The Continuity of Life I: Reproduction

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The Continuity of Life I Reproduction

• Jacob Murphy
• Period 1

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Sexual Reproduction

• 2 events- meiosis and fertilization
• In Vertebrates- almost always diploid, meiosis
  produces gametes, the only haploid forms in the
  life cycle.
• Gametes specialized for motility (sperm) or for
  production and storage of nutrients(eggs)
  produced in gonads of individuals of the 2
  separate sexes (male and female).

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Sperm and Egg Production
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Sexual Reproduction Continued

• In Invertebrates- (insects in particular) the
  generations are nonoverlapping, (ex. Annual
  plants) in fish and amphibians, and many
  invertebrates, fertilization is external.
• Organisms that lay amniote eggs (reptiles, birds,
  and monotreme mammals) fertilization is internal.
  The outer protective shell, produced as the egg
  moves through the female reproductive tract is
  laid down after the egg cell is fertilized,
  enclosing the embryo and its membranes.
• For Marsupial and placenta animals fertilization
  is also internal, embryo nourished inside the
  mother.

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The Male Reproductive System

• Sperm cells, male gametes, are produced in the
  testes(sing. Testis)
• Testes are developed in the abdominal cavity of
  the male embryo. In humans descend in to an
  external sac, the scrotum.
• A temperature of 3 degrees lower than that of the
  body necessary to produce sperm.

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Spermatogenesis

• Each testes divided into about 250 compartments
  (lobules) and each is packed with tightly coiled
  seminiferous (seed-bearing) tubules. Sperm
  producing regions.
• Between the tubules are the interstitial cells,
  source of the male sex hormone testosterone.
• Tubules contain 2 types of cells spermatogenic
  cells and Steroli cells

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Spermatogenic Cells

• Pass through several stages of differentiation.
  Once production of sperm begins at puberty in the
  human male, it goes on continuously, so it is
  possible to find cells in all of the stages of
  spermatogenesis.
• Unusual to find all of the stages in a single
  cross section because spermatogenesis occurs in
  waves.
• In the the first stage of spermatogenesis,
  spermatogonia line the basement of the membrane
  of each seminiferous tubule.
• The secondary spermocytes undergo the second
  meiotic division to produce spermatids, each of
  which contains the haploid number of single
  chromosomes.

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Spermatogenic Cells cont.

• Spermatogonia are diploid and have, in humans, 44
  autosomes and 2 sex chromosomes, X and Y.
• Divide continuously
• Primary spermocytes undergo the second meiotic
  division to produce spermatids, each of which
  contains 22 autosomes and either X or Y each of
  the 23 chromosomes consist of 2 chromotids.

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Differentiation of Spermatids

• A spermatid is a small spherical or polygonal
  cell that develops into a sperm cell.
• 1st visible sign of differentiation of a
  spermatid is the appearance, within the Golgi
  complex, of vesicles containing small, dark
  granules. The vesicles enlarge and coalesce into
  a single vesicle, the arcosome.
• The arcosomes contains enzyme which aids the
  sperm in penetrating the surface of the egg.
• Position of the vesicle determines the polarity
  of the sperm, that is, it establishes where the
  anterior (head) is going to be.

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Differentiation of Spermatids (continued)

• During early stages of acrosome formation, a
  centriole from the cell appears to initiate the
  assembly of tubulin dimers into micro tubules-
  the beginning of the sperm flagellum.
• One sperm eventually forms part of a connecting
  piece within the neck of the sperm, linking the
  flagellum to the nucleus.
• As the flagellum grows, the axial filament
  appears that the 92 structure characteristic of
  eukaryotic cilia and flagella.
• Mitochondria aggregate about its basal end,
  forming a continuous spiral, providing a ready
  energy source (ATP) for flagellar movement.

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Differentiation of Spermatids-cont.

• The rest of the axial filament is surrounded by
  nine additional protein fibers tightly coiled in
  a helix that forms a fibrous sheath.
• During this period the nucleus condenses,
  apparently by eliminating water.
• In its final form, the sperm cell consists of the
  acrosome, the tightly condensed nucleus, the
  connecting piece in the neck, the mitochondrial
  sheath, and the long, powerful flagellum itself,
  all bounded by the cell membrane.
• The DNA and its associated protein are condensed
  and coiled in the sperm head.

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HUMAN SPERM CELL
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Pathway of the Sperm

• From the testis the sperm are carried to the
  epidermis, which consists of a coiled tube 7
  meters long, overlying the testis.
• The sperm are nonmotile when they enter the
  epidermis and gain motility only after some 18
  hours there.
• From the epidermis, the sperm pass to the vas
  deferens, where most are stored.
• The vas deferens are covered with a heavy, three
  layered coat of smooth muscle whose contractions
  propel the sperm along.

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Male Reproductive System
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Pathway of Sperm- cont.

• The vas deferens merge with the ducts of the
  seminal vesicles. The vas deferens from each
  testis then enters the prostate gland and merges
  within the urethra, which extends the length of
  the penis.
• The urethra serves both for the excretion of
  urine and the ejaculation of sperm.

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Erection of the Penis and Orgasm in the Male

• The Penis is formed of three cylindrical masses
  of spongy erectile tissue, each of which contains
  a large number of small spaces, each about the
  size of a pinhead.
• Erection, which can be elicited by a variety of
  stimuli, occurs as a consequence of an increased
  flow of blood that fills the spongy, erectile
  tissues of the penis.
• Blood flow is controlled by parasympathetic nerve
  fibers to the blood vessels leading into the
  erectile tissues.

