Embryology

Question 1

What happens between implantation and day 5-7?

Answer 1

Sperm fertilises ovum, ovum divides multiple times to produce morula (free floating in uterus), develops into ball of cells called blastocyst (have trophoblast and inner cell mass)

Question 2

What do the inner cell mass and trophoblast turn into?

Answer 2

• Inner cell mass becomes the embryo. Yellow cells eventually form a complete lining
• Trophoblast becomes the placenta.

Question 3

What do the blue cells and yellow cells become?

Answer 3

• Blue cells develop into the outside of the embryo (ectoderm cell)
• Yellow cells forms inner layer of gastrointestinal tract, the lungs, the tubes of the liver (endoderm cell = inside skin) = day 8

Question 4

What happens on day 10?

Answer 4

Extracellular matrix laid down between blue + yellow mass and green mass of placenta

Question 5

What else happens on day 10?

Answer 5

Cells leak chemicals, these break down to form holes between endoderm and forming placenta. Chemical eventually breaks down completely, left with circular plate of blue cells with a circular plate of yellow cells in a jelly-like substance. Jelly-like substance = connected to placenta with a stalk. Stalk will develop into umbilical cord. Blue cells develop into number of structures, e.g. skin and CNS. Yellow develops into lining of bowl, lungs etc.

Question 6

What is the bilaminar disc?

Answer 6

Blue and yellow cells from 2 layers = bilaminar disc. The earliest time we recognise an embryo

Question 7

What happens to the bilaminar disc as the embryo grows?

Answer 7

Groove forms on ectoderm (primitive streak). The primitive streak spawns cells that fill up space between ectoderm + endoderm = mesoderm. By day 17, there is a trilaminar disc.

Question 8

In which two places does the mesoderm not separate the ectoderm and the endoderm?

Answer 8

• Head of embryo = ectoderm adheres to endoderm, will form the mouth
• Back of embryo = will form anal canal

Question 9

What does the ectoderm develop into?

Answer 9

• Epidermis of skin, hair, nails
• Mammary, sweat + sebaceous glands
• CNS
• PNS
• Pituitary gland
• Enamel of teeth
• Lens of eye + parts of inner ear
• Sensory epithelium of nose, ear + eye

Question 10

What does the endoderm develop into?

Answer 10

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

Question 11

What does the mesoderm develop into?

Answer 11

Everything else:

• All of musculoskeletal system
• Deep layers of skin
• Abdominal and chest walls and lining
• Walls of bowel (but not lining)
• Urogenital system

Question 12

How is the notochord formed?

Answer 12

• Endoderm touches ectoderm where groove is forming. Chemical come across, changes behaviour of endoderm. Develops into different type of cell, folds up into tube and separates from endoderm. This tube separates ectoderm from endoderm

Question 13

What happens to the ectoderm as cells grow?

Answer 13

There isn’t enough space on the outer surface of the embryo to accommodate all cells, so there is folding. Takes place along midline as this is where notochord is attached to ectoderm. Blue lines eventually fuse, will form skin. Green will from sensory part of nervous system. Purple form a tube, will form brain + spinal cord

Question 14

What are somites?

Answer 14

Formed from mesoderm. Notochord producing chemicals, diffusing from midline, stimulating mesoderm to from somites. Each somite supplied by a single spinal nerve

Question 15

What happens to somites? What happens if fusion does not extend to the bottom or top of the embryo?

Answer 15

• Somites become myotomes (muscles)
• If fusion does not extend all the way to caudal end, child has spina bifida
• If fusion does not extend all the way to cranial end, child born with cerebral cortex, so dies

Question 16

Picture of embryo cut in half with bilimanar disc.

Answer 16

Question 17

Pictures of mesoderm forming.

Answer 17

Intra-embryonic body cavity forms (will form the space surrounding the lungs, heart + bowel)

Question 18

What does the paraxial mesoderm form?

Answer 18

Paraxial mesoderm forms somites (form myotome, cartilage and bone and dermis of skin (superficial layer of skin is from ectoderm)). Each somite supplied by a single spinal nerve (nerve that comes off between two separate vertebra)

Question 19

What does the intermediate mesoderm form?

Answer 19

Urogenital system - kidneys, gonads + their respective duct systems

Question 20

What are the two layers of the lateral mesoderm? What do they form?

Answer 20

• Somatic (parietal) layer = forms inside of chest + abdominal walls
• Splanchnic (visceral) layer = forms organs in thorax + abdomen

Question 21

Diagram showing lateral folding of the embryo.

Answer 21

Ectoderm grows faster than endoderm, so folding occurs. The most lateral part of blue layer begins to fold round and surround mesoderm and the CNS

Question 22

What eventually happens to the blue layer?

Answer 22

Completely surrounds the embryo. Amniotic sac (extra-embryonic coelom) surrounds this (embryo). Intra-embryonic coelom develops into cavity that surrounds the lungs, heart and all of the bowel)

Question 23

Image of cephalo-caudal folding

Answer 23

Different perspective

Question 24

Image of growth

Answer 24

Question 25

What does the cranial area of the embryo entail?

Answer 25

Ectoderm + endoderm fused together. Heart developing in front of that. Cranial flexion = as brain grows, it will hold heart underneath

Question 26

Summary of early embryology

Answer 26