The explanation of the theory of development is divided into ten categories:
- Identifying the nature of growth, reproduction and developmental biology: The science of development is the study of any living organism, and how it is produced, develops, and then reproduces. The modern understanding of the science of development identifies all of the activities, which arise under the control of genes, including the development and differentiation of the cell, the formation of tissues and organs, and the processes of reproduction and maturing.
- The sperm and the ovary: Here we are going to discuss the formation and the development of the human body. However, to understand this concept we should generate some questions in our minds as to how the fertilization of one small egg by a sperm results in the very complex structure that is the human body. What are the methods that guide and regulate this process? What are the factors that help to form the organ system of the body?
The DNA in the cell nuclei is the chief controller that changes the nature and function of any cell; and it is like a library storing all of the aforementioned information. Thus, to access and use the stored information, the cell requires proteins. The proteins undertake all of the functions of cells. In that case, which protein undertakes what function at the external and internal of the cells?
The other questions that naturally arise relate to how and where the sperm and eggs are produced in the male and female body. Figure 18 shows the production of an egg in the female gonads and sperm in male testes. Firstly, the eggs develop beyond the primary oocyte stage in the womb when females reach 12 years of age. Figure 19 shows that under the control of genes, the pituitary gland (part of the amygdala of the brain) releases two hormones directly into the bloodstream to the gonads.
These chemical signals are catalysts for the reaction leading to the maturation of the egg cell in the gonad. At birth, females have about 400,000 egg cells that are ready to mature in response to the appropriate hormone signaling. Once females have matured, they will experience monthly menstrual cycles consisting of the maturation of the egg cell and its degradation.
Similarly, the male human body undergoes formation of sperm in the testes. The male reproductive system includes the testes, urethra, vas deferens, prostate gland, seminal vesicle, and penis. About 200 million sperm cells are generated by the testes each day.
As aforementioned, genes control the pituitary gland, which is part of amygdala of the brain and releases hormones directly into the bloodstream. These hormones are the catalysts that stimulate the generation of sperm. The hormones also help to mature immature sperm, and carry them into the epididymis for nutrition and storage.
3. The formation of the embryo in the womb
Spermatogonia reach the female reproductive organ (མོ་དབང་)and fuse with the egg
within 12 to 44 hours. Each ejaculation releases about 300 million spermatogonia, which survive in the female reproductive system for 4-5 days. There are only several hundred of these that actually reach the female egg, and the [acidic] environment of the embryo kills most of them. Only one among the several hundred sperms that reach the egg can enter inside of female egg. To enter the the sperm must perforate the thin wall of the female egg. The acrosome (top part of sperm) releases acid to perforate the wall, which also immediately renders the wall of female egg impermeable to other sperm.
The fertilized egg cell that results from the union of a female egg with a male sperm is called a zygote. After forming a zygote, the single cell divides into two smaller cells each containing the same genetic information. At this stage, the zygote is called a morula and travels through the ventricle. The morula then continues to divide into multiple cells until it becomes a trophoblast and surrounds the embryoblast developing internally on one side.
Liquid creates a space between the two different cells, and the three parts compose a blastocyst. The complete organ system of the fetus will develop from the inner cells to the outer cells, becoming a thin layer covering the fetus in the womb. The inner cells, now known as embryonic stem cells which can differentiate into any cell type, are then ready and may travel to the surface of the organs or tissues.
4. Cell Division
Special activity is necessary for the development of the embryo cells into various states; this process is called cellular differentiation (the process by which a cell changes from one cell type to another.) In the process of cellular differentiation, the initial cell type specializes into another cell type, and each cell has a specific function. For instance, the embryo cells will gradually differentiate into blood cells, organ cells, skin cells and so on. The collection of one cell type will form a specific tissue and different tissues form a specific organ. This process allows the fetus’ organ structure to adapt quickly and efficiently, therefore if there is a problem with cellular differentiation then there is the potential for disability.
