Human Reproductive System

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Human Reproductive System
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Purpose of Reproduction

• Reproduction is the process by which existing
  cells or organisms produce new cells or organisms
  to replace those that die
• There are 2 main types of reproduction
• Asexual Reproduction
• Sexual Reproduction

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Asexual Reproduction

• Simple organisms tend to reproduce asexually by
  processes such as budding and binary fission
• Asexual reproduction involves only one parent
  cell/organism
• The resulting organism is genetically identical
  to the organism from which it came
• Cloning is a type of asexual reproduction

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Sexual Reproduction

• Complex organisms (ex. animals and plants) tend
  to reproduce sexually
• Sexual reproduction involves the fusion of 2
  different types of cells
• The resulting organism is genetically different
  from either of the 2 cells that united
• Genetic recombination occurs due to the
  combination of 2 different sets of genes/DNA
• The offspring from sexual reproduction may have
  characteristics that make them more likely to
  survive

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Sexual Reproduction (continued)

• In most sexually reproducing organisms, the cells
  that fuse are called gametes, or sex cells
• The male gametes are sperm the female gametes
  are ova (or eggs)
• Gametes are produced in the gonads, or sex organs
• The male gonads are the testes the female gonads
  are the ovaries

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Sexual Reproduction (continued)

• The cell that results from the union of sperm and
  egg is called a zygote
• The process by which the 2 sex cells unite is
  called fertilization

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Sexual Reproduction (continued)

• In humans, gametes have the haploid chromosome
  number of 23
• A zygote has the diploid chromosome number of 46
• Gametes are produced by meiosis
• A zygote divides by mitosis as it becomes an
  embryo
• During embryonic development, the cells of the
  new organism become different from one
  anothermore specialized, by a process called
  differentiation

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Twins

• Occasionally, as a zygote divides repeatedly by
  mitosis, the daughter cells formed separate from
  one another
• If these cells continue to divide and
  differentiate into separate individuals,
  identical twins (or triplets, etc.) result these
  twins have identical DNA, so they are the same
  gender
• Fraternal twins result when two eggs are
  fertilized (by two sperm) these twins are no
  more alike than normal siblings and may not even
  be of the same gender

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Male Reproductive System

• The male gonads (or sex organs) are the testes
• The testes produce sperm and the hormone
  testosterone
• Testosterone is the primary male sex hormone it
  stimulates spermatogenesis and development of the
  secondary sex characteristics
• Male secondary sex characteristics include the
  growth of facial and body hair, muscle
  development, deepening of the voice, and the
  growth of the male accessory sex organs

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Male Reproductive System (continued)

• Testes develop within the abdominal cavity of a
  male embryo, but about 2 months before birth they
  descend into the scrotum
• The scrotum is a sac that suspends the testes
  from the groin and maintains the temperature of
  the developing sperm at about 2C below normal
  human body temperature
• Sperm cells cannot develop at temperatures as
  high as normal body temperature

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Male Reproductive System (continued)

• The testes are composed primarily of a tangled
  mass of hollow tubes called seminiferous tubules
• These tubules are the site of spermatogenesis,
  the meiotic process by which sperm are produced
• Once a male reaches puberty, this process
  proceeds continuously millions of sperm are
  produced each day

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Spermatogenesis

• The seminiferous tubules are lined with
  undifferentiated diploid cells called
  spermatogonia
• New spermatogonia are constantly produced by
  mitosis
• When puberty is reached, testosterone is produced
  at a level high enough to stimulate the
  production of sperm from spermatogonia

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Spermatogenesis (continued)

• Some of the spermatogonia mature into diploid
  cells called primary spermatocytes
• Primary spermatocytes are the cells that enter
  Meiosis I and divide into 2 daughter cells
• These daughter cells are haploid and are called
  secondary spermatocytes

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Spermatogenesis (continued)

• Each secondary spermatocyte enters Meiosis II and
  divides into 2 haploid cells called spermatids
• Spermatids mature and become sperm cells
• 4 sperm result from division of 1 primary
  spermatocyte

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Sperm Cell Structure

• Sperm cells look very different from spermatids
• Each sperm cell consists of 3 regions
• Head
• Middlepiece
• Tail

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Sperm Cell Structure (continued)

