Gastrulation in the mouse
Embryonic development refers to the process of growth and development of embryo. Stage of embryonic development is a phase within the period of embryonic development that is marked by distinct growth changes in the embryo. Gastrulation is a stage in the development of embryos in the mouse. The reason for gastrulation is to form a body plan that serves in the succeeding embryo morphogenesis. In the Mus musculus (house mouse), gastrulation is like a continually changing process, three-dimensional puzzle that has specific characteristics when comparing it to non-mammals and other mammalian embryos.
In mouse, gastrulation starts after a blastula implants into the uterine walls of the mother, and is immediately followed by development of different organ systems also knows as organogenesis. Organogenesis is characterized by the formation of vital organs such as brain and spinal cord, coelom, muscle tissues, notochord and neural tube (Ibim, 2010). The coordinated cell movements result in a spatially organized embryo, and puts together the framework on which other features are to develop from into building the body. Gastrula is the term used in defining the process in which embryo undergoes gastrulation.
In mouse a two-day gap exist between implantation and starting of gastrulation; there is an implantation of blastocyst in the uterine wall and this occurs after two and half days after coitum (DPC), and gastrulation commences six and half DPC. In the two-day span, cells undergo transformation to develop a fertilized egg called zygote. In the cleavage process, cells are united together by adhesive proteins, which must be deactivated to permit movements of specific cells. The relaxed cellular bonds allow the inner cell mass to enlarge and reorganize into specific layers. One the layers, the epiblast, is layer of cells, which are the precursors of all embryonic cells. As epiblast continues to grow, the shape become cup like, with the rim situated on dorsal side of embryo.
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 blastocyst. 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).
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.
Prenatal development is the process that occurs during pregnancy whereby an embryo gestates from fertilization to birth. Fertilization occurs when a sperm penetrates fully into the egg. The end product is called a zygote (Sagan 1997 p.164). There are various stages of development that a zygote undergoes before birth namely; the germinal, embryonic and fetal stage. The embryonic period is that stage at fertilization that is, at the second week of gestation and continues up to the tenth week of gestation. It starts after the germinal stage when the blastocyst attaches itself to the uterine wall a process called implantation.
The stage is marked by the zygote’s attachment to the uterine wall and the first occurrence of ossification, that is, the formation of solid bone. Also, the placenta and the umbilical cord fundamental in supporting the embryo develop.
It is worth noting that at this particular stage, there is evidence of lots of growth of the embryo and cell differentiation. Cells differentiate into various body systems including blood cells, kidney cells and nerve cells. Differentiation of organ systems takes place in three layers (Miller 1967).
a). Ectoderm -This is when the nerve system and the sensory cells develop. At this point, the hair, nails and the outer layer of the skin and the skin glands develop.
b). Mesoderm -This layers is characteristic of the muscles, skeleton, the excretory and the circulatory system develop. The inner skin layers also develop.
c). Endoderm -The gastrointestinal tract, glands, the lungs, pancreas and liver develop.
During the fourth week of gestation, the embryo is approximately the size of the human thumbnail. It is interesting to note that even at this small size, there is a pulse by a small vessel which eventually becomes the heart, at 200 heartbeats a minute.
At the eighth week, nearly at the end of this period, the head is as large as the body of the embryo. It is worth noting that at this time, some features including the face, fingers, toes, eyes and the external genitalia are recognizable (Harris & Butterworth 2002).
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. Fetal developments in the growth phase are described in the next section.
Gastrulation commences with the formation on primitive node on epiblast’s posterior side. Primitive node is a collection of cells that secretes cellular signals in forms of proteins for instance fibroblast growth factor (FGF). The signals assist the cells to move within embryo during gastrulation. When node appearances it indicates the existence of tail and head distinction or posterior and anterior polarity. From the node, a structure termed as primitive streaks develops. This is a groove that comes from embryonic ventral side. As primitive streak increases in size, epiblast cells situated in the inner sides of cup enters into streak. As cells moves into primitive streak, it relates with cellular signals, which regulates the tissue type the cells can form. The cells that go through streak become mesendoderm, which are endoderm and mesoderm precursors. After they leave the primitive streak, these cells scatter and form a wave of mesendoderm that protrudes to cover the exterior of embryo.
In the mouse, mesendoderm covers the ectoderm, which later form the nervous system and epidermis. The mesendoderm splits into mesoderm; which becomes the muscles, skeleton, and different internal organs. On the other hand, endoderm separates to become gastrointestinal and respiratory systems. The mesoderm forms in the anterior portion of embryo, and endoderm on the posterior side where primitive streak initiated. The two, mesoderm and endoderm fully separate from each other around sixteen DPC.
As various researchers normally employ gastrulation in mouse as model for gastrulation in other mammals, the ordering of the germ cells is precisely opposite as in majority of the mammals for instance in human beings. 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). In humans, the gastrula arranges in flat shape, known as planar arrangement, whereby the ectoderm situated on the dorsal side de of endoderm and mesoderm. 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.
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