Embryogenesis

Question 1

Define the pre-embryonic, embryonic and foetal periods of human development

Answer 1

-Pre-embryonic = First 2 weeks
-Embryonic = 2-8 weeks
-Foetal = Post 8 weeks

Question 2

What are the changes that take place as the zygote travels down the Fallopian tube towards the uterus

Answer 2

• Cells in the zygote undergo cleavage
• Cells become smaller with each division (blastomeres). However, cell size does not increase due to the zona pellucida.
• Creates a ball of cells known as the morula.
  -Spaces between the cells of the morula merge to form the blastocoele, which converts the morula into the blastocyst.
  This takes about 4 days

Question 3

What is a zygote

Answer 3

A zygote is a fertilised oocyte

Question 4

What is cleavage

Answer 4

Cleavage is cell division that begins after fertilisation that form blastomeres

Question 5

What is a morula

Answer 5

A morula is a group of blastomeres packed together. All the cells at this point are totipotent.c

Question 6

What is an ovary

Answer 6

Ovaries are small oval shaped glands that produce and store eggs. They are found on either side of your uterus

Question 7

What is a Fallopian tube

Answer 7

The Fallopian tubes are bilateral tubes between the ovaries and the uterus, which function as channels for oocyte transportation and fertilisation

Question 8

What is a uterus

Answer 8

The uterus is a hollow organ which zygotes implant themselves onto

Question 8

What is a uterus

Answer 8

The uterus is a hollow organ which zygotes implant themselves onto

Question 9

Discuss the formation of the blastocyst and the initial stages of implantation

Answer 9

The blastocyst forms as the spaces between the cells of the morula merge to form the blastocoele, and the morula turn to the blastocyst. The blastocyst then hatches from the zona pellucida, so it is no longer constrained and then interacts with the uterine surface to implant. The conceptus, (around 100 cells), will split as some become the embryo and some become the placenta

Question 10

Define the sites of ectopic pregnancy and placenta praevia and how to distinguish between the two.

Answer 10

Ectopic pregnancy - When the implantation is at a site other than the uterine body, most commonly the fallopian tube, which can very quickly become a life-threatening emergency.
Placenta praevia - When the implantation is in the lower uterine segment, which can cause a haemorrhage and requires a C-section delivery.

Question 11

What is a zona pellucida

Answer 11

A glycoprotein ‘shell’ which prevent other sperm from fertilising the egg.

Question 12

What is a blastocyst

Answer 12

Blastocyst is a rapidly dividing ball of cells, (which are pluripotent), made up of the trophoblast and embryoblast

Question 13

What is the trophoblast

Answer 13

The trophoblast is the outer cell mass, provides nutrients to the embryo and develops into a large part of the placenta

Question 14

What is the inner cell mass

Answer 14

The embryoblast (inner cell mass), is the part of the blastocyst, that turns into the embryo, amniotic cavity and the yolk sac

Question 15

What is the cytotrophoblast

Answer 15

The cytotrophoblast is the inner part of the trophoblast.

Question 16

What is the synctiotrophoblast

Answer 16

The synctiotrophoblast is the outer layer of the trophoblast that determines what structures cross the placenta, (such as nutrients), and which do not, (such as maternal hormones).

Question 17

What is implantation

Answer 17

Implantation is when the cells attach to the uterine wall during the pre-embryonic period

Question 18

Describe the formation of the embryonic disc and how the embryoblast differentiates

Answer 18

At 8-9 days the inner cell mass differentiates to the epiblast and the hypoblast, (two layers). This layer is known as the bilaminar disc, which has a dorsal (ectodermal), and ventral (endodermal) surface.

Question 19

What is the germ layer

Answer 19

The germ layer is a group of cells in the embryo. The layers interact with each other as the embryo develops which eventually forms the organs and tissues.

Question 20

What is the ectoderm

Answer 20

The ectoderm is the outermost layer of the trilaminar disc, which comes from the epiblast

Question 21

What is the mesoderm

Answer 21

The mesoderm is the middle layer of the trilaminar disc coming from the epiblast.

