Embryology: the Process of Fertilisation and Cell Differentiation - Essay Example

EMBRYOLOGY AND FOETAL DEVELOPMENT By of the of the of the School 13 February This paper covers the following learning outcomes in the topic of embryology and foetal development; the stage of embryonic development, the stages of foetal development, causes of foetal challenges and abnormalities and the principles of homeostatic mechanisms. Explain the process of fertilisation and cell differentiation The fertilization process in human reproduction occurs when a sperm succeeds to penetrate the female ovum. Normally, it is preceded by the ovulation process where the matured egg from the ovarian follicle is released for fertilization. The matured ovum moves through the fallopian tube into the ampulla section where fertilization would occur. It only takes 12 to 24 hours for the egg to live after which shedding of the unfertilized egg occurs (menstruation), compared to the sperms that can last up to 48 hours depending on individuals (Hatasaka, n.d.). Unlike one egg that is released in each ovulation, there are many sperms released during the sexual intercourse. The sperms ascend into the ampulla section of the fallopian tube to the matured egg. The egg is covered by protective layer called zona pellucid, mainly composed of glycoproteins, which the sperm that comes into encounter with the ovum penetrates through biochemical events. It entails the release of a digestive enzyme onto the layer that the sperm plasma membrane penetrates to fuse with that of the egg, causing the sperm nucleus to move into the ovum. When the nuclear membrane of the sperm and egg fuse, their nuclear genomes combine together to form a zygote. The zygote is a diploid cell and has to undergo differentiation as the development of the embryo occurs. The zygote is moved down to the uterus for implantation and within few days, the zygote undergoes cell division to form a group of cells called blastula, whose cavity is filled with blastocoel fluid. It is termed the morula stage and comprises of first cell differentiation where the inner cells of the blastula separate to form the embroblast and outer the trophoblast, which develop into the embryo and placenta respectively (Embryology.ch, n.d). More hundreds of cells are cleaved from the blastula, some of which contain maternal chemicals, while others do not. The presence of chemicals influences the gene expression of each cell in the embryo’s development. Certain genes in the cells are turned on while others off, and depending on the location of the cell during the early embryo development, the type of the cell is determined. As the patterns of gene expressions are altered, the cells are differentiated into brain, blood, bone and muscle cells among others, but ensure that all have the same number of chromosomes as the zygote. Describe the embryonic developmental stages relating organ and structure development to each embryonic stage The first two months after fertilization of the ovum show the development of the human embryo in stages. They are a total of 23 morphological stages divided into the first ten, with least changes and second half where embryos increase in size and develop other physical features. The 1st carniege stage entails a unicellular embryo (zygote) formed after fertilization, the 2nd stage is the cleaving embryo migrating to the uterus to form a blastocyte, and in stage 3, the cells continue to divide as earlier explained. The trophoblast and embroblast are already segregated in stage 3 and implantation begins at stage 4, where the endometrium enters the secretory phase to attach the blastocyst. The separated trophoblast cells further divide into syncytiotrophoblast whose cells form cords that infiltrate into the endometrium epithelium. At stage 5, the blastocyst has successfully penetrated the endometrium; it is characterized by solid trophoblast (syncytiotrophoblast /syncytium) with cavities that later penetrate into the maternal capillaries for them to be filled with maternal blood (Drews, 1995). The umbilical vesicle and amniotic cavity emerge and looking at the structure, a vascular circle is formed. Stage 6 is marked by development of chorionic villi and later the primitive streaks. To this far, the embryos’ length are at a maximum of 0. 2mm. According to Ulijaszek, Johnston and Preece the “embryo now has the left and right side, as well as rostral and caudial ends; and has attained a stable ontological human identity” (1998, p.161). Further structural development occurs during stage 7 in the third week with an increase in the embryonic disk. It is characterised by the notochordal process where the mesodermal hollow tube elongates with the migration of the primitive node cells to the proximal end of the tube. Here the embryo is about 0.4mm and the amniotic sac is removed due to its cavity expansion. Stage 8 of the embryo is presomitic with an ovoid shaped embryonic disk. “On the germinal disk, the notochordal, neurenteric canals and primitive pits can be discerned” and the tertiary villi occur (Benirschke, Burton and Baergen, 2012, p.148). The first somites and neural groove are visible at stage 9 and the diameter of the chorionic sac increases. At stage 10 between 22 and 25 days, the