Embryology 1 & 2 Flashcards
What is the name of the cell in males that divides to form sperm?
Spermatogonium
What type of cell is spermatogonium, diploid or haploid?
Diploid
What happens to the spermatogonium?
It undergoes meiosis (two phases), to produce 4x sperm cells.
What type of cell is sperm?
Haploid
What is the name of the cell in females that divides to form an ovum?
Oogonium
What type of cell is an oogonium, and how many chromosomes does it have?
Diploid, 46 chromosomes.
What happens to the oogonium?
It undergoes meiosis in two phases, the first in utero but the second does not take place until roughly teenage age, whereby an ovum and three polar bodies are produced.
What type of cell is an ovum, and how many chromosomes does it possess?
Haploid, 23 chromosomes.
What is the name given to sperm and ovum cells?
Gametes
What name is given to the production of sperm?
Spermatogenesis
What name is given to the production of an ovum?
Oogenesis
What are the three prenatal periods of embryology?
- Pre-embryonic phase (week 1-3)
- Embryonic phase (week 4-8)
- Foetal phase (week 9-40)
Embryogenesis
Fetogenesis
What happens during fertilisation?
What new cell is formed?
The pro-nucleus of a sperm enters penetrates the ovum and fuses with the pro-nucleus of the ovum to form a zygote.
Why do males with low sperm count struggle to fertilise a female egg?
In order for fertilisation to take place, there must be lots of sperm surrounding the egg, even though only one penetrates (normally).
What is a zygote?
Where about is it formed, and in which week of pregnancy?
The first cell formed after fertilisation.
It is formed from the sperm and ovum fusing together, it is a diploid cell with 46 chromosomes.
It is formed in the uterine tube, in week one.
Why is the zygote genetically unique?
It contains genetic information from both mum and dads chromosomes. Also, each sperm and ovum is genetically unique thus the reason siblings have different physical and personal traits.
What is the morula?
After fertilisation, the zygote begins to undergo a process of division (mitosis), it divides into 2, then 4, then 8 and so on.
This produces a solid ball of cells called the morula, which can then go on to form the blastocyst.
Why is the fact that only the nucleus of the father fuses with the ovum clinically important?
It means that all organelles are inherited from the mother so in genetic abnormalities carried in the mitochondrial DNA will only be passed on by the mother.
Why does the morula form a blastocyst?
As the morula grows in size, the cells become compressed and struggle to receive nutrition.
To get around this, a blastocystic cavity is formed where nutrients can reach the growing mass of cells.
Describe the structure of a blastocyst.
There is an inner blastocystic cavity, an inner mass of cells which accumulates at one end and an outer layer called the trophoblast.
What does the trophoblast differentiate to form?
The placenta and the membranes around sacs around the foetus (amniotic sac), and also the umbilical cord.
Describe the movement of the zygote from the ovaries to the uterus and the changes that it undergoes?
How long does this process take?
The ovum is usually fertilised by sperm to form the zygote shortly after release from the ovary at the distal end of the uterine tube.
The zygote begins to undergo mitosis as it travels along the uterine tube toward the uterus, it becomes the morula in the process. The morula is aided by the ciliated squamous cells which surface the fallopian tubes.
By the time the cell reaches the uterus, it is a blastocyst. This process takes about one week.
What issues in the uterine tube can lead to serious complications?
Uterine tubes can become seriously affected by infection (e.g. chlamydia), pelvic inflammatory disease, fibrous/scar tissue etc.
This can affect the cilia, and the transport of the zygote to the uterus. If the zygote/blastocyst becomes lodged in the uterine tube, it can lead to an ectopic pregnancy which could rupture and cause serious complications.
What is the next stage that happens, once the blastocyst has reached the uterus?
Roughly, when does this happen?
Implantation
At the end of week one (day 6)
Where does the blastocyst implant in the uterus?
Uterine endometrial layer (endometrium)
What happens to the trophoblast during implantation?
It divides to form two layers. It ultimately forms the chorion.
What three main things does the chorion do?
- Facilitates implantation into the endometrium (chorionic villi).
- Goes on to form the placenta.
- Releases hormone HCG (Human Chorionic Gonadotropin).
What does the hormone HCG do?
The chorion secretes HCG into the bloodstream, this hormone sends a messenger to the ovaries to continue secreting oestrogen which maintains the endometrium with implanted blastocyst.
It is also what is used to detect pregnancy in urine.
What is the decidua basalis?
Decidua = endometrium Basalis = basal layer
It is part of the endometrium deep to the implanted conceptus, it becomes the mothers side of the placenta.
What does the inner mass of the blastocyst form on implantation?
The cells of the inner cell mass form a 2–layered flat disc called the bilaminar disc.
What are the two layers of the bilaminar disc?
Epiblast
Hypoblast
What is the cavity above and below the bilaminar disc known as?
Above - amniotic cavity
Below - yolk sac
What is the cavity called into which metabolic waste products of the feotus drain into?
What does this cavity go on to form in the embryo?
Allantoic cavity
Bladder etc.
Why is it possible for mother and baby to have different blood groups?
