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Embryonic and fetal development of man

Source: lunar caustic, www.flickr.com, licencja: CC BY-SA 2.0.

Link to the lesson

Before you start you should know
  • that gametes are involved in the fertilization process;

  • that the functioning of the female reproductive system is based on cycles;

  • that ovulation determines the fertile days of a woman.

You will learn
  • to characterise the human embryonic and foetal development;

  • to recognise foetal membranes in a schematic and describe their functions;

  • to indicate when the mother’s behaviour has a harmful effect on the development of the foetus;

  • to explain what childbirth is.

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Nagranie dźwiękowe abstraktu

Fertilization

Fertilization (conception) can occur when the woman is in the fertile phase. An egg cell is released from the ovarian follicle into the fallopian tube and transported by the cilia of the cells lining the tube towards the uterus. During the intercourse, the man ejaculates semen, with about 200 million sperm cells in it, into the vagina. Thanks to their tails and contractions of the uterine muscles, the sperm cells reach the oviduct in search of an egg cell. Along the way, most sperm cells die. Only a few thousand reach the female reproductive cell and only one participates in fertilization. Enzymes in the head cap of the sperm (acrosome) dissolve the ovum shell. After the penetration, the egg shell hardens, creating a barrier that is inaccessible to other sperm cells. The moment of connection of the egg cell nucleus with the nucleus of the male reproductive cell is called fertilization. In the moment of fertilization, the pregnancy begins. It normally lasts 280 days.

Embryonic and foetal development

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Embryo implantation
Source: Andrzej Bogusz, licencja: CC BY 3.0.

The development of a fertilized egg in the womb includes two phases: the embryo and the foetus. The first one lasts from the moment of conception up to the 8th week of life of the embryo. As a result of fertilization, a zygote is formed. After 24 hours it begins to divide rapidly, forming a multicellular embryoembryoembryo. As it keeps dividing, the embryo moves for about 5 days through the fallopian tube towards the womb (uterus). At first, the developing embryo uses the nutrients accumulated in the egg cell. When it leaves the fallopian tube, the embryo already consists of a large number of cells. In this form, around the 7th day since conception, it adheres to the vast, blood‑rich mucous membrane. Its cells gradually begin to differentiate, creating tissues and organs.

Later, the fertilized egg becomes not only of the body of the future child, but also the extraembryonic (foetal) membranesextraembryonic membranesextraembryonic (foetal) membranes: amnion, allantois and chorion. Amnion forms a cavity directly surrounding the embryo. It is filled with amniotic fluid. It absorbs shocks and allows the embryo to move freely. Allantois collects products of the embryo’s metabolism for a part of the embryonic period. Chorion, the outermost membrane, is adjacent to the wall of the uterus. Around the 4th week, finger‑like appendages are formed that penetrate deep into the mucous membrane. This is how the placenta is formed.

The placenta participates in the exchange of respiratory gases, nutrients and metabolic waste between the mother’s blood and foetal blood. This way, the mother transfers the antibodies present in her blood to the foetus. The placenta is a protective barrier against most pathogens. However, some harmful substances can still pass through it: alcohol, drugs, nicotine and some small‑sized viruses (e.g. rubella, chickenpox, HIV). The placenta also fulfils the function of an endocrine gland, producing hormones such as progesterone and oestrogens.

At the end of the first stage of pregnancy, the embryo is about 3 cm long, weighs about 10 g, has all the organs and looks like a human. From now on, the embryo is referred to as a foetusfoetusfoetus. The foetus is connected to the placenta by the umbilical cordumbilical cordumbilical cord.

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The third month of pregnancy
Source: Tomorrow Sp. z o.o., licencja: CC BY 3.0.

The foetus grows to about 52‑55 cm and its weight reaches about 3.5 kg. A properly developing pregnancy ends 40 weeks after fertilization with childbirth, during which the child leaves through the mother’s birth canal the vagina.

Pregnancy and childbirth

The time of pregnancy is accompanied by a series of changes in the woman’s body. During the first 3 months of pregnancy, hormonal changes cause breast enlargement and such ailments as nausea or vomiting. In the following months, the developing placenta and foetus as well as the increasing amount of the amniotic fluid cause a gradual increase in the mother’s weight and a change in the centre of mass of the body. This in turn often leads to a body posture which is unnatural to the spine, especially when standing.

To ensure a proper course of pregnancy and childbirth, it is important for the mother‑to‑be to stay physically active, as far as her condition allows (unless there are medical contraindications). Proper nutrition is essential as well. It should be adjusted to the body’s increased need for vitamins and mineral salts (mainly iron, calcium and phosphorus). A pregnant woman must at all times refrain from drug use, alcohol consumption, smoking and being in the company of smokers, taking any medicines without consulting a gynaecologistgynaecologygynaecologist. Alcohol may cause developmental defects, mental disorders, miscarriage. Toxic substances found in smoke may cause placental vasoconstriction, which may lead to foetal hypoxia as well as heart and central nervous system defects.