Question 22

What is the primitive streak

Answer 22

The primitive streak is a linear band of thickened epiblast, that appears in the caudal end of the epiblast, that leads to migration and invagination of the epiblast cells and is the site where gastrulation occurs

Question 23

What is gastrulation

Answer 23

Gastrulation is the process that establishes all three germ layers in the embryo.

Question 24

What is the trilaminar embryonic disc.

Answer 24

The trilaminar disc is made up of the three germ layers: ectoderm, mesoderm and endoderm

Question 25

What are the ectodermal derivatives

Answer 25

The ectoderm differentiates to form the epithelial and neural tissues (spinal cord, e.g., skin, linings of the mouth, etc

Question 26

What are the mesodermal derivatives

Answer 26

The mesoderm derivatives are supporting tissues, such as the skeletal system of muscles, bones and cartilage. Also, a key derivative is the heart and blood vessels

Question 27

What are the endodermal derivatives

Answer 27

The endoderm differentiates to form internal structures such as the epithelial lining of the GI tract, respiratory tract and the parenchyma of glands

Question 28

How does the mesoderm differentiate into the: notochord, somite’s, intermediate mesoderm and lateral plate

Answer 28

The notochord forms from an aggregation of the mesoderm.
Somites are organisations of the paraxial mesoderm into segments, which form the vertebral column.
The intermediate mesoderm is one of the derivates from the mesoderm.
The lateral plate is the most lateral part of the mesoderm, which is a sheet that splits into two layers, somatic and splanchnic with a space, the coelom between them.

Question 29

Describe the formation of the intra-embryonic coelom

Answer 29

The intra-embryonic coelom is formed by the fusion of small spaces that arise between the two layers of the lateral plate mesoderm.

Question 30

List the derivatives of each region of the mesoderm

Answer 30

Intermediate mesoderm - The urogenital system, (e.g. kidneys, gonads)
Paraxial mesoderm - Somites which then make up the 31 pairsof spinal nerves.
Somatic mesoderm - Bones, ligaments, blood vessels, connective tissue
Splanchnic mesoderm - Mesothelial covering of the visceral organs

Question 31

Describe the formation of the neural plate and the development of the neural tube.

Answer 31

The neural plate is formed during gastrulation when the epiblast cells, and the primitive streak respond to signals from the node. This then turns to the neural tube. The notochord signals cause the overlaying ectoderm to thicken. The edges of the neural plate elevate and curl towards each other creating the neural tube.

Question 32

Describe the formation of the head fold, tail fold

Answer 32

Head fold - Bending of the cranial aspect of the embryo due to the rapid growth of the forebrain and stiffness of the notochord. Brings the mouth and heart to their ventral positions.
Tail fold - Bending of the caudal aspect of the embryo ventrally to reposition the cloaca and allantois. The cloaca will sub-divide into the urinary and anal openings.

Question 33

Describe the formation of the lateral body folds

Answer 33

Lateral fold - Lateral edges of the embryonic disc flex sharply ventral. Edges of each germ layer make contact at head and tail regions. Ectoderm covers the entire body surface except the umbilical region where the connecting stalk and yolk sac merge

Question 34

Describe the differentiation of somite’s, into dermatome, myotome and sclerotome and the fate of each one.

Answer 34

Sclerotome - Organised degeneration as ventral walls of somite’s break down leading to the formation if the sclerotome. Forms hard tissue section (Bones)
Dermatome - From the dermomyotome which forms further organisation of the dorsal portion. Dermatome disperses from the dermomyotome. Forms the skin section (dermis)
Myotome - From the dermomyotome, the myotome forms as it proliferates and migrates. Form the muscle section (muscles).

Question 35

Why do we use the terms dermatome and myotome when describing the innervation of skin and muscles in adult.

Answer 35

Derma - Skin
Myo - muscles
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