Fertilisation & blastocyst development phys.

Question 1

Define ‘ontogenetic development’

Answer 1

Development of an individual to maturity

Question 2

What are the 2 stages ontogenetic development is split into?

Answer 2

1. Prenatal stage - begins with fertilisation off egg, ends at birth
2. Post-natal stage - begins at birth, ends when animal sexually mature

Question 3

What are the stages of the pre-natal stage?

Answer 3

• Fertilisation
• Cleavage
• Gastrulation
• Organogenesis
• Foetal growth & cell differentiation
• Gametogenesis

Question 4

What occurs during fertilisation?

Answer 4

1. Cumulus cells surrounding zona pellucida expand, substance between cells becomes gel like
2. Sperm penetrate this layer, bind to zona via ZP3 protein
3. Once fused, acrosomal reaction takes place causing deterioration of sperm outermembrane, laying bare inner membrane. Proteolytic enzymes are released
4. Inner membrane contains receptor for ZP2 protein
5. Proteolytic enzymes break down zona, sperm binds to ZP2
6. Zona breaks down, sperm & egg membranes fuse
7. Sperm nucleus enters egg cytoplasm, fertilisatin occurs
8. Entry of nucleus triggers Ca wave within egg
9. Cortical granule reaction occurs, hardening zona pellucida preventing more than 1 sperm fertilising the egg
10. Egg completes 2nd mitotic division
11. Sperm nucleus decondenses, female & male pronuclei develop - 1st cleavage division

Question 5

When does the 1st mitotic division occur?

Answer 5

20-24hrs after fertilisation

Question 6

Define ‘embryo cleavage’

Answer 6

The development of the single fertilised cell into a multicellular complex within the zona pellucida

Question 7

Describe what happens during embryo cleavage

Answer 7

• Zygote undergoes mitotic divisions to form blastomeres which are totipotent
• Cell gets progressively smaller
• After 3-4 divisions are 8-16 cells, blastomere becomes morula
• Morula enters uterus at day 4

Question 8

Draw a table to illustrate the different stages of embryo cleavage including no. of cells, stage and time after fertilisation

Answer 8

* once enters morula stage cells are pluripotent

Question 9

Describe development from a morula to blastocyst

Answer 9

• Blastomeres become compressed against each other, form tight junctional complexes with adjacent cells
• Morula draws in uterine fluid until embryo has central fluid filled cavity with a clump of cells pressed against one pole
• Occurs from 5 days after fertilisation
• Is now blastocyst

Question 10

Describe the structure of a blastocyst

Answer 10

• Trophectoderm - ectoderm like epithelium
• Inner cell mass (ICM) - clump of pluripotent cells at one pole
• Blastocoele - fluid filled space within blastocyst

Question 11

How can twins be formed?

Answer 11

1. Dizygotic - arise from 2 seperate ova, each fertilised by a seperate sperm
2. Monozygotic - arise from a single ova fertilised by a single sperm which splits at the blastomere stage/duplication of ICM within blastocyst

Question 12

Describe what occurs during implantation

Answer 12

• Blastocyst grows in size until zona breaks
• Trophoblast cells contact maternal tissue, initiates implantation for nutrition
• Trophoblast provides signals for maternal recognition of pregnancy
• ICM proliferate to form embryonic disc

Question 13

What occurs during expansion of the blastocyst?

Answer 13

1. Ruminants and pigs
   * after hatching from zona blastocytes expand enormously in 2nd week (upto 1cm/hr in pigs)
   
   2. Mares
      
      • hatch but are covered in glycoprotein capsule that remains spherical

Question 14

How early can pregnancy be detected in the mare?

Answer 14

14 days after ovulation due to embyronic vesicle. Appears black on ultrasound.

Question 15

Describe what happens during gastrulation

Answer 15

• Rearrangement of ICM cells by morphometric movement to form 3 germ layers of embryo
• First formation is of bilaminar embryo
• Second formation is of trilaminar embryo

Question 16

What makes up the bilaminar embryo?

