What is Embryogenesis explain in detail?

Embryogenesis in humans consists of the first eight weeks of development of the zygote. Embryogenesis in animals is generally divided into four main stages: cleavage, gastrulation, neurulation, and organogenesis. The later stages of cleavage and gastrulation are greatly modified in higher animals, including man.

Cleavage :  Cleavage, the initial stage of cell division, begins while the egg is still in the Fallopian tube. During this stage, various gene-regulating factors segregate to determine later development of each cell. Human cells undergo radial cleavage; that is, a solid ball of cells is formed. This group of cells is called the morula, and it has radial symmetry. No cell growth occurs during this stage, which involves only cell division. That is to say, the cell becomes subdivided without increasing in size. Morula cells secrete substances that result in formation of a central cavity, forming a hollow fluid-filled sphere. This stage is called a blastula in lower animals, and is modified in mammals to form a structure referred to as a blastocyst.

The blastocyst contains an inner mass of cells adherent to the thin outer wall at one point of the sphere. All of the three primary germ layers of the embryo--the ectoderm, mesoderm, and endoderm--will come from this inner cell mass. The ectoderm will develop to form what ultimately becomes the skin and associated sweat, sebaceous, and mammary glands, mucous membranes of the mouth and anus, the nervous system, lens of the eye, and the inner ear. The mesoderm will form what ultimately becomes the connective tissue, dermis of the skin, muscles, the circulatory system, skeletal system, outer layers of the gastrointestinal and respiratory systems, and reproductive and urinary systems except for the urinary bladder. The endoderm will form most of what becomes the digestive tract, most of the respiratory tract, the liver, pancreas, and the bladder.

The thin outer layer of the blastocyst, called the trophoblast, secretes enzymes to initiate the process of implantation of the embryo in the endometrium of the uterus. The trophoblast forms four membranes that develop into distinct parts of the embryo. The outermost membrane, the chorion, is already formed by the sixth day following fertilization. It develops fingerlike projections called chorionic villi that interact and grow into the endometrium to form the placenta, the organ through which the embryo receives nutrients from the maternal blood supply and eliminates wastes. The placenta also secretes hormones to sustain the pregnancy. Chorionic gonadotropic hormone activates the corpus luteum to produce estrogens and progesterone, which stimulate growth of the endometrium. High estrogen levels also inhibit the pituitary from production of follicle-stimulating hormone and luteinizing hormone, preventing ovulation and menstruation during pregnancy.

Beneath the chorion, the allantois forms a cavity that receives wastes early in development, and later becomes the umbilical cord, the conduit through which blood vessels extend from the embryo and the placenta. The membrane immediately surrounding the embryo, the amnion, becomes a fluid-filled sac which serves to cushion and protect the embryo and provide a fluid environment for the embryo. The fourth membrane, the yolk sac, provides nutrients to the embryo before implantation.

Gastrulation :  After about fifteen days, the embryo begins the second stage of embryogenesis, gastrulation. At the beginning, a double layer of cells called the embryonic disk separates the amniotic cavity from the yolk sac. The cell layers then split apart, and a slit called the primitive streak develops in the center of the upper layer. Cells of the upper layer migrate through the slit into the interior of the embryo, forming the mesoderm. The lower layer of cells, the endoderm, forms the notochord, the primitive structure that will become the backbone.

Beginning at gastrulation, differentiation of the embryo is controlled by a process called induction, by which adjacent cells and tissues alter the fate of other cells. The mechanism of induction is not well understood, but apparently the cells that are induced to form a structure must originate in the proper germ layer for it to occur. For instance, invaginations called optic vesicles in the embryo induce ectoderm to form an eye lens. If the optic vesicles are transplanted from the head to elsewhere on the body, they can still produce a lens from overlying ectoderm. They cannot produce lenses from mesoderm or endoderm.

Cells destined to become endoderm, mesoderm, and ectoderm form originally in the gastrula by primary embryonic induction, again, a process not well understood.

Neurulation :  Neurulation, or forming of the neural tube, occurs from the 17th to the 25th days. This formation of the precursor of the spinal cord and brain begins by pinching off a tube of cells in the ectoderm alongside the notochord. At this stage, the embryo is only about 2 mm long.

Organogenesis :  After neurulation, development of the body organs, or organogenesis, occurs. The heart, which is still shaped like a swelling in a tube, begins to beat. The nervous system, gut, and blood vessels develop. By the end of the fifth week, the embryo's head makes up one half of the entire body mass. Arm and leg buds, eyes, ears, and nasal organs begin to develop. By the sixth week, the embryo has gill slits and a prominent tail, reminding us of its evolutionary ancestry. These structures are reabsorbed at later stages of development.

Following organogenesis, at the eighth week of pregnancy, the embryo is distinctly human and is called a fetus. These rapidly developing structures are most susceptible to harmful environmental influences during this period of embryogenesis. Drugs and alcohol, which easily reach the fetus through the placenta, are especially dangerous.