human reproduction

• Reproduction


  The fundamental biological purpose of all organisms is to reproduce and make more of themselves
• Reproductive strategies


  • Asexual reproduction
  • Sexual reproduction
• Sexual reproduction


  • Creates offspring with a slightly different genome, helping the new generation stay one step ahead of pathogens or competitors
• Gametes


  Haploid cells with one set of chromosomes, formed by meiosis
• Types of gametes


  • Ovum/egg
  • Sperm
• Fertilization


  The fusion of a sperm and an egg to create a single diploid cell with all the instructions to make a new organism
• The only consistent biological difference between males and females is that females produce large, less mobile gametes, while males produce smaller, more mobile gametes</b>
• Females


  • Tend to be pickier about mate quality, as they have a limited number of eggs and want them fertilized by high quality genes
• Males


  • Generally louder, larger, brighter, and more combative than females in order to attract mates
• Sex determination in mammals

  1. XX chromosomes = female

  2. XY chromosomes = male
• In mammals, the default sex is female, and the presence of a Y chromosome causes the development of male characteristics
• Secondary sex characteristics


  Physical traits that develop at puberty and distinguish males and females, like facial/body hair, muscle/fat distribution, antlers, etc.
• Female reproductive system


  1. Ovaries produce eggs

  2. Eggs travel through fallopian tubes to uterus

  3. Uterine lining builds up and sheds during menstrual cycle if egg is not fertilized
• Male reproductive system


  1. Testes produce sperm

  2. Sperm mature in epididymis

  3. Sperm travel through vas deferens and mix with fluids from seminal vesicles and prostate to form semen

  4. Semen is ejaculated through penis
• Components of the reproductive system

  • Primary, internal sex organs (gonads like testes and ovaries)
  • Sex hormones
  • Gametes (sperm and eggs)
  • Glands
  • Ducts
  • External genitalia
  • Particular brain parts
• All animals have their own particular and fascinating anatomical methods for getting their gametes together
• Vulva

  Includes the mons pubis over the pubic bone, and labia majora and labia minora — the elongated skin folds that surround the vestibule, which contains both the urethral and vaginal openings
• Vagina


  How menstrual blood and babies leave the body, and how sperm gets in
• Ovaries


  Their main job is to produce and release female gametes and sex hormones like estrogen and progesterone
• Gametes

  Haploid cells, meaning that they only have one set of chromosomes, and are formed by meiosis
• When a sperm fuses with an egg, they make a diploid cell, which has all the genetic instructions required to make a baby
• Ovarian follicles


  Tiny-sac-like structures that each hold a single primary oocyte — a sort of incomplete proto-egg — along with a bunch of supporting follicle cells around it
• Females are born with essentially all of the primordial follicles they will ever have -- around 1 million at the time of birth
• The actual process of egg creation, or oogenesis, is delayed until puberty
• Human eggs mature one-by-one, almost constantly, doled out so that every month or so, a mature egg is either fertilized, or dies to make way for a new egg
• Menstrual cycle

  What happens in the uterus to prepare for a fertilized egg
• Ovarian cycle


  The maturation of the follicle and egg, and it's what drives the menstrual cycle
• Ovarian cycle


  1. Primordial follicles begin maturation

  2. One follicle is selected to fully mature

  3. Follicle ruptures and releases the mature oocyte (ovulation)

  4. Ruptured follicle becomes corpus luteum
• Hypothalamus and pituitary


  Set up the ovarian and menstrual cycles by secreting hormones
• Ovulation


  1. Follicle-stimulating hormone stimulates follicle growth

  2. Luteinizing hormone triggers oocyte to complete meiosis I and move to metaphase II

  3. Mature follicle ruptures and releases oocyte
• Corpus luteum


  The structure that forms from the ruptured follicle, and secretes progesterone and other hormones
• Menstrual cycle


  1. Proliferative phase (days 6-14): Estrogen stimulates endometrium regeneration

  2. Secretory phase: Progesterone from corpus luteum thickens endometrium

  3. If no fertilization, corpus luteum degenerates and endometrium sheds (menstruation)
• If the egg is fertilized, the corpus luteum continues to secrete progesterone to support the thickened endometrium for implantation
• Sex is a big gamble: 'I mean, the mental risks alone that we take with our relationships are taxing enough: When is he going to text me back? Am I good enough? What if she doesn't like the real me? But the emotional price we pay for intimacy can almost seem paltry compared to the physical investments that our bodies make in order to reproduce.'
• Female gamete


  A big bet. It's like putting all your chips on black. An ovum and follicle take over a year to develop, and, as human cells go, it's huge -- about a tenth of a millimeter, just large enough to be seen with the naked eye. Plus, it requires a whole network of supporting tissues in the uterus while it waits for fertilization, which -- if that doesn't happen -- then have to be shed and re-built a month later.
• Male reproductive strategy


  The opposite of the female. Male gametes are basically the penny slots. Human sperm are only about one-hundred-thousandth the mass of an egg. And they're almost as easy to produce as skin cells -- actually, the process by which they mature is pretty similar. That's because sperm are small and stripped down -- not invested in much beyond a nucleus, and a tail, and some extra mitochondria to fuel their journey. All they have to do is drop off their DNA, and fall apart.
• The risk of failure is still there. In fact, with male gametes, the odds are overwhelmingly against any single sperm actually accomplishing its mission.
• While the female reproductive system meets this risk head-on, with a single, big play, the male's tries to beat the odds with the sheer force of numbers.
• Testes


  The gonads of the anatomical male, tasked with making male gametes, sperm, and the androgen hormone testosterone. They are located outside the abdominal cavity, in the scrotum, because sperm are sensitive to temperature and require a lower temperature for proper production.
• Testes


  • Divided into about 250 sections called lobules, which are loaded with tightly-coiled seminiferous tubules. These tubules are the sperm factories, made of a stratified epithelium surrounding a central fluid-filled lumen. Supported by sertoli cells inside the tubes, which nourish developing sperm cells, and Leydig cells that secrete testosterone.