Introduction to Embryology (use slides for images) 20.10.22

Question 1

What is an oocyte?

Answer 1

ovum, egg. Female germ cell

Question 2

What is a sperm?

Answer 2

Male germ cell

Question 3

What is a zygote?

Answer 3

cell resulting from union of oocyte and sperm at fertilisation

Question 4

What is a blastomere?

Answer 4

early embryonic cells

Question 5

What is a morula?

Answer 5

Solid mass of 12-32 blastomeres

Question 6

What is a blastocyst?

Answer 6

Stage where morula has entered uterine cavity and blastocystic cavity develops within

Question 7

What is a gastrula?

Answer 7

Stage after gastrula when neural tube forms

Question 8

What is an embryo?

Answer 8

Developing human during embryonic stage (to the end of the 8th week)

Question 9

What is a fetus?

Answer 9

Period from 9th week to birth

Question 10

What happens in stages 1-3?

Answer 10

• Stage 1: fertilisation
• Stage 2: Zygote forms (diploid, fusion of 2 haploid)
• Stage 3 (day 4): Zygote to Morula to Late blastocyst

Question 11

What happens in stage 3-4?

Answer 11

Blastogenesis occurs
- The trophoblast becomes the embryonic part of the placenta
- The inner cell mass (composed of blastomeres) becomes the embryo

Question 12

What is inside the endometrial epithelium? (stage 3-4)

Answer 12

• Endometrial glands
• Endometrial capillary
• They produce substances needed for survival and development of placenta and foetus
• It undergoes structural and histological changes during menstruation ready for implantation

Question 13

What is a trophoblast? And give two types of them.

Answer 13

A layer of tissue on the outside of a blastocyst supplying the embryo with nourishment and later forming the major part of the placenta
Synctiotrophoblast - specialised epithelial cells and determines what enters the cell and what doesn’t e.g. oxygen = yes
Cytotrophoblast - The inner layer of the trophoblast

Question 14

What happens in stages 4-6?

Answer 14

Stage 4 - Trophoblast
Stage 5 (day 8)- Here the inner cell mass differentiates into two layers separated by a basement membrane. ectoderm and endoderm.
Day 9 - formation of yolk sac and amniotic cavity
Day 10 - Lacunae appear (spaces)

Question 15

What happens on day 13?

Answer 15

The formation of the umbilical cord:
- The chorionic vesicle hangs in the chorionic cavity
- The chorionic vesicle joined to placenta cells by connecting stalk (becomes umbilical cord)

Question 16

What happens in week 3?

Answer 16

Gastrulation:
- Bi-laminar disk to tri-laminar disk
- Tail end (caudal)
- Head end (cranial)
- As ectoderm grows a folding occurs along the caudal midline which causes the Primative streak

Question 17

What are the three germ layers called?

Answer 17

Endoderm, ectoderm, mesoderm
- Cells from the base of the primative streak detach and migrate to lie between the layers of the ectoderm and endoderm
- This creates a new layer: mesoderm

Question 18

What is the oropharyngeal membrane?

Answer 18

Septum between primative mouth and the throat.

Question 19

What does the ectoderm (outer layer) differentiate to?

Answer 19

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

Question 20

What does the endoderm (inner layer) differentiate to?

Answer 20

• Epithelial lining of the 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 21

What does the mesoderm (middle layer) differentiate to?

Answer 21

• 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 22

How is the notochord formed?

Answer 22

• Tube develops from end of Primative streak and extends towards the cranial end
• The tube fuses with endoderm to become a groove
• Plate fold (curls in to create a circular tube) to become a tube once again (the notochord)
• Notochord has a central role in further midline development: provide mechanical and signally queues to the developing embryo

Question 23

What is neurulation (week 4)?

Answer 23

1. Neuroectodermal tissues (arises from ectoderm and first step in forming the nervous system) differentiate from the ectoderm and thicken into the neural plate, separated by the neural plate border
2. Neural plate bends dorsally (to the back) until the two plates convergence to form the neural crest (like a u shape)
3. The neural crest is disconnected from the epidermis by the closure of the neural tube. The cells of the neural crest differentiate to form most of the peripheral nervous system
4. The notochord degenerates, only persisting as the nucleus pulposus (fluid in spinal cord) of the intervertebral discs. Mesoderm cells differentiate into somites, precursors of the axial skeleton and skeletal muscle

Question 24

What happens if the fusion does not extend to the caudal end?

Answer 24

The embryo or child is born with spina bifida

Question 25

What happens if the fusion does not extend to the cranial end?

Answer 25

The child is born without the cerebral cortex (brain outer most layer of tissue/ grey matter)

Question 26

What is spina bifida?

Answer 26

• There is a protruding and visible sac from the column.
• Part of the spinal cord and nerves are damaged from protruding sac.
• The name changes with increasing severity of spina bifida:
• Occulta
• Meningocele
• Myelomeningocele

Question 27

What happens when the mesoderm differentiates?

Answer 27

• This process occurs simultaneously to neurulation
• Intermediate mesoderm, lateral mesoderm (differentiates to cardiovascular system, heart and blood etc) , paraxial mesoderm (differentiates and divides into paired cuboidal bodies known as somites)

Question 28

What is a somite?

Answer 28

• First appear in the future occipital region of the embryo (smallest of the lobes in cerebral hemisphere)
• Develop cranio-caudally (head to tail)
• they give rise to axial skeleton, associated musculature (myotome), Adjacent dermis of skin (dermatome)

Question 29

What does the intermediate mesoderm differentiate to?

Answer 29

Urogenital system
- Kidneys
- Gonads
- Respective duct system

Question 30

What does the lateral mesoderm differentiate to?

Answer 30

Splits to two layers
- Somatic (parietal) layer: outer layer covers inside of chest and abdominal walls
- Splanchnic (visceral) layer: second layer which covers the organs in the thorax and abdomen

Question 31

What happens with embryo folding (in week 4)

Answer 31

For this imagine the baby on all fours
- Lateral folding (horizontal plane)
- Cephalic to Caudal folding (median plane)
Folding in both planes occurs at the same time

Question 32

What is the lateral folding?

Answer 32

• Starting with the developing trilaminar discs
• Axial or transverse slice (across embryo)
  1. Flaps of lateral plate mesoderm fold towards ventral midline and merge everywhere except at umbilical cord
  2. As the flaps fold, space is maintained which becomes intraembryonic coelom
  3. Amnion and ectoderm continue to undergo rapid growth
  4. Amnion now surrounds entire embryo
  5. Endodermal layer meets at midline and fuses to form gut tube
  6. Fusion is complete except at midgut level
  7. Yolk sac and midgut have a wide connection until lateral folding is complete

Question 33

What is the Cephalo - Caudal folding?

Answer 33

• Starting with the bilaminar disc
• Sagittal slice along length of embryo
  1. Cranial area contains oropharyngeal (or bucchopharyngeal) membrane, cardiogenic area (the heart) and the septum transversum
  2. Cranial flexion brings these areas ventrally
  3. These areas form the surface of the future face, neck and chest
  4. The heart is brought to the thoracic position
  5. The septum transversum is brought to the diaphragm
  6. New position under heart separates coelom into thoracic and abdominal cavities
  7. Caudal flexion brings the cloacal membrane to the ventral surface of the embryo
  8. Connecting stalk is moved ventrally so it merges with the neck of the yolk sac and attaches to the ventral surface of the embryo
  9. Parts of the endodermal germ layer are incorporated into the embryo as the hindgut
  10. The primitive streak moves from lying cranially to caudally in relation to the cloacal membrane
  11. Allantois is partially incorporated into the embryo