What controls the cells gradual growth? How does one type of cell develop into another type of cell? How do different cells form different tissues, and how do these
form different organs? When the cells are in contact with one another, the genes in the cells activate and then produce different proteins. These different proteins are conditioned for developing different cells. The growth factor proteins (of the cell) do signal transduction, signaling to the surrounding cells, and transcription factors proteins (of the cell) transcribe specific genes from specific proteins. Proteins interfere with the intracellular and extracellular functions of the cell while completing their own functions.
There are many levels of embryo development and in the beginning of gastrulation the outer surface cells move around to adjust the structure and order of the embryo. During this structuring and ordering known as embryogenesis, three important layers form: endoderm, mesoderm, and ectoderm. Of these three layers, the endoderm forms the innermost layer from which the internal organs develop such as the five nonvisceral organs of the human body (heart, lungs, liver, spleen and kidney) and the six visceral organs (stomach, colon, intestine, bile, urinary bladder, and gonads). The ectoderm generates the outer layer of the embryo and produces tissues of the epidermis, aids in the formation of neurons within the brain, and
constructs melanocytes. The mesoderm germ layer forms in the embryos of triploblastic animals and is a substantial component for the development of the muscle system, skeletal system, and circulatory system.
- Cell signaling in multicellular organisms
The conception of a baby in the mother’s womb begins with a single cell, which can only differentiate with the help of signaling. The differentiation process also occurs due to interactions between the cells. If the cell relies on cell signaling for differentiation, what steps lead to cell signaling? The signal comes from the environment the cell exists in or may come from other surrounding cells. The reaction that leads to signal transduction allows the signal to be carried through the cell membrane (cell wall; only found in plant cells) and then amplified.
Some chemical signals, whether they have a receptor or not, enter inside the cell by breaking through the cell membrane. The majority of signals signal receptors on the outer surface of cells. Signal receptor proteins’ most important function is for intracellular and extracellular communication. Even if there is a disturbance of signal transmitters extracellularly, the receptor proteins only bind their specific chemical signal carriers and not others.
Once these signals enter the cell, they continue to travel from protein to protein. The ultimate destination of the signal depends on the signal’s entity. Some signals enter the nucleus from the inside of the cell, some exit the nucleus, and some travel to specific parts of the cell membrane. When the signal travels from one protein to another, the previous protein changes the following proteins and so forth; this is called the signaling pathway (cascade).
There are different types of signaling pathways, some of which spread into many different directions signaling many places in the inner cell. The signal can increase as it travels from one protein to another, or it can decrease during cell division; either way, the signal generates a huge reaction. After the signals reach their ultimate destination, they change the behavior of the cell. However, the behavior changes all depend on which proteins interfere and participate in the process of signal transduction.
6. Reproduction: a study model
Biologists are studying the development of human beings on numerous models. The animals that are simple and most accessible to study are mice, bees, frogs, chickens, water insects and so on. During the development in the womb of these animals, biologists carefully examine the functions of proteins and genes.
Furthermore, biologists observe the influence of these functions on the organ system through gene changes, regulation, and movement. Depending on these models, the biologists explain the protein and gene activities during human development.
7. Changes in the womb at each week
The process of the embryo formation in the mother’s womb and the developmental stages are referred to as embryogenesis. After five weeks, the embryo begins to form a brain, spinal cord, and heart. Other organ systems related to the stomach and small intestine also begin to develop. During the sixth and seventh week, limb bones form and the brain further develops by dividing into five parts. Some of these parts begin to show nerve formation and begin to form tissues that generate the eyes, ears, vertebra, and other bones. The heart further develops and begins to beat.
The blood begins to circulate in the main blood vessels of the fetus. During the eighth week, the limb extremities (fingers and toes) lengthen and show as protruding shapes, and the lungs begin to form. During the ninth week, hair nerve cells begin to form, limbs become more prominent, and some important organs begin to develop. During the tenth week, the eyelids and outer ears start to appear. By the end of tenth week of development, the initial embryonic period ends.
