Embryology

Question 1

LO E1.2 Discuss the changes taking place as the fertilised human ovum (zygote) travels down the Fallopian (uterine) tube to the uterus

Answer 1

Fertilisation

• Oocyte is released from the ovary
• Travels along the Fallopian (uterine) tube
• Is fertilized by sperm in the ampulla
• Fertilised oocyte is called the zygote
• Ideal site for implantation is the posterior uterine wall

Question 2

LO 1.3 Define a zygote

Answer 2

Zygote - A fertilised ovum/oocyte

Question 3

LO 1.3 Define Cleavage

Answer 3

Cleavage - The dividing of cells into two masses known as blastomeres. First cleavage happens ~30 hours after fertilisation (resting period)

Question 4

LO 1.3 Define Zona Pellucida

Answer 4

The glycoprotein ‘shell’ to prevent polyspermy

Question 5

LO 1.3 Define Morula

Answer 5

The result of cleavage of the fertilised oocyte, each cell is Totipotent

Question 6

LO 1.3 Define Ovary

Answer 6

The female reproductive organ producing oocytes/eggs for fertilisation

Question 7

LO 1.3 Define Fallopian tube

Answer 7

A tube the oocyte travels along to reach the uterus. The egg is fertilised at the ampulla of the fallopian tube.

Question 8

LO 1.3 Define Uterus

Answer 8

Female reproductive organ where implantation of the zygote and growth of the embryo occurs

Question 9

LO 1.3 Define Blastocyst

Answer 9

Formed from the compaction of cells that make up the morula

Question 10

LO 1.3 Define Trophoblast

Answer 10

(Outer cell mass) will later form support structures for the embryo (placenta)

Question 11

LO 1.3 Define Embryoblast

Answer 11

(Inner cell mass) will later become the bilaminar disk

Question 12

LO 1.3 Define Implantation

Answer 12

The attachment of the Blastocyst to the wall of the uterus at day 6 – 7

Question 13

LO 1.3 Define Cytotrophoblast

Answer 13

(Derivative of Trophoblast) Placental membrane around the yolk sac

Question 14

LO 1.3 Define Syncytiotrophoblast

Answer 14

(Derivative of Trophoblast) Cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation.

Question 15

LO 1.4 Discuss the formation of the Blastocyst