number of visible somites increases in the embryo, neural groove deepens and neural folds begin to fuse while extending bidirectionally. The Chorionic sac further extends while first foetal capillaries start occurring. Meanwhile, the crown rump length increases gradually from 1 to 1.5 and 1.8 mm in between stage 8 to 10 (Yamada and Takakuwa, n.d.). Towards the end of this stage, the pharyngeal membrane ruptures and the first two pharyngeal arches become visible. At the second half of the embryo development, the rostral neuropoles close up and the body segments along the axis are much conspicuous in the 11th stage. Contrary, the caudal neuropoles closes up and the upper limbs appear; the embryo folds and there are also 21-29 pairs of somites compared to the 13-21 available in the last stage (Benirschke, Burton and Baergen, 2012). The respiratory and cardiovascular development has begun inside the embryo at this stage, which are not easily detected. At stage 13, the numbers of somites in the embryo exceed losing count, the optic vesicle and all the buds for the four limbs are visible enough and the chorionic sac is approximately 25mm wide. The embryo in stage 14 consists of elongated upper limbs and an open lens pit that would invaginate into the visible optic cup. Pancretic and ureteric buds for internal body organs develop; cerebellar plates and the cerebral hemispheres of the brain are also visible (Yamada and Takakuwa, n.d.). In the next stage, the embryo has developing arms in the hands plates, the lens vesicle closes, the lobal buds and cerebral vesicle become clear, while the nasal plates invaginate for the formation of a nasal pit. Carnegie stage 16 and 17 consist of the early face development. The foot plate emerges, the embryo increases weight, retina pigments are visible and it can be possible to monitor the embryo’s movement at stage 16. The finger rays can be seen at the hand plates, but more so, the relative size of the head enlarges in the next stage. The foot plate toe rays appears, elbows at the hand plates, the eye lid folds, nipples occur at stage 18 and the trunk straightens, with much limbs and head development occurring in stage 19. The development of the upper and lower limbs is in parallel with evidence of the larger toe and thumb location being easily situated. Through the umbilical cord, parts of the developing intestines can be detected. At stage 20, the focus goes to the lengthening of the upper limbs, which bend at the hand joints and elbows. The fingers also separate and curve, while the embryo is said to make spontaneous movement at this stage. The size of the fingers lengthens and the hands pull closer together towards the heart area in stage 21. Similar to the hands, the feet also turn inwards. The facial features (eyelids and ear lobes) further develop to give them a distinct shape in stage 22 and in the last stage the head becomes rounded, limbs length increases as well as that of the body, while further development of body organs occurs. By the last stage, the embryo is approximately 30 mm in CRL and the chorionic sac about 63mm (Benirschke Burton and Baergen, 2012). The size of the chorionic sac, as well as the weight increases gradually at each stage depending on the embryo. Explain the transition from embryo to foetus The ninth week to the 12th week marks the transitioning period of the embryo to the foetus. It is a delicate period from which miscarriages occurs. The basic physiology of the foetus is in place with essential body parts already accounted for. The foetus genitals begin to form though impossible to tell the sex yet, and the developed placenta is functional to cater for delivery of waste products from the foetus and nutrients from the maternal system. The external structures and most internal organs continue to develop. Between week 10 and 11, there is rapid growth of the placenta, foetus jawbones, facial bones and most internal organs. For example, the ear canals develop internally and the heartbeat of foetus can be detected. The foetus forehead bulges out and the size of the head is almost half the foetus size. The foetus can make small random movements. The finger and toes of the limbs have separated and their small nails have formed by the end of the 12th week. The nerve cells also multiply rapidly, which enhance the foetal sensory. As a result, the foetus looks closer to human as the facial features move closer to their strategic locations in the face, and the internal organs become functional but continue to develop in the subsequent trimesters. Foetal development stages (foetal anatomy, organ and structure maturation) The 4th month (week 13) begins the second trimester of the foetus. The period is characterised by the rapid movement of the foetus especially in the mid second trimester. In the first month, the head of the foetus becomes more erect, the limbs, neck and trunk increase rapidly and develop downy hair that covers the body. With the help of screening, the developing bones of the foetus can be detected. The second month of the trimester has slow growth of the foetus, but known for secretion of vernix caseosa by sebaceous glands that protect the skin against the effect of the amniotic fluid (Fritsch and Kuhnel, 2008). The hair on the