FHMP 025 Formation of the body plan (gastrulation and neurulation)

Question 1

what are the 3 germ layers called?

Answer 1

• mesoderm
• endoderm
• ectoderm

Question 2

what does the ectoderm give rise to in the foetus?

Answer 2

• nervous system ( e.g. brain, spinal cord, neurons, ganglia, retina
• epidermis (e.g. cornea, sweat glands, lens, keratinocytes, nails, hair follicles
• small bones and cartilage ( e.g. facial bones, auditory ossicles, teeth, hyoid cartilage)

NESB = nervous, epidermis, small bones

Question 3

what does the mesoderm give rise to in the fetus?

Answer 3

• big bones (e.g. limbs, vertebrate, ribs)
• connective tissue
• muscle
• blood / vasculature
• reproductive organs (e.g. ovaries, testes)
• somites

BBC might be rude sometimes

Question 4

what does the endoderm give rise to in the foetus?

Answer 4

• digestive tract epithelium
• respiratory organs
• gall bladder
• liver
• pancreas

DR GLP

Question 5

what is gastrulation?

Answer 5

• gut formation in the fetus
• week 3 of development
• It involves a coordinated series of cell movements that place the endoderm and mesoderm inside the embryo where they give rise to internal organs

Question 6

what occurs on day 14 of development?

Answer 6

• a ridge called the primitive streak appears in the caudal (tail end) half of the epiblast layer (in the amniotic cavity)
• after the development of a primitive streak, lab work on embryos is not licensed

Question 7

what occurs on day 15 of development?

Answer 7

• a mound of cells called the primitive node appears on the rostral (head end) end of the primitive streak

Question 8

what is the cloacal membrane?

Answer 8

• forms the anus of the foetus

Question 9

what is the buccopharyngeal membrane?

Answer 9

• forms the mouth of the foetus

Question 10

what does rostral mean?

Answer 10

• head end of the fetus

Question 11

what does caudal mean?

Answer 11

• tail end of the foetus

Question 12

what is ingression?

Answer 12

• when epiblast cells ingress through the primitive streak and node after been attracted by a signal
• the first cells invade the hypoblast and displace it to the periphery of the bilaminar disk - these cells form the endoderm
• the next cells invade the space between the epiblast and endoderm, forming the mesoderm
• the remaining epiblast cells form the ectoderm
• cells ingressing through the node form the prechordal mesoderm and notochord (aka axial mesoderm),
• axial mesoderm determines the order of mesoderm cells from the primitive streak: paraxial mesoderm, then intermediate mesoderm, and then lateral plate mesoderm

Question 13

what is axial mesoderm?

Answer 13

• the prechordal mesoderm and notochord

Question 14

what is the notochord?

Answer 14

• flexible rodlike structure of mesodermal cells that is the principal longitudinal structural element of chordates and of the early embryo of vertebrates
• plays an organizational role in nervous system development
• In later development, it becomes part of the vertebral column.

Question 15

what is paraxial mesoderm?

Answer 15

• somites
• somites form myotome, sclerotome and dermatome

Question 16

what are somites?

Answer 16

• Somites are segmental axial structures of vertebrate embryos that give rise to vertebral column, ribs, skeletal muscles, and subcutaneous tissues
• gives rise to myotome ( skeletal muscles supplied by a single spinal nerve), sclerotomes (vertebrate and ribs), dermatome ( area of skin supplied by a single spinal nerve)

Question 17

what does intermediate mesoderm form?

Answer 17

• kidneys
• gonads (ovaries, testes…)

Question 18

what does lateral plate mesoderm form?

Answer 18

• splanchnopleuric: smooth muscle and connective tissue of digestive tract, heart and vasculature
• somatopleuric: connective tissue of body walls and limb bones
• coelom: pericardial, pleural, peritoneal cavities
• lateral plate mesoderm forms 2 layers (somatopleuric and splanchnopleuric) and central cavity (coelom)

Question 19

how many pairs of somites do embryos and human adults have?

Answer 19

• embryo = 40-44 pairs
• adult = 38 pairs

Question 20

what happens to the notochord after development?

Answer 20

• most degenerates and replaced by the axial skeleton
• but retained in the nucleus pulposus (centre of the intervertebral discs)

Question 21

describe somite development

Answer 21

• somites are formed of a simple epithelium surrounding a central cavity
• cells closest to the notochord form the sclerotome, which breaks away from the somite before migrating around the notochord and neural tube
• the dermomyotome breaks apart to form the myotome and dermatome

Question 22

what does the dermatome form?

Answer 22

• deep layer of skin innervated by a single spinal nerve

Question 23

what does myotome form?

Answer 23

• skeletal muscle innervated by a single spinal nerve

Question 24

what does the sclerotome form?

Answer 24

• vertebrate
• ribs
• sacrum
• coccyx
• occipital bone

Question 25

what is neurulation and when does it happen?

Answer 25

• formation of the neural tube from the dorsal ectodorm which is a continuation of gastrulation
• develops the nervous system
• 4th week of development

Question 26

what is the neural plate and what is its structure?

Answer 26

• the ectoderm rostral to the primitive node forms the neural plate
• it is the precursor of the nervous system
• cells of the neural plate elongate to form a psuedostratified columnar epithelium while cells of the epidermis remain cuboidal
• as the embryo expands through cell proliferation, the primitive streak becomes restrictive to the posterior margin of the epiblast - called regression

Question 27

what is regression of the primitive streak?

Answer 27

• when the embryo grows and expands but the primitive streak is restricted to the posterior margin of the epiblast so it stays the same size

Question 28

describe the formation of the neural tube

Answer 28

• the neural plate folds along the midline causing the lateral edges to rise
• forms fold from where cells constrict their apical surfaces, becoming wedge shaped, known as hinge points
• the lateral edges of the nerual plate folds up more towards each other until they fuse together and form the neural tube
• the edges first meet at the boundary of the hindbrain and spinal cord (the middle) and eventually it ‘zips’ upwards and downwards until the cranial/rostral neuropore closes (day 24) and then the caudal neuropore closes (day 26)
• the neural tube dissociates from the rest of the ectoderm and the remaining ectoderm becomes epidermis
• lateral edges of the neural tube form the neural crest which are pluripotent stem cells which break free from the dorsal root of the neural tube and migrate throughout the body to form neurons and glia of parasympathetic nervous system

Question 29

what is the neural crest?

Answer 29

• the neural crest forms at the lateral edges of the neural plate, at the boundary with the epidermis
• crest cells migrate away from the neural tueb to form parts of the peripheral nervous system and craniofacial bones
• they are highly invasive and can move anywhere

Question 30

what does the neural crest form?

Answer 30

• ganglia
• neurons
• thymus
• face cartilage/bones
• teeth
• schwann cells
• melanocytes
• smooth muscle of the heart
• adiposcytes

Question 31

what are the 2 neural tube defects?

Answer 31

• anencephaly
• spina bifida

Question 32

what is anencephaly?

Answer 32

• when the cranial/rostral neuropore of the neural tube doesn’t close
• brain is underdeveloped and often exposed
• usually miscarried or stillborn
• not common 2.6 per 1000 births

Question 33

what is spina bifida?

Answer 33

• when the caudal neuropore fails to close on the neural tube
• bottom of the spinal cord can be exposed
• usually paralysed, bowel and bladder problems
• not lethal but low quality of life (usually terminated before)
• bit more common than anecephaly

Question 34

what can help reduce the risk of neural tube defects?

Answer 34

• adding 0.4mg of folic acid to daily diet when concieving