Intro to Embryology

Question 1

Oocyte (ovum,egg)

Answer 1

female germ cell

Question 2

sperm

Answer 2

male germ cell

Question 3

zygote

Answer 3

cell resulting from union of oocyte and sperm at fertilistaion

Question 4

Blastomeres

Answer 4

early embryonic cells

Question 5

Morula

Answer 5

Solid mass of 12-32 blastomeres

Question 6

Blastocyst

Answer 6

stage where morula has entered uterine cavity and blastocytic cavity develops within

Question 7

Gastrula

Answer 7

stage after trilaminar embyronic disc formation

Question 8

Neurula

Answer 8

Stage after gastrula when neural tube forms

Question 9

Embryo

Answer 9

Developing human during embryonic stage (to end of 8th week)

Question 10

Fetus

Answer 10

Period form 9th week to birth

Question 11

Stages 1-3 (Fertilisation and Early cell division)

Answer 11

Day 1: Fertilisation

Day 2: Zygote
Zygote is a diploid cell produced from fusion of two haploid gametes (fertilised ovum)

Day 3: Morula
Dividing mass of 20-30 cells

Question 12

Stages 3-4: Blastogenesis

Answer 12

Day 4: Stage 3
Inner cell mass (made of blastomeres)- becomes embryo

Tropoblast- becomes embryonic part of placenta

Day 5: Hatching of blastocyte/ shedding of the zona pellucida
Zona pellucida is a membrane which surrounds the ocum before implantation to then produce the late blastocyst

Day 5: Endometrial epithelium- undergoes structural and histological changes throughout the menstraul cycle in preperation for implantation
Endometrial glands- provide substances for survival, development and implantation for the cunceptus (fertizilised egg)

Day 6: Stage 4
Syncytiotrophoblasts- special layer of epitheliul cells through which substances cross the placenta

Question 13

Stages 4-6: Formation of the Bilaminar Disc and Implantation

Answer 13

Day 8: Stage 5
Bilaminer disc- here the inner cell mass differenciates into two distinct layers. They are seperated here by an extracellular basement membrane;

External layer is called Epiblast
Internal layer is called the Hyperblast
Together both these layers form the bilaminar disc

Day 9: Formation of yolk sac which provides nutrition and gas exchange between the mother and cunceptus before the placenta is formed
Amnion is a membrane that covers the blastocyste and will eventually fill with fluid to form the embryonic sac

Day 10- Lucunae appear
These are spaces filled with maternal blood

Question 14

Day 8-9: Formation of the Placenta and Embryo

Answer 14

Day 8: Cytotrophoblast- inner layer of the trophoblast
Becomes embryonic part of placenta

Day 9: Becomes embryo
Endoderm (inner)- constructs the digestive tube, primitive gut, respiratory tube. These are cuboidal cells
Mesoderm (middle)
Ectoderm (outer)- becomes the central nervous system, skin, hair and nails. These are column cells.

Question 15

Day 11-13: Formation of the Bilaminar Disc and Extraembryonic Cavity

Answer 15

Day 11-12:
Exocoelomic cavity- inside the extraembryonic mesoderm
Important transfer interphase and a transfer of nutrients for the embryo

Day 13:
Exocoelomic cyst- reminisence of the primary yolk sac

Cavities become confluent to form new space called extraembryonic coelom or chorionic cavity

Question 16

Day 13: Formation of the umbilical cord

Answer 16

Maternal capillaries nearby expand to form the maternal sinusoids

1) Chorionic vesicle hangs in the chorionic cavity
2) Chorionic vesicle joined to placenta cells by connecting stalk
3) Becomes umbilical cord

Question 17

Week 3: GastrulationBilaminar Disc ————> Trilaminar Disc

Answer 17

Caudal: Tall end
Cranial: Head end

As the ectoderm grows a folding occurs along the caudal midline which creates the Primitive Streak

