Human life cycle
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A full term pregnancy, from conception to birth, usually lasts for around nine months and is more commonly known as the gestation period. This gestation period can be split into three phases, known as the three trimesters. The first trimester is concerned with the development of the embryo, the second trimester is where the embryo is known as a foetus and foetal development continues and the third trimester is where the baby finishes developing and gains the bulk of what will be the birth weight. In this essay we are going to look at the developmental stages the fertilised egg goes through to become a foetus, known as embryonic development.
BBC GCSE BITESIZE (2010)
Each individual pregnancy needs to begin with a process called fertilisation; this is the term we use to best describe the exact moment the male (sperm) and female (egg) gametes fuse together. Each gamete contains 23 chromosomes which is exactly half the full number of chromosomes found in humans. Fertilisation takes place in the fallopian tube and is carried out by joining the male and female gametes together to create a new cell known as a zygote.
As mentioned earlier, the first trimester is concerned mainly with the development of the embryo, this happens in three distinct and separate phases known as cleavage, gastrulation and organogeny. Cleavage is the process during which the zygote undergoes mitotic division to form a solid ball of cells, gastrulation happens when these cells continue to divide and then rearrange themselves to form special layers in the embryo, called germ layers and organogeny is when the germ layers undergo further division and cell specialisation to form the organs in the embryo.
During cleavage, the first phase, the zygote immediately begins to divide through mitosis to develop into an embryo. The zygote passes along the fallopian tube and within 24-36 hours it has divided into two cells, called blastomeres. Twelve hours later it divides again, this time into four cells, and so on until it consists of 16 cells. This solid ball of cells is known as the 16 cell morula or pre-embryo.
About four days after conception, the morula leaves the fallopian tube and enters the uterus and at about 6 days, the morula forms a hollow cavity known as a blastocele. With the appearance of this blastocele, the morula becomes known as a blastocyte. The blastocyte consists of: an inner cell mass, called the embryoblast, from which the embryo develops; and outer cells, called the trophoblast, which is "concerned with the future nourishment of the embryo" (Baker et al, 2007) and forms the placenta.
The blastocyte remains free in the uterus for about 48 hours before depositing itself into the endometrium (lining of the uterus). Once the blastocyte has embedded itself into the endometrium, the inner cell mass will begin to develop into the embryo itself and the outer cells will begin to develop the placenta.
The placenta is an organ formed partly from the endometrium and partly from membranes, which surround the developing foetus. It is connected to the foetus by the umbilical cord which contains two umbilical arteries, that carry blood from the foetus to the placenta, and one umbilical vein, that carries blood from the placenta to the foetus.
In the placenta, the blood from the mother and the blood from the foetus come very close together, but do not mix. The barrier separating the foetus's blood from the mother's blood is very thin and is known as the chorion membrane. The placenta has numerous finger-like chorionic villi which are found in blood spaces and contain blood capillaries, which are connected to the foetus by the arteries and vein in the umbilical cord.
As the foetus's blood flows through the placenta it picks up oxygen and dissolved food substances from the mother's blood, via the chorionic villi. At the same time it sheds carbon dioxide and excretory waste into the mother's blood. The allantois is the structure that is primarily responsible for this process of nutrition and excretion.
After implantation has occurred, the inner cell mass becomes subdivided into two tissue layers, the endoderm and the ectoderm. The embryo develops inside a fluid filled sac called the amniotic cavity. The fluid found inside the amniotic cavity is known as amniotic fluid and this fluid offers protection to the foetus from getting bumped or damaged by cushioning it. The cavity is enclosed by a thin membrane called the amnion, which folds around to cover the embryo.
After about 14 days the embryo becomes an elongated, oval shape resembling a disc like structure and is more commonly known as the Embryonic disc. A few of the ectoderm cells migrate to the middle of the disc, forming a thickened line called the "primitive streak" and this primitive streak will later develop into the foetus's central nervous system (Visembryo, 2009). Some of the ectoderm cells carry on through the primitive streak and materialize through the ectoderm and endoderm as a new germ layer, called mesoderm. The embryo now has three layers, the ectoderm, endoderm and mesoderm and this process is known as gastrulation. The embryo is now ready to enter the third and final embryonic phase called organogeny.
From around the third week, up until the end of the trimester, the embryo will take on many different shapes and sizes while the organs begin to develop. One of the very first things that will happen is that the heart will start to beat.
The embryo begins to take on the shape of a tadpole at around four weeks. Its head will start to develop and it has gill like structures in the area that will later develop into a throat. It also has a very noticeable tail. (Tribhuwan, 2009)
Around five weeks, tiny buds begin to emerge which later form into the arms and legs. It has hands with webbed fingers that have formed at the end of the arm buds. The face "has a distinctly reptilian aspect" and "...the embryo still has a tail and cannot be distinguished from pig, rabbit, elephant, or chick embryo." (Tribhuwan, 2009)
At six weeks the face has two eyes, one on each side of its head, and tiny openings have appeared which will eventually become the mouth and nose. (Tribhuwan, 2009)
The tail eventually disappears at around seven weeks. The sense of touch and pain sensors will start to develop. The embryo is not able to feel pain yet though, as the higher functions of the brain, i.e. the pathways that transfer pain signals from the pain sensors to the brain, have not yet developed. (Tribhuwan, 2009)
By the two month mark, the embryo's face begins to resemble that of a primate's, but is not fully human in appearance. The brain will start to form and will function throughout the human's life. The embryo will start to respond to prodding, although it has no consciousness at this stage of development. The higher functions of the brain will only start to develop much later on in pregnancy. (Tribhuwan, 2009)
During the second trimester, the embryo is transformed into a foetus. Its face looks human, its gender may be detectable via ultrasound and the foetus will begin to develop fingernails and bones. The female may also start to feel the foetus' movements and the heartbeat can usually also be detected. Hair and lungs also develop during this stage and interlinking of the brain's neurons begins. The higher functions of the foetal brain turn on for the first time and some rudimentary brain waves can be detected. The foetus has become conscious of its surroundings and will also be able to feel pain for the very first time. It is at this point in the development of the foetus that the foetus is regarded as a living human life and not just a developing gamete. (Tribhuwan, 2009)
During the third trimester regular brain waves, similar to those in adults, are detectable. During the last few months the foetus will gain its birth weight and will move into position, ready and waiting enter the world. (Tribhuwan, 2009)
This is the process every human being will go through to physically become who they are today.
- (Tribhuwan, 2009)
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