Session 2 ILO's - Embryology and early pregnancy

Question 1

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

Answer 1

Pre-embryonic: 1-2 weeks after fertilisation (Cleavage, compaction, implantation - is essential in allowing us to embark on the embryonic period)

Embryonic: 3-8 weeks inclusive post fertilisation (All of the systems of the body are built - embryo will have brain, kidney, heart ect…)

Fetal: From 9 weeks post fertilisation to birth (Functional maturation of those structures built in embryonic phase)

Question 2

Describe the fertilisation of the oocyte

Answer 2

• The oocyte gets fertilised by the sperm in the ampulla of the Fallopian tube
• The sperm passes through the zona pellucid and the plasma membrane
• It merges it’s genetic information with the ovum, to form a zygote

Question 3

Describe the zona pellucida

Answer 3

• Thick glycoprotein shell

Question 4

How long is a sperm viable for? How long is an oocyte viable for?

Answer 4

Sperm - up to 3 days
Oocyte - 1 day

Question 5

Discuss the changes taking place as the fertilised human ovum (zygote) travels down the Fallopian (uterine) tube towards the uterus

Answer 5

• The zygote, is transported along the fallopian tube to the uterus, by the specialised cilia in the oviduct under the actions of progesterone.
• The zygote travels from the ampulla end of the Fallopian tube to the junction with the uterus (called the isthmus). This takes about 4 days.
• As it travels through the Fallopian tube, the zygote undertakes successive cell divisions, a process known as cleavage.
• Cleavage (week one):
• Begins 30 hours after fertilisation
• Cell divisions occur without an increase in cell size and cells are now known as blastomeres
• First division occurs to create two totipotent blastomeres of equal size
• Cells become smaller with each cleavage division
• Cells keep dividing, all within zona pellucida

Compaction:

• After the third cleavage blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions.
• The process of compaction, segregates inner cells from the outer cells and they communicate extensively by gap junctions.
• Compaction creates a ball of cells known as the morula.
• The spaces between the cells of the morula merge to form one large central space (blastocoele or blastocyst cavity) converting the morula into a hollow sphere of cells, which is now called the blastocyst.
• The blastocyst comprises two cell groups with different fates and functions:
• The inner cell mass and the outer cell mass

Hatching (week 1):

• Blastocyst hatches from zona pellucida
• No longer constrained and now free to enlarge
• Can now interact with the uterine surface to implant

Implantation begins next?

• The inner cell mass becomes the embryo itself.
• At 8-9 days, the inner cell mass shows the first evidence of cell differentiation, where the blastocyst has formed two layers, the epiblast and the hypoblast.
• At this stage, this layer is known as a bilaminar embryonic disc. It now has dorsal (ectodermal) and ventral (endodermal) surfaces, but as yet no real head to tail orientation.
• Whilst the outer cells collectively become the trophoblast, which is the major component of the fetal membranes, especially the placenta, that surround and sustain the developing embryo and fetus, respectively, until birth.
  o The trophoblastic placenta develops a special cell layer, the syncytiotrophoblast, which first adheres to, and then invades, the modified (decidualised) endometrium.
  o This process is known as implantation. The preimplantation stages of pregnancy are the most hazardous for life, with up to 30% of zygotes failing to develop to the blastocyst stage.

COME BACK TO

Question 6

State the steps that occur before implantation begins

Answer 6

1) Fertilisation
2) Cleavage
3) Compaction
4) Hatching
5) Implantation begins

All these stages occur in week 1

Question 7

Where does the zygote travel from and to before implantation? How long does this take?

Answer 7

• The zygote travels:
• From the ampulla end of the Fallopian tube to the junction with the uterus (called the isthmus). This takes about 4 days.

