How Embryo Develops Into Human Being
Embryology is defined as the study of the changes that occur during the development of an embryo into a new born child. In humans, like in other species, new life starts as a single cell which is a fertilized egg resulting from the union of a sperm and an ovum. During development, the fertilized egg divides repeatedly to produce many different cells and tissues in a final complex pattern, which is determined by its genome. Special feature of mammalian embryo is that its development is hidden within the uterus. Fertilization takes place in the fallopian tubes to form a zygote, which then moves into the uterus for implantation.
The embryonic period
This period extends from the third week of development to eighth week during which the three germ layers, i.e., ectoderm, mesoderm and endoderm give rise to specific tissues, structures and organs. By end of the embryonic period, the main organ systems have developed giving an external form to the body of the embryo.
The primary germ layers of the embryo give rise to the organs in a process called organogenesis. Organogenesis, therefore, is the main event of the embryonic development.
One of the first events of organogenesis is the formation of the central nervous system (the spinal cord and brain). It arises from the ectoderm of the ICM. Early in the embryonic development, the ectoderm along the back of the embryo folds inward. This creates a long trench, the neural groove that runs the length of the back surface of the embryo. Over the next few weeks, the neural groove deepens and eventually closes off, creating the neural tube. The walls of the neural tube thicken and form the spinal cord. In the head region, the neural tube expands to form the brain.
Numerous nerves that attach to the spinal cord and brain (spinal nerves and cranial nerves respectively) develop from small aggregations of the ectodermal sells, the neural crest, lying on either side of the neural tube. These cells give rise to axons that grow into the body and attach to organs, muscle, bone and skin. The ectoderm also gives rise to the outer layer of the skin, the epidermis.
The middle germ layer, the mesoderm, gives rise to deeper structures, the muscle, cartilage, bone and others. Much of the mesoderm first aggregates in blocks, called the somites. The somites form the backbone and the muscles of the neck and trunk. Mesoderm lateral to the somites becomes the dermis of the skin, connective tissues and the bones and muscles of the limbs.
The ectoderm which is the lowermost germ layer of the ICM, forms a large pouch under the embryo called the yolk sac. The upper most part of the yolk sac becomes the lining of the intestinal tract. The yolk sac also gives rise to blood cells and primitive germ cells. During organogenesis, the germ cells migrate from the wall of the yolk sac to the developing testes and ovaries, which are located near the kidneys. These cells become spermatogonia in males or oogonia in females.
Thus, the edodermal germ layer forms the epithelial lining of the primitive gut, the intraembryonic portion of the allantois and vetelline duct. With further growth, it also gives rise to the epithelial lining of the respiratory tract, the parenchymal of the thyroid, parathyroid, liver and pancreas, the reticular stroma of the tonsils and thymus, lining epithelium of gall bladder, urinary bladder and urethra and epithelial of the tympanic cavity and auditory tube.