| |
Chapter 1 Introduction
|
| |
| The dynamic process by which
the single-cell human zygote
becomes a 100-trillion-cell
adult is perhaps
the most remarkable
phenomenon in all of nature.
|
| Researchers now know that many
of the routine functions
performed by the adult body
become established
during pregnancy -
often long before birth.
|
| The developmental period
before birth
is increasingly understood
as a time of preparation
during which
the developing human
acquires the many structures,
and practices
the many skills, needed
for survival after birth.
|
Chapter 2 Terminology
|
| |
| Pregnancy in humans normally
lasts approximately 38 weeks
as measured from the time
of fertilization,
or conception, until birth.
|
| During the first 8 weeks
following fertilization,
the developing human
is called an embryo,
which means "growing within."
This time, called
the embryonic period,
is characterized
by the formation
of most major body systems.
|
| From the completion of 8 weeks
until the end of pregnancy,
"the developing human
is called a fetus,"
which means "unborn offspring."
During this time,
called the fetal period,
the body grows larger and
its systems begin to function.
|
| All embryonic and fetal ages
in this program
refer to the time
since fertilization.
|
The Embryonic Period (The First 8 Weeks)
Embryonic Development: The First 4 Weeks
Chapter 3 Fertilization
|
| |
| Biologically speaking,
"human development
begins at fertilization,"
when a woman and a man
each combine 23
of their own chromosomes
through the union
of their reproductive cells.
|
| A woman's reproductive cell
is commonly called an "egg"
but the correct term is oocyte.
Likewise, a man's
reproductive cell
is widely known as a "sperm"
but the preferred term
is spermatozoon.
Following the release of
an oocyte from a woman's ovary
in a process called ovulation,
the oocyte
and spermatozoon join
within one
of the uterine tubes,
which are often
referred to as Fallopian tubes.
The uterine tubes
link a woman's ovaries
to her uterus or womb.
The resulting single-celled
embryo is called a zygote,
meaning "yoked or
joined together."
|
Chapter 4 DNA, Cell Division, and Early Pregnancy Factor (EPF)
|
| |
| The zygote's 46 chromosomes
represent the unique
first edition
of a new individual's
complete genetic blueprint.
This master plan resides
in tightly coiled
molecules called DNA.
They contain the instructions
for the development
of the entire body.
DNA molecules
resemble a twisted ladder
known as a double helix.
The rungs of the ladder are
made up of paired molecules,
or bases, called guanine,
cytosine, adenine, and thymine.
|
| Guanine pairs
only with cytosine,
and adenine with thymine.
Each human cell contains
approximately 3 billion
base pairs.
The DNA of a single cell
contains so much information
that if it were represented
in printed words,
simply listing the first letter
of each base
would require over 1.5 million
pages of text!
If laid end-to-end,
the DNA in a single human cell
measures 3 1/3 feet
or 1 meter.
|
| If we could uncoil
all of the DNA
within an adult's
100 trillion cells,
it would extend
over 63 billion miles.
This distance reaches from
the earth to the sun and back
340 times.
|
| Approximately 24 to 30
hours after fertilization,
the zygote completes
its first cell division.
Through the process
of mitosis,
one cell splits into two,
two into four, and so on.
|
| As early as 24 to 48 hours
after fertilization begins,
pregnancy can be confirmed
by detecting a hormone
called "early pregnancy factor"
in the mother's blood.
|
Chapter 5 Early Stages (Morula and Blastocyst) and Stem Cells
|
| |
| By 3 to 4 days
after fertilization,
the dividing cells of the embryo
assume a spherical shape
and the embryo is called
a morula.
By 4 to 5 days, a cavity forms
within this ball of cells
and the embryo is then called
a blastocyst.
The cells inside
the blastocyst
are called
the inner cell mass
and give rise to the head, body,
and other structures
vital to the developing human.
Cells within
the inner cell mass
are called embryonic stem cells
because they have the ability
to form each
of the more than 200 cell types
contained in the human body.
|
Chapter 6 1 to 1½ Weeks: Implantation and Human Chorionic Gonadotropin (hCG)
|
| |
| After traveling
down the uterine tube,
the early embryo
embeds itself
into the inner wall
of the mother's uterus.