Question 8

Describe the process of cleavage

Answer 8

• As the zygote travels through the Fallopian tube, the it undertakes successive cell divisions, a process known as cleavage.
• Cleavage (week one):
• Begins 30 hours after fertilisation
• Cell divisions occur without an increase in cell size and cells are now known as blastomeres
• First division occurs to create two totipotent blastomeres of equal size
• Cells become smaller with each cleavage division
• Cells keep dividing, all within zona pellucida

Question 9

Describe the process of compaction

Answer 9

Compaction (week 1):

• After the third cleavage blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions.
• The process of compaction, segregates inner cells (embryoblast) from the outer cells (trophoblasts) and they communicate extensively by gap junctions.
• Compaction creates a ball of cells known as the morula (16+ cells)
• The spaces between the cells of the morula merge to form one large central space (blastocoele or blastocyst cavity) converting the morula into a hollow sphere of cells, which is now called the blastocyst.
• The blastocyst comprises two cell groups with different fates and functions:
• The inner cell mass and the outer cell mass

Question 10

How do cells before compaction differ to cells after compaction?

Answer 10

Before compaction - Totipotent (the capacity to become ANY cell type)

After compaction - Pluripotent (the capacity to become one of MANY cell types - multi lineage potential)

Question 11

What helps the zygote get transported from the Fallopian tube to the uterus?

Answer 11

• The zygote, is transported along the fallopian tube to the uterus, by the specialised cilia in the oviduct under the actions of progesterone.

Question 12

Describe the process of Hatching

Answer 12

Hatching (week 1):

• Blastocyst hatches from zona pellucida
• No longer constrained and now free to enlarge
• Can now interact with the uterine surface to implant

Question 13

Describe the meaning of the following term zygote

Answer 13

A fertilised ovum/oocyte

Question 14

Describe the meaning of the following term cleavage

Answer 14

The division of cells in the early development of the embryo, following fertilisation

Question 15

Describe the meaning of the following term zona pellucida

Answer 15

The thick glycoprotein shell surrounding a mammalian ovum(/zygote?) before implantation.

Question 16

Describe the meaning of the following term morula

Answer 16

A solid mass of blastomeres (16+) resulting from a number of cleavages of a zygote

Question 17

Describe the meaning of the following term ovary

Answer 17

A female reproductive organ in which eggs are produced, present in humans and other vertebrates as a pair

Question 18

Describe the meaning of the following term Fallopian tube

Answer 18

Either of a pair of tubes along which eggs travel from the ovaries to the uterus.

Question 19

Describe the meaning of the following term uterus

Answer 19

A hollow, pear-shaped organ located in a woman’s lower abdomen between the bladder and the rectum, where offspring are conceived and in which they gestate before birth (AKA the womb)

Question 20

Discuss the formation of the blastocyst

Answer 20

• The spaces between the cells of the morula merge to form one large central space (blastocoele or blastocyst cavity) converting the morula into a hollow sphere of cells, which is now called the blastocyst.
• The blastocyst comprises two cell groups with different fates and functions:
• The inner cell mass and the outer cell mass

(Compaction is what leads to formation of the blastocyst)

Question 21

What is the difference between pluripotent and totipotent cells?

Answer 21

A totipotent cell has the potential to divide until it creates an entire, complete organism. Pluripotent stem cells can divide into most, or all, cell types in an organism, but cannot develop into an entire organism on their own.

CHECK DISCUSSION BOARD

Question 22

Describe the initial stages of implantation

Answer 22

Hatching (week 1):

• Blastocyst hatches from zona pellucida
• No longer constrained and now free to enlarge
• Can now interact with the uterine surface to implant
• The inner cell mass becomes the embryo itself.

Question 23

What stages of pregnancy are the most hazardous for life and why?

Answer 23

The preimplantation stages of pregnancy are the most hazardous for life, with up to 30% of zygotes failing to develop to the blastocyst stage.

Question 24

Define sites for ectopic pregnancy and placenta praevia and distinguish between these two implantation defects * Why does it require C section delivery?

