Lecture 13: Early human development Flashcards
What is embryogenesis?
The period between fertilisation and the fully formed embryo
What is fertilisation?
The merging of a haploid sperm cell and a haploid secondary oocyte to form a single diploid nucleus
What is corona radiata?
The outermost layer surrounding the secondary oocyte made up of granulosa cells
What is the zona pellucida?
Glycoprotein layer between the corona radiata and the membrane of the oocyte
What is the pathway of the sperm cell into the secondary oocyte?
Corona radiata -> Zona pellucida -> Plasma membrane -> cytoplasm
How does the sperm penetrate the seconadry oocyte?
Hydrolytic enzymes in the head of the sperm
What is cleavage?
Rapid mitotic cell division of the zygote occurring after fertilisation
What happens 30 hours after fertilisation occurs?
The first division of the zygote via cleavage forming the two-cell stage
What happens by the second day of fertilisation?
Second cleavage completed forming a 4 cell zygote
What happens by the third day of fertilisation?
Four cell stage of the zygote
What is a morula?
A solid sphere of cells surrounded by the zona pellucida -> formed from cleavage of cells up to 4th day
What is a blastocyst?
Formed from a blastocyst cavity (large fluid-filled), a blastocyst mass contains hundreds of cells and is the same size as the original zygote
What are the components of a blastocyst?
Embryoblast (inner cell mass)
Trophoblast
What are the steps to implantation of the zygote?
Ovulation - release of eggs from ovary
Fertilisation occurs
Zygote moves down fallopian tube undergoing cleavage to form morula as it travels through tube
Fully formed blastocyst enters the uterine cavity
Implantation of blastocyst on the endometrium (about 6 days after fertilisation
What is the trophoblast/.
A group of cells in the blastocyst that give rise that allow the blastocyst to be implanted into the endometrium and stimulates development of placenta
What is the embryoblast?
Group of cells in the inner part of the blastocyst that form the bilaminar embryonic disc
What is the bilaminar embryonic disc?
Formed in the zygote in the endometrium from the embryoblasts
Divided into two layers:
Hypoblast - endoderm
Epiblast - ectoderm
What are the 3 germ layers in foetal development
Ectoderm (outer)
Mesoderm (middle)
Endoderm (inner)
What is gastrulation?
The rearrangement and migration of cells from the epiblast
The establishment of the 3 germ layers of the organism
How does gastrulation occur?
Formation of the primitive streak on the epiblast of the bilaminar embryonic disk (head and tail established)
Formation of the ectoderm, mesoderm and endoderm
What is embryonic folding?
Conversion of the two-dimensional trilaminar embryonic disc into a three dimensional cylinder
Occurs as the ectoderm folds around the lower layers forming a tube
Organs begin to form as embryonic folding occurs
What are stem cells?
Cells that divide to form one daughter that goes on to differentiate, and one daughter that retains its stem-cell properties
Why are stem cells important?
Growth
Renewal (skin)
Repair
What are totipotent cells?
Capable of giving rise to all cell types of the body and extra-embryonic tissues (placenta)
The only totipotent cell is zygote in the early days of fertilisation
What are pluripotent cells?
Capable of giving rise to all types of cells but not extra-embryonic tissue
What are multipotent cells?
Capable of giving rise to all cell types of a particular tissue or organ
What are nullipotent cells?
Not capable of giving rise to other cell types
What happens as cells divide?
They lose renewability - ability to reform original cell
What are unipotent cells?
Cells in the skin that allow for constant renewal
What are the unipotent cells in the skin called?
Keratinocytes
How do keratinocytes renew skin?
They exist as a layer in the epidermis of the skin and divide forming differentiated cells that move towards the outer surface and make up the skin
List all the stages up to formation of fetus
- Fertilisation
- Cleavage
- Implantation (as blastocyst)
- Bilaminar embryonic disk formed
- Gastrulation occurs
- Organogensis