• The head contains most of the cytoplasm of the
  cell, the nucleus, and the DNA
• The middlepiece contains numerous mitochondria,
  which produce ATP energy by cellular respiration
• The tail is actually a flagellum, which enables
  the sperm cell to swim
• At the front end of a sperm cell is an acrosome,
  an enzyme packet that contains chemicals to help
  the sperm penetrate an egg

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Other Cells

• Sertoli cells are nurse cells located in the
  seminiferous tubules (along with the developing
  sperm cells) which nourish the sperm
• In between the seminiferous tubules are
  interstitial cells, which secrete the hormone
  testosterone

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Male Accessory Sex Organs

• Once sperm are produced by spermatogenesis within
  the seminiferous tubules of the testes, they pass
  into a long, coiled tube called the epididymis
• Further sperm maturation occurs within the
  epididymis over about an 18-hour time period the
  sperm become motile during this time

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Male Accessory Sex Organs (continued)

• From the epididymis, the sperm enter another long
  tube, the vas deferens, which passes into the
  abdominal cavity
• The vas deferens is actually the main sperm
  duct
• During a vasectomy (a male sterilization
  procedure), a portion of each vas deferens is cut
  and the open ends are tied this prevents sperm
  from passing through the remainder of the male
  reproductive tract

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Male Accessory Sex Organs (continued)

• The vas deferens from each testis joins with one
  of the ducts from a pair of glands called the
  seminal vesicles
• The seminal vesicles produce a fluid which
  nourishes sperm
• This fluid is part of semen, which also contains
  sperm cells and secretions from the prostate
  gland

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Male Accessory Sex Organs (continued)

• Next, the sperm ducts pass through the prostate
  gland, which is located at the base of the
  urinary bladder
• The prostate gland is normally about the size of
  a golf ball
• It contributes about 60 of the composition of
  semen

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Male Accessory Sex Organs (continued)

• Beyond the prostate gland, the sperm duct merges
  with the urethra, the duct that exits the urinary
  bladder (which stores urine)
• The urethra carries the semen out of the body
  through the penis during coitus, or sexual
  intercourse
• The penis is the copulatory organ of the male it
  delivers sperm to the female reproductive tract
  during internal fertilization

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Female Reproductive System

• The female gonads (or sex organs) are the ovaries
• The ovaries produce eggs, or ova, and the
  hormones estrogen and progesterone
• Eggs develop within small sacs in the ovaries
  called follicles

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Oogenesis

• Within the follicles of the ovaries,
  undifferentiated diploid cells called oogonia
  mature into primary oocytes (also diploid)
• This occurs before birth when a female is born,
  she has all the primary oocytes (future eggs) she
  will ever produce
• These cells enter a rest period that lasts
  until puberty

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Oogenesis (continued)

• When puberty is reached, estrogen is produced at
  a level high enough to stimulate the further
  development of the primary oocytes
• The activities that occur in the ovaries happen
  over the course of about a month and are cyclic
  in nature
• These activities make up the menstrual cycle

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Oogenesis (continued)

• Each month a few follicles are stimulated to grow
  and mature
• At the same time, the primary oocyte within these
  particular follicles divides during Meiosis I to
  produce two haploid daughter cells
• This division occurs unevenly, though, producing
  one large cell and one small cell
• The large cell is called a secondary oocyte the
  small cell is called a polar body

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Oogenesis (continued)

• The polar body usually disintegrates because it
  has very little cytoplasm
• The secondary oocyte begins Meiosis II, but stops
  in metaphase II
• The cell is discharged from the mature ovarian
  follicle during ovulation
• It only completes division if fertilization
  occurs

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Oogenesis (continued)

• In this case, the secondary oocyte divides into
  another polar body and and a haploid ovum
• Occasionally, the first polar body survives and
  divides also, forming two more polar bodies
• 4 cells may result from the division of 1 primary
  oocyte, but only one of these cells can become an
  egg

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Female Accessory Sex Organs

• The Fallopian tubes, or oviducts, are tubes with
  muscular walls and cilia to help transport an egg
  from the ovaries to the uterus
• Fertilization usually happens in the first third
  of the Fallopian tube
• The uterus, or womb, is a muscular, pear-shaped
  organ in which an embryo grows and develops
• The cervix is the tapered, neck-like entrance to
  the uterus it projects slightly into the birth
  canal, or vagina, which opens to the outside of
  the body

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Hormonal Control of Reproduction