head, eye lashes and eye brows appears, while nails and buds for permanent teeth further develop. The brain organ grows with the cortical plate and germinal matrix having the highest cellular density. The muscles are well developed and the foetal kidney can now function fully. The foetus is in a position to make insulin, and the kidney can secrete urine into the amniotic fluid for excretion. Foetal skin grows rapidly in the 3rd month and the respiratory system matures up towards the end of the trimester as the alveoli form in the lungs. It is until week 25 when the foetus becomes viable. Brain’s neurons are formed and the foetus can respond to the mother’s strenuous moves. The 3rd trimester occurs from around the 26th to 38th week of pregnancy. The foetus rapidly grows and gains lot of weight. In case it is born early, it has high chances of survival since its organs can support individual life. The foetus can regulate the body processes like swallowing and breathing among others. By end of 7th week, eyelids can open and close since their appearance in the 1st trimester and the foetus’ bone marrow can manufacture blood. There is active brain development with the enlargement of the cerebral cortex along the trimester. According to Ricci and Kyle, there are increased body functions by the CNS, rhythmic breathing movements, and the foetus has the capacity to store its own nutrients (calcium, phosphorous and irons) at around week 8 (2009). The foetus’s head shifts downwards in preparation for birth, the skin hair would disappear and the skin colour turns from reddish in the last gestational age. Nails extend to the tip of the toes, the pulmonary blood flow increases and testes descend for the male foetus. All organs have developed in the last month with the exception of the lungs that goes up to the delivery. A fatter layer develops under the skin for regulation of the foetal temperature, and through the placenta, the foetus receives antibodies from the mother before delivery. Explain the relationship between exposure to toxins and disruption in growth and development within the foetus Toxins are exposed to the foetus through the mother’s placenta. Some result from viruses in the maternal body that release toxic substances, while others are due to maternal ingestion and consumption behaviours like toxic drugs and chemicals. Whatever the mother takes in is transmitted to the foetus, and hence the child could be exposed to environmental toxins. Toxic chemicals cause mental retardation and impair foetal development in the prenatal period; such that, toxins from alcohol intake or rubella virus in the maternal system interfere with chemical signals which control normal foetal brain development, or exhibit foetal alcohol syndrome (Carlson, 2013). Due to toxins, the foetus could develop slowly, and even at birth, it could be born smaller in size or with certain facial abnormalities. Environmental toxins like mercury and lead that find their way to the maternal system and eventually the foetus could lead to depletion of stored nutrients like vitamins, calcium, and iron, which certainly affect foetal growth. Different exposure to toxins could create certain abnormalities, some of which could be rectified while others remain permanent. Evaluate maternal influence on foetal development Maternal behaviour in the course of pregnancy influences foetal growth and development pattern. Besides the toxicity influence, maternal nutrition largely affects the growth of foetus. Unhealthy diets during pregnancy influence the type of nutrients transferred to the foetus. Epigenetic changes in the foetus are commonly influenced by the level of fat and protein intake by the mothers. Poor or maternal under-nutrition affects the placental foetal development and affects the birth weight. Malnutrition of the foetus could impair brain development, organs development and affect the immune system. Maternal physical exercises cause increase in the FHR in response to the activity and low birth weights (Carballo, Sterling, Zakynthinaki and Mulas, 2008). The emotional stress of the mother causes stress hormones from the maternal system to pass to the foetus, leading to premature labour and enhancing the foetal heartbeats and other activities that could be difficult to regulate in the foetus. Another risky maternal influence arises from the incompatibility of the Rhesus factor that could cause destruction of foetus red blood cell, mental retardation, or even infants death along the pregnancy. Identify strategies in utero which redresses the balance To respond to intrauterine nutrition and modify the epigenetic status of the foetus while ensuring maternal health, various internal and external processes occur in the maternal system. Foetal dietary supply depends on the maternal metabolic process to provide the nutrients required for growth. In cases where the maternal system cannot meet the demand, the placenta mobilizes fatty acids from earlier on deposited maternal tissue, to enhance uptake and retention of the nutrient supplied for the foetus (SACN, 2011). Another obstruction of the intrauterine nutrition balance is lack of enough blood supply to the placenta. To keep up with the blood supply required by the foetus, the mother needs to expand her