Embryology : Early human development

Question 1

Describe the phases of fertilisation.

Answer 1

1. Penetration of the corona radiata
2. Penetration of the zona pellucida
3. Fusion oocyte and sperm cell membrane + Secondary oocyte in 2nd meiotic division

Question 2

Name and describe features of the sperm molecule.

Answer 2

-Acrosome- sack which carries enzymes to allow fertilisation to take place
Midpeice – engine of sperm
Flagellum- beats for movement of sperm

Question 3

Does corona radiata and zona pellucida provide a barrier to sperm?

Answer 3

Corona radiata- does not provide barrier for sperm

Zona pellucida- does provide a barrier

Question 4

what is a zygote?

Answer 4

single cell embryo

Question 5

what does sperm entry trigger?

Answer 5

Sperm entry triggers cortical granule release in egg cortex triggering formation of fertilisation membrane, which is impenetrable to sperm

Question 6

what does fusion of the male and female pronuclei restore?

Answer 6

restores the diploid number of chromosomes = zygote, day 0 of development

Question 7

How long does the zona pellucida remain during development?

Answer 7

first few days

Question 8

what is cleavage?

Answer 8

Period of rapid cell division (no size increase)

Question 9

is there an increase in cell size during cleavage?

Answer 9

No because the cell size is limited by zona pellucida

Question 10

what happens to the morula under pressure?

Answer 10

changes into the blastocyst

Question 11

Describe the structure of the blastocyst.

Answer 11

• outer wall- trophoblast
• inner cell mass- embyoblast
• blastocyst cavity

Question 12

Describe the blastocyst hatching.

Answer 12

• Inner Cell Mass gives rise to the embryo, remaining cells form the trophoblast which contributes to the placenta
• Embryonic Stem (ES) cells are derived from the ICM
• Following hatching from the zona pellucida the embryo now interacts directly with the endometrial lining of the uterus

Question 13

where does the blastocyst normally implant?

Answer 13

dorsal wall of uterus

Question 14

where are abnormal implantation sites?

Answer 14

1. mesentery
2. ampula
3. uterine tube
4. internal os of uterus
5. ovaries

Question 15

what is the maternal response after implantation?

Answer 15

new blood vessels form -maternal response

Question 16

What happens during implantation?

Answer 16

• blastocyst makes contact with uterine epithelium
• Trophoblast sends projections into epithelium – breakdown connecting cells and crawl in
• trophoblast is now differentiated
• leading edge - syncytiotrophoblast
• Cytotrophoblast –remaining part of trophoblast

Question 17

what ways does the cell face ?

Answer 17

• Emybroblast – 2 cells: one facing into cavity of blastocyst and one adjacent to trophoblast (epiblast)
• epiblast are proliferating – producing early cells of amnioablast

Question 18

what happens to the implanted embryo at 9-12 days.

Answer 18

• The syncytiotrophoblast has extended and by erosion contacts maternal blood supply.
• The amniotic cavity is present above the bilayered embryonic disc and the definitive yolk sac below
• The chorionic cavity has developed and encircles the entire embryo

Question 19

what is gastrulation?

Answer 19

formation of the germ layers

Question 20

what are the 3 germ layers?

Answer 20

• Ectoderm : nervous system and skin
• Mesoderm: Muscle, bone and internal organs
• Endoderm: gut

Question 21

where do cells migrate through?

Answer 21

primitive streak

Question 22

Describe the process of gastrulation.

Answer 22

-Surface of epiblast is not uniform surface – impression at one end –buccopharyngeal membrane (cranial end) and primitve streak at one end – -drives prosess of gastrulation for 3 germ layers
Primitive streak sends signal – some epiblasts will come towards primitive streak and go deep into it and some go into hypoblast and some fill space (3 germ layers)

Question 23

what is neurulation?

Answer 23

formation of the neural tube

Question 24

what is the first step of neurulation?

Answer 24

An area of the ectoderm is induced to form Neural Tissue

Question 25

when does neurulation start?

Answer 25

once gastrulation finishes

Question 26

how is the brain formed during neurulation

Answer 26

In response to signals from mesoderm, some ectodermal cells differentiate and become neural
Then neural plate expands –produces brain - the different areas of brain i.e dicephalon

Question 27

what happens at day 19/20 of neurulation?

Answer 27

• Creates hinge point along midline-pulling edges of neural tube upwards – pulls ectoderm with them as they are connnected
• The groove gets deeper over time
• Midpoint of embyro- make contact with one another – the neural edges fuse with one another and pulling ectoderm with them so will fuse with each other over the top and so neural tube is hidden
• Fiusion contines in both direction to enclose the entire neural tube

Question 28

what happens during neural tube formation at day 23?

Answer 28

Anterior part close first

Question 29

Neural crest

Answer 29

Undergo Epithelial to Mesenchymal transition and leave the neuroectoderm to enter the mesoderm

Question 30

Name neural crest derivatives.

Answer 30

• Schwann cells and Meninges
• Dorsal root ganglia
• Autonomic ganglia
• Bones and connective tissue of craniofacial structures
• Melanocytes
• Chromaffin cells (adrenal medulla)
• Odontoblasts

Question 31

Describe the cranial folding of the 3D embryo.

Answer 31

buccopharyngeal membrane, heart primordia and septum transversum arise in front of the brain and are displaced ventrally forming the foregut and defining the thoracic cavity

Question 32

Describe the caudal folding of the 3D embryo.

Answer 32

cloacal membrane and connecting stalk are displaced ventrally alongside the neck of the yolk sac, forming the hindgut, anus and the umbilical cord

Question 33

Describe the lateral folding of the 3D embryo.

Answer 33

converts the open coelom into a closed cavity, creating thoracic and abdominal cavities and closing fore and hind gut… mid gut is the last to close

Question 34

what do pharyngeal arches contribute to in humans?

Answer 34

face and neck

Question 35

when do you develop pharyngeal arches?

Answer 35

in weeks 4 and 5 :
Arch 1 -> Day 22
Arches 2 and 3 -> Day 24
Arches 4 and 6 -> Day 29

Question 36

what does each arch consist of?

Answer 36

• core of mesenchyme → musculature of face and neck
• neural crest cells → skeletal components of face
• cranial nerve component
• artery

Question 37

when happens when arch 4 and 6 begin to form?

Answer 37

earlier arches begin to regress

Question 38

what ventral facial features are formed by pharyngeal arch 1?

Answer 38

• Maxilla, Mandible, Zygomatic and Temporal Squamous bones

- Muscles of mastication

Question 39

what ventral facial features are formed by pharyngeal arch 2?

Answer 39

Muscles of Facial Expression

Question 40

Describe the formation of upper and mid-facial features.

Answer 40

Proliferating mesenchyme ventral to developing brain

Question 41

what is the stomodeum?

Answer 41

future mouth structure

Question 42

Describe the changes of nasal swellings.

Answer 42

By week 5 :

• Nasal placodes invaginate to gives nasal pit
• tissues surrounding invagination
• Maxillary Prominences push nasal placode towards midline
• Medial nasal swelling extends downwards to make contact with maxillary swelling

Question 43

Describe the components of the face.

Answer 43

Upper Lip:
2 medial nasal prominences
2 maxillary prominences

Lower Lip:
Mandibular prominences

Nose:
Fontonasal prominence
Bridge and nasal septum
Medial nasal prominences
Crest and Tip
Lateral nasal prominences
Alae (sides)