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Enbryology / All About Embryology
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Embryology is the process encompassing the period from the formation of an embryo, through the development of a fetus, to birth.
The human body , like that of most animals, develops from a single cell produced by the union of a male sex cell and a female sex cell. Human development follows closely the basic vertebrate pattern, and it departs only in certain details from the type specifically characteristic of mammals. A prenatal period, in which most of the developmental advances occur, is followed by a long postnatal period. Only at about the age of 25 are the last progressive changes completed.
The development and liberation of the male and female gametes are steps preparatory to their union through the process of fertilization. Random movements first bring some spermatozoa into contact with follicle cells adhering to the secondary oocyte, which still lies high in the uterine tube. The sperm then propel themselves past the follicle cells and attach to the surface of the gelatinous zona pellucida enclosing the oocyte. Some sperm heads successfully penetrate this capsule by means of an enzyme they secrete, hyaluronidase , but only one sperm makes contact with the cell membrane and cytoplasm of the oocyte and proceeds farther. This is because the invading sperm head releases a substance that initiates surface changes in the oocyte cytoplasm that other competitors cannot master.
The successful sperm is engulfed by a conical protrusion of the oocyte cytoplasm and is drawn inward. Once within the periphery of the oocyte, the sperm advances toward the centre of the cytoplasm; the head swells and converts into a typical nucleus, now called the male pronucleus, and the tail detaches. It is during the progress of these events that the oocyte initiates its final maturation division. Following the separation of the second polar body, the oocyte nucleus reconstitutes typically and is then called the female pronucleus of the ripe egg. It is now ready to unite with its male counterpart and thereby consummate the total events of fertilization.
The two pronuclei next approach, meet midway in the egg cytoplasm, and lose their nuclear membranes. Each resolves its diffuse chromatin material into a complete, single set of 23 chromosomes . Centrioles (complex particles involved in cell division), apparently supplied by the sperm, appear, and a mitotic spindle organizes with the two sets of chromosomes arranged midway on it—ready to proceed with a typical mitosis. This climax in the events of fertilization creates a joint product named the zygote. It contains all the essential factors for the development of a new individual.
The fundamental results of fertilization are the following: (1) reassociation of a male and female set of chromosomes (thus restoring the full number and providing the basis for biparental inheritance and for variation); (2) establishment of the mechanism of sex determination for the new individual (this depending on whether the male set of chromosomes included the X or the Y chromosome); (3) activation of the zygote, initiating a beginning toward the production of a new individual.
 The onset of mitosis (ordinary cell proliferation by division) in the activated zygote is a first step toward development in the ordinary sense of that term, and the cells so produced are the first external sign of future body building. To this end, the relatively enormous zygote directly subdivides into many smaller cells of conventional size, suitable as early building units for the future organism. This process is called cleavage and the resulting cells are blastomeres (figure to left). The tendency for the progressive increase in cell numbers to follow a doubling sequence is soon disturbed and then lost. Each blastomere receives the full complement of paternal and maternal chromosomes.
Subdivision of the zygote into blastomeres begins while it is still high in the uterine tube. The cohering blastomeres are transported downward chiefly, at least, by muscular contractions of the tubal wall. Such transport is relatively rapid until the lower end of the tube is reached, and here cleavage continues for about two days before the multicellular cluster is expelled into the uterus. The full reason for this delay is not clear, but it serves to retain the cleaving blastomeres until the uterine lining is suitably prepared to receive its prospective guest.
Since the human egg contains little inert yolk material, and since this is distributed rather evenly throughout the cytoplasm, the daughter cells of each mitosis are practically equal in size and composition. This type of cleavage is known as total, equal cleavage. The sticky blastomeres adhere and the cluster is still retained for a time within the gelatinous capsule—the zona pellucida—that had enclosed the growing and ovulated oocyte. There is no growth in the rapidly dividing blastomeres, so that the total mass of living substance does not increase during the cleavage period.
By the fourth day after ovulation, a cluster of about 12 blastomeres passes from the uterine tube into the uterus. When the cluster numbers 12 to 16 blastomeres it is called a morula (Figure 1D). By the time some 30 blastomeres have been produced, pools of clear fluid accumulate between some of the internal cells, and these spaces soon coalesce into a common subcentral cavity. The resulting hollow cellular ball is a blastula of a particular type that occurs in mammals and is called a blastocyst; its cavity is the blastocoel (see figure above).
An internal cellular cluster, eccentric in position and now named the inner cell mass, will develop into the embryo. The external capsule of smaller cells, enveloping the segregated internal cluster, constitutes the trophoblast. It will contribute to the formation of a placenta. During its stay within the uterine cavity, the blastocyst loses its gelatinous capsule, imbibes fluid, and expands to a diameter of 0.2 millimetre (0.008 inch); this is nearly twice the diameter of the zygote at the start of cleavage. Probably several hundred blastomeres have formed before the blastocyst attaches to the uterine lining. Take this preliminary to see if your condition could respond to treatment.
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