Answer 24

Placenta praaevia
- Placenta previa is a problem during pregnancy when the placenta completely or partially covers the opening of the uterus (cervix):

• Implantation of embryo in lower uterine segment
• Can result in placenta positioning itself across the cervix
• During labour, this can cause haemmorage as the placenta is a vascular structure
• Requires C-section delivery because…
• High risk but are still viable

Ectopic Pregnancy
- Implantation of embryo at a site other than the uterine body (most commonly Fallopian tube)
- Uterus has a muscular wall which can expand as the foetus develops
- Fallopian tube and other non-uterine areas don’t have the ability to expand, which can lead to rupture of Fallopian tube and haemmorage
- Can be peritoneal (abdomen) or ovarian
- Can quickly become life-threatening emergency
- Usually not viable

Question 25

Define the meaning of the term blastocytst

Answer 25

By the fifth or sixth day, the fertilized egg is known as a blastocyst — a rapidly dividing ball of cells

Question 26

Describe the meaning of the following term trophoblast

Answer 26

A thin layer of cells that helps a developing embryo attach to the wall of the uterus, protects the embryo, and later forms the major part of the placenta.

Question 27

Describe the meaning of the term inner cell mass

Answer 27

The inner cell mass (ICM) is the part of the blastocyst that is fated to become the embryo, amnion, and yolk sac.

Question 28

describe the meaning of the term syncytiotrophoblast

Answer 28

The syncytiotrophoblast, the outermost layer of the human placenta, is the main site of exchange for drugs and metabolites, nutrients, waste products, and gases between the maternal and fetal circulations

Question 29

Describe the meaning of the term cytotrophoblast

Answer 29

Cytotrophoblast is the name given to both the inner layer of the trophoblast (also called layer of Langhans) or the cells that live there.

Question 30

Describe the meaning of the term implantation

Answer 30

Implantation is when a fertilized egg cell attaches to the wall of the uterus in early pregnancy.

Question 31

Describe the formation of the embryonic disc in the inner cell mass and initial cell differentiation within it (and implantation)

Answer 31

• At 8-9 days, the inner cell mass shows the first evidence of cell differentiation, where the blastocyst has formed two layers, the epiblast and the hypoblast.
• At this stage, this layer is known as a bilaminar embryonic disc. It now has dorsal (ectodermal) and ventral (endodermal) surfaces, but as yet no real head to tail orientation.
• Whilst the outer cells collectively become the trophoblast, which is the major component of the fetal membranes, especially the placenta, that surround and sustain the developing embryo and fetus, respectively, until birth.
• The trophoblastic placenta develops a special cell layer, the syncytiotrophoblast, which first adheres to, and then invades, the modified (decidualised) endometrium.
• This process is known as implantation.

Question 32

Define the term bilaminar disc

Answer 32

The bilaminar embryonic disc is formed when the inner cell mass forms two layers of cells, separated by an extracellular basement membrane. The external layer is called the epiblast and the internal layer is called the hypoblast. Together, they compose the bilaminar embryonic disc.

Question 33

Define the term germ layer

Answer 33

A germ layer is a group of cells in an embryo. Those layers interact with each other as the embryo develops and will eventually give rise to all of the organism’s organs and tissues. Almost all animals initially form two or three germ layers.

Question 34

Define the term ectoderm

Answer 34

The outermost of the three primary germ layers of an embryo that is the source of various tissues and structures (such as the epidermis, the nervous system, and the eyes and ears)

Question 35

Define the term mesoderm

Answer 35

The mesoderm is a germ layer that arises during gastrulation, and is present between the ectoderm, which will turn into skin and central nervous system cells, and the endoderm, which will produce the gut and the lungs

Question 36

Define the term primitive streak

Answer 36

• The primitive streak is a linear band of thickened epiblast that first appears at the caudal end of the embryo and grows cranially. At the cranial end its cells proliferate to form the primitive knot (primitive node).
• Appears on day 15 of human development, marks the start of gastrulation

Question 37

Define the term gastrulation

Answer 37

Gastrulation is a crucial time in the development of multicellular animals. During gastrulation, several importance things are accomplished: The three primary germ layers are established. The basic body plan is established, including the physical construction of the rudimentary primary body axes.

Question 38

Define the term trilaminar embryonic disc

Answer 38

A trilaminar embryonic disc is an early stage in the development of triploblastic organisms, which include humans and many other animals. It is an embryo which exists as three different germ layers - the ectoderm, the mesoderm and the endoderm.

