Early human development

Question 1

What is the site of fertilisation?

Answer 1

• Ampulla region of uterine (fallopian) tube

Question 2

What are some of the most common Ectopic (abnormal) sites of implantation?

Answer 2

• Ovary
• Ampulla
• Uterine tube
• Outside of uterine tube, within peritoneum
• Lower part of uterine body or cervix

Question 3

Where should implantation normally occur?

Answer 3

• Posterior uterine wall

Question 4

What is the basis for several assisted reproduction techniques?

Answer 4

The ability to manipulate gametes and embryos in culture

Question 5

Describe the term inner cell mass

Answer 5

• Refers to one of the 2 types of cells that form the blastocyst
• Inner cell mass are a group of 20-30 pluripotent cells located one side of the blastocyst, and this is what gives rise to the definitive structures of the foetus
• The inner cell mass forms the epiblast and amnionic ectoderm during implantation

Question 6

Describe the term trophoblast

Answer 6

• Single layer of flattened cells within the blastocyst
• Stems from morula’s outer mass
• Gives rise to the placenta
• Forms extra-embryonic tissues

Question 7

Describe the process of formation of oocytes and spermatozoa

Answer 7

• Germ cells begin meiosis - Diploid genome duplication results in 2 sets of homologous chromosomes (the duplicate versions of the paternal and maternal chromosomes are called sister chromatids).
• During meiosis I, homologous chromosomes undergo recombination, or crossing over.
• Homologous chromosomes are then separated by cellular and nuclear division and another round of division separates the sister chromatids from one another to give rise to 4 daughter cells, each now containing a haploid genome (1 copy of each chromosome). At this point, each of those nuclei is packaged into 4 spermatids that then differentiate into 4 spermatozoa
• However, the oocytes arrest during meiosis I, and meiosis won’t resume until a single oocyte has been selected to mature within an ovarian follicle.
• As the first meiotic division proceeds, one set of homologous chromosomes is excluded from the oocyte and remains within the zona pellucida - this is the first polar body
• The oocyte is paused in meiosis again and the second meiotic division won’t resume until the oocyte has been fertilised. One set of genetic material (this time, one set of sister chromatids) is excluded from the oocyte and this is the second polar body

Question 8

Describe the cleavage stage of fertilisation

Answer 8

• Haploid pronuclei of sperm and oocyte fuse, forms a zygote containing both maternal and paternal DNA
• Zygote undergoes mitotic division forming a 2-cell zygote, and then the 2-cell zygote divided to form a 4-cell zygote

Question 9

Describe morula and blastocyst formation

Answer 9

• 4- cell zygote undergoes mitotic division to form 8-cell zygote
• 3 days after fertilisation, cells of compacted embryo divide again forming a cluster of cells - This is a mulberry-shaped 16-cell morula (can range from 12-16 cells) - It’s composed of 2 zones: inner, outer cell mass
• After the third cleavage, blastomeres (cells outside of the morula) form compact balls of cells, connected by tight junctions (compaction) - This causes fluid to accumulate within the internal cavity (this is called the blastocoele - a fluid-filled cavity) of the zygote, and the morula becomes a blastocyst
• The zona pellucida begins dissolving
• 4-5 days, embryo consists of approx 100 cells

Question 10

Describe the structure of the blastocyst

Answer 10

Fluid filled hollow cell, 2 zones:
- Trophoblast - single layer of large flattened cells, stemming from morula’s outer cell mass; gives rise to placenta
- Embryoblast (inner cell mass) - 20-30 pluripotent cells located on one side stemming from inner cell mass; gives rise to embryo

Question 11

Describe hatching

Answer 11

Enzymes within the embryo and uterine environment are secreted to break down the zone pellucida protein coat surrounding the blastocyst so that it’s able to implant within the posterior uterine wall

The blastocyst has 2 poles, the embryonic pole (this is the side of the inner cell mass) and the abembryonic pole

The embryonic pole is the side that fuses with the posterior uterine wall

Question 12

Describe the process of implantation on Day 7

Answer 12

• Trophoblast binds to uterine wall with L-selectin, integral receptors- penetrates between epithelial cells
• High progesterone released from corpus luteum, develops endometrium for implantation
• Blastocyst implants into decidua basalis, along superior posterior wall of uterus

Question 13

What happens to the trophoblast on Day 8?

Answer 13

• Proliferates, forms 2 layers
  
  • Cytotrophoblast (cellular trophoblast):
    
    • Inner layer of mononucleated cells
    • Produces primary chorionic villi, protrudes into syncytiotrophoblast - This villi increases the SA available for products from the maternal blood to be absorbed into the placenta
  • Syncytiotrophoblast:
    
    • Outer multinucleated mass of cells (without distinct cell boundaries)
    • Invades decidua basalis with finger-like processes, make enzymes that erode uterine cells; blastocyst burrows into decidua basalis surrounded by pool of blood leaked from degraded blood vessels
    • Secretes hCG protein- maintains viability of copus luteum, increases secretion of progesterone and oestrogen

Question 14

What happens to the inner cell mass (embryoblast) on day 8?

