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Stages of Embryonic Development
Embryonic development refers to the process of growth and development of embryo. On the other hand, stage of embryonic development is a phase within the period of embryonic development that is marked by distinct growth changes in the embryo. Also known embryogenesis, the process of embryonic development starts after fertilization of an ovum by a single spermatozoon. The spermatozoon produces enzymes which allow it to penetrate through a layer of follicle cells covering the ovum (corona radiata) and fuses with the egg’s plasma membrane to form a diploid, called zygote (Campbell & Reece, 2005). In human beings, the process of fertilization takes place in the ampulla of the uterine tube. There are four main stages of embryonic development: cleavage, patterning, differentiation, and growth.
The cleavage or cell division stage starts approximately 18-36 hours after fertilization. The zygote formed after fertilization divides into multiple cells through a process known as binary fission (Campbell & Reece, 2005). The resultant cells after fission carry equal number of chromosomes and genes. After the 8-cell stage, the cells bind to form a compact sphere. Further divisions within the sphere lead to the formation of a 32-cell mass known as morula. Each of the cells within the molula is known as blastomere. This is followed by a process known as cavitation in which the outer cells secret fluid containing proteins into the sphere, facilitating rapid division of the cells (Forgacs & Newman, 2005). Despite the increase in the number of cells in this phase, the volume of the zygote remains the same. The zygote lands into the uterus during the 6th or the 7th day, after which developmental changes lead to the formation of three membranes: placenta, chorion and amnion (Forgacs & Newman, 2005). Further development of the morula leads to formation of a ball containing about 2000 cells known as blastcyst. Also a fluid-filled cavity called blastocoele is also formed within the embryo. The developmental changes end with the formation of a structure known as blastula. The blastula is surrounded by a layer of single sells called trophoblast. However, it contains a thick inner layer of cells at one end of the inner cavity, from which the embryo will develop (Campbell & Reece, 2005)
Patterning or morphogenesis is the second stage in embryonic development, during which the outer membrane (zona pellucida) of the molula starts disintegrating and the zygote starts to increase in volume. Approximately eleven days after implantation of zygote on the uterus, some cells from inner mass migrate into the fluid-filled cavity (blastocoele) and organize to form masses and layers. The process of migration is called gastrulation. The space from which the cells migrate forms omniotic cavity (Campbell & Reece, 2005). Three major germ layers are also formed in the process: endoderm, mesoderm and ectoderm. The ectoderm forms the neural tube and the epidermis. The mesoderm forms vascular elements, muscles and tissues. The endoderm develops to form lining of the gut and associated organs. This stage is also characterized by the formation of belly and back sides, right and left sides and front and the rear. By the end of the patterning stage, the genes of the embryo are fully formed and expressed (Campbell & Reece, 2005).
Differentiation is the third phase in which specialized structures that perform more functions start to form. About 21 days after the implantation of the embryo onto the uterus, the endoderm, mesoderm and ectoderm grow further and differentiate to form neutral tube and notocord, among other vital organs of the body. The process of formation of body organs from the germ layers is called neurulation (Campbell & Reece, 2005). Notocord develops from mesoderm and undergoes through a series of developments to form the vertebral column. However, notochord in human beings disappears before birth. The development of neural groove in this stage leads to the formation of brain, and spinal cord (Campbell & Reece, 2005). The mesoderm also undergoes through a series of developments leading to the formation of vertebrae, muscles, and connective tissues. In addition, the mesoderm grows to form gonads, adrenal glands, and kidneys. Furthermore, coelom is formed by the end of differentiation phase.
Growth is the last phase in embryo development. During this phase, the body goes through a period of growth characterized by formation of new cells, extra cellular matrix, and new organs (Campbell & Reece, 2005). The growth phase starts between the eighth and ninth months after conception and continues until birth. Foetal developments in the growth phase are described in the next section.
