| define fertilisation | fertilisation is the fusion of two gametes to form a zygote |
| where does internal fertilisation take place | in the oviduct - Fallopian tubes in humans |
| what is an oviduct | it is where unfertilised eggs reside in terrestrial life forms and it is where fertilisation takes place |
| what are the four main steps of fertilisation in humans | → the sperm dissolves and penetrates any protective layer around egg reaching the cellular membrane
→ molecules on the sperm surface bind to receptors (specialised proteins) on the eggs plasma membrane to ensure both gametes are from the same species – sperm nucleus enters the egg
→ the surface of the egg changes so that no other sperm can enter
→ the eggs and the sperms nuclei’s fuse to form a zygote |
| what occurs after fertilisation | cleavage |
| what is cleavage | it is the stage after fertilisation and it is when the egg proliferates for three days turning in to 8 cells (morula) |
| what happens after three days of cleavage divisions | the zygote becomes a morula which contains approximately 8 cells |
| what is a zygote | a fertilised egg |
| what is a blastocyste | it is the stage of fertilisation after morula and occurs 5-7 days after fertilisation |
| what are the three components of a blastocyte | trophoblast, blastocoel and inner cell mass |
| what is the trophoblast | a single layer of cells forms around blastocyte called a trophoblast which allows it to implant into the uterus – trophoblast will later form the placenta |
| what is a blastocoel | there is also a fluid filled cavity in the blastocyte called a blastocoel which turns in to the yolk (humans have small ones – provides nutrient) |
| what is the inner cell mass | the inner cell mass will turn in to the embryo (inner cell mass cells are pluripotent – embryonic stem cells) |
| when does the embryo undergo gastrulation | after 12 days of fertilisation the embryo undergoes gastrulations and becomes a gastrula |
| when does a gastrula become an embryo | after three weeks of fertilisation |
| when does an embryo become a foetus | after 8 weeks of fertilisation |
| define gastrulation | The process when a blastocyte becomes a gastrula – occurs over week 2 and 3 of conception |
| what is the process of gastrulation | Involves the blastocyte folding in on itself to form 2 cell layers - the third is created from one of the other 2 layers – the layers created are called germ layers |
| what are germ layers | they are the layers created once a blastocyste undergoes gastrulation |
| what are organisms called if they have only two germ layers | diploblastic – jelly fish |
| what does it mean if an organism is diploblastic | it means it only has two germ layers |
| what are organisms called if they have three germ layers | triploblastic (most higher animals including humans) |
| what does it mean if an organism is triploblastic | it means it has three germ layers |
| what are there three germ layers called | the inner layer is called the endoderm
the middle layer is the mesoderm
the outer layer in the echoderm |
| what cells does the endoderm give rise to (innermost layer) | the inner most layer (endoderm) gives rise to lung cells, thyroid cells, and pancreatic cells |
| what cells does the mesoderm give rise to (middle layer) | the middle layer (mesoderm) gives rise to cardiac muscles, skeletal muscle cells, tubule cells of the kidney, red blood cells, and smooth muscle cells (found in the gut) |
| what cells does the ectoderm give rise to (outermost layer) | the outer layer (ectoderm) gives rise to epidermal cells, neurons, and pigment cells |
| what two membranes are the germ layers protected by | the yolk sac and the amnion sac |
| what is a yolk sac | The yolk sac is a small, membranous structure situated outside of the embryo with a variety of functions during embryonic development
- very small in humans and other animals are viviparous)
- larger in animals that are oviparous and ovoviparous as the embryo gets its nutrients from the sac |
| list the yolk sacs main two functions in viviparous organisms | primary purpose is to provide nutrient to the embryo early in development
- responsible for blood circulation for the first 12 weeks before the embryo develops its own circulatory system, and delivers nutrients through a primitive aorta |
| what is the amnion sac | it is a sac that surrounds the embryo and it is full of amniotic fluid |
| what are two functions of the amnion sac | - protect the embryo from impacts
- allows the embryo to move as it equalises external pressure |
