
One pair of chromosomes is different in males
and females. These are called the sex
chromosomes, and are non-homologous in one
of the sexes. In humans the sex chromosomes
are homologous in females (XX) and nonhomologous in males (XY). (In birds it is the
other way round!) The non-sex chromosomes
are sometimes called autosomes, so humans
have 22 pairs of autosomes, and 1 pair of sex
chromosomes.
Diploid Cells

A cell is said to be diploid if it contains 23
pairs of chromosomes.
2N = 46
The Sex Chromosomes
The pair of sex chromosomes determines
whether an individual is female (XX) or
male (XY).
 One member of each pair of chromosomes
is inherited from each parent.

Gametogenesis
Begins with meiosis.
 Produces secondary oocytes in the female.
 Produces sperm in the male.

Meiosis

A type of cell division that starts off with a
diploid parent cell and produces haploid
daughter cells (sperm or eggs/ova).
Meiosis 1
Meiosis results in the formation of
gametes (sex cells).
 In meiosis I, homologous chromosomes
are separated after synapsis and crossing
over occurs.
 In meiosis II, sister chromatids are
separated in a sequence of phases that
resembles mitosis.

Interphase
•This is when the cell is not dividing, but is
carrying out its normal cellular functions.
•chromatin not visible
•DNA, histones and centrioles all replicated
•Replication of cell organelles e.g.
mitochondria, occurs in the cytoplasm.
Prophase 1
Homologous, double-stranded chromosomes
in the parent cell form pairs (synapsis).
 The actual pair of homologous chromosomes
is called a tetrad.
 Crossing over occurs between the maternal
and paternal chromosomes.

Prophase
•chromosomes condense and become
visible – this prevents tangling with other
chromosomes.
•Due to DNA replication during interphase,
each chromosome consists of two identical
sister chromatids connected at the
centromere
•centrioles move to opposite poles of cell
•nucleolus disappears
•phase ends with the breakdown of the
nuclear membrane
Metaphase 1
The homologous pairs of chromosomes
line up above and along the equator of the
cell.
 Forms a double line of chromosomes.
 Alignment is random with respect to
maternal or paternal origin.

Metaphase
•spindle fibres (microtubules) connect
centrioles to chromosomes
•chromosomes align along equator of cell
and attaches to a spindle fibre by its
centromere.
Anaphase 1

Pairs of homologous chromosomes
separate and are pulled to the opposite
ends of the cell.
Anaphase
•centromeres split, allowing chromatids to
separate
•chromatids move towards poles,
centromeres first, pulled by kinesin (motor)
proteins walking along microtubules (the
track)
•Numerous mitochondria around the
spindle provide energy for movement
Telophase 1 & Cytokinesis
Nuclear division finishes and the nuclear
envelopes re-form
 The cytoplasm divides
 Two new haploid cells are produced

Telophase
•spindle fibres disperse
•nuclear membranes from around each set
of chromatids
•nucleoli form
•In animal cells a ring of actin filaments
forms round the equator of the cell, and
then tightens to form a cleavage furrow,
which splits the cell in two.
Cytokinesis
•In plant cells vesicles move to the
equator, line up and fuse to form two
membranes called the cell plate. A new
cell wall is laid down between the
membranes, which fuses with the existing
cell wall.
Prophase II
Resembles the prophase stage of mitosis.
 In each of the two new cells, the nuclear
membrane breaks down, and the
chromosomes collect together.
 Crossing over does not occur in this
phase.

Metaphase II
The double-stranded chromosomes form a
single line in the middle of the cell.
 Spindle fibers extend from the centrioles at
the poles to the centromere of each
double-stranded chromosome.

Anaphase II
The sister chromatids of each double
stranded chromosome are pulled apart at
the centromere.
 Each chromatid (single strand) is pulled to
the opposite pole of the cell.

Telophase II & Cytokinesis
The single-stranded chromosomes arrive
at opposite ends of the cell.
 A cleavage furrow forms and the
cytoplasm in both cells divides, producing
a total of four haploid daughter cells.
 These daughter cells mature into sperm
in males or oocytes in females.

Oogenesis
In females, the sex cell produced is called
the secondary oocyte.
 This cell will have 22 autosomes and one
X chromosome.

Oogenesis
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The parent cells that produce oocytes are called
oogonia and they reside in the ovaries.
Oogonia are diploid cells.
All the oogonia start the process of meiosis and
form primary oocytes prior to birth.
They are arrested in Prophase I and remain this
way until the female reaches puberty.
Each month usually only one becomes a
secondary oocyte.
Oogenesis
Only the secondary oocyte has the
potential to be fertilized.
 The secondary oocyte is ovulated
 The corona radiata and the zona pellucida
form protective layers around the
secondary oocyte.

Oogenesis
If the secondary oocyte is not fertilized, it
degenerates about 24 hours after
ovulation, still arrested in metaphase II.
 If the secondary oocyte is fertilized, it first
finishes the process of meiosis. Two new
cells are produced, and as before, the
division of the cytoplasm is unequal.

The cell that receives very little cytoplasm
becomes another polar body and
eventually degenerates.
 The cell that receives the majority of the
cytoplasm becomes an ovum which can
be fertilized.

Oogenesis
Typically, only one secondary oocyte is
expelled (ovulated) from one of the two
ovaries each month.
 The left and right ovaries alternate
ovulation each month.

Spermatogenesis
The parent or stem cells that produce
sperm are called spermatogonia.
 Spermatogonia are diploid cells that reside
in the the testes.
 Each one first divides by mitosis to make
an exact copy of itself called a primary
spermatocyte.

