Principles of Medical Embryology Flashcards
Revision
How long does in utero human development last?
9 months
(38-40 weeks)
3 Trimesters
What are the names for each of these time periods?
Weeks 0-3
Weeks 3-8
Weeks 9-40
Weeks 0-3 Conceptus/Embryo Weeks 3-8 Embryonic period Weeks 9-40 Foetal period
What the time at which the chance of having a congenital defect increases?
The first trimester is the point at which the chance of having a congential defect increases.
Week 5 is critical.
What are the 6 phases of embryogenesis?
- Gametogenesis
- Fertilisation
- Cleavage
- Gastrulation
- Formation of a body plan
- Organogenesis
What is a gamete?
A cell with a compliment of 23 chromosomes (half of the typical somatic cell).
How many chromosomes does a haploid cell have?
23 chromosomes
What is the definition of gametogenesis?
The formation of gametes.
What processes does gametogenesis consist of that lead to genetic variation?
Gametogenesis consists of mitosis and meiosis which leads to genetic variation.
What is the process o meiosis?
- Pairing begins
- Pairing of chromosomes
- Chiasma formation
- Pulling apart of double-structured chromosomes
- Anaphase of 1st meiotic division
- Cells resulting from 1st meiotic division. Cells contain 23 double-structured chromosomes.
- Cells resulting from 2nd meiotic division. Cells contain 23 single chromosomes.
What is the definition of spermatogenesis?
It is the formation of male gametes.
At what age does spermatogenesis occur?
It occurs at puberty and continues throughout life.
Each cycle takes around 64 days.
What is the production rate of spermatogenesis?
Around 6.5 million sperm per gam of testicular tissue/day.
Decreases with age.
What is the outcome of spermatogenesis?
22+X or 22+Y
What is Oogenesis?
It is the formation of female gametes.
At what age does Oogenesis occur?
It starts in development and ends at menopause.
By week 28-30: Primary oocytes begin meiosis but arrest in prophase until puberty.
When is a female’s reproductive life span determined.
Reproductive life span is determined in foetal life.
(Females start with 1 million eggs. When they run out of eggs, a person goes into menopause.
How does Down Syndrome occur?
This occurs when there is an additional copy of chromosome 21.
What typically is the cause of there being an additional copy of chromosome 21 which leads to down syndrome?
Nondisjunction.
75% of nondisjunction cases occur during oocyte formation.
What is the definition of Fertilisation?
The union of gametes (Sperm and oocyte) and is the formation of a zygote.
What is a zygote?
It is diploid, single cell embryo, with a sex determination (46,XX or 46XY)
The release of what enzymes allow sperm to penetrate zona pellucida?
acrosomal enzymes
Sperm penetration triggers completion of what?
Completion of 2nd meiotic division.
How does sperm penetration stop other sperm cells from entering the egg?
Sperm penetration causes cortical granules to release their contents, rendering zona pellucida impenetrable to others.
What is the definition of cleavage?
It is a period of rapid cell division.
Up until how many cells is it called a zygote?
16 cells.
From 16 cells and up what is this bundle of cells called?
Morula
What is the definition of gastrulation?
This is the formation of the germ layers which occurs in week 3.
Body axes established
What are the 3 layers formed during gastrulation called?
Ectoderm (Ecto = outer) This is the outer layer and forms e.g. the nervous system, hair, nails and skin
Mesoderm (Meso = middle)
Endoderm (Endo = inner) This forms the lining of the gut
What is embryonic folding?
This is where there is a tube-within-a-tube.
Lateral folding occurs, where we are a flat disc and are rolled up.
The head has to be folded down and the tail folded up to form a foetal position.
What is the definition of organogenesis?
This is the formation of organs and organ system.
The basis of the formation of organs and organ systems are all in place by when?
The end of the embryonic period.
Organs and organ systems will continue to develop during when?
The foetal period.
During what trimester does the baby mainly grow in length?
The 2nd trimester
During what trimester is the most weight gained?
3rd trimester
14g at the end of 2nd month to 3500g at birth.
During the foetal period what are the main processes that occur?
Growth and weight gain. Tissues mature and become functional. Overt sexual differentiation. Bone laid down; Connections made in CNS. (Not everything in the baby is fused together).
The terminology and landmarks used in adults are different for those used in foetus’. what are they for both?
Adults - Superior - Posterior - Anterior - Inferior Foetus - Cranial = Superior - Dorsal = posterior - Ventral = anterior - Caudal = inferior
From becoming a zygote to a human, are there a large/small number of processes?
Relatively small number of processes.
The processes interact with each other producing more complex outcomes.
What are the primary cell processes that occur during development?
Cell division Differentiation Cell attachment Apoptosis Induction Cell migration
What is the role of cell division?
It increases the number of cells
What is the role of differentiation?
Specialisation
Change in appearance/structure
Adoption of new functions
What is the role of cell attachment?
Physical/functional linkages
Formation of tissues
What is the definition of apoptosis?
Programmed cell death
What is the definition of induction?
It is the ability of one cell type to cause another to differentiate (directly or at a distance).
What is the definition of cell migration?
Movement of cells from one location to another.
What are the secondary processes that occur during development?
Axis formation/polarity
Folding/Rotation
What is the role of axis formation/Polarity?
Which way is up?
Cells need to know where they are in relation to each other and the embryo as a whole.
What is the role of folding/rotation?
The entire embryo or structures within it folds and rotates.
This gives the embryo its 3D form
It allows the formation of complex organ structures e.g. heart, gut
(Developing heart - starts as a simple tube. In the developing hear, cells will develop faster at the bottom of the heart to form the shape of the heart).
(We fold ourselves up into a tube and then the head is folded down and the tail is folded up).
What controls development?
Genetic
Epigenetic
Enviromental
How is development genetically controlled?
Gene expression - tightly regulated in time and space
How is development epigenetically controlled?
Preferential expression of either the maternal or the paternal copy of a gene.
Epigenetics is a science in it’s very early stages).
Deleted region chromosome 15 contains 2 paternally expressed genes and 1 maternally expressed gene. What is the difference if the maternal or paternal chromosome is deleted?
Angelman Syndrome
Deletion on maternal chromosome 15 mental retardation; poor motor development
Prader-Willi Syndrome
Deletion on paternal chromosome 15 Mental retardation; Obesity; Hypogonadism (where the external genetalia is much smaller).