Primitve streak is a transient strcuture and its formation marks the start of gastrulation and establishes the anterior posterior body axis

Question 18

Define Gastrulation

Answer 18

This is where the embryo transforms from a 1D layer of epithelial cells, the blastula, and reorganises into a multilayered multidimensional strcuture called the castella

Question 19

Week 3: Formation of Germ layers

Answer 19

Cells from the base of the primitive streak detach and migrate to lie between the layers of the ectoderm and endoderm

The invagination of these cells creates a new layer called the mesoderm (middle)

The disc is now known as the trilaminar disc

-At the caudial end there is the Primitive streak
-At the cranial end theres the oropharyngeal membrane (aka buccal pharyngael membrane)- This forms a septum between the primitive mouth and the throat pharynx

Question 20

Differentiation of Germ layers

Answer 20

Each of the three germ layers differentiate into specific tissues and organ systems

Question 21

Ectoderm- outer layer

Answer 21

Epidermis at the skin, hair nails,

Various glands (mammary, sweat, sebaceous glands)

Central nervous system

Peripheral nervous system

Pituitary gland

Enamel of the teeth

Lens of the eye and parts of the inner ear

Sensory epithelium of nose, ear and eye

Question 22

Endoderm- inner layer

Answer 22

Differenciates into specific tissues and organ systems

-Epithelial lining of the gastrointestinal tract, respiratory tract and urinary bladder

-Parenchyma of the thyroid gland, parathyroid gland, liver and pancreas

-Epithelial lining of the tympanic cavity and auditory tube

-Plays a part in the development of the notochord

Question 23

Mesoderm- middle layer

Answer 23

All of the musculoskeletal system

Deep layers of the skin

Abdominal and chest walls and lining

The walls of the bowel (but not the lining of the bowel)

The urogenital system

Question 24

Formation of the Notochord

Answer 24

1) Tube develops from end of primitive streak extending towards cranial end

2) Tube fuses with endoderm to become a groove

3) Plate fold to become a tube once again – the notochord

4) Notochord has a central role in further midline development. This is because it provides both mechanical and signalling keys to the developing embryo

Question 25

Weel 4: Neurulation

What is Neurulation?

Answer 25

This is a process in which the neural plate, a thickened portion of the ectoderm along the midline of the embryo, bends up and fuses to form a hollow tube that will eventually differenciate into the brain, spinal cord and central nervous system

Question 26

Process of Neurulation

Answer 26

1. Neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate, separated by the neural plate border
2. Neural plate bends dorsally until the two plates convergence to form the neural crest
3. The neural crest is disconnected from the epidermis by the closure of the neural tube.
   The cells of the neural crest differentiate to form most of the peripheral nervous system
4. The notochord degenerates, only persisting as the nucleus pulposus of the intervertebral discs.
   Mesoderm cells differentiate into somites, precursors of the axial skeleton and skeletal muscle

Question 27

Clinical revelance

Answer 27

Caudal end:
If fusion does not extend all the way to the caudal end of the embryo the child is born with spina bifida

Cranial end:
If fusion does not extend all the way to the cranial end of the embryo the child is born without the cerebral cortex (anencephaly)

Question 28

Spina Bifida

Answer 28

Occulta: Mildest type of spina bifida (aka hidden spina bifida)-
With this there’s a small gap in the spine but there’s no opening or sack on the back so the spinal cord and nerves are usally fairly normal

Meningocele: Next more severe spina bifida
Here there’s a visible sack protruding from the spinal column

Myelomeningocele: Most severe and most serious type of spina bifida
Here a sack of fluid comes through an opening in the baby’s back and part of the spinal cords and nerves in the sack are damaged

Question 29

Differenciation of Mesoderm

Answer 29

Intermediate mesoderm- generates the urogenital system
Including; kidneys, the gonads, reproductive tract

Paraxial mesoderm-Differentiates and divides into paired cuboidal bodies known as somites

