Life Before Birth Flashcards
List the three developmental stages of life before birth and give their timings.
Week 1: preimplantation stage
Week 2-8: embryonic stage - implantation, organogenesis, development of placenta
Week 9-38: foetal stage - growth and development
What is an oocyte?
An unfertilised egg
What is a zygote?
An egg that has been fertilised with a sperm.
What is the narrowest part of the uterine tube?
The isthmus
What triggers cell division of the zygote?
The fusion of the maternal and paternal pronuclei.
What happens during cleavage in the preimplantation stage?
Mitotic divisions of the zygote where the overall size of the embryo remains the same so it can travel through the uterine tube without becoming lodged.
What is the zona pellucida?
A tough, glycoprotein coat that surrounds the zygote. It helps to maintain the size of the zygote so that it does not get lodged before implantation and also prevents premature implantation in an unsuitable region such as the uterine tube.
What is the morula?
This forms around day 4 after fertilisation. The cells maximise contact with each other and form a cluster held together by tight junctions.
When does the zygote enter the uterus?
Around day 4 when the morula is formed.
What is the blastocyst composed of?
The inner cell mass (becomes embryo), the trophoblasts (contribute to placenta) and the fluid filled blastocyst cavity.
When does the first cell differentiation of human life occur?
During blastocyst formation as cells differentiate either in to the inner cell mass or a trophoblast.
What does the inner cell mass go on to form?
The embryo and extra-embryonic tissues.
How does fluid enter the blastocyst to form the fluid filled cavity?
Via the zona pellucida
What are the trophoblasts also known as?
Trophectoderm
Describe how the blastocyst ‘hatches’ from the zona pellucida.
The inner cell mass undergoes proliferation, fluid builds up in the blastocyst cavity and it bursts out of the zona pellucida.
Does the inner cell mass or trophectoderm attach to the uterine lining for implantation?
Trophectoderm/trophoblasts.
Summarise week 1 of life before birth.
- fertilised embryo travels down uterine tube
- embryo undergoes cleavage
- cells compact against each other to form morula
- first differentiation occurs to form blastocyst
- blastocyst hatches out of zona pellucida
What is the key process that occurs during week 2 of development?
Implantation
What is implantation?
The interaction between the embryo and the endometrial layer of the uterus so that the embryo can access nutrients and dispel waste.
When does decidualisation occur?
When the blastocyst makes contact with the endometrium of the uterus.
Where does decidualisation occur?
In the stromal cells of the uterus.
What happens during decidualisation?
- several changes occur in the endometrium to prepare it for pregnancy
- the production of several molecules is triggered e.g. glycogen
- the trophoblast cells become invasive
- endometrial cells become enlarged and elongated
What do the trophoblast cells differentiate into during implantation?
The cytotrophoblast and the syncytiotrophoblast.
What is the cytotrophoblast?
A single layer of cells closest to the inside of the embryo
What is the syncytiotrophoblast?
The outer, invasive layer of the differentiating blastocyst. It is more extensive than the cytotrophoblast.
What 2 layers does the inner cell mass form at implantation?
The epiblast and hypoblast.
What is the bilaminar disc made up of?
The epiblast and hypoblast.
What hormone is produced by the syncytiotrophoblast by the end of week 2 of development?
Human chorionic gonadotrophin (hCG)
Summarise week 2 of life before birth.
- implantation occurs so a connection is established between embryo and maternal endometrium
- inner cell mass forms two layers (epiblast and hypoblast) to become bilaminar disc
- trophoblast layer differentiates into two placental layers, cytotrophoblast and syncytiotrophoblast
What is ectopic implantation or pregnancy?
When implantation occurs in abnormal sites.
What may cause an ectopic pregnancy?
- slow transit in the uterine tube
- premature hatching of the blastocyst
- more common in IVF pregnancies
List some abnormal implantation sites.
- uterine tube
- peritoneal cavity
- surface of the ovary
- wall of GI tract
Why do ectopic embryos not develop to full term?
They cannot form a fully functional placenta.
If an ectopic pregnancy does not spontaneous miscarry, why will surgery or drugs be required to stop the pregnancy progressing?
Because the invasive syncytiotrophoblasts can cause a haemorrhage.
List the four extra embryonic or foetal membranes.
The amnion, the chorion, the yolk sac and the allantois.
What is the amnion continuous with?
The epiblast of the bilaminar disc.
What does the amnion do?
Lines the amniotic cavity which is filled with fluid and acts to protect the developing embryo.
How long is the amnion present for?
Up until birth.
Describe the chorion.
A double layered membrane formed by the trophoblast and extra embryonic membranes.
What does the chorion do?
Lines a structure called the chorionic cavity which is present in early pregnancy but disappears due to the expansion of the amniotic cavity. It also forms the foetal component of the placenta.
What is the yolk sac continuous with?
The hypoblast of the bilaminar disc.
What does the yolk sac do?
Important in nutrient transfer, blood cell and gut formation.
When does the yolk sac disappear?
By week 20
Describe the allantois.
- forms as an outgrowth of the yolk sac
- contributes to umbilical arteries
- connects to foetal bladder
What does gastrulation mean? (Greek)
Formation of the gut
Where does the trilaminar embryo stem from?
The epiblast of the bilaminar disc.
List three important structures of the trilaminar embryo.
Primitive streak, notochord and neural tube.
What are the three germ layers?
Ectoderm, mesoderm and endoderm.
What signals that gastrulation is about to begin?
The formation of the primitive streak.
What is the primitive streak?
A thickened area of cells which appears at tail end of embryo. Has a bulge called primitive node.
Which molecule controls the movement of cells during gastrulation?
Fibroblast growth factor 8 (FGF8) made by primitive streak.
How does gastrulation occur?
The epiblast invaginates via the primitive streak and FGF8 to create 3 new layers of cells.
Which germ layer does the epiblast become?
Ectoderm
Which germ layer pushes the hypoblast out of the way as it forms?
Endoderm
List the derivatives of the ectoderm. (8)
- epidermis and derivatives e.g. sweat glands, hair follicles
- epithelial lining of mouth and anus
- cornea and lens of eye
- nervous system
- sensory receptors in epidermis
- adrenal medulla
- tooth enamel
- epithelium of pineal and pituitary glands
List the derivatives of the mesoderm. (10)
- notochord
- skeletal system
- muscular system
- muscular layer of stomach and intestine
- excretory systems
- circulatory and lymphatic systems
- reproductive systems (except germ cells)
- dermis of skin
- lining of body cavity
- adrenal cortex
List the derivatives of the endoderm. (7)
- epithelial lining of digestive tract
- epithelial lining of respiratory system
- lining of urethra, urinary bladder and reproductive system
- liver
- pancreas
- thymus
- thyroid and parathyroid glands
Summarise the key stages of weeks 1-3 of life before birth.
Week 1-2: preimplantation - fertilisation, cleavage, morula, blastocyst
Week 2: implantation - bilaminar disc
Week 3: gastrulation - trilaminar embryo
What does cell potency mean?
The more potent a cell is, the more types of cell it can differentiate in to.
What is the cell potency of a zygote?
Totipotent as it is able to differentiate into all cell types
What is the cell potency of embryonic stem cells?
Pluripotent as they can differentiate in to many but not all cell types.
What is the cell potency of Haematopoietic stem cells?
Multipotent as they can make multiple but not many cell types.