blood volumes through behavioural adaptations; eating healthy and encouraging lifestyles that would increase blood flow or pressure to meet foetal needs. Other changes entail placenta and foetal hormones that promote and control growth. Through modification of stress physiology, mothers can undertake physical actions that reduce stress translated to the foetus that increase demand in nutrients. Through physiological strategies, the mother can control the psychological impact caused by the stress and improve nutrients balance across the placenta. Common structure between foetus and uterus and their relevance in foetal development and Homeostasis The placenta organ in the uterus is shared by the mother and the foetus for gaseous exchange, nutrients and waste products from the foetus. It is implanted at the wall of the uterus on the mother and to the foetus through the umbilical cord. The maternal and foetal blood vessels in the placenta lay close together facilitating diffusion of products, though the blood never mixes. The foetal blood vessels circulate the oxygenated blood through its developing organs and bring back the waste products in the retuning blood veins to the maternal veins adjacent to it in the placenta to be excreted. In addition, as the foetus develops inside the womb, some of its cells migrate to the placenta and can remain in the mother’s body for many years. There are certain genetic compositions and hormones that have already formed in the foetus and exist in the uterus or placenta. The genes influence the chemical composition of the cell types and essentially the outcome of the foetal feature development. The foetus and maternal body need a balance of body fluids, which is critical to their health. Hence the foetus can be able to regulate the absorption of salts and water into their system. The placenta plays an active role in intrauterine functions and the transportation of ions to the foetus for its development. Other nutrients such as amino acids and lipids use specialized parts of the placenta to get to the foetus. In calcium homeostasis, parathyroid hormone is required in regulating calcium across the placenta and “placental expression of genes involved in calcium and other solute transfer, and may directly stimulate placental calcium transfer” (Res, 2010). Other hormones like aldesterone and vasopressin modify the foetal renal process and aid in its ions and water re-absorption during the body fluid homeostasis. Describe how homeostasis is maintained in the developing embryo and growing foetus Through sweating and excretion, the foetus can maintain its body temperature. However, its temperature is always higher than that of the mother, considering that it’s surrounded by the amniotic fluid that helps retain the temperature. The circulatory system of the mother assists the foetus in regulating temperature, but still remains higher. According to Blateiss, “during heat stress, blood flow to and from the placenta increases and other foetal peripheral tissues” to enhance heat loss and control its temperature (1998, p.157).When the foetus can make its own insulin, the excess produced is excreted in form of waste products back to the maternal system. Similarly, before it can produce its own, it takes up glucose, amino acids and other substrate energies across the placenta to facilitate its growth process and development of organs. Through electrolyte concentration, both maternal and foetal body fluids are balanced with the help of hormonal responses. Appendix FHR- Foetal Heart rate CNS – Central Nervous system Reference List  Benirschke, K., Burton, G. J. and Baergen, R. N., 2012. Pathology of the Human Placenta. 6th Ed. New York: Springer. Blatteis, C. M., 1998. Physiology and Pathophysiology of Temperature Regulation. London: World Scientific Publishing. Carballo, R.B., Sterling, J.R., Zakynthinaki, M. and Mulas, A. L., 2008. Acute Maternal Exercise during the Third Trimester of Pregnancy, Influence on Foetal Heart Rate [online] Available at: [Accessed on 5 March 2014]. Carlson, N. R., 2013. Physiology of Behavior. New Jersey: Pearson Education Inc. Drews, U., 1995. Color Atlas of Embryology. New York: Thieme Medical Publishers. Embryology.ch, n.d. Early Development and Implantation [online]. Available at: [Accessed on 6 March 2014]. Fritsch, H. and Kühnel, W. ed., 2008. Color Atlas of Human Anatomy: Internal organs. New York: Thieme. Hatasaka, H.H., n.d. Fertilization, Early Pregnancy and It’s Disorders [online]. Available at: [Accessed on 6 March 2014]. Res, J. B. N., 2010. Parathyroid Hormone Regulates Foetal-Placental Mineral Homeostasis [online] Available at: [Accessed on 6 March 2014]. Ricci, S. S. and Kyle, T., 2009. Maternity and Paediatric Nursing. Philadelphia, PA: Lippincott Williams & Wilkins. SACN, 2011. The Influence of Maternal, Fetal and Child Nutrition on the Development of Chronic Disease in Later Life [online] Available at: [Accessed on 6 March 2014].  Ulijaszek, S. J., Johnston, F. E. and Preece, M. A. eds., 1998. The Cambridge Encyclopedia of Human Growth and Development. Cambridge: Cambridge University Press. Yamada, S. and Takakuwa T., n.d. Introduction – Developmental Overview of the Human Embryo [online] Available at: [Accessed on 5 March 2014]. Read More