Between the eleventh and fourteenth week of development, the facial details form and limbs lengthen. The development of reproductive system begins as well as the formation of red blood cells in the lungs. During this time, the baby’s head alone composes half the weight of the whole body. From that point until the twenty-eighth week of development, the baby’s eyes will remain closed. Between the fifteenth to eighteenth weeks of development, the skin remains transparent and hair begins to grow on the head. The mouth is able to move to suck and inhale, and fluids are produced (or released) by the liver and gallbladder. Muscular tissues are formed and bones become stronger. The baby begins to move. From the nineteenth week, the baby is able to hear sound.
During the twenty-second week the head develops a layer of fine hair, eyelashes, and eyelids. At the fingertips, the nails begin to develop. As the fetus develops muscles, there is an increase in movement and the mother can feel the fetus’ movements. The rapid heartbeat of the fetus can be heard with a stethoscope. Between the twenty third to twenty fifth weeks, the bone marrow starts generating blood stem cells. Although the lower parts of the lungs appear, the fetus cannot breath due to the incompletion of its air pathways. During the twenty-sixth week of development, there is the completion of growth of the eyebrows, eyelashes, all parts of the eyes, palms and soles, and (diaphragm containing the lungs).
During the twenty-seventh to thirtieth weeks of development, there is rapid development of the brain and nervous system that controls the activities of the body. From that point onwards the eyes are able to open and close, and respiratory system further develops. Between the thirty-first and thirty-third weeks, fatty tissue (?) develops in the lungs and the breathing process begins although the lung development is incomplete. Although the skeletal formation is complete, the fetus is soft and flexible. The fetus’ body begins to store iron, calcium, and phosphate.
During the thirty-eighth week of development, the fetus become fatter as it gradually loses its hairs. Between the thirty-ninth and forty-second weeks, only the upper parts of the hands and shoulders retain their hair while the rest of the body sheds its hair. In contrast, the hair on top of the fetus’ head becomes thicker, and its nails lengthen. In accordance with the mother’s body, the baby is born between the forty-second and forty-fourth week.
8. The menstrual cycle and fetus development
The average period of a menstrual cycle is twenty-eight days but varies depending on the female body. Depending on the female body, a middle-aged woman would have a menstrual cycle of twenty-one to thirty-five days while a teenage woman can have a menstrual cycle of twenty-one to forty-five days. The first day a female loses menstrual blood is considered the beginning of the menstrual cycle. Then, the ovarian follicle (a place for oocyte development) starts to develop.
At the end of the menstrual cycle, there is a decline of catalysts leading to oocyte development. Menstruation ends after five days and because of the catalyst enzymes some follicle growth occurs in the ovary during menstrual period. Oocytes are then activated in the ovarian follicle. Normally, there is a continuous development or growth of the ovarian follicle for the seven or eight days following the end of menstrual bleeding. After fourteen days, one of the many ovarian follicles becomes significantly larger and further develops. At this point, the embryonic layer thickens and readies itself for embryogenesis. The embryonic layer is filled with blood and vitamins.
On the fourteenth day of embryonic development, with the help of catalysts or enzymes, the developed eggs are released from the gonads, rupture the follicle, and enter fallopian tube. The process of rupturing and release is called ovulation. For a few days after ovulation, the egg travels and if the egg meets and fuses with a sperm then it will continue to travel until it binds the surface of the uterus. It is possible to become pregnant through sexual contact between the eleventh to eighteenth day of the menstrual period. If the female egg does not meet a sperm cell, then ovulation gradually ends on the twenty-fifth day. When an egg is released and does not meet a sperm cell, then the egg will disintegrate and descend through the fallopian tube.
9. The benefits of a multicellular cell
As discussed earlier, any organism has unicellular or multicellular cells. There are significantly more unicellular organisms than multicellular organisms. Therefore, multicellular organisms have a division of labor rendering them more effective in their function. A variety of multicellular organisms evolved due to this characteristic. What are the benefits of being multicellular organisms?
