Human Embryo Development Extended Study

Learning Objectives List the sequence of development of an embryo Explain the term fertilized egg Explain the term blastocyst Explain the term amnion Explain how the placenta is formed Explain how the embryo develops up to the third month of gestation

Sequence of development from fertilised egg Early stages

Sequence of development from fertilised egg The zygote contains 46 chromosomes, twenty three from the egg and 23 from the sperm

It divides rapidly by mitosis to produce 2 cells, then 4, then 8, 16 etc. and continues to divide

At this point the developing individual is referred to as the morula

Around 5 days after fertilisation the morula forms a hollow ball of cells called the blastocyst

The 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

The 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 tissue Inner cell mass

Stem cells Huge research potential to renew or repair damaged body parts.

Learning 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?

The morula/blastocyst is pushed along the fallopian tube until it enters the uterus

Here it will implant into the uterus wall Here it will implant into the uterus wall. The endometrium now provides nourishment for the developing blastocyst Connections with the mother will begin to form (placenta and umbilical cord)

This point marks the beginning of pregnancy

Sequence of development from fertilised egg Development of the embryo

About 10 days after fertilisation the inner cell mass forms the embryonic disc This usually consists of three layers called germ layers Ectoderm (outside) Mesoderm (middle) Endoderm (inside)

Each of these layers gives rise to specific structures in the developing embryo In humans the mesoderm is split by a layer called the Coelom This allows space for more complex organs such as heart, lungs and kidneys to develop

Ectoderm – skin, nervous system Coelom – heart, lungs Mesoderm – muscles, skeleton Endoderm – inner lining of digestive system

The Amnion

The Amnion When 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

Four to five weeks after fertilisation The heart forms and starts to beat The brain also develops The limbs have started to form

By the 6th week Eyes are visible The mouth, nose and ears are forming The skeleton is at the early stages of development

By the 8th week the major body organs are formed Sex glands have developed into testes or ovaries Bone is beginning to replace cartilage

By the 8th week At this stage the embryo has taken on a recognisably human from. From this point it is referred to as a foetus The foetus continues to grow. No new organs are formed from this point

By the 12th week (3 months) The nerves and muscle become co-ordinated allowing the arms and legs to move The feotus sucks its thumb, urinates and even releases feaces into the amniotic fluid

By the 12th week (3 months) The gender of the foetus can be seen in scans

The time from fertilisation to birth (the gestation period) lasts about 38 weeks (9 months)

Learning Check What are germ layers? Name them What features have already appeared by the fifth week? At what stage is the developing individual referred to as a feotus?

Placenta formation

Placenta formation

Placenta formation The placenta forms from a combination of the tissues of the uterus and the embryo Soon after implantation a membrane called the chorion completely surrounds the amnion and embryo

The chorionic villi emerge from the chorion and invade the endometrium allowing the transfer of nutrients from maternal blood to fetal blood

This combination of the chorionic villi and the endometrium will eventually form the placenta which becomes fully operational about three months into the pregnancy

The Placenta Chorion Embryo Placenta Mother’s blood Amnion Nutrients, Oxygen, antibodies Wastes, Carbon Dioxide, Water Amnion Amniotic fluid Umbilical cord Embryo’s blood Embryo

Placenta allows gases, nutrients, waste, antibodies, some drugs, hormones and micro-organisms to be exchanged between mother and baby Placenta also produces hormones

Blood supplies of mother and embryo do not mix Blood types may not be compatible Mother’s blood pressure might damage embryo

Umbilical cord connects the embryo with the placenta it takes blood from the embryo to the placenta and back again It must be cut at birth to separate mother and baby

45 seconds after birth!

Syllabus

Learning check Name the structures that move into the endometrium and eventually become part of the placenta At what point does the placenta become fully operational? Why is it important that the blood of the mother and baby do not mix?

Depth of treatment Sequence of development from fertilised egg, morula, blastocyst, existence of amnion, placenta formation from embroyonic and uterine tissue. Development of embryo up to third month.