This process, called
implantation, begins 6 days
and ends 10 to 12
days after fertilization.
Cells from the growing embryo
begin to produce a hormone
called human chorionic
gonadotropin, or hCG,
the substance detected
by most pregnancy tests.
HCG directs maternal hormones
to interrupt the normal
menstrual cycle,
allowing pregnancy to continue.
|
Chapter 7 The Placenta and Umbilical Cord
|
| |
| Following implantation,
cells on the periphery
of the blastocyst
give rise to part of
a structure called the placenta,
which serves as an interface
between the maternal
and embryonic
circulatory systems.
The placenta delivers
maternal oxygen, nutrients,
hormones, and medications
to the developing human;
removes all waste products;
and prevents maternal blood
from mixing with the blood
of the embryo and fetus.
The placenta also
produces hormones
and maintains embryonic
and fetal body temperature
slightly above that
of the mother's.
The placenta communicates
with the developing human
through the vessels
of the umbilical cord.
The life support capabilities
of the placenta rival those
of intensive care units
found in modern hospitals.
|
Chapter 8 Nutrition and Protection
|
| |
| By 1 week,
|
| cells of the inner cell mass
form two layers called
the hypoblast
and epiblast.
The hypoblast gives rise
to the yolk sac,
which is one of the structures
through which
the mother supplies nutrients
to the early embryo.
Cells from the epiblast form
a membrane called the amnion,
within which the embryo
and later the fetus
develop until birth.
|
Chapter 9 2 to 4 Weeks: Germ Layers and Organ Formation
|
| |
| By approximately 2 1/2 weeks,
the epiblast has formed
3 specialized tissues,
or germ layers,
called ectoderm,
endoderm,
and mesoderm.
Ectoderm gives rise
to numerous structures
including the brain,
spinal cord,
nerves,
skin,
nails,
and hair.
Endoderm produces the lining
of the respiratory system
and digestive tract,
and generates
portions of major organs
such as the liver
and pancreas.
Mesoderm forms the heart,
kidneys,
bones,
cartilage,
muscles,
blood cells,
and other structures.
|
| By 3 weeks
the brain is dividing
into 3 primary sections
called the forebrain,
midbrain,
and hindbrain.
Development of the respiratory
and digestive systems
is also underway.
|
| As the first blood cells
appear in the yolk sac,
blood vessels form
throughout the embryo,
and the tubular heart emerges.
Almost immediately,
the rapidly growing heart
folds in upon itself
as separate chambers
begin to develop.
The heart begins beating
3 weeks and 1 day
following fertilization.
The circulatory system
is the first body system,
or group of related organs,
to achieve a functional state.
|
Chapter 10 3 to 4 Weeks: The Folding of the Embryo
|
| |
| Between 3 and 4 weeks,
the body plan emerges
as the brain,
spinal cord,
and heart of the embryo
are easily identified
alongside the yolk sac.
Rapid growth causes folding
of the relatively flat embryo.
This process incorporates
part of the yolk sac
into the lining
of the digestive system
and forms the chest
and abdominal cavities
|
Embryonic Development: 4 to 6 Weeks
Chapter 11 4 Weeks: Amniotic Fluid
|
| |
| By 4 weeks
the clear amnion
surrounds the embryo
in a fluid-filled sac.
This sterile liquid,
called amniotic fluid,
provides the embryo
with protection from injury.
|
Chapter 12 The Heart in Action
|
| |
| The heart typically beats
about 113 times per minute.
Note how
the heart changes color
as blood enters and leaves
its chambers with each beat.
The heart will beat
approximately
54 million times before birth
and over 3.2 billion times
over the course
of an 80-year lifespan.
|
Chapter 13 Brain Growth
|
| |
| Rapid brain growth is evidenced
by the changing appearance
of the forebrain,
midbrain,
and hindbrain.
|
Chapter 14 Limb Buds
|
| |
| Upper and lower limb
development begins
with the appearance
of the limb buds by 4 weeks.
|
| The skin is transparent
at this point
because it is only
one cell thick.
As the skin thickens,
it will lose this transparency,
which means that we will only
be able to watch internal organs
develop for about another month.
|
