M6.1

• Stages of development


  • Ovum
  • Zygote
  • Embryo
  • Fetus
  • Conceptus
• Age of viability


  The earliest age at which fetuses survive if they are born is generally accepted as 24 weeks or at the point a fetus weighs more than 500-600 g
• Fertilization


  The union of an ovum and a spermatozoon
• Fertilization usually occurs in the outer third of a fallopian tube, termed the ampullar portion
• Usually, only one ovum reaches maturity each month
• Fertilization must occur fairly quickly because an ovum is capable of fertilization for only about 24 hours (48 hours at the most)
• After 24 hours, the ovum atrophies and becomes nonfunctional
• The total critical time span for successful fertilization is about 6 days (5 days before ovulation plus 24 hours afterward)
• Zona pellucida


  A ring of muco-polysaccharide fluid surrounding the ovum
• Corona radiata


  A circle of cells surrounding the ovum that serves as protective buffers against injury
• Ovum transport


  1. Ovum propelled into fallopian tube

  2. Spermatozoa reach cervix within 90 seconds

  3. Spermatozoa reach fallopian tube within 5 minutes
• An ejaculation of semen averages 2.5 mL of fluid containing 20 to 200 million spermatozoa per milliliter
• Fertilization depends on at least three separate factors
• Factors for successful fertilization


  • Equal maturation of both sperm and ovum
  • Ability of the sperm to reach the ovum
  • Ability of the sperm to penetrate the zona pellucida and cell membrane
• Under ordinary circumstances, only one spermatozoon is able to penetrate the cell membrane of the ovum
• Once a spermatozoon penetrates the ovum, the cell membrane changes composition to become impervious to other spermatozoa
• Zygote


  The single-cell fertilized ovum
• The fertilized ovum has 46 chromosomes
• If an X-carrying spermatozoon fertilizes the ovum

  The resulting child will be assigned female at birth (XX)
• If a Y-carrying spermatozoon fertilizes the ovum

  The resulting child will be assigned male at birth (XY)
• Zygote migration


  1. Zygote migrates to uterus

  2. Mitotic cell division begins

  3. First cleavage occurs at about 24 hours

  4. Zygote consists of 16 to 50 cells by the time it reaches the uterus
• Blastocyst


  The structure formed when the zygote consists of 16 to 50 cells
• Trophoblast cells


  Cells in the outer ring of the blastocyst that will later form the placenta and membranes
• Embryoblast cells


  The inner cell mass of the blastocyst that will form the embryo
• Implantation occurs approximately 8 to 10 days after fertilization
• About 8 days after ovulation, the blastocyst sheds the last residues of the corona and zona pellucida
• Implantation is an important step in pregnancy because as many as 50% of zygotes never achieve it
• Occasionally, a small amount of vaginal spotting appears on the day of implantation
• The decidua is the term for the endometrium during pregnancy
• Chorionic villi


  Probing structures that reach into the uterine lining and are involved in placental hormone production
• Human Chorionic Gonadotropin (hCG)


  The first placental hormone produced
• hCG can be found in maternal blood and urine as early as the first missed menstrual period
• The purpose of hCG is to ensure the corpus luteum continues to produce progesterone and estrogen
• The structure of hCG is similar to that of luteinizing hormone of the pituitary gland
• The syncytial (outer) layer of the chorionic villi develops into a separate and important hormone-producing system
• Human Chorionic Gonadotropin (hCG)


  The first placental hormone produced, found in maternal blood and urine as early as the first missed menstrual period
• Levels of hCG vary throughout pregnancy
• The pregnant person's blood serum will be completely negative for hCG within 1 to 2 weeks after birth
• Finding no serum hCG after birth can be used as proof that placental tissue is no longer present in the uterus
• Purpose of hCG


  To ensure the corpus luteum of the ovary continues to produce progesterone and estrogen