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Chapter 18

Chapter 18. Biology 25: Human Biology Prof. Gonsalves Los Angeles City College Based on Mader’s Human Biology ,7 th edition and Fox’s 8 th ed Powerpoints. Fertilization. Ejaculation 300 million sperm, 100 reach (uterine) fallopian tube. Fertilization occurs in the uterine tubes

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Chapter 18

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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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