Embryology 1 & 2 Flashcards
What are the phases of development of the human?
before birth
Pre-embryonic (0-3 weeks)
Embryonic (4-8 weeks)
Foetal (9-40 weeks)
Spermatogenesis is the process of sperm formation
From what cells do sperm form from?
Spermatogonium
What is the process of egg formation?
Oogenesis
Describe the process of egg formation
Oogonium cell in ovaries
Undergoes meiosis
Forms 1 ovum and 3 polar bodies
What is the collective term for the sites of gamete formation?
(testes and ovaries)
Gonads
Describe the process of fertilisation
Several sperms surround ovum in uterine/fallopian tube, only 1 penetrates
Pronucleus of sperm fuses with pronucleus of ovum
This produces diploid cell - zygote
From a zygote, what is the next stage of development?
Morula
Zygote divides by mitosis several times to form a solid ball of cells, a morula
“The morula will develop into a ______”
Blastocyst
Describe the structure of a Blastocyst
Trophoblast - outer lining of cells
Inner cell mass - accumulation of cells at one side
Blastocyst cavity - hollows it out
Why is it necessary for the morula to develop into a blastocyst?
As the morula becomes larger, it becomes difficult to get nutrients to cells in the middle
Forming the cavity and inner cell mass allows nutrients to reach these cells - allowing further growth
Once a zygote is formed, it begins to divide (morula, blastocysts etc)
What else happens?
It moves from the uterine tube to the uterine cavity
What facilitates movement through the uterine tubes?
Cilia
How long does it take for a zygote to first divide?
36 hours
After the first division of a zygote, how does the time taken for successive divisions change?
Successive divisions are faster
1: 36 hours
2: 24 hours
3: 12 hours
What happens in the 2nd week of the pre-embryonic phase?
3
Implantation of blastocyst into endometrium
Formation of bilaminar disc
Sacs, membranes and cord used for nourishment, form
“In order for implantation to take place, the _______ must develop into _______”
Trophoblast
(must develop into)]
Chorion
How does chorion form, and what does it do?
Trophoblast divides to form 2 layers
Ultimately develops into chorion
Chorionic villi form - these are finger like processes that implant and eventually form part of the placenta
It also releases HCG
What is HCG?
Human chorionic gonadotropin
It is secreted to maintain the endometrium, which is normally shed
What is the muscle that surrounds the uterine cavity?
Myometrium
After implantation, what name is given to the endometrium?
Decidua
What is the decidua basalis?
Part of the endometrium (decidua) that is ‘deep’ to the implanted conceptus
This is where the interface between maternal blood vessels and the placenta will be
After implantation, what happens to the inner cell mass of the blastocyst?
They form a 2-layered flat disc called the bilaminar disc:
- Epiblast (faces the endometrium)
- Hypoblast (faces away)
This splits the blastocyst cavity:
- Amniotic sac (epiblast)
- Yolk sac (hypoblast)
Once implanted, what are the sources of nutrition for the embryo?
Yolk sac
Maternal nutrition via placenta
“Waste products from embryonic development are secreted into the _______”
Allantoic sac
What sac is the embryo contained in?
Amniotic cavity
What cavity encases the amniotic cavity, the yolk sac, the allantoic sac and the umbilical cord?
Chorionic cavity
The placenta has 2 faces
What is the structure of the foetal face?
Smooth
Contains foetal blood vessels
Start of the umbilical cord
How is the maternal face of the placenta different from the foetal face?
Maternal face is rough, and contains maternal blood vessels instead of foetal blood vessels
What causes the development of fraternal/dizygotic twins?
2 ovum are released, and fuse with 2 sperm
Thus 2 zygotes are formed
2 placentas
What causes identical/monozygotic twins to develop?
1 ovum and 1 sperm fuse
Zygote splits to form 2 embryos
Shared placenta and may share same amniotic and chorionic sac
Summarise what happens in the 3rd week of gestation
Gastrulation (formation of germ layers)
Neuralation (formation of neural tube)
Development of somites
Early development of cardiovascular system
What happens to the bilaminar disc before gastrulation can happen?
Epiblast and hypoblast must separate
Primitive streak must form
This forms in the midline of the epiblast through the dipping of the cells
What information does the direction of the primitive streak tell us?
Axis of the embryo
Describe the process of gastrulation
Epiblast cells migrate into the space between epiblast and hypoblast layers
These cells then displace the hypoblast
Trilaminar disc is then formed and cells have become specialised
What are the layers that make up the trilaminar disc?
Ectoderm
Mesoderm
Endoderm
From which layer of cells does the notochord form?
Which part of this layer does it form from?
Ectoderm
Forms from the primitive streak
What is the notochord?
Solid tube of cells
What is the function of the notochord?
It induces the ectodermal cells in the primitive streak to form the neural plate, which then separates and sinks down to form the neural tube
Describe what happens after the neural tube is formed?
NT induces the mesoderm to thicken and separate into 3 parts:
Paraxial
Intermediate plate
Lateral plate
“The lateral plate mesoderm splits to form a ________ and a ________”
Somatic mesoderm and a splanchnic mesoderm
What is the name given to the space between the somatic mesoderm and the splanchnic mesoderm?
Intraembryonic coelom
What do the somites form from?
Paraxial mesoderm
What forms from the intermediate plate mesoderm?
Urogenital system (kidneys and repro)
What forms from the lateral plate mesoderm?
Body cavity and coverings
What is the appearance of the somites compared to the paraxial mesoderm?
The PM segments to form the somites
They look like a row of teeth
When is the organogenetic period?
4th - 8th week
How many somites are formed from the paraxial mesoderm?
43 pairs
These differentiate further
What happens in the organogenetic period?
Lateral folding completes
Forebrain, midbrain, hindbrain and spinal cord develop
Heart starts to beat on day 24
Gut formation from endoderm
Urogenital system forms
Body cavities form
Somites form and differentiate
Limb buds form
Neck develops - pharyngeal arches
What do the somites develop into?
Dermatome
Myotome
Sclerotome
What do dermatomes form?
Dermis of skin
What do myotomes form?
Muscles
What do sclerotomes form?
Bones (inc. vertebrae)
What is a teratogen?
Environmental factor that causes abnormal development
German measles is an example of a teratogen
What is it’s effect?
It can cause congenital rubella
Give a famous teratogen and it’s effect
Thalidomide
Causes malformation of limbs
What environmental agents can harm a baby’s development?
Alcohol
Tobacco
Other drugs
Radiation
Infectious agents
What are the ToRCH agents and why are they significant?
Toxoplasma, Rubella, Cytomegalovirus, Herpes
These can be transferred through the placenta and affect the baby’s development
Give examples of genetic factors that could affect the development of a baby
Too many/few chromosomes
Structural changes/mutations of chromosomes
When is the risk of teratogenesis greatest?
weeks 3 - 8
When is the risk of death highest?
Weeks 1 - 2
Low teratogen risk though
What are the methods of diagnosing malformations prenatally?
Blood - AFP
Ultrasound scan
Invasive tests such as chorionic villus sampling and amniocentesis
What are the methods of diagnosing malformations post-natally?
Hip stability
Testes (descent)
Fingers and toes
Hearing
