Embryology Flashcards
What occurs during the first trimester ?
- 0-13 weeks
- embryonic development
What occurs during the 2nd & 3rd trimester ?
- 2nd =14-26 weeks
- 3rd = 27-40 weeks
- foetal development
Describe Germ Cells
- contain 1 copy of each chromosome
- this is because the nuclei of 2 germ cells will fuse at fertilisation to produce a 2n zygote
What are germ cells derived from?
- they’re derived from diploid cells called primordial germ cells (PGCs) which are produced by the embryo
When do PGCs commence meiosis in males?
- after puberty
Describe when PGCs begin meiosis in females
- PGCs begin meiosis before birth in order to form a primary follicle
- these are then held in an arrested state mid-meiosis till after puberty
When is meiosis arrested during female development ?
- during prophase of the 1st division
What happens to the primary follies during the menstrual cycle ?
- 5-12 primary follicles resume development
- eventually a single oocyte becomes dominant & completes development
Define oocyte
egg
What happens when the primary follicle matures?
- called a Graafian follicle
- contains a fluid-filled cavity = antrum
What happens to the oocyte on day 13/14 of menstruation ?
- it resumes meiosis & completes it
- creating a polar body
- beings meiosis II but it arrested midway - will remain this way till after fertilisation
What happens to the follicle at ovulation ?
- it ruptures to release the oocyte into the Fallopian tube
- the oocyte is carried to the womb by flow created by beating cilia
What happens to the ruptured follicle ?
- it forms the corpus luteum which acts as an endocrine organ
- signals the endometrium
What hormones are released by the corpus luteum ?
- oestrogen
- progesterone
What hormones are released by the pituitary gland during menstrual cycle?
- follicle stimulating hormone
- Luetenising hormone
What is the term for male germ cell production ?
Spermatogenesis
Describe what happens to dormant male PCGs after puberty
- proliferate via mitosis to form spermatogonia
- spermatogonia undergo both rounds of meiosis concurrently
- this produces spermatocytes –> haploid spermatids
What happens when spermatids mature ?
- maturation produces mature spermatozoa
What are the 3 anatomical regions of Spermatozoa ?
- Head = contains condensed nucleus & acrosome
- midpiece = contains mito.
- tail = contains microtubules for propulsion
What is an acrosome ?
a vesicle containing hydrolytic enzymes
Describe the process of fertilisation
- multiple sperm are attracted to the oocytes & penetrate the cumulus cell layer
- sperm bind to glycoprotein receptor called ZP2
- binding triggers enzymes to digest ZP
- sperm membrane fuses with oocyte
Define Polyspermy
- fertilisation by more than 1 sperm
How is Polyspermy prevented ?
- via changes triggered by the membrane fusion including generation of calcium wave & release of cortisol granules
Define Zygote
- fertilised egg
- single cell from which all other cells of the body are derived from
What are the major stages of embryo development ?
- Day 0 = fertilisation
- Day 2-4 = cleavage
- Day 5 = Blastula
- Day 8-9 = Implantation
Describe the Cleavage Stage of embryo development
- Day 2-4
- large oocyte is divided into smaller cells
- results in an undifferentiated ball of cells called a Morula
Describe the Blastula stage of embryo development
- Day 5 embryo becomes a Blastula - specific structures & regional differences can be seen
- coelum (cavity) forms inside morula
- 2 types of cells = inner cell mass & trophoblasts
Describe the implantation stage of embryo development
- embryo attaches then embed into wall of the endometrium
- trophoblasts differentiates to form the placenta
- endometrium is induced to differentiate to produce blood vessels
Describe the Development of the Placenta
- begins at implantation when trophoblasts invade maternal endometrium
- maturation reached at 2nd trimester
Describe the role of the placenta
- mediates essential communication between embryo/mother via close contact between maternal & foetal blood vessels
- mediates uptake of nutrition & oxygen
- endocrine & immune functions
Describe the Gastrula Stage of embryo development
- Day 14-21
- Gastrulation is one of the most important stages
- 3 major events occur during ;
1. cells become fate-restricted
2. cells become motile
3. embryo is patterned
Describe what happens when cells become fate-restricted during gastrulation
- they-re fate restricted to become one of the three germ layers
- Ectoderm, mesoderm or endoderm
Describe what happens when the embryo is patterned during gastrulation
- forms the major axes which will determine the body plan
Describe the Neurula stage of embryo development
- Days 21-28
- immediately after gastrulation, neuralisation begins
- nervous system is formed as flat plate on embryo surface and later this plate folds/rolls to form a neural tube
How is the CNS built from the neural tube ?
- Brain is formed by expansion of part of neural tube followed by tube folding
What are the 3 morphological divisions of the brain that can be seen from early stages of development ?
- Forebrain
- Midbrain
- Hindbrain
What are the 2 fundamental processes occurring during development ?
- differentiation
- morphogenesis
Define differentiation
the process of producing mature cells with a define function
Define morphogenesis
- the acquisition of shape and form
- new cells must be precisely organised to form tissues & organs
How does differentiation occur ?
- embryo contains stem cells which remain undifferentiated & repeatedly divide
- cell will stop dividing and begins differentiation to acquire mature form
How does morphogenesis occur ?
- cells communicate with each other
- embryos contain patterning centres which release signals that organise & pattern the body
How are tissues and organs developed in the 2nd stage of embryogenesis ?
- fields of precursor cells are first patterned -
- they receive/respond to signals which give them an identity –> this tells cells where they are and where they need to be
- this information its used to build tissues/organs
What are the 2 key properties of stem cells?
- replicate indefinitely in undifferentiated state
- under correct conditions they can differentiates into mature cell types
What is potency ?
- the ability of a stem cell to differentiate into 1 or more different cell types
What are the 4 different levels of potency ?
- decreasing in potency
1. totipotency
2. pluripotency
3. multipotency
4. unipotency
Define Totipotency
- ability to differentiate into any cell both within the body & within the placenta
- maintained through cleavage stage but lost when embryo becomes a blastula
What level of potency does a zygote have ?
totipotency
What level of potency do blastulas have?
- pluripotency –> this is because they contain 2 distinct cell types
1. trophoblasts
2. embryonic stem cells
How were pluripotent stem cells orginally obtained ?
- from the inner mass of blastula embryos
- embryos from IVF clinic can be used but this raises ethical concerns
Describe multipotent stem cells
- mid-range potential and can differentiate into a limited number of cell fates
- examples = mesenchymal stem cells which can form adipocytes, chondrocytes or osteocytes
Describe unipotent stem cells
can propagate indefinitely in an undifferentiated state but when they differentiate they can only adopt a single fate
Describe the concept of cell fate
- it describes the differentiated state a cell adopts at the end of development
- fate specification is the process by which cells choose what fate they will adopt
Define Cell Fate Determination
- describes the state of being irreversibly committed to adopting a particular cell fate
How is a stem cells differentiation potential reduced during development ?
- its reduced because certain regions of the genome are silenced