Answer 14

• Differentiates into 2 layers, forms flat disc:
  • Hypoblast - small cuboidal cells adjacent to blastocyst
  • Epiblast - columnar cells
    • Cavity forms inside → amniotic cavity
    • Lined with amnioblasts

Question 15

What do the syncytiotrophoblast cells do?

Answer 15

• Finger-like projections that fuse blastocyst to posterior uterine wall
• Multinucleated cells, degrade extracellular matrix and tissue of uterine wall, allow embryo to implant
• Implantation site then sealed off by a fibrin plug

Question 16

What do trophoblast cells secrete during implantation?

Answer 16

• hCG (human chorionic gonadotropin) protein - This is the basis for pregnancy tests
• Reaches follicle from which the oocyte was released in the ovary, and allows the maintenance of the corpus luteum, which secretes progesterone
• Progesterone functions in a feedback loop to maintain the embryo implanted into the uterine wall, develops endometrium for implantation

Question 17

What happens to the embryo during implantation?

Answer 17

• Inner cell mass splits - becomes epiblast (ectoderm) and hypoblast (extra embryonic endoderm):
  • Different genes required: maternal- embryo ; paternal - trophoblast (imprinting)
  • Hydatidiform moles - rare, mass or growth with no embryo, all paternal (trophoblast)
• Two cavities form:
  
  • Amniotic - derived from epiblast
  • Chorionic - derived from trophoblast
• Ready for gastrulation

Question 18

What is the bilaminar embryo?

Answer 18

When the inner cell mass splits into 2 layers, the epiblast and hypoblast:

• Differentiates into 2 layers, forms flat disc
  
  • Hypoblast - small cuboidal cells adjacent to blastocyst
  • Epiblast - columnar cells
    
    • Cavity forms inside → amniotic cavity
    • Lined with amnioblasts
• These together are the bilaminar embryo

Question 19

Describe the process of gastrulation

Answer 19

Happens in week 3

• Begins with formation of primitive groove (narrow depression into centre of epiblast layer)
  
  • Starts at caudal end, grows towards cranial end → cranial-caudal axis
  • Groove forms on dorsal side of embryo → dorsal-ventral axis
  • 2 sides of groove: left, right side of body (bilateral symmetry)
• Primitive node forms at cephalic end of primitive groove
  
  • Contains primitive pit, surrounded by slightly elevated area of ectoderm
• Primitive groove, node, pit → form primitive streak
• Epiblast cells migrate towards primitive groove → move to bottom, slide under (invagination)
• After invagination, 3rd germ layer forms (mesoderm, from ectoderm) and migrates between the other 2 (trilaminar disc)
• Migration is axial, paraxial, intermediate and lateral
• Cells of trilaminar disc multipotent (ability to differentiate into many tissues, organs)
  
  • Some epiblast cells displace ventral hypoblast layer, form endoderm
  • Invaginated epiblast cells between newly formed endoderm, epiblast→ mesoderm layer
  • Rest of epiblast forms ectoderm layer

Question 20

What is the trilaminar embryo?

Answer 20

An embryo which exists as three different germ layers – the ectoderm, the mesoderm and the endoderm

Question 21

What does the ectoderm form?

Answer 21

• Outer surface of embryo
• Neurectoderm:
  
  • Neural tube (CNS + spinal cord)
  • Neural crest
• Epidermis, hair, nails, internal ear, lens, enamel

Question 22

What does the endoderm form?

Answer 22

• Digestive tube
• Pharynx
• Respiratory tube
• Linings: epithelium of GI, urinary and respiratory tracts, epithelial parts of liver, pancreas, thyroid

Question 23

What does the mesoderm form?

Answer 23

• Dorsal
• Paraxial: somites - muscle, skeleton
• Intermediate - urogenital
• Lateral - Heart, spleen, blood
• Head - Skull, dentine in teeth

Question 24

Describe neurulation

Answer 24

• Formation of brain, spinal cord, cranial and spinal nerves from neural tube and cranial and spinal ganglia from neural crest
• Takes place at end week 3, all week 4, often prior to detection of pregnancy
• Neural plate develops neural groove, from groove, neural folds elevate, meet in middle and fuse to form tube, as tube is closing, neural crest cells migrate out
• Neural plate derived from neuroectoderm, induced from epiblast by surrounding tissue
• Non-closure of neural tube leads to spina bifida, or anencephaly (neural tube defects)

Question 25

From what germ layer do the main bodily tissues/ organs form?

Answer 25

- Ectoderm Forms: - Outer surface of embryo - Neurectoderm (neural tube, neural crest) - Epidermis, hair, nails, internal ear, lens, enamel

Question 26

How can chromosomal anomalies result in spontaneous abortion and malformation?