The cells of multicellular organisms demonstrate division of labor and responsibility. Each cell has its unique specialized responsibility, and if there is a deficiency of any cell then there is no way to complete its work. Each cell completes its function, and cannot take on the function of another. For example, the nucleus will not complete the function of the membrane and the membrane will not complete the function of the nucleus.
Why are there different functions for different kinds of cells? All the cells have to complete their unique activity so as to form specific tissues. For instance, the cells which develop into lung tissue cannot develop into other organ tissues such as heart or brain tissue. Different tissue cells create different shapes and structures depending to their function. Therefore, different cells have different responsibilities and are trained for specific functions.
We can understand that the reason for having different shapes and sizes is to complete different functions and responsibilities. For example, neurons are tiny and long for they help transmit messages to all parts of the body. Skin cells are flat and tightly connected because they help to defend the body from foreign invasions. Many activities are needed for the completion of whole body system, and the collection of similar cells can function as a specialized cell.
As mentioned earlier stem cells can irreversibly change into specific cells under the influence of chemical signals. Once it becomes a specialized cell, it can begin to develop into each organ. Why are cells with the same kind of DNA able to differentiate into cells with different characteristics? Although the DNA of the cells is the same, due to the different expression or activation of genes the cells express different characteristics. According to the timing and what function must be completed, the same genes of cells may be expressed differently.
If we think of our own bodies- skin, bones, blood, and muscles- all around our body there are different cells. These different cells sum to a total of over 3 trillion cells in our bodies, and all these cells arise from a single cell. With different functions of different kinds of cells, aggregations of cells form tissues and the assembly of many tissues leads to the formation of organs. Each organ completes a unique task, and to complete the whole task the organ works with other organs to form an organ system. The complete function of the body comes from the cooperation of all the organ system’s function.
All living beings have tissues and cells to live continuously. If we take the example of the human being, many organ systems form with the function to allow the body to live life continuously. All the organs are engaged in their unique functions and actively complete their role. For example, the function of the nervous system is to receive signals from all parts of the body and to carry them to the brain. The brain will understand the signals and respond accordingly.
The blood circulatory system is an organ system that permits blood to circulate and transports oxygen and nutrients to different parts of the body while also carrying waste collected from different parts of the body to the kidney. Also, the integumentary system or immune system acts as a barrier to protect the body against disease and maintain the welfare of the body. The reproductive system ensures that humans are able to procreate. Any activity, from the movement of a thumb to the jumping into the air was conditioned by the muscular system. All of these body systems complete their unique tasks, and therefore allow for the continued survival of human beings.
10. Two mutational processes of genes
The other concept that must be explained is that faulty genotype during embryogenesis may lead to an abnormal fetus (for example a fetus with two heads, incomplete limbs, twins sharing one body, chest limbs, or deafness and blindness from birth. There are so many conditions; what are the causative agents and how do they arise? To know the answer, we need to understand the theory of gene mutation. A gene mutation is a mutation that occurs in the DNA sequence that comprises the gene. Gene mutations can be classified in two major categories:
Hereditary mutations are inherited from a parent and are present throughout a person’s life in virtually every cell in the body. These mutations are also called germline mutations because they are present in the parent’s egg or sperm cells, which are also called germ cells. This germline mutation is present for the whole life in all cells of the child’s body.
Acquired (or somatic) mutations occur at some time during a person’s life and are present only in certain cells, not in every cell in the body. These changes can be caused by environmental factors such as ultraviolet radiation from the sun, or can occur if a mistake is made as DNA copies itself during cell division. Acquired mutations in somatic cells (cells other than sperm and egg cells) cannot be passed on to the next generation. This type of mutation occurs in a single cell early in embryonic development.
Mutations cause a deformation of the proteins, which leads to a change in cell function resulting in an abnormal or defective body. For example, flies (bees?) may have three different types of mutations before and after the embryonic development. When a deletion mutation occurs, there is the danger of losing part of the body. When a point or missense mutation occurs, then there is controlled fault. When a frameshift mutation occurs, each part of the inside of section of body.