2Learning Objectives List the sequence of development of an embryo Explain the term fertilized eggExplain the term blastocystExplain the term amnionExplain how the placenta is formedExplain how the embryo develops up to the third month of gestation
3Sequence of development from fertilised egg Early stages
4Sequence of development from fertilised egg The zygote contains 46 chromosomes, twenty three from the egg and 23 from the sperm
5It divides rapidly by mitosis to produce 2 cells, then 4, then 8, 16 etc. and continues to divide
6At this point the developing individual is referred to as the morula
7Around 5 days after fertilisation the morula forms a hollow ball of cells called the blastocyst
8The outer layer of the blastocyst forms the trophoblast The outer layer of the blastocyst forms the trophoblast. This will later develop into the layer of membranes that surround the embryo (placenta and amnion)Trophoblast
9The inner cells (called the inner cell mass) of the blastocyst will eventually form the embryo. These cells are not yet specialised. They have a phenomenal ability to differentiate – divide to give rise to many different types of tissueInner cell mass
10Stem cellsHuge research potential to renew or repair damaged body parts.
11Learning check How many chromosomes in a fertilised egg? Is a fertilised egg haploid or diploid?What is the developing individual referred to when it is made up of 8 cells?What is it referred to after a number of days?What is unusual about the cells of the inner cell mass?
12The morula/blastocyst is pushed along the fallopian tube until it enters the uterus
13Here it will implant into the uterus wall Here it will implant into the uterus wall. The endometrium now provides nourishment for the developing blastocystConnections with the mother will begin to form (placenta and umbilical cord)
15Sequence of development from fertilised egg Development of the embryo
16About 10 days after fertilisation the inner cell mass forms the embryonic disc This usually consists of three layers called germ layersEctoderm (outside)Mesoderm (middle)Endoderm (inside)
17Each of these layers gives rise to specific structures in the developing embryo In humans the mesoderm is split by a layer called the CoelomThis allows space for more complex organs such as heart, lungs and kidneys to develop
18Ectoderm – skin, nervous system Coelom – heart, lungsMesoderm – muscles, skeletonEndoderm – inner lining of digestive system
20The AmnionWhen first formed the amnion is in contact with the embryo, but at about the fourth or fifth week fluid begins to accumulate within it (amniotic fluid)The primary function of the amnion and amniotic fluid is the protection of the embryo for its future development
21Four to five weeks after fertilisation The heart forms and starts to beatThe brain also developsThe limbs have started to form
42Learning checkName the structures that move into the endometrium and eventually become part of the placentaAt what point does the placenta become fully operational?Why is it important that the blood of the mother and baby do not mix?
43Depth of treatmentSequence of development from fertilised egg, morula, blastocyst, existence of amnion, placenta formation from embroyonic and uterine tissue.Development of embryo up to third month.