Spermatogenesis
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Primary spermatocytes then undergo meiosis
and produce haploid cells called spermatids.
Spermatids contain 23 chromosomes, but they
still must undergo further changes to form a
sperm cell.
In spermiogenesis, spermatids lose much of
their cytoplasm and grow a long tail called a
flagellum.
Spermatogenesis
The newly formed sperm cells are haploid
cells that exhibit a distinctive head, a
midpiece, and a tail.
 From a single spermatocyte, four new
sperm are formed.
 All sperm have 22 autosomes and either
an X chromosome, or a Y chromosome.

Fertilization
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Two sex cells fuse to form a new cell containing
genetic material derived from both parents.
Restores the diploid number of chromosomes.
Determines the sex of the organism.
Initiates cleavage.
Occurs in the widest part of the uterine tube (the
ampulla).
Fertilization
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Millions of sperm cells are deposited in the
female reproductive tract during intercourse.
Only a few hundred have a chance at
fertilization.
Only the first sperm to enter the secondary
oocyte is able to fertilize it.
The remaining sperm are prevented from
penetrating the oocyte.
Cleavage
Shortly after fertilization, the zygote begins
to undergo a series of divisions.
 Divisions increase the number of cells in
the pre-embryo, but the pre-embryo
remains the same size.
 During each succeeding division, the cells
are smaller and smaller.

Cleavage

Before the 8-cell stage, cells are not tightly
bound together, but after the third
cleavage division, the cells become tightly
compacted into a ball called a morula.
Implantation
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By the end of the first week after fertilization, the
blastocyst enters the lumen of the uterus.
The zona pellucida around the blastocyst begins
to break down as the blastocyst prepares to
invade the endometrium.
Implantation is the process by which the
blastocyst burrows into and embeds within the
endometrium.
Amnion
Eventually encloses the entire embryo in a
fluid-filled sac called the amniotic cavity to
prevent desiccation.
 The amniotic membrane is specialized to
secrete the amniotic fluid that bathes the
embryo

Chorion
The outermost extraembryonic membrane,
is formed from rapidly growing cells.
 These cells blend with the functional layer
of the endometrium and eventually form
the placenta.

The Placenta
Functions in exchange of nutrients, waste
products, and respiratory gases between
the maternal and fetal bloodstreams.
 Transmission of maternal antibodies to the
developing embryo or fetus.
 Production of hormones to maintain and
build the uterine lining.

Gastrulation
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Occurs during the third week of development
immediately after implantation.
One of the most critical periods in the
development of the embryo.
Cells of the epiblast migrate and form the three
primary germ layers which are the cells from
which all body tissues develop.
The three primary germ layers are called
ectoderm, mesoderm, and endoderm.
Organogenesis
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Once the three primary germ layers have
formed, and the embryo has undergone folding,
organogenesis begins.
The upper and lower limbs attain their adult
shapes, and the rudimentary forms of most
organ systems have developed by week 8.
By the end of the embryonic period, the embryo
is slightly longer than 2.5 centimeters (1 inch),
and yet it already has the outward appearance
of a human.
WEEK 7
The embryo begins to move spontaneously.
The jaw forms, including teeth buds in the
gums. Soon the eyelids will seal to protect
the embryo's developing light sensitive eyes,
and will reopen at about the seventh month.
At week 8, the embryo is a little more than
an inch long and is now called a fetus, Latin
for "young one" or "offspring."
Everything is now present that will be
found in a fully developed adult.
A thin membrane separates the embryo's
blood in the villi from the mother's blood that
flows through the space surrounding the villi
(intervillous space). This arrangement allows
materials to be exchanged between the blood
of the mother and that of the embryo.
The embryo floats in fluid (amniotic fluid),
which is contained in a sac (amniotic sac). The
amniotic fluid provides a space in which the
embryo can grow freely. The fluid also helps
protect the embryo from injury. The amniotic
sac is strong and resilient.
MONTH 9
Toward the end of this month, the baby
is ready for birth. The average
duration of pregnancy is 280 days from
the first day of the mother's last
menstrual period. By this time the
infant normally weighs between 6 and
9 pounds and his heart is pumping
about 250 gallons of blood a day. He is
fully capable of life outside the womb.
Tracking Fetal Development
Fetal
Fetal Organs
Development
First Month
(Embryo)
Vital organs are forming and the brain and beginning of the spine are
evident.
Fifth Week
Heart begins to beat and circulate blood; arm and led buds emerge;
brain, spinal cord, and nervous system are established.
Sixth Week
Digestive system is forming and arms and legs begin to grow.
Seventh Week
The umbilical cord joins the embryo to the placenta; long bones and
internal organs are developing.
Second
Month
(Fetus)
Human face, arms, legs, fingers, toes, elbows, knees, eyelids and bone
cells are forming.
Sex is distinguishable; fingers and toes are moving; teeth buds are
Twelfth Week present and the kidney and bladder form. Baby is 2-4" long and weighs
an once or two.
Sixteenth
Baby moves and kicks, sleeps and wakes, swallows; hair forms,
20 Weeks
Spurt in baby's growth; internal organs are maturing; hair, eyebrows and lashes
are present; baby increases storage of iron. Baby is 8-12" long and weighs 1/2
pound.
24 Weeks
Baby's skin is wrinkled; covered by lanugo and vernix; and baby has an audible
heartbeat.
28 Weeks
Most rapid growth; red and wrinkled; eyelids can open and close; baby storing
large amounts of calcium and iron; fetus has a chance of surviving if born.
Baby is 15" long and weighs 3 pounds.
32 Weeks
Weight gain and rapid growth; settles in favorite position; valuable fat increases.
36 Weeks
Baby gains 1/2 pound per week; bones of head are soft and flexible; baby has
developed immunities. Baby is 18" long and weighs 6 pounds.
Birth
38-42 Weeks
Organs developed; respiratory system is mature.