Answer 16

1. Outer trophoblast cells covering ICM degenerate
2. ICM cells exposed and proliferate to form embryonic disc
3. Hypoblast layer forms on underside of ICM as a continuous sheet lining the interior of blastocyst
4. Emrbyonic disc made up of 2 layers; epiblast & hypoblast
5. Extra-embryonic endoderm surrounds blastocoele forming yolk sac

Question 17

Define gastrulation

Answer 17

The rearrangement of the 2 layers (epiblast/hypoblast) & proliferation with morphic movements to form 3 germ layers that are intra-emrbyonic

Question 18

What are the 3 germ layers formed by gastrulation?

Answer 18

1. Ectoderm (outer) - becomes epithelial (skin) & neural tissue
2. Mesoderm (middle) - becomes muscle, bone, blood, heart & connective tissue
3. Endoderm (inner) - becomes gut lining & associated structures (organs)

Question 19

Define the ‘primitive streak’

Answer 19

• Thickening caused by epiblast cells that migrate & proliferate along the diameter of the embryonic disc causing elongation
• Forms 2 ridges with a depression in the centre called the primitive groove
• Epiblast cells migrate down the primitive groove to the cranial end and form a mound of cells known as the primitive knot/Henson’s node

Question 20

What are the functions of the primitive streak and primitive node?

Answer 20

• Streak - form mesoderm
• Node - form notochord (neuroectoderm) and mesoderm of the head

Question 21

What happens during formation of mesoderm?

Answer 21

1. epiblast cells proliferate & migrate into the primitive streak
2. starting caudally, cells migrate ventrally through the groove
3. forms a loose arrangement of cells between the epiblast & hypoblast called the mesoderm
4. these layers expand laterally to form the trilaminar embryo - NOT into buccopharyngeal plate or cloacal membrane
5. epiblast becomes ectoderm, hypoblast becomes endoderm

Question 22

Where does the notocchord form?

Answer 22

• in front of the primitive streak
• expands cranially as a solid rod of mesoderm
• marks the cranio-caudal axis of embryo
• underlies & induces differentiation of future nervous system

Question 23

What leads to differentiation of the neural ectderm?

Answer 23

The fact that the embryonic disc grows much faster than the primitive streak

Question 24

What are the 3 derivatives of the mesoderm?

Answer 24

1. Paraxial mesoderm - develops into skeleton, muscles
2. Intermediate mesoderm - develops into urogential tract
3. Lateral mesoderm - immediately splits into 2 cell layers

Question 25

Describe properties of the paraxial mesoderm

Answer 25

* forms somites * week 4 of development somites differentiate into 3 different types of functional structures; dermatome (CT under skin), myotome (muscles), schlerotome (vertebrae, ribs, CT)

Question 26

Describe the properties of the intermediate mesoderm

Answer 26

Forms parts of urogenital system including: * embryonic kidneys * adult kidneys * urinary bladder & duct system * gonads & repro. tract

Question 27

Describe the properties of the lateral mesoderm

Answer 27

Splits to form intra- & extraembryonic coelum (fluid accumulation between 2 layers of mesoderm) 1. Extraembryonic: * consists of outer & inner layer of mesoderm * outer = somatic, forms somatopleure * inner = splanchnic layer, fuses with endoderm to form splanchnopleure * exocoelum = fluid filled space in between 2. Intraembryonic: * forms within intraembryonic portion of embryo * fluid filled spaces form, splitting mesoderm into somatic & splanchnic layers * eventually forms 3 main body cavities; pericardial, pleural, peritoneal

Question 28

Which structure separates the thoracic & abdominal cavities and forms the future diaphragm?

Answer 28

Septum transverse

Question 29

What are the 2 stages involved in establishing body form?

Answer 29

Tubulation and flexion

Question 30

Describe what occurs during tubulation

Answer 30

1. disc shaped embryo folds into a tube * outer = body wall * inner = GIT * inbetween = intraembryonic coelum cavity 2. gut formation * intrembryonic endoderm fuses & forms the hind & fore guts * midgut still continous with yolk sac via umbilical stalk * begins where 1st somites appeared 3. neuroectoderm development * neural plate folds, fuses into neural tube - neuralation * neural crest cells form head, facial structures, heart CT, autonomic NS

Question 31

Describe what occurs during flexion

Answer 31

1. head area (with cardiogenic area cranial to it) folds down & under itself, bringing heart beneath head 2. embryo forms 'C' 3. head and tail areas become recognisable