Download Embryonic and Fetal Development

Embryonic and Fetal Development
The transformation from a single cell to an amazingly beautiful baby (to their mom and dad, at least) in 40 weeks
is one the most amazing processes in biology. It’s cool for any organism, but we’re human so of course we must be
anthropocentric. This time period consists of incredible growth, proliferation, and differentiation of cells that result in
tissues and organs of intricate complexity.
The First Trimester
At implantation the embryo consists of the trophoblast and an inner mass of cells. The inner cell mass gradually gives rise
to the organs and tissues of the embryo. The cells of this inner mass are also called stem cells.
Gastrulation Approximately three weeks after fertilization, a process called gastrulation takes place. Gastrulation forms
three cell layers:
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the ectoderm (outer layer) forms the outer part of the embryo's skin and the central nervous system
the endoderm (inner layer) forms the digestive tract and lungs
the mesoderm (middle layer) forms most of the other organs
Membranes During the first few weeks, membranes form that protect and nourish the embryo
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the amnion, forms a fluid-filled sac that protects the embryo from physical impact
the yolk sac membrane produces the first blood cells and is the source of cells that eventually form gametes
the chorion is a third membrane that becomes the embryo's portion of the placenta
the allantois forms part of the umbilical cord that connects the embryo to the placenta.
Placenta Soon after implantation, trophoblast cells and cells from the uterus form an important structure called the
placenta. The placenta develops inside the uterus and surrounds the embryo. This structure enables nutrients and waste
products to be transferred between the mother and developing baby (Figure 33-14). By the end of the third month, the
placenta is fully formed and functional. In the wall of the placenta, the mother's blood and baby's blood remain isolated in
separate circulatory systems. However, the mother's blood vessels release pools of blood very close to the baby's blood
vessels. Nutrients and waste products are able to diffuse back and forth through interstitial fluid from one blood system to
the other.
From week 9 until birth, the developing human is called a fetus. At nine weeks, the fetus is approximately 3 cm long,
about the length of a paper clip. It has all of its organs and major body parts, including a head that is large in proportion to
its body. By the end of the first trimester, the fetus can move its arms and legs, turn its head, frown, and make sucking
motions.
The Second Trimester
During weeks 12 through 23, the period of the second trimester, the fetus increases in length to about 25 cm. By the end
of this period, the fetus weighs about 0.5 kg and has the face of an infant, complete with eyebrows and eyelashes. At this
point, the fetal heartbeat is easily detected and the fetus can be quite active. The mother can usually feel this activity,
which ranges from flutters to kicks and pokes. Because the fetus is now filling up much of the space in the uterus, it curls
forward into what is referred to as the "fetal position."
During this period there is also an important change in hormone secretion by the placenta. Early in the second trimester,
the placenta starts secreting progesterone, rather than receiving it from the corpus luteum. No longer needed to secrete
progesterone, the corpus luteum disintegrates.
The Third Trimester
From week 24 until birth, the period of the third trimester, the fetus experiences rapid growth, becoming larger and gaining
stronger bones and muscles. The respiratory and circulatory systems undergo changes that will enable the baby to start
breathing air and perform other vital functions outside the mother's uterus when it is born. As it fills more and more of the
available space in the uterus, the fetus becomes less active.
Childbirth
At birth, the average baby is 40-50 cm long and weighs 2.7-4.5 kg (7-12 lbs), or about the size of a small watermelon. The
baby is born as a result of a series of strong, rhythmic contractions of the muscles of the mother's uterus—a condition
called labor. Once again, hormones of the endocrine system regulate the process. During the final weeks of pregnancy,
estrogen levels peak. This causes cells to form in muscles of the uterus that serve as receptors for a hormone called
oxytocin. As it reaches the final stages of development, the fetus and the mother's pituitary gland start to secrete oxytocin.
This secretion causes the muscles of the uterus to begin contracting. These muscle contractions stimulate the secretion of
more oxytocin, which causes more contractions, in an increasingly intense cycle that results in the baby being forced out
of the uterus (Figure 33-18).
Within moments of entering its new world, the baby gasps its first breaths of air. Television and movies have not painted a
very accurate picture of what a newborn really looks like. Immediately after birth, most infants are bluish in color and are
covered with blood and mucus. Often the facial features are distorted from having been forced through the narrow
opening of the cervix and vagina. Usually, these distortions disappear within a few hours, and of course each baby looks
beautiful to its parents.
Shortly after birth, additional contractions force the placenta out of the uterus. This is sometimes called the afterbirth. The
umbilical cord is tied off and cut. The remnant of the cord will eventually dry up and fall off on its own, leaving a familiar
scar known as the navel, or belly button.
A morula is an embryo at an early stage of embryonic development, consisting of cells (called blastomeres) in
a solid ball contained within the zona pellucida (a glycoprotein membrane surrounding the plasma
membrane of an oocyte)
In humans, blastocyst formation begins about 5 days after fertilization, The blastocyst is a sphere of about
150 cells, with an outer layer (the trophoblast), a fluid-filled cavity (the blastocoel), and a cluster of cells on the
interior (the inner cell mass).
Gastrulation
Gastrulation - The point in embryogenesis where the basic organization of the organism is established.
Establishment of the three basic germ layers
Ectoderm, Mesoderm, Endoderm
Groups of cells that will give rise to specific organ systems and tissues are moved into the right position both externally
and internally. Groups of cells that will influence each other’s differentiation are positioned so they can have an effect on
each other
.
The three germs layers are the endoderm, the ectoderm, and the mesoderm.
 the ectoderm gives rise to the nervous system and the epidermis;
 the mesoderm gives rise to the muscle cells and connective tissue in the body
 the endoderm gives rise to columnar cells found in the digestive system and many internal organs.
http://www.ivfcliniclondon.com/en/treatments/blastocyst-transfer/what-is-a-blastocyst.html
http://cwx.prenhall.com/bookbind/pubbooks/martini10/chapter28/custom3/deluxe-content.html
"It is not birth, marriage, or death, but gastrulation, which is truly the most
important time in your life."
Lewis Wolpert (1986)