Embryology

Question 1

What are the different stages of embryonic development

Answer 1

Weeks 0-3 : conceptus / embryo
Weeks 3-8 : Embryonic period
Weeks 9-40 : fetal period

Question 2

What does gametogenesis consist of?

Answer 2

Mitosis and meiosis

Question 3

Describe meiosis

Answer 3

• chromosomes double and pair up then chiasmata form
• the nucleus divides and chromosome pairs divide
• the daughter nuclei divide again and each chromosome is singular

Question 4

What is spermatogenesis?

Answer 4

• Formation of male gametes
• occurs at puberty and continues throughout life
• each cycle takes 64 days
• production rate decreases with age

Question 5

What is oogenesis?

Answer 5

• formation of female gametes
• starts in development and ends at menopause
• by week 28-30 of embryonic development primary oocytes begin meiosis but arrest in prophase until puberty
• reproductive life span determines in fetal life.

Question 6

What is trisomy 21?

Answer 6

=down syndrome

• usually caused by nondisjunction leading to an additional copy of chromosome 21
• 75% of nondisjunction cases occur during oocyte formation

Question 7

What is cleavage?

Answer 7

A period of rapid cell division with no increase in size

It results in the formation of morula then a blastocyst

Question 8

What is gastrulation?

Answer 8

The formation of germ layers (week 3 embryonic development)

Question 9

What are the three germ layers?

Answer 9

Endoderm (inner layer)
Mesoderm (middle layer)
Ectoderm (outer layer)

Question 10

How is the body initially formed in embryos?

Answer 10

Embryonic folding leads to a tube within a tube ( one tube being the gut tube and the other being the skin/body wall)

Question 11

What is organogenesis?

Answer 11

The formation of organs and organ systems
The basis of all organs is in place by the end of the embryonic period (week 8) and will continue to develop through the foetal period (40 weeks)

Question 12

What happens in the foetal period (weeks 9-40)

Answer 12

• Mainly grows in length during the 2nd trimester and adds weight in the 3rd trimester.
• tissues become mature and functional
• overt sexual differentiation
• bone laid down and connections made in the CNS

Question 13

What are the anatomical terms used in embryology ?

Answer 13

Nearer the head = cranial
Nearer the feet = caudal
Nearer the front = ventral
Nearer the back = dorsal

Question 14

What are the primary process which take place from zygote formation to embryo?

Answer 14

Cell division, differentiation, cell attachment, apoptosis, induction (ability to cause another cell to differentiate) and cell migration

Question 15

What are secondary processes in embryonic development?

Answer 15

Axis formation/ polarity (cells knowing which way is up)

Folding/ rotation (gives embryo 3D form and formation of organs)

Question 16

What is epigenetic control ?

Answer 16

Preferential expression of either the maternal or paternal copy of a gene

Question 17

Describe the sperms role in fertilisation

Answer 17

1- the sperm binds with the zone pellucida glycoprotein (ZP3)
2- acrosomal enzymes released from the sperm head, the sperm digests its way into egg
3- egg and sperm plasma membrane fuse and sperm contents enter the egg
4- sperm entry triggers completion of meiosis 2 and the realease of cortical granules by oocyte

Question 18

When are blastomeres thought to be totipotent ?

Answer 18

Up until the 8 cell stage

Question 19

What is mosaicism?

Answer 19

Individuals with two or more cell lines with different chromosome complements due to mitotic disjunction (trisomy 21)
In a zygote with trisomy 21 some cells will have a normal complement of cells and some with trisomy making the features less prominent

Question 20

Describe the compaction of the morula

Answer 20

Cells are confined within the zona pellucida (ZP) and maximise available space by coming into closer contact with each other and begin to form cell junctions.
Outer cells form extensive gap junctions and fluid enter through the ZP

Question 21

What are the parts of the blastocyst?

Answer 21

The inner cell mass = embryoblast
Outer cell mass =trophoblast
Blastocyst cavity

Question 22

When does blastocyst hatching occur?

Answer 22

When the embryo interacts directly with the endometrial lining of the uterus for implantation

Question 23

When and where does implantation occur?

Answer 23

Typically by day 6 on the posterior or anterior uterine wall (usually posterior)

Question 24

What happens around day 7 of embryonic development?

Answer 24

(Bilaminar embryo)
The active process is driven by the trophoblast which divides into the cytotrophoblast (many individual cells) and the syncytiotrophoblast (single multinucleated cell)

Question 25

What happens around day 9 of embryonic development?

Answer 25

• Formation of two cavities; the amniotic cavity (epiblast) and the primitive yolk sac (hypoblast)
• implantation complete and closure by the fibrin Coagulum
• vacuoles appears in the syncytiotrophoblast and unite to form lacunae (small space)

Question 26

What happens around day 12 of embryonic development?

Answer 26

Establishment of the uteroplacental circulation where maternal blood flows into lacunae to diffuse to the embryoblast
The extraembryonic mesoderm develops and eventually degenerates forming the chorionic cavity

Question 27

What happens on day 13 of embryonic development?

Answer 27

Further development of the chorionic cavity and presence of the connecting stalk (eventually umbilical cord)
Second wave of hypoblast cells migrate to form the definitive (secondary) yolk sac

Question 28

What happens at week 3 of embryonic development?

Answer 28

Formation of the primitive streak =appears in the midline at caudal end of epiblast
At the cranial end of the primitive streak is the primitive pit and node

Question 29

What is gastrulation?

Answer 29

Movement of epiblast cells through the primitive streak to form three primary germ layers and body axes are established.
Three layers = endoderm, mesoderm and ectoderm

Question 30

What does the ectoderm do?

Answer 30

It is responsible for the epidermis of skin and the nervous system

Question 31

What are the layers of the mesoderm?

Answer 31

The mesoderm becomes divided into 3 regions either side of the notochord - the paraxial mesoderm, intermediate mesoderm and the lateral plate mesoderm (somatic layer and visceral layer)

Question 32

What does the paraxial mesoderm do?

Answer 32

Responsible for the axial skeleton, voluntary skeletal muscle and parts of the dermis (back)

Question 33

What does the intermediate mesoderm do?

Answer 33

Forms urogenital systems (kidney, ureter, gonads etc)

Question 34

What does the somatic part of the lateral plate mesoderm become?

Answer 34

Most of the dermis, lining of body wall and parts of limbs

Question 35

What does the visceral part of the lateral plate mesoderm become?

Answer 35

Cardiovascular system, mesothelial covering of organs and smooth muscle

Question 36

What does the endoderm become?

Answer 36

Lining of the gut tube, lining of the respiratory tract and lining of the bladder and urethra

Question 37

what happens around day 8 of embryonic development?

Answer 37

The embryo organised into 2 cell layer; the epiblast (dorsal surface) and the hypoblast (ventral surface) and the amniotic cavity begins to form

Question 38

when is the risk of birth defects being induced the most likely?

Answer 38

day 5

Question 39

describe the process of heart development to a single tube.

Answer 39

• cardiac progenitor cells migrate through the primitive streak into the visceral layer of lateral plate mesoderm
• two endocardial tubes come together to form one tube. proliferation of mesoderm leads heart into thorax.
• the bulbus cordis moves ventral, caudual and to the right. the atrium moves dorsal and cranial. looping and folding occurs.

Question 40

describe how the heart goes from a single tube to a four chambered heart.

Answer 40

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