Which embryonic period is the most perilous for the developing child?
Embryonic (weeks 3-8)
What is week 3 known as, and why?
‘Week of threes’
- Three cavities have formed (amniotic, chorionic and yolk sac)
- Three germ layers produced (Ectoderm, mesoderm and endoderm)
Define gastrulation (in human embryos):
The process by which the bilaminar disc (epiblast and hypoblast layers) is converted into the trilaminar disc (ectoderm, mesoderm and endoderm)
What marks the start of gastrulation?
The appearance of the primitive streak
What drives and coordinates gastrulation?
The primitive streak
How does the appearance of the surface of the epiblast layer differ from the end of the second week to the end of the third week?
At the end of the second week it is a uniform disk, but by the end of the third week there is a primitive streak which forms on the dorsal surface.
Describe the appearance of the primitive streak:
- Narrow groove
- Bulging edges
- Forms node at the cranial end of the streak
Where is the primitive pit located?
It forms at the center of the primitive node, located at the cranial end of the primitive streak
How can you tell the cranial end from the caudal end of the embryo during gastrulation?
The primitive node first appears at the caudal end of the embryo, and grows towards the cranial end, forming the primitive node at its cranial end.
During what process can you first tell the cranial end from the caudal end of the embryo?
Gastrulation
What type of cells form the trilaminar disc?
Epiblast cells
How do the epiblast cells form the 3 layers?
The epiblast cells begin to migrate and differentiate, gathering around the primitive streak.
They then push through the epiblast layer, creating a new layer between the epiblast layer and the hypoblast layer, the endoderm.
After that more cells migrate out of the epiblast layer, to form the mesoderm.
Which layer does the hypoblast layer become in the trilaminar disc?
The hypoblast layer does not contribute any cells to the trilaminar disc.
Name the 3 layers of the trilaminar disc:
1) Ectoderm
2) Mesoderm
3) Endoderm
Why isn’t the trilaminar disc consistent across the cell?
There is no mesoderm at 2 places in the trilaminar disc, producing avascular tissue which will form openings at the mouth and anus
What structure is the basis of the axial skeleton, and drives the formation of the nervous system?
Notochord
What is the basic 2 functions of the notochord?
1) Basis for axial skeleton
2) Drives the formation of the nervous system
What is the origin of the cells which form the notochord?
Epiblast cells
How do some epiblast cells form the notochord?
Prenotochordal cells migrate through the cranial part of the primitive pit, and form a solid rod of cells
Describe the appearance of the notochord in the trilaminar disc:
A solid rod of cells forming the midline of the disc, in the middle of the mesoderm layer
What does the notochord become in a adult human?
The inner core of the vertebral discs - ‘nucleus pulposus’
What are the 6 main directional anatomical terms (of adult)?
1) Superior/cranial
2) Inferior/caudal
3) Anterior/ventral
4) Posterior/dorsal
5) Right
6) Left
What sets the axes of the adult human?
The notochord in the developing embryo
What is the difference in the meaning of ‘anterior’ when describing an embryo vs an adult?
Anterior in the embryo refers to the ‘head end’, or rostral,
Anterior in the adult refers to the front, or belly
Name the 2 main structures which are formed from the ectoderm:
1) Epidermis
2) Nervous system
Name 2 types of tissues which derive from the mesoderm:
1) Muscular tissue
2) Connective tissue - bone/cartilage/blood/lymph/adipose
What is the main role of cells which derive from the endoderm?
Epithelial cells which line:
- GI tract
- respiratory system
- urinary system
- many glands
Which layer of the trilaminar disc gives rise to the largest variety of tissues?
Mesoderm layer
How does the primitive node denote left-right asymmetry of the embryo?
Ciliated cells at the primitive node beat in such a way, to send signalling molecules to the left or right, promoting differentiation in a particular area of the developing embryo.
What usually causes complete mirror-image of the viscera (situs inversus)? (ie internal organs mirror image to usual placement)
Immotile cilia at the primitive node.
Why does the gastrulation process complete faster at the cranial end compared to the caudal end of the developing embryo?
Because development proceeds ‘cephalocaudally’ (from cranial end to caudal end)
What does the term ‘monozygotic twins’ mean?
A single fertilised oocyte has given rise to two infants
What does the term ‘dizygotic twins’ mean?
Two fertilised oocytes produce two infants, during the same pregnancy
What type of twins are identical?
Monozygotic
What are the 2 types of twins?
1) Monozygotic
2) Dizygotic
What are the 2 ways that monozygotic twins may be produced?
1) Initial cleavage after fertilisation causes the zygote to completely split, forming 2 embryos, with separate membranes and placenta’s
2) Two inner cell masses form in one blastocyst, forming 2 embryos with the same membranes and placenta
What process may produce conjoined twins?
When two inner cell masses form in one blastocyst, as the 2 embryos will form from the same membranes, which may not complete separation before birth
Will conjoined twins always be identical, and why (not)?
Yes, as they form from the same zygote, sharing the placenta and amniotic sac
Define teratogenesis:
The process by which normal embryonic development is disrupted, leading to congenital malformations.
Which period of embryonic development is most sensitive to teratogenesis?
Embryonic (weeks 3-8)
What is a teratogen?
An agent or factor which causes malformation of an embryo, by interfering with development
Why is the pre-embryonic period of development not susceptible to teratogenesis?
If teratogenesis occurs here, death of the embryo and abortion occurs.
Name 3 main teratogens:
1) Thalidomide
2) Alcohol
3) Rubella