• The pituitary gland, which is located at the base
  of the brain, secretes two hormones that regulate
  reproductive function in both males and females
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)

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Hormonal Control of Reproduction (continued)

• FSH and LH are released beginning at puberty
• In females, FSH stimulates the growth of ovarian
  follicles and the secretion of estrogen LH
  stimulates ovulation
• In males, FSH stimulates development of the
  seminiferous tubules and spermatogenesis LH
  stimulates the interstitial cells to secrete
  testosterone

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The Female Menstrual Cycle

• At puberty, the ovaries begin to secrete the
  major female sex hormone estrogen
• Estrogen stimulates the formation of the female
  secondary sex characteristics and the beginning
  of the monthly menstrual cycle
• A typical menstrual cycle lasts 28 days, but the
  number of days may vary

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The Menstrual Cycle (continued)

• Each month from puberty until menopause, this
  cycle runs its course to prepare the body for a
  possible pregnancy
• Menopause occurs at about age 50 at this time,
  the ovaries stop producing estrogen and eggs
  for fertilization

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The Menstrual Cycle (continued)

• Day 1 of the menstrual cycle is the first day of
  menstruation
• During menstruation the lining of the uterus, the
  endometrium, is sloughed off, accompanied by
  bleeding
• The endometrium is an epithelial cell layer of
  the uterus that thickens each month in
  preparation for the possible implantation and
  development of an embryo

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The Menstrual Cycle (continued)

• If fertilization does not occur, this layer
  degenerates and is discharged through the vagina
• Menstruation lasts about 5 days
• At this time, the pituitary releases FSH, which
  travels through the bloodstream to the ovaries
• FSH stimulates several follicles in the ovaries
  to develop/mature also, oogenesis of a primary
  oocyte occurs, resulting in the production of a
  secondary oocyte (and a polar body)

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The Menstrual Cycle (continued)

• The developing follicles of the ovaries secrete
  estrogen, which causes the endometrium of the
  uterus to begin thickening again
• The increase in the level of estrogen in the
  blood signals the pituitary gland to secrete a
  surge of LH
• Due to this LH surge (on about day 14), a mature
  follicle ruptures and releases the secondary
  oocyte in the process called ovulation
• Once ovulation occurs, the secondary oocyte has
  about 24 hours to be fertilized if it is not
  fertilized, it dies and is flushed from the body
  during the next menstruation

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The Menstrual Cycle (continued)

• The ruptured ovarian follicle begins to heal and
  becomes a corpus luteum
• The corpus luteum begins to secrete the hormone
  progesterone
• Progesterone maintains the endometrium so an
  embryo can implant
• If fertilization and implantation do not occur,
  eventually progesterone secretion stops and
  menstruation begins

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The Menstrual Cycle (continued)

• If fertilization does occur, a zygote is formed,
  the endometrium is not sloughed off, and
  menstruation does not occur
• The zygote begins dividing by mitosis to form an
  embryo
• About a week after fertilization, the embryo
  implants in the endometrium of the uterus, where
  it grows and develops until birth

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The Menstrual Cycle (continued)

• The period of time prior to ovulation is called
  the preovulatory phase the period of time after
  ovulation is called the postovulatory phase
• Normally, ovulation occurs on day 14 and the
  preovulatory and postovulatory phases each last 2
  weeks

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Fertilization and Pregnancy

• Once ovulation occurs, a secondary oocyte has
  about 24 hours to be fertilized (or it dies)
• A sperm cell has 2-3 days (48-72 hours) to
  fertilize a secondary oocyte (or it dies)
• Fertilization is the union of a sperm cell and an
  egg (actually a secondary oocyte) to form a
  zygote
• This process generally occurs in one of the
  Fallopian tubes

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Fertilization and Pregnancy (continued)

• A zygote reaches the uterus about a week after
  fertilization at this time it actually is an
  embryo, consisting of a ball of many cells
• The embryo implants in the endometrial lining of
  the uterus
• Membranes that develop around the embryo secrete
  the hormone human chorionic gonadotropin (hCG)

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Fertilization and Pregnancy (continued)

• This hormone signals the corpus luteum to
  continue to secrete progesterone so the
  endometrium will remain
• hCG is the pregnancy hormone detected in both
  blood and urine pregnancy tests its level
  increases consistently during early pregnancy