How an embryo transitions to foetus
In human beings, the development of foetus from conception to birth can be divided into three stages: first trimester, second trimester and third trimester. Each of the stages consists of three months. The process of transformation of embryo to foetus is a gradual process that takes place within the first trimester. Transition starts during the cleavage phase, characterized by rapid division of cells within the embryo. This is followed by the formation of germ layers (endoderm, mesoderm and ectoderm), chorion and amnion within three to six weeks after conception. Organogenesis follows in the next six to eight weeks (differentiation phase) characterized by the formation of vital organs such as brain and spinal cord, coelom, muscle tissues, notochord and neural tube (Ibim, 2010).
However, the transition from embryo to foetus continues after the differentiation phase. Other changes occur from the ninth week, which make the embryo to look more like a human being (Ibim, 2010). The changes include the formation of head, ears, nose, heart, hands and legs. Also, the placenta is fully formed by the end of the ninth week. By the end of the first trimester, the embryo is fully transitioned into foetus. The chorion is fully established, the embryo is in the amniotic sac and the sex of the foetus can be determined (Ibim, 2010).
The second trimester is characterized by more growth and refinement activities and less developmental activities. The placenta plays the biggest role in maintaining homeostasis through secretion of progesterone and transfer of nutrients and wastes. The foetus is approximately 6cm by the beginning of the first trimester. At 20 weeks, the foetus has grown to approximately half a kilogram. At this time, heartbeat can be heard and the legs, head, face, and hands are prominent (Ibim, 2010).
Further growth and development continues during the third trimester. A series of changes and developments take place in the circulatory and respiratory systems which facilitate air-breathing after birth. Additional changes and developments occur, which enable the foetus to maintain constant body temperature. Changes also occur such as increase in size and weight, reduction in the size of head, thickening of the muscle and hardening of bones and skin ((Ibim, 2010).
Maturation processes within developmental stages in the foetus
After birth, the foetus is required to adapt to the environmental changes by establishing and maintaining physiological homeostasis, without the assistance of the placenta. Therefore, the survival of the foetus after birth is dependent upon maturation of structures and organs that interface with the new environment. Examples of essential organs and structures are lungs, immune system, gut, liver, pancreases and kidney (Strauss & Barbieri, 2013). Various studies have shown that maturation processes during foetal development are induced by glucocorticoids (Strauss & Barbieri, 2013). Critical maturation processes that occur during foetal development include deposition of glycogen in the liver; activity of enzyme systems in the foetal brain, thyroid gland, pancreases, retina, and gut; and production of surfactant by foetal lungs. Maturation of the foetal lungs is particularly important since inability to breathe due pulmonary immaturity has been found to be a leading cause of mortality among preterm infants as well as neonatal morbidity (Norris & Lopez, 2010). However, the key functions of glucocorticoids in the process of maturation are not yet clear. As Norris and Lopez (2010) highlight, some studies have shown that glucocorticoids do not initiate maturation of cells; they simply accelerate the process of maturation.
Structures linking mother and foetus
The placenta is the main structure that connects the foetus to the mother. During the first four weeks of foetal development, prior to development of the placenta, the embryo relies on simple diffusion of oxygen and nutrients. Once the placenta and associated organs develop, it starts releasing hormones (progesterone, estrogen, placental lactogens, relaxin, and chorionic gonadotrophins) into the circulation system of the mother, which mobilizes nutrients and modifies maternal appetite (British Nutrition Foundation, 2013). It plays a key role in transferring nutrients (such as amino acids, glucose, lipids, minerals, water, and oxygen) from the mother to the foetus and wastes (such as uric acid, carbon dioxide and bilirubin) from the foetus to the mother. Harmful substances (such as nicotine, cocaine and alcohol) and infectious (such as cytomegalovirus and rubella) are also transferred from the mother to the foetus (British Nutrition Foundation, 2013). It also prevents the transfer of some noxious agents from maternal system to the foetus. The hormones released by the placenta also help to maintain homeostasis. The placenta maintains homeostasis through regulation of nutrients and processes within the body of the foetus, in response to changes in maternal system (British Nutrition Foundation, 2013). However, there are some challenges that are associated with process of maintaining homeostasis. For instance, higher concentration of glucose in the body of the mother is reflected by a rise in concentration of glucose in the body of the foetus. The placenta is unable to regulate the movement of glucose molecules, which may be detrimental to the foetus in cases where concentration of glucose in the body of the mother is too high or too low (Sinha, Miall & Jardine, 2012).