Question 39

List the important ectodermal, mesodermal and endodermal derivatives

Answer 39

Ectoderm - Organs and structures that maintain contact with the outside world:

• Nervous system
• Skin (epidermis)

Mesoderm - Supporting tissues:

• Muscle
• Cartilage
• Bone
• Vascular system (including heart and vessels)

Endoderm - Internal structures:

• Lining of GI tract
• Respiratory tract
• Parenchyma of glands

Question 40

Describe the differentiation of mesoderm into the notochord, somites, intermediate mesoderm, and lateral plate.

Answer 40

Later development of the mesoderm creates an axial rod, the notochord, and a series of segments on either side of it, the somites.
 The notochord precedes the vertebral column.
 The somites form the vertebral column, ribs and extensor muscles of the
spine and muscles of the body wall.

Lateral to the somites there is a cord of mesoderm, the intermediate mesoderm that forms the kidneys. The most lateral part of the mesoderm is a sheet, the lateral plate. This splits into two layers, somatic and splanchnic, with a space, the coelom, between them.
The somatic layer forms the diaphragm and limb muscles

The splanchnic layer forms the muscle of the heart and viscera

The coelom becomes the body cavities.

Question 41

Describe the formation of the intra-embryonic coelom

Answer 41

The intraembryonic coelom first appears as many small isolated coelomic spaces in the lateral mesoderm and cardiogenic mesoderm (between the 2 layers of the lateral plate mesoderm) which coalesce to form a horseshoe-shaped cavity, the intraembryonic coelom, which is lined by flattened epithelial (mesothelial) cells. It will become the pleuropericardial-peritoneal cavity.

Question 42

List the derivatives of each region of the mesoderm

Answer 42

Notochord:
- Preceedes/comes before Vetebral column

Somites:
- Vertebral column
- Ribs
- Extensor muscles of the
spine
- Muscles of the body wall.

Intermediate mesoderm:
- Kidneys

Lateral plate:
This splits into two layers, somatic and splanchnic, with a space, the coelom, between them.

• The somatic layer forms the diaphragm and limb muscles
• The splanchnic layer forms the muscle of the heart and viscera
• The coelom becomes the body cavities.

Question 43

Describe the formation and fate of the neural plate

Answer 43

At the same time that the mesoderm differentiates, changes affect the ectoderm. An oval thickening forms in the area overlying the notochord. It is the neural plate, the origin of the nervous system.
 Its wide anterior end becomes the brain.
 Its narrower posterior end the spinal cord.

Question 44

Outline the development of the neural tube.

Answer 44

At the same time the endoderm becomes the lining of the primitive gut, the neural plate, on the dorsal aspect of the embryo, is folding into the neural tube. This process is known as neurulation. The embryo now possesses the cell types necessary for the differentiation of all specialised tissues of the adult, organised in a basic human body plan.

Question 45

Describe the formation of the head fold, tail fold and lateral body folds.

Answer 45

Before day 14, the embryo is still disc shaped, with ectoderm dorsally, endoderm ventrally, and mesoderm sandwiched between them. The next stage converts it into a 3-dimensional, roughly cylindrical configuration, with ectoderm
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outside, endoderm inside, and mesoderm between. Head, tail and lateral body folds form, closing the embryo off from the yolk sac.

(watch lecture 4)

Question 46

Describe the differentiation of somites into dermatome, myotome and sclerotome and the fate of each of these in later development.

Answer 46

The somites undergo further changes which underlie the pattern of outgrowth of spinal nerve roots from the spinal cord, the formation of the vertebral column and the innervation of muscles and skin.

Each somite separates into a dermatome, which gives rise to an area of skin, a myotome, which gives rise to a block of skeletal muscle (myotomes become the muscles of the thoracic and abdominal walls.) and a sclerotome, which gives rise to a vertebra and rib. Spinal nerve roots growing out from the neural tube enter and innervate the dermatomes and myotomes. The sclerotomes are rearranged to lie between the nerve roots and become the vertebrae.

Question 47

Explain the use of the terms “dermatome” and “myotome” in describing the innervation of skin and muscles in the adult.

Answer 47

The terms ‘ dermatome’ and ‘ myotome’ are used clinically to define an area of skin or muscle supplied from a single spinal nerve root, and the spinal cord is described as having ‘ segments’, each of which gives rise to a nerve root.