12. Reproduction

• Insemination


  Ejaculation of sperm in semen into the vagina
• Fertilisation usually occurs in the uterine tube when the ovum is ~1/3 of the way down the tube
• Zygote


  The result of the fertilisation of a sperm and an ovum
• Fertilisation process


  1. Sperm travels through cervix

  2. Sperm travels through uterus

  3. Sperm reaches uterine tubes

  4. Sperm fertilises ovum
• Secondary oocyte


  • Released at ovulation
  • At metaphase II
  • Surrounded by corona radiata and zona pellucida
• Acrosomal reaction


  Release of enzymes in acrosome when sperm reaches zona pellucida
• Once sperm enters oocyte, its tail is absorbed and head forms male pronucleus
• Secondary oocyte completes Meiosis II and nucleus develops into female pronucleus
• The pronuclei fuse forming a single nucleus with diploid number of chromosomes
• The fertilised oocyte is now called a zygote
• After fertilisation, the zygote travels down the uterine tube and begins dividing by mitosis
• Approx. 6 days after fertilisation, the zygote has reached the uterus and developed into a blastocyst
• Blastocyst


  A hollow ball of cells that surround a fluid-filled cavity
• Inner cell mass is composed of stem cells that will differentiate into different body cells to form the embryo
• Blastocyst formation and implantation


  1. Blastocyst forms ~6 days after fertilisation

  2. Inner cell mass has ~30 cells

  3. Blastocyst remains free in uterus for 2-3 days

  4. Blastocyst sinks into endometrium and attaches to uterine wall
• High oestrogen and progesterone in the blood stop the endometrium breaking down
• In early pregnancy, the corpus luteum produces hormones until the placenta takes over after approx. 8-12 weeks
• Primary germ layers


  Three layers of cells called ectoderm, mesoderm, and endoderm
• Primary germ layers will differentiate into different tissues and organs
• Functions of the placenta

  • Secretes hormones
  • Transports wastes
  • Transports antibodies
  • Transports nutrients
  • Transports oxygen
• The placenta is an organ that forms from both foetal and maternal tissues during the first 3 months of gestation
• The placenta supplies nutrients to, and removes wastes from, the foetus
• Foetal blood and mother’s blood do not mix
• Exchange of materials occurs by diffusion and active transport
• Umbilical cord contains two umbilical arteries and one umbilical vein
• Chorionic villi contain lots of blood vessels and grow from the chorion into the endometrium
• Chorion becomes the main part of the foetal portion of the placenta
• Amnion surrounds the embryo and secretes amniotic fluid
• Amniotic fluid protects the embryo against physical injury and helps maintain constant temperature
• Amniotic fluid allows embryo/foetus to move freely
• Amniotic fluid expands as growth occurs and ruptures before birth
• Chorionic villi


  Finger-like projections that contain blood vessels
• Umbilical cord


  Structure that connects the placenta to the foetus
• Placenta


  Organ that supplies nutrients and removes wastes from the foetus