Tuesday, 17 February 2015

Human embryo development



The union of sperm with egg cell  takes place in fallopian tube of female reproductive system. After this union  diploid Zygote is formed with two sets of chromosome one set  of  chromosomes from sperm and another set   from ova.The two sets of genetic material from two sources recombine to recreate and a unique genetic combination, which unfolds gradually during embryonic development forming a baby. The baby inherits some traits from father and some from mother. The period in which the fertilized zygote develops in to fully matured baby in side uterus is called gestation period. The gestation period for  human is 9 months.
The fertilized zygote undergoes rapid cell divisions called Clevage which starts 30 hrs after fertilization. The zygote undergoes first mitotic division and forms two identical cells called Blastomeres. The clevage continues to form mass of 16 cells called Morula.The size of morula is same size as that of zygote. By 72 hrs Morula reaches uterus. Morula continues dividing and forms ball of 100 cells called Blastula. It is hollow at centre as is called Blastocoel. It  gets implanted in endometrial tissue. In rare conditions one out of 300 pregnancies, Blastula some where other than uterus in fallopian tube or abdominal cavity (Ectopic pregnancy) causes severe complications. 



The next stage in embryonic development is the formation of the body plan.The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation.  Each of these layers is called a germ layer, which differentiate into different organ systems.The three germs layers are the  outer ectoderm, , and middle mesoderm  and inner most endoderm


Organogenesis is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism by cell differentiation, the process by which a less-specialized cell becomes a more-specialized cell type. This must occur many times as a zygote becomes a fully-developed organism. In vertebrates, one of the primary steps during organogenesis is the formation of the neural system.


The ectoderm

It forms epithelial cells and tissues, as well as neuronal tissues.It gives rise to the nervous system and the epidermis. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate.
 The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube. In further development, the neural tube will give rise to the brain and the spinal cord .

The endoderm

It consists of flattened cells, which subsequently become columnar. It forms the epithelial lining of the whole of the digestive tube. It also forms the lining cells of all the glands which open into the digestive tube, including those of the liver and pancreas; the epithelium of the auditory tube and tympanic cavity; the trachea, bronchi, and air cells of the lungs; the urinary bladder and part of the urethra; and the follicle lining of the thyroid gland and thymus. Additionally, the endoderm forms internal organs including the stomach, the colon, the liver, the pancreas, the urinary bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the parathyroid, and the intestines.


The mesoderm gives rise to the muscle cells, bones and connective tissue in the body.

Week 3:

The embryo grows to a length of about 2 millimeters long  with rounded head and a tapering tail. There are four extra-embryonic membranes which protect the developing embryo they are the Yolk sac, the Allantois , the Amnion, and the Chorion.

 Amnion surrounds the body of the embryo like a fluid-filled balloon.It allows fetus to be suspended in amniotic fluid, protecting from mechanical injuries, allows normal fetal movement and accommodates growth.The normal amount of amniotic fluid  is typically 500 to 1000 ml.Amniocentesis is procedure of removing a small amount of amniotic fluid by inserting a needle through the mother’s abdomen and into the amniotic cavity.Fetal cells present in the amniotic fluid can be cultured and examined for various chromosomal and metabolic defects.Traditionally after birth of baby, amnion and and placental tissues were discarded, but these tissues were found quite promising for stem cell therapy as they contain pluripotent stem cells that  can form variety of tissues.

Yolk sac usually contains reserve food  'Yolk'  for developing embryo. It is well observed in embryos of fishes and amphibians. In human embryo Yolk sac is devoid of yolk and is vestigial as there is ample supply of nutrients from mother's blood.

 Allantois is large sac which is vestigial again in human embryos but in most aves and reptiles it stores urinary wastes and serves respiration.Allantois is continuous with forming urinary bladder of embryo.

Blood vessels begun to form and by day 20, the embryo has developed arteries and veins which circulates embryos blood. Cells aggregate along the embryo's dorsal surface to form neural tube. Neural tube eventually develop into the brain and spinal cord.




Week 4:

The embryo takes curved C shape as  eye stalks and ear pits appear. Upper and lower limb buds become noticeable. The umbilical cord and  facial areas develop. It`s size is approx 3-4 mm in this stage. Body tapers to spiraled tail which is prominent in 4 week embryos. Lung, liver, pancreatic, and gall bladder buds emerge in the form of mesodermal somites which are group of differentiating cells.Fourth week is marked by  rapid neural and cardiac development. On either sides of neural tube feature spinal cord primitive bones develop.  Nerves, muscle, and connective tissues emerge around the primitive bone formations.





Week 5:

the embryo is about 7 to 9 millimeters long and has all of its internal organs. The mouth, stomach, and urinary bladder are present. Nose pits and eye lenses are visible. A few days after upper limb bud extension, the lower limb buds evolve further.The external ears takes their form, and upper limb buds extend to form paddle like hands.  Extensive brain development occurs at this time, and the head grows big, causing it to  appear large compared to the body. The umbilical cord becomes more clearly defined.

Week 6:

The trunk of embryo straightens and upper limb continues developing. Bone, cartilage, and muscles become defined around the spinal cord and in the embryonic chest. Early in this week, tooth buds appear. These buds will become the baby teeth that are lost in childhood. Rib cells line up horizontally along the trunk sides, and skin.The regions of the brain that will become the cerebral hemispheres are very prominent at this time. The embryo appears more human by this point. It is about 11 to 14 millimeters long, and its rate of heart beat is 140 to 150 beats per minute.


Week 8:

Nerve cells in the brain form at a rate of about 100,000 a minute. The top of the head becomes more rounded and erect. Between day 52 and day 56, the fan shaped toes go from being webbed to separated. The fingers are entirely distinct. The eyelids close over the eyes and become fused shut until about the twenty-sixth week. External genital sex organ differences begin to develop. All appearances of the tail are gone. By day 56, the embryo is roughly 27 to 31 millimeters long.

The first three months of embryonic development are known as the first trimester. At the end of the first trimester, the embryo looks like an adult, with all major organs having been formed. It is about 7.5 cms long.  Additional time in the mother's womb is needed to permit further development of the organs.
At the beginning of the second trimester, the growing organism is no longer called an embryo, but a fetus. Fetal development continues through the second and third trimesters until it is ready for birth at the end of the ninth month.When fetus reaches the end of 9th month, uterus contractions begin due to Oxytosin hormone associated with labor pains. Child birth takes place by rupture of embryonic membranes by dilation of  cervical muscles and vagina. parturition. The process of normal childbirth is occurs in three stages the  dilation of the cervix,  birth of the baby fallowed by detachment  and exit of placenta  through vagina. The umbilical cord is clamped in two places and is cut in between.Respiration initiates normally by usual first baby cry. The newly born baby given warm water bath and swallowed amniotic fluid is removed out using a sterilized tube. The baby is allowed to feed on mother`s first milk called colostrum which has high concentrations of nutrients and antibodies. Antibodies in colostrum protect the mucous membranes in the throat, lungs, and intestines of the baby.








The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/
The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/
The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/
The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems

Source: Boundless. “Cleavage, the Blastula Stage, and Gastrulation.” Boundless Biology. Boundless, 03 Jul. 2014. Retrieved 17 Feb. 2015 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/animal-reproduction-and-development-43/fertilization-and-early-embryonic-development-242/cleavage-the-blastula-stage-and-gastrulation-899-12150/

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