Lateral mesoderm- Intercellular cavities in lateral plate (mesoderm). This forms progeny cells that constitue the heart, cardiovascular system, blood, kidneys, smooth muscle, lineage and limb skeleton in the developing vertebrate embryo

Question 30

Somites

Answer 30

First appear in future occiptal the smallest of the four lobes in the cerebral hemisphere

Somites are the precursor populations of cells that give rise to important structures associated with vertebrate body, so associated with the body plan
This will eventually differenciate into the dermis, skeletal muscle, cartilage, tendons and vertebrae

The development is craniocaudally here

Question 31

Intermediate mesoderm

Answer 31

Gives rise urogenital system

Kidneys
Gonads
Respective duct system

Question 32

Lateral mesoderm

Answer 32

This splits into two layers:

Somatic (parietal) layer mesoderm – Outer layer
Covers inside of the chest and abdominal walls

Splanchnic (visceral) layer mesoderm – Second layer
Covers organs in the thorax and abdomen

Question 33

Week 4: Embryo folding

Answer 33

Two types: lateral and head to tail caudial folding

Both these types of folding in both planes is actually occuring simultaneously

Question 34

Lateral folding

Answer 34

Starting with the developing trilaminar disc

Axial or transverse slice (across embryo)

1) Flaps of lateral plate mesoderm fold towards ventral midline and merge everywhere except at umbilical cord

2) As the flaps fold, space is maintained which becomes intraembryonic coelom

3) Amnion and ectoderm continue to undergo rapid growth

4) Amnion now surrounds entire embryo

5) Endodermal layer meets at midline and fuses to form gut tube

6) Fusion is complete except at midgut level

7) Yolk sac and midgut have a wide connection until lateral folding is complete

Question 35

Lateral folding summary

Answer 35

A portion of the yolk sac (which is lined with endoderm) is incorporated into the embryo to form the primitive gut

The remaining part of the yolk sac and allantois remain outside the embryo, connected through the umbilicus

Question 36

Formation of Body Cavities

Answer 36

Somatic layer mesoderm- outer layer

Superficial layers of the skin formed from ectoderm

Intra-embryonic coelom forms body cavities - abdomen and chest

Question 37

Cephalo – Caudal Folding

Answer 37

Starting with the bilaminar disc
There’s a Sagittal slice (along length of embryo)- a vertical plane running from front to back, dividing the
body or any of its parts into the right and left sides

Question 38

Day 22- Part 1 of Caudal Folding

Answer 38

1) Cranial area contains oropharyngeal (or bucchopharyngeal) membrane, cardiogenic area (the heart) and the septum transversum

2) Cranial flexion brings these areas ventrally

3) These areas form the surface of the future face, neck and chest

4) The heart is brought to the thoracic position

5) The septum transversum is brought to the diaphragm

6) New position under heart separates coelom into thoracic and abdominal cavities

Question 39

Day 23- Part 1 of Caudal Folding

Answer 39

1) Caudal flexion brings the cloacal membrane to the ventral surface of the embryo

2) Connecting stalk is moved ventrally so it merges with the neck of the yolk sac and attaches to the ventral surface of the embryo

3) Parts of the endodermal germ layer are incorporated into the embryo as the hindgut

4) The primitive streak moves from lying cranially to caudally in relation to the cloacal membrane

5) Allantois is partially incorporated into the embryo

Question 40

Summary

Answer 40

Week 1 —Fertilisation, formation of the morula and blastocyst

Week 2—Implantation of blastocyst and formation of bilaminar embryonic discand early placenta

Week 3—Differentiation of cell layers to form the trilaminar embryonic disc

Week 4—Folding of the embryoand continuing development of the three germ layers (ectoderm, endoderm and mesoderm)

Week 5 – Week 8— Development of all external and internal structures (organogenetic period)

End of Week 8— Embryo looks like an ‘adult’ and is called the fetus. Growth becomes the predominant feature from this point onwards