The placenta consists of a maternal and a foetal component. The maternal component is known as decidua and develops from a layer of endometrium. Endometrium is a layer that forms on the uterine wall after implantation. The foetal component is known as chorion and is connected to the foetus by the umbilical cord (British Nutrition Foundation, 2013). The chorion has villi which aid the process of transfer of essential nutrients between the mother and the foetus. The umbilical cord contains two arteries and one vein. The arteries transport de-oxygenated blood from the foetus to the mother while the vein carries oxygenated blood from the mother to the foetus (British Nutrition Foundation, 2013).
Relationship between foetal development and exposure to toxins
The first three months of foetal development involve numerous changes in division of cells and development of structures and cells in the embryo. Exposure of the embryo to toxins during this period may interfere with growth and development processes and may lead to deformities, irreparable damage or miscarriage (Ibim, 2010). Examples of harmful compounds are nicotine (cigarette smoking), some recreational drugs, cocaine, heroin, mercury, lead, some pesticides, air pollutants, alcohol, high levels of radiation, and viral infection (Ibim, 2010). The compounds may not harm the mother but they may pass through the placenta and cause harm to the foetus. The compounds are most injurious during the first six to eight weeks after fertilization (a period during which organogenesis takes place).
Maternal influence on foetal development
The growth and development of the foetus depends on the conditions to which the mother is exposed to during pregnancy. As mentioned, consumption or exposure of the mother to toxic substances may cause harm to the foetus. Maternal system responds by trying to get rid of the toxic substances from the body. However, some toxic substances are quick to cause harm on the foetus before they are removed. The body of the mother may also be unable to protect the foetus where the concentration of the toxic substances is too high (DeWit & O’Neill, 2013). Severe deficiencies of nutrients such as glucose and vitamins in the maternal body system have also been shown to affect mental and physical development of the foetus (Semrud-Clikeman & Ellison, 2009). The body of the mother responds to nutrients deficiency through oxidation of fats stored in the body. In extreme cases, the little available nutrients are used in maintaining vital processes between the mother and the foetus, such as placenta development. At the same time, over-nutrition during pregnancy may be detrimental to the foetus. However, over-nutrition is usually solved through homeostasis processes in the mother’s body. Extreme maternal stress has been found to increase level of stress in foetus and to affect mental and physiological development of the foetus. Maternal corticotrophin responds by releasing hormones that help to reduce stress (DeWit & O’Neill, 2013). The health condition of the mother is also vital to the development of foetus. Chronic diseases such as diabetes and rheumatic heart disease may cause harm to the foetus. Studies have shown that the immune system of the mother may not be effective in protecting the foetus from danger caused by such diseases. It has been proposed that pregnant women diagnosed with such chronic diseases should rely on medical therapy (DeWit & O’Neill, 2013).
Examples of foetal abnormalities related to homeostatic challenges, exposure to toxins and maternal influence
Homeostatic challenges and maternal conditions leading to deficiency in nutrients may cause abnormalities such as retarded prenatal and postnatal growth, postnatal leptin surge, and hypertension. Over-nutrition may cause abnormalities such as obesity, lipid peroxidation, and postnatal hipppocampal neurogenesis (Johnson, 2003). Examples of abnormalities associated with exposure of the foetus to toxins are foetal alcohol syndrome (FAS) ,and lightweight babies. FAS occurs as a result of consumption of alcohol during the critical periods of foetal development (Ibim, 2010). On the other hand, smoking of alcohol, marijuana and consumption of high concentrations of caffeine before and during pregnancy have been shown to lead to birth of lightweight babies. Homeostatic challenges and factors such as maternal under-nutrition and stress and age (below 16 and above 40 years) have also been associated with birth of lightweight infants.
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