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Fertilization, Implantation, and Placental Development in Human Biology

This chapter discusses the process of fertilization, implantation, and placental development in human biology. Key topics include the acrosomal reaction, blastocyst formation, placental changes, placental hormones, critical stages of embryonic and fetal development, parturition, pregnancy and human development, and lactation.

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Fertilization, Implantation, and Placental Development in Human Biology

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  1. Chapter 18 Biology 25: Human Biology Prof. Gonsalves Los Angeles City College Based on Mader’s Human Biology,7th edition and Fox’s 8th ed Powerpoints

  2. Fertilization • Ejaculation 300 million sperm, 100 reach (uterine) fallopian tube. • Fertilization occurs in the uterine tubes • Acrosomal reaction: • Acrosome of sperm contains hyaluronidase, an enzyme that digests a channel through zona pellucida. • Sperm fuses with ovum cell membrane.

  3. Fertilization • As fertilization occurs, secondary oocyte completes 2nd meiotic division. • Sperm enters ovum cytoplasm. • Ovum nuclear membrane disappears, zygote formed.

  4. Blastocyct Formation • Cleavage: • 30-36 hrs. after fertilization the zygote divides by mitosis. • Blastocyst develops: • Inner cell mass: fetus. • Surrounding chorion: trophoblasts form placenta.

  5. Implantation • 6th day after fertilization, blastocyst attaches to uterine wall. • Blastocyst secretes enzymes that allow blastocyst to burrow into endometrium. • Trophoblast cells secrete hCG.

  6. Amnion • Envelop the embryo. • Amnionic fluid contains sloughed cells of the fetus.

  7. hCG • Human chorionic gonadotropin. • Trophoblast cells secrete hCG. • Signals corpus luteum not to degenerate until placenta secretes adequate [hormone]. • Effects similar to LH. • Basis of pregnancy test.

  8. Placenta • Syncytiotrophoblast secretes enzymes that create blood filled cavities in the maternal tissue. • Cytotrophoblast then from projections (villi) that grow into the venous blood. • Producing chorion frondosum on the side that faces the uterine wall. • Other side of chorion bulges into the uterine cavity.

  9. Placental Changes • Decidual reaction: • Endometrial growth. • Accumulation of glycogen. • Decidua basalis: maternal tissue in contact with the chorion frondosum. • Maternal and fetal blood do not mix.

  10. Placenta Function • Gas exchange: • 02 and C02. • Nutrient exchange. • Waste exchange. • Synthesis of proteins and enzymes.

  11. Placental Hormones • hCS: • Chorionic somatomammotropin. • GH effects. • Diabetic-like effect: • Glucose sparing effect. • Polyuria. • Lipolysis.

  12. Placental Hormones • Fetal-placental unit: • Placenta must cooperate with the adrenal cortex in the fetus to produce estrogen. • Estrogen stimulates: • Endometrial growth. • Inhibit prolactin secretion. • Growth of mammary ducts. • Enlargement of mother’s uterus.

  13. Placental Hormones • Progesterone: • Suppresses uterine contractions. • Stimulates uterine growth . • Suppresses LH and FSH. • Stimulate development of alveolar tissue of the mammary gland.

  14. Critical Stages of Embryonic and Fetal Development

  15. Formation of Testes • First 40 days after conception the gonads of males and females are similar in appearance. • Spermatogonia and oogonia migrate from yolk sac to developing embryonic gonads. • TDF (testis-determining factor) promotes the conversion to testes: • Seminiferous tubules: • Germinal cells: sperm. • Nongerminal cells: Sertoli cells. • Leydig cells: • Appear about day 65.

  16. Formation of Testes • Leydig cells secrete testosterone. • Begins 8th week and peaks at 12-14th week. • Masculinizes embryonic structures. • Testosterone then declines to very low levels until puberty. • Testes descend into scrotum shortly before birth.

  17. Formation of Ovaries • Absence of Y chromosome and TDF, female develop ovaries. • Ovarian follicles do not appear until 2nd trimester.

  18. Development of Accessory Sex Organs and Genitalia • Presence or absence of testes determines the accessory sex organs and external genitalia. • Male accessory organs derived from wolffian ducts. • Sertoli cells secrete MIF. • Female accessory organs derived from mullerian ducts.

  19. Parturition • Estrogen in late pregnancy: • Increases amount of oxytocin stored. • Stimulate production of oxytocin receptors in myometrium. • Stimulate prostaglandin production. • Uterine contractions: • Oxytocin. • Prostaglandins.

  20. Pregnancy and Human Development • Gestation: In humans starts at conception and lasts approximately 38 weeks (266 days). • Due date typically 40 weeks after last menstruation. • First Trimester: By the end of week 13 • Sex distinguished externally • Fetus weighs about 150 grams • Second Trimester: By the end of week 26 • Fetus weighs about 1000 g • May survive if born at this point. • Third Trimester: By the end of week 38 • Fetus weighs about 3500 g (7 ½ lb.) • Testes have descended in males.

  21. Lactation • Hypothalamus releases PRH. • Anterior pituitary releases prolactin: • Stimulate milk production. • Oxytocin needed for “milk letdown”.

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