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Wykres liniowy przedstawia ryzyko urodzenia dziecka z zespołem Downa w zależności od wieku matki. Na osi X zaznaczono wiek matki (mother's age), na osi Y ryzyko wystąpienia zespołu Downa w procentach (the risk of Down syndrome [%]). W wieku 20 i 25 lat jest to 0,1%, następnie ryzyko zaczyna rosnąć. Wśród 30-letnich matek ryzyko wynosi 0,2%, wśród 35-letnich to 0,5%, 40-letnich 0,8%. Po 40. roku życia ryzyko gwałtownie wzrasta i wynosi 3,6% wśród 45-letnich matek.
The risk of having a baby with Down's syndrome
Source: Dariusz Adryan, licencja: CC BY 3.0.

Prenatal testsprenatal testsPrenatal tests are used to assess foetal development. The most commonly recommended and performed test is ultrasound (ultrasonography). It is completely safe for the mother and the child. It allows to assess foetal development and detect any abnormalities. In some special cases, the doctor may order other, which often allow to discover any diseases or conditions in time. With today’s medicine, treatment is possible at a very early stage of development, including foetal surgery inside the womb.

Pregnancy ends with childbirth, which consists of three stages and on average lasts 12‑16 hours. In the first stage, uterine contractions occur with gradually increased frequency. They slowly guide the child towards the cervix. The amniotic membrane breaks and the amniotic fluid is released. During the second stage, strong contractions of uterine and abdominal muscles push the child out of the mother’s body. In the third stage of labour, the mother’s body removes the foetal membranes and the placenta.

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Exercise 1
Ćwiczenie Wyjaśnij kluczowe pojęcia związane z tematem abstraktu.
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Exercise 2
Ćwiczenie przyporządkuj elementy. Match the factors affecting the health of an unborn child to the correct group. Harmful factors: Możliwe odpowiedzi: 1. moderate physical activity of the mother, 2. exposure to radiation, 3. taking medications without consulting a doctor, 4. regular check-ups, 5. healthy and balanced diet, 6. regular eating, 7. drug use, 8. alcohol consumption, 9. smoking and being in the company of smokers, 10. mother's illness. Positive factors: Możliwe odpowiedzi: 1. moderate physical activity of the mother, 2. exposure to radiation, 3. taking medications without consulting a doctor, 4. regular check-ups, 5. healthy and balanced diet, 6. regular eating, 7. drug use, 8. alcohol consumption, 9. smoking and being in the company of smokers, 10. mother's illness

Summary

  • Fertilization occurs in the upper part of the fallopian tube, when the sperm head penetrates inside the egg cell.

  • The moment of fertilization is when the period of pregnancy begins. Pregnancy covers two stages of child’s development: embryonic and foetal.

  • The embryonic period lasts from the moment of fertilization up to about 8 weeks of pregnancy.

  • The embryonic period is when cells are differentiated, tissues and organs are created and extraembryonic membranes and placenta are formed.

  • Extraembryonic membranes: amnion, allantois and chorion provide optimal conditions for the development of the child.

  • Placenta allows the exchange of substances between the mother’s and the child’s blood.

  • Foetal development covers the period from the 9th week until the birth of the child.

  • During the foetal period, organs form and prepare for their functions outside the mother’s body.

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Keywords

fertilization, pregnancy, extraembryonic membranes, placenta

Glossary

prenatal tests
prenatal tests
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Nagranie dźwiękowe słówka prenatal tests

badania prenatalne – badania płodu pozwalające na ocenę jego rozwoju i wykrycie ewentualnych wad rozwojowych

extraembryonic membranes
extraembryonic membranes
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Nagranie dźwiękowe słówka extraembryonic membranes

błony płodowe – błony otaczające zarodek takich kręgowców, jak gady, ptaki i ssaki

gynaecology
gynaecology
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Nagranie dźwiękowe słówka gynaecology

ginekologia – dział medycyny zajmujący się funkcjonowaniem i chorobami żeńskiego układu rozrodczego

foetus
foetus
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Nagranie dźwiękowe słówka foetus

płód – zarodek ssaka znajdujący się na takim etapie rozwoju, kiedy można już w jego budowie wyróżnić cechy charakterystyczne dla danego gatunku

umbilical cord
umbilical cord
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Nagranie dźwiękowe słówka umbilical cord

sznur pępowinowy – pępowina; przewód łączący płód z łożyskiem, w którym znajdują się naczynia krwionośne: tętnice odprowadzające krew od płodu oraz żyły pępowinowe doprowadzające krew do płodu

embryo
embryo
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Nagranie dźwiękowe słówka embryo

zarodek – organizm we wczesnym stadium rozwoju, powstały w wyniku podziałów zygoty; obejmuje okres od momentu zapłodnienia do 8. tygodnia życia nowego organizmu

conception (fertlization)
conception (fertlization)
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Nagranie dźwiękowe słówka conception

zapłodnienie – proces połączenia jąder komórki jajowej i plemnika, w wyniku którego powstaje zygota rozwijająca się w nowy organizm