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Erection- cont.

• Erection is accompanied by the discharge into the
  urethra of a small amount of fluid from the
  bulbourethral glands. (pea-shaped organs at the
  base of the penis)
• This fluid serves as a lubricant, enabling the
  movement of spermazoa along the male urethra and
  aiding penetration of the penis into the female.

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The Role of Hormones

• Testis are major source of male hormones, known
  collectively as androgens.
• Principle androgen is testosterone, which is
  necessary for sperm production.
• It is a steroid, produced primarily by the
  interstitial cells of the testes.
• Others are produced in the adrenal cortex.
• Testosterone not only affects production of
  sperm, but also growth of the larynx (deepening
  of the voice), increase in skeletal size, and a
  characteristic distribution of body hair.

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Continued

• Androgens stimulate the biosynthesis of proteins
  and so of muscle tissue.
• They stimulate the apocrine sweat glands, whose
  secretions attract bacteria and so produce the
  body odors associated with sweat after puberty.
• Often cause sebaceous glands of the skin to over
  activate, causing acne.
• Secondary sex characteristics are sex hormones
  but not directly involved in reproduction.

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Secondary sex characteristics

• Example the cocks comb and spurs, and the bright
  plumage of many adult male birds.

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Regulation of Hormone Production

• Testosterone is regulated by a negative feedback
  system involving a gonadotropic hormone called
  luteinizing hormone, which is produced in the
  pituary gland, under the influence of
  hypothalamus.
• Inhibin is secreted by the Steroli cells which
  regulates FSH (follicle-stimulating hormone)
• Testosterone is dramatically affected by the
  synthetic compounds known as anabolic steroids.
  Which were developed in Germany in the 1930s to
  produce muscle-building effects.

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The Female Reproductive System

• The gamete producing organs are the ovaries, each
  a solid mass of cells about 3 centimeters long.
• They are suspended in the abdominal cavity by
  ligaments and mesenteries.
• The oocytes, from which the eggs develop, are in
  the outer layer of the ovary.
• The uterus is a hollow, muscular, pear-shaped
  organ slightly smaller in size than a clenched
  fist in non pregnant female.
• The smooth muscles in the walls move in a
  continuous waves.

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Female Reproduction cont.

• The muscular sphincter guarding the opening of
  the uterus is the cervix. The sperm pass through
  this opening on their way to the oocyte.
• The vagina is a muscular tube about 7.5
  centimeters long that leads from the cervix of
  the uterus to the outside of the body.
• The lining of the vagina is rich in glycogen,
  which bacteria normally present in the vagina
  convert to lactic acid. Also mildly acidic,
  between pH 4 and 5.
• The vulva and the clitoris, composed chiefly of
  erectile tissue, are the external genital organs.

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Female Reproduction -cont.

• The labia are two flaps of skin, which enclose
  and protect the underlying more delicate
  structures.

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Female Reproductive System
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Oogenesis

• Primary oocytes remain in prophase until the
  female matures sexually.
• Then, under the influence of hormones, the first
  meiotic division of a primary oocyte resumes,
  resulting in a secondary oocyte and a polar body.
  About the time of ovulation.
• Maturation involves both meiosis and a great
  increase in size.
• Oocytes do not divide into two equal sized cells,
  one very large cell is produced. The other nuclei
  are then discarded.
• As the nucleus divides, the cytoplasm of the
  oocyte bulges out.

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Oogenesis-cont.

• One set of chromosomes move into the bulge, which
  pinches them into a small cell, the first polar
  body.
• The second meiotic division does not take place
  till after division, which produces an ovum and
  another small polar body.
• All the polar bodies eventually die.

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Oocytes

• Develop near the surface of the ovary. An oocyte
  and the specialized cells surrounding it are
  known as an ovarian follicle.
• The follicle supplies nutrients to the growing
  oocyte and also secretes estrogens.

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Pathway of the Oocyte
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Orgasm in the Female

• When under the influence of a variety of stimuli,
  the clitoris and its bulbs become engorged and
  distended with blood.
• Orgasm is marked by rhythmic muscular
  contractions, followed by the expulsion into
  veins of the blood trapped in the engorged tissue.

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Hormonal Regulation in Females

• The menstrual cycle is the recurring pattern of
  varying hormone levels and tissue changes.
• It is timed and controlled by the hypothalamus.
• The onset of menstruation marks the beginning of
  puberty in the female human.

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Menstrual Hormone Levels
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Estrus

• Females of most mammalian species, except homo
  sapiens, will mate only during this fertile
  period, during which oocytes are released.
• Estrus may occur only once a year, wolves and
  dear, about once a month, cows and horses, or
  every few days, rats and mice.

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Contraceptive Techniques.

• With no contraceptive techniques, 80 percent of
  women of childbearing age having regular sexual
  intercourse will become pregnant with in a year.
• In order of effectiveness vasectonomy(no sperm
  released), tubal ligation(prevents passage of
  oocyte to uterus), The pill(inhibits secretion
  of FSH and LH, prevents follicle maturation and
  ovulation)

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The End -Chap. 44 Slide Presentation

• Presented by Jacob Murphy