Answer 26

Approximately 50% of spontaneously aborted embryos are chromosomally abnormal, due to the many meiotic and post zygotic mitotic divisions involved within the process of fertilisation

Question 27

Explain the formation of dizygotic twins

Answer 27

- Form from 2 separate oocytes - Approx 2/3 of all twins

Question 28

Explain the formation of monozygotic twins

Answer 28

- Form from 1 oocyte that can **split** at various stages: - Before blastocyst, occurs less than 5 days after fertilisation, accounts for 33% of MZ twins, results in 2 chorions, 2 amnions, 2 placentas (DCDA) Inner cellular mass, occurs 5-7 days after fertilisation, accounts for approx 66% MZ twins results in 1 chorion, 2 amnions, 1 placenta (MCDA) Epiblast (embryonic disk), occurs 10 days after fertilisation, accounts for approx 1% of MZ twins, results in 1 chorion, 1 amnion, 1 placenta (MCMA) Primitive streak, occurs 14 days after fertilisation, very rare, results in 1 chorion, 1 amnion, 1 placenta (MCMA)

Question 29

How can human fertilisation and preimplantation be supported in culture?

Answer 29

- Human fertilisation and preimplantation can be supported in vitro-somatic nucleus put in denucleated unfertilised egg (SCNT)

Question 30

Explain the process of IVF

Answer 30

1. Hormonal stimulation = several mature follicles 2. Collect oocytes from follicles with aspirator during laparoscopy 3. Place in Petri dish with capacitated sperm = IVF 4. Cleave zygotes in medium until 4-8 cell stages reached and then transfer 2-3 cleaving embryos to uterine cavity by catheter in vagina and cervical canal

Question 31

Why are blastomeres used for genetic screening?

Answer 31

- They're totipotent - an give rise to any cell type

Question 32

What product of IVF be used to culture embryonic stem cells?

Answer 32

Inner cell mass (embryo blast) Can be removed, cultured as embryonic stem cells, genetically manipulated, replaced into embryo to form a transgenic

Question 33

How are embryonic stem cells important, therapeutically?

Answer 33

Can differentiate into many adult cells = human therapeutic clones for diseases of cell loss/damage

Question 34

What is the order of the stages of early development?

Answer 34

1. Fertilisation 2. Cleavage 3. Morula 4. Blastocyst formation 5. ICM differentiation 6. Full implantation 7. Gastrulation 8. Neurolation 9. Somitogenesis

Question 35

List the three germ layers, what are the main tissues that arise from each of these?

Answer 35

- Endoderm - linings, e.g. epithelium of GI, urinary, respiratory, epithelial of liver, pancreas, thyroid - Ectoderm - Neurectoderm, e.g. neural tube CNS and spinal cord. Surface ectoderm e.g. epidermis, hair, nails, internal ear, lens, enamel - Mesoderm - paraxial (somites i.e. muscle and skeleton), intermediate (urogenital), lateral (heart, spleen, blood) and head (skull, dentine)

Question 36

How does ectopic implantation occur?

Answer 36

Zona pellucida doesn't lyse at correct time, causes ectopic pregnancy

Question 37

What drives the spermatozoa to the oocyte?

Answer 37

Chemoattractants

Question 38

Describe capacitation and acrosomal reaction

Answer 38

These are the 2 processes required for fertilisation: - Capacitation - Epithelial interactions b/w sperm and uterine wall. Glycoprotein coat, seminal plasma proteins covering acrosomal region removed, easier enzyme release, acrosomal reaction - Acrosomal reaction - After binding to zone pellucida - Release of enzymes needed to penetrate zona pellucid

Question 39

What happens to the ectoderm and endoderm layers on Day 15?

Answer 39

- 2 areas of ectoderm (cranial, caudal) push ventrally, fuse with endoderm (exclude mesoderm layer) and forms 2 bilaminar regions in the trilaminar disc - Cranial bilaminar region develops into oropharyngeal membrane - Disintegrates in week 4 to form mouth opening - Caudal bilaminar region develops into cloacal membrane -> disintegrates in week 7 to form anal opening, GU tracts

Question 40

What do the group of mesoderm cells form?

Answer 40

Notochord - Transient embryonic structure, solid, helps influence how embryo folds Secretes protein called Sonic HedgeHog (SHH) which guides tissue differentiation

Question 41

What do the mesoderm cells around notochord differentiate into?

Answer 41

3 specialised types of cells: - Paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm These make different tissues, organs

Question 42

Explain the process initiated by the notochord

Answer 42

Neurulation - Stimulates cells of ectoderm to form neural plate Neural plate folds, forms neural groove with edges called neural folds Neural plate continues to grow, neural folds come together, pinch off from surface of ectoderm to form neural tube b/w ectoderm and mesoderm

Question 43

Describe vasculogenesis

Answer 43

Trophoblast continues developing: - Primary villi - Made of cytotrophoblastic core covered by syncytial layer - Secondary villi - Form when extraembryonic somatic mesoderm cells penetrate the primary villi -> grow toward decidua - Tertiary villi - Form when mesodermal cells differentiate into small blood vessels - Form villus capillary system -> fetal contribution to placenta