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:
· 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
· 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.
Amorulais anembryoat an early stage ofembryonic development, consisting ofcells(calledblastomeres) in a solid ball contained within thezona pellucida (aglycoprotein membrane surrounding theplasma membraneof anoocyte)
In humans, blastocyst formation begins about 5 days afterfertilization, The blastocyst is a sphere of about 150 cells, with an outer layer (thetrophoblast), a fluid-filled cavity (theblastocoel), and a cluster of cells on the interior (theinner cell mass).
Gastrulation
Differentiation of germ layers
The three germ layers give rise to different cell types in the animal body: the ectoderm forms the nervous system and the outer layer of skin, the mesoderm gives rise to muscles and connective tissues, and the endoderm gives rise to the lining of the digestive system and other internal organs.
The three germs layers are the endoderm, the ectoderm, and themesoderm. The ectoderm gives rise to thenervous systemand the epidermis; the mesoderm gives rise to the muscle cells and connective tissue in the body; and the endoderm gives rise to columnar cells found in the digestive system and many internal organs.
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
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, butgastrulation, which is truly the most important time in your life."
Lewis Wolpert (1986)