Flashcards in L10-11. Embryology Deck (70)
Describe the formation of the single cell that forms the human. What is the term for this cell?
The sperm and ova meet at the most distal part of the fallopian tube in a process called FERTILISATION.
This forms the zygote which moves into the uterus bounding along the ciliated wall of the fallopian tube.
Describe the process between the zygote and the cavitated blastocyst
The zygote undergoes numerous divisions:
1 division per 24 hrs initially and accelerating with time
By 3 days = 16 cells the embryo is known as the MORULA
By 4 days = The BLASTOCYST forms and begins to look asymmetric (cells take on different shapes) as cavitation forms (hole is called the blastocoele)
What is a congenital disorder?
What are the two major types?
A disorder that exists at birth or before birth
1. Structural Deformities
2. Functional disorders
What are some causes of congenital abnormalities?
Describe the structure of the blastocyst?
It has 2 cell types:
Outer ring layer = TROPHOBLAST = forms the extraembyronic structures (including the placenta)
INNER CELL MASS = holds the future embryo
When does the blastocyst implant into the uterine wall? Describe this process
At around day 5-10
It sinks into the wall and is enveloped by the uterine epithelium forming the interface between the mother (waste and nutrient transport)
Describe what happens to the inner cell mass as it implants into the uterine wall
It begins to compact (cells get closer together) and also cells begin to DIFFERENTIATE
they differentiate into a two layered structure called the epiblast on top and the hypoblast below
What is the primitive streak and where and how is it formed?
It is formed just after the differentiation of the two inner cell mass layers at the centre of the embryo (dividing the embryo in half in the saggital plane.
It is formed by cells of the epiblast migrating towards the centre and into one another forming a small bump
Describe the process of gastrulation
When the cells of the epiblast migrate to form the primitive streak, these cells eventually push against each other and burrow into the hypoblast and in between the two layers.
This forms a third layer in between the bilaminar disc.
As the cells migrate, they differentiate further and there is the formation of the 3 germ layers
What are the three germ layers?
Each layer gives rise to distinct lineages of tissues in the adult body
What is neurulation? What is the first structure to form to begin this process?
It is the formation of neural elements in early embryogenesis.
The first structure to appear is the NOTOCHORD - by the further differentiation of the cells in the centre of the mesoderm = a core just under the primitive streak
What is the purpose and fate of the notochord?
Its main purpose is in differentiation (doesn't have much role or function in the adult except in some formation of the IV discs)
In embryogenesis: the notochord is important to organise the embryo into left, right, top and bottom
What occurs as a consequence of the formation of the notochord?
The notochord induces a change/differentiation in the ectodermal cells just above it (causes a 'thickening' of sorts) leading to the formation of a NEURAL PLATE
What happens to the neural plate cells of the ectoderm?
They dive in and burrow into the mesoderm and form a ting like structure called the neural tube along the length of the embryo
The neural tube zips up on itself to form a closed tube. As it does so what happens to some of the cells of the ectoderm?
They break off in the process and fly out into the mesoderm and away from the neural tube (acts like a fourth germ layer).
They are known as the neural crest cells and they form and differentiate into their own tissues.
What does the neural tube go on to eventually form?
The brain and the spinal chord
What are some derivatives of the neural crest?
Dorsal root ganglia
Sympathetic and parasympathetic ganglia
Melanocytes (pigment cells of the skin)
Dentine (central part of the teeth)
The MUSCLE, CARTILAGE and BONE of the SKULL, JAW, FACE and PHARYNX
Describe the origin and formation of the face
As the neural tube closes (from the top to bottom) the neural crest cells migrate from the back of the head to across the lateral sites to meet each other in the anterior midline.
Describe neural crest defects: cleft lip and palate
Occurs in 1:1000 live births
Migration of cells is very difficult (long distant migration that requires very tightly regulated signalling)
If something goes wrong then the cells are unable to meet in the midline and form an incomplete face
What are some derivatives of the mesodermal layer?
Dermis (not epidermis)
Heart and blood vessels
Wall of the gut and respiratory tract
Haemopoetic tissue (blood)
Pleura, pericardium and peritoneum
What is the difference between the terms mesoderm and mesenchyme?
Mesoderm describe from which germ layer tissues are derived.
Mesenchymal refers to the shape and behaviour of cells - loose and independently moving, free with no attachments to its neighbouring cells (which is opposite to epithelial which means connected to one another)
Mesoderm cells often form mesenchymal tissue but they are not mutually exclusive terms
What are the three zones of the mesoderm?
They are distinct zones and they also behave differently to one another:
1. Paraxial (medial) - close to midline
2. Intermediate - narrow zones
3. Lateral - on the edges
What structures arise from the paraxial mesoderm?
Dermis of the skin
Axial limb and muscles
What structures arise from the intermediate mesoderm?
What structures arise from the paraxial mesoderm?
Ventrolateral body wall
Visceral pleura, peritoneum and pericardium
Blood vessels and blood forming tissue
Wall of the gut and respiratory tube
What forms in the flat sheet of paraxial mesoderm on either side of the neural tube?
Bumps or swellings of cells called SOMITOMERES that appear progressively down the length of the mesoderm (from rostral to caudal end)
What happens at the 20 somitomere stage?
The 8th pair of somitomeres becomes larger and cells that make it up differentiate to the point that the somitomere separates itself for form a SOMITE
Once this occurs there is progressive replacement of the somitomeres into somites in the caudal direction
What is the importance of this somite formation only occuring from the 8th pair of somitomeres onwards?
Somitomeres form the head whilst the somitomeres (somites) from the 8th onwards form the neck and body (vertebral column)
There is a further differentiation of the somites into the three layers. Describe this zone formation
The somite splits in two: sclerotome medially and the dermomyotome laterally
The dermomyotome then splits into two zones: dermatome laterally and myotome medially (that separates it from the sclerotome)