Mother blood vessels and foetal vessels in the placenta do not touch one another due to the chorionic villi.
What are the three main functions of the placenta?
- Foetal nutrition
- Transport of waste and gas
- Immune protection
Describe the foetal end of the placenta?
Smooth with foetal blood vessels and end of umbilical cord.
Describe the maternal end of the placenta?
Decidua basalis of endometrium.
Rough and has maternal blood vessels.
What separates the placenta between mother and foetus?
Chorionic villi
What are the two different types of twins?
Dizygotic (non-identical)
Monozygotic (identical)
Explain how dizygotic twins form.
- 2 ovum
- 2 sperm
- 2 zygotes (genetically unique)
- 2 placenta
Explain how monozygotic twins form.
- 1 ovum
- 1 sperm
- 1 zygote (initially
- Zygote splits and goes on to form two embryos (identical)
- 1 placenta
- May or may not share amniotic and chorionic sacs.
When can a monozygotic pregnancy be problematic?
If the division of the zygote happens late on in pregnancy (e.g. conjoined twins)
Summarise what happens in first two weeks in terms of conceptus.
Fertilisation - zygote - morula - blastocyst - inner cell mass - bilaminar disc - epiblast and hypo blast - amniotic cavity and yolk sac
Summarise what happens in first two weeks in terms of location.
Morula in uterine tube - cilia - blastocyst reaches uterus - implantation in endometrium - outer cell mass (trophoblast) - chorionic villi faciltates - deciduas basalis formed - placental formation begins
What is the primitive streak?
What does it form in terms of the embryo?
It is the midline of the epiblast, viewed from above. It is formed by the dipping of cells through invagination.
It forms the axis for the embryo.
What is the formation of germ layers in week 3 called?
Gastrulation
What happens during gastrulation?
Epiblast cells migrate into space between epiblast and hypoblast layers.
They then form three layers (trilaminar disc).
What are the three layers formed in the trilaminar disc, from top to bottom?
Ectoderm
Mesoderm
Endoderm
Summarise the process of blastocyst to specialisation.
Blastocyst - inner cell mass - bilaminar disc - epiblast and hypo blast - primitive streak invagination - ectoderm, mesoderm and endoderm - cells become specialised.
Cells from the ectoderm sink down into the trilaminar disc to form what?
Notochord - a solid tube
Explain the process of neurulation.
The notochord induces the ectodermal cells in the primitive streak to thicken and form a neural plate.
This then sinks down to form the neural tube.
What week does gastrulation and neurulation take place in?
Week 3
Explain how the neural tube interacts with the mesoderm, and what is produced from this.
Neural tube induces mesoderm to thicken.
Mesoderm splits into three:
Paraxial mesoderm
Intermediate plate mesoderm
Lateral plate mesoderm
Lateral plate mesoderm splits into somatic and splanchnic mesoderm.
What is the space generated in the lateral plate mesoderm called?
Intraembryonic coelom
Where do somites originate?
From segmentation of paraxial mesoderm.
What does the intermediate plate mesoderm go on to form?
Urogenital system (kidneys and reproductive organs)
What does the lateral plate mesoderm go on to form?
pleura, peritoneum and body cavities.
What does the neural tube develop into?
The brain and spinal cord
Where does gut formation develop from?
Endoderm
Where does uritogenital system develop from?
Intermediate plate mesoderm
Where do body cavities develop from?
Lateral plate mesoderm (i.e. the somatic and splanchnic mesoderm)
How many pairs of somites from in the paraxial mesoderm?
43
How does each somite further differentiate?
It divides into three:
Dermatome (dermis)
Myotome (muscle)
Sclerotome (bone)
Where does respiratory system develop from?
Endoderm
What occurs in the embryonic period, in terms of developing of the neck?
Pharyngeal arches form
What is teratology?
Study of when things go wrong during development.
What are teratogens?
Environmental factors that cause abnormal development.
Give example of some teratogens?
Drugs - thalidomide
Infection - measles
Radiation
Alcohol/tobacco
Give example of some genetic factors that lead to abnormality?
Chromosomal number (too many, or too few)
Structural changes in chromosomes
What does ToRCH stand for (for infectious teratogens)?
toxoplasma
rubella
CMV
herpes
In what stage of pregnancy is the foetus most sensitive to teratogens?
Weeks 3-8 (embryonic phase)
What are the three factors that constitute to the risk of teratogenesis?
- Time of exposure
- Dosage of teratogen/exposure length
- Genetic constitution of embryo ie some more susceptible than others at equivalent doses etc.
How are malformations identified and diagnosed in prenatal phase?
Blood - AFP
Ultrasound scan – 12 week anomaly scan
Invasive tests: chorionic villus sampling and amniocentesis
How are malformations identified and diagnosed in postnatal phase?
Hip stability
Testes (descent)
Fingers and toes
Hearing
Summarise the first four weeks of embryonic development.
Week 1 - fertilisation, zygote formation, uterine tube to uterus, blastocyst
Week 2 - implantation, bilaminar disc, chorionic villi, placenta formation
Week 3 - primitive streak invagination, gastrulation, neurulation, somite formation
Week 4 - organogenesis, lateral folding
