introduction to embryology

Question 1

formation of bilaminar disc and implantation

Answer 1

trophoblast growing into the uterus, forming the foetal part of the placenta
ectoderm (which later develop into outside of embryo)
endoderm (later forms inner layer of gastro-intestinal tract, lungs, tubes of liver

Question 2

formation of the placenta and embryo

Answer 2

• extracellular layer between ectoderm and trophoblast layer
• cells leave chemicals which break down to form holes between the endoderm and the forming placenta
• eventually chemicals break down completely, left with circular plate of ectoderm cells with a circular plate of endoderm cells in a jelly like substance which is connected to the placenta with a stalk – develops into umbilical cells
• bilaminar disc is the earliest time we recognise the embryo – only this part develops into the live birth

Question 3

bilaminar disc → trilaminar disc

Answer 3

• as the ectoderm grows a folding occurs along the caudal midline; this is the Primitive streak
• cells from the base of the primitive streak break off and migrate to lie between the layers of the ectoderm and endoderm; this new layer is the mesoderm
• by day 17 mesoderm separates the whole of the ectoderm from the endoderm except for two small areas, one at each end of the trilaminar disc. These areas will eventually breakdown to form the mouth/nose and the anus
differentiation of the three germ layers – ectoderm, mesoderm and endoderm each differentiate into their own tissues and organ systems

Question 4

ectoderm develops into

Answer 4

• Epidermis of skin, hair, nails
• Mammary, sweat and sebaceous glands
• Central and peripheral nervous system
• Pituitary gland
• Enamel of the teeth
• Lens of the eye and parts of the inner ear
• Sensory epithelium of nose, eye and ear

Question 5

endoderm develops into

Answer 5

• Epithelial lining of gastrointestinal tract, respiratory tract and urinary bladder
• Parenchyma of the thyroid gland, parathyroid gland, liver and pancreas
• Epithelial lining of the tympanic cavity and auditory tube
• Plays a part In the development of the notochord

Question 6

mesoderm develops into

Answer 6

• All of the musculoskeletal system
• Deep layers of the skin
• Abdominal and chest walls and lining
• The walls of the bowel (but not the lining of the bowel)
• The urogenital system

Question 7

formation of the Notochord

Answer 7

• A tube develops from the end of the primitive streak and extends towards the cranial end
• This tube fuses with the endoderm to become a groove in the endoderm layer
• The plate then folds to become a tube again – the notochord
• The notochord plays a central role in further developments in the midline
• Chemicals from notochord cause cells in ectoderm to change/develop

Question 8

Differentiation of the germ layers ectoderm - neurulation

Answer 8

1. Neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate. the neural plate border separates the ectoderm from the neural plate
2. the neural plate bends dorsally, with the two ends eventually joining at the neural plate borders, which are now referred to as the neural crest
3. the closure of the neural tube disconnects the neural crest from the epidermis. Neural crest cells differentiate to form most of the peripheral nervous system

neural plate forms a tube which is the brain and spinal cord ( if it doesn’t fuse at the bottom of the spine it causes spina bifida)

somites become the myotomes - the ectoderm overlying them are the dermatomes
if the fusion does not extend all the way to the cranial end of the embryo the child is born without the cerebral cortex - anencephally

Question 9

somites form the

Answer 9

a) Myotome (muscle tissue)
b) Sclerotome (cartilage and bone)
c) Dermatome (dermis of the skin; NB the superficial layer of skin is from ectoderm)
each somite is supplied by a single spinal nerve

Question 10

intermediate mesoderm

Answer 10

uridenital system: kidneys, gonads, and their respective duct systems

Question 11

lateral mesoderm

Answer 11

splits into two layers;
The outer layer covers the inside of the chest and abdominal walls; the somatic (parietal) layer mesoderm
The other layer covers the organs in the thorax and abdomen; the splanchnic (visceral) layer mesoderm

Question 12

Cephalo-causal folding

Answer 12

• The cranial area of the embryo contains the buccopharyngeal (oropharyngeal) membrane, the cardiogenic area (the heart) and the septum transversum
• Cranial flexion brings the buccopharyngeal membrane, cardiogenic area and septum transversum ventrally, forming the surface of the future face, neck and chest. It brings the heart into its thoracic position and septum transversum to the diaphragm.
• Caudal flexion – brings the cloacal membrane onto the ventral surface of the embryo