Fertilization and intra-ulterine development

Q.14. Describe briefly the fertilization and intra-uterine development of human being?

Ans. Fertilization takes place only when the sperms enter the female genital tract near about the time of ovulation. Muscular contractions of the uterus and the fallopian tube create a suction for the sperms in the female tract. Each sperm is about 0.05 millimeter in length and consists of a head, neck, body and tail. The head contains the nucleus having dense deoxyribose nucleic acid (D.N.A.) as genetic material. The neck is small having a basal body that encloses the centrioles. The body is rich is mitochondria while the tail has energy rich adenosinetriphosphate (A.T.P.). As soon as the sperms are released in the female genital tract, the tail particularly its flagellum moves rapidly causing the sperm to move at a maximum velocity of half a centimeter in a minute. A sperm has 23 unpaired chromosomes with 22 autosomes and one sex ethics chromosome (X or Y). The sex chromosome of the sperm determines the superior gender of the new life. The ovum has 23 unpaired chromosomes having 22 autosomes and one sex chromosome (all X). As all the ova have X chromosomes they have no role in determining the gender of the zygote. It takes less than an hour for the sperms to move from the place of discharge in the deep vagiua to the place of fertilization in the lateral part of the fallopian tube, covering a distance of 15 to 20 centimeters at an average speed of 3 m.m. per minute. Some sperms even reach the ampulla of fallopian tube, the place of fertilization within ten minutes. Presence of adequate quantity of prostaglandins and hyaluronidase in the spermatic fluid is essential for fertilization. Only one sperm which is one of the strong and active sperms can penetrate the ovum. The head and some part of the neck enter the ovum while the body and tail degenerate. The pronuclei of the sperm and the ovum fuse to form the zygote which carries 23 pairs of chromosomes as either 44 + XX or 44 + XY.

It takes about 265 days or nearly 38 weeks for the zygote to develop in the uterus from the time of fertilization after which it delivers out. It takes about 3 days for the zygote to move from the fallopian tube to reach the uterus by which time it develops into twelve cells called morula stage. The first three weeks after fertilization is called the germinal period, from three weeks to six weeks the embryonic stage and beyond six weeks till parturition the foetal stage. The embryo develops from the embryonic plate. The amniotic cavity has liquor amino fluid which bathes the factus and lined by a double layer amnion which in turn has an outer membrane called chorion at the end of the six weeks of intrauterine sex. Various parts of embryo are differentiated at one month of intra uterine life. The embryo has a germ-disc comprising of three germ layers as endoderm, mesoderm and ectoderm that constitute the various parts of the embryo. The sex cells originate from the primitive endodermal cells of the stem cell at three weeks after fertilization. The gonads differentiate as testes or ovaries after six weeks to form the genital ridge. The placenta becomes well defined by ten weeks of fertilization. The external genitalia appears at ten weeks and becomes clearly differentiated at twelve weeks of intrauterine life. The penis as phallus develops in the presence of male sex hormones while the clitoris develops in the absence of male sex hormones. Genital swellings appear at lower part of phallus to form the scrotal folds which fuse together to from the scrotum. The testes come into the scrotum at thirty four weeks of intrauterine life.
Placental circulation ceases at the time of birth control. Parturition takes place after thirty eight weeks when the neonate is able to survive outside the womb of the mother.