Biology 25: Human Biology

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Chapter 18

• Biology 25 Human Biology
• Prof. Gonsalves
• Los Angeles City College
• Based on Maders Human Biology,7th edition and
  Foxs 8th ed Powerpoints

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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.

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Fertilization

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

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Blastocyct Formation

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

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Implantation

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

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Amnion

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

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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.

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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.

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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.

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Placenta Function

• Gas exchange
• 02 and C02.
• Nutrient exchange.
• Waste exchange.
• Synthesis of proteins and enzymes.

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Placental Hormones

• hCS
• Chorionic somatomammotropin.
• GH effects.
• Diabetic-like effect
• Glucose sparing effect.
• Polyuria.
• Lipolysis.

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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 mothers uterus.

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Placental Hormones

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

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Critical Stages of Embryonic and Fetal Development
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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.

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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.

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Formation of Ovaries

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

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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.

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Parturition

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

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• 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.

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Lactation

• Hypothalamus releases PRH.
• Anterior pituitary releases prolactin
• Stimulate milk production.
• Oxytocin needed for milk letdown.

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