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  2. 2. THE DEVELOPING HUMAN Introduction • Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male • Cell division, cell migration, programmed cell death, differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, a zygote, into a multicellular human being • From a single cell to a baby in 9 months, the study of the developmental processes that take place is called Embryology Note: • Development does not stop at birth, • Although most developmental changes occur during the embryonic and fetal periods, • important changes occur during later periods of development: infancy, childhood, adolescence, and early adulthood 2
  3. 3. Terminologies • Oocyte ( ovum /egg): refers to the female germ or sex cells produced in the ovaries • Sperm (spermatozoon): refers to the male germ cell produced in the testes (testicles) • Zygote: This cell results from the union of an oocyte and a sperm during fertilization  A zygote or embryo is the beginning of a new human being Developmental Periods: can be divided into  prenatal (before birth)  postnatal (after birth) period.  Stages of prenatal development: • begins at fertilization and embryonic development ends on day 56 (8th week ) • The fetal period begins on day 57 and ends when the fetus is completely outside the mother 3
  4. 4. Prefertilization Events  Sexual Reproduction • Sexual reproduction occurs when female and male gametes (oocyte and spermatozoon, respectively) unite at fertilization. • Gametes are direct descendants of primordial germ cells, which are first observed in the wall of the yolk sac at week 4 of embryonic development and subsequently migrate into the future gonad region. • Gametes are produced by a process called gametogenesis (formation of gamete) • In males, this process is called spermatogenesis • In females, it is called oogenesis  note : • The sequence of gametogenesis is the same, but the timing of events during meiosis differs in the two sexes. 4
  5. 5. Primodial germ cells in wall of yolk sac Yolk sac 5
  6. 6. • In preparation for fertilization, germ cells undergo gametogenesis. Which include:  meiosis, to reduce the number of chromosomes  and cytodifferentiation to complete their maturation  Chromosomes • A single chromosome consists of TWO characteristic regions called arms. These include:  Short arm/ p arm  Long arm/ q arm • These two arms are separated by a centromere • During meiosis I, single chromosomes undergo DNA replication, which essentially duplicates the arms. • This forms duplicated chromosomes, which consist of two sister chromatids attached at the centromere. 6
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  8. 8. 1) Ploidy and N number • Ploidy refers to the number of chromosomes in a cell • The N number refers to the amount of DNA in a cell  In humans, somatic cells (cells of an organism other than the germ cells) contain 46 single chromosomes  The chromosomes occur in 23 homologous pairs, of which one member (homologue) of each pair is of maternal origin, and the other is of paternal origin to form the diploid number of 46 Note: The term “diploid” is classically used to refer to a cell containing 46 single chromosomes  The 23 homologous pairs of chromosomes is made up of:  22 pairs of matching chromosomes called autosomes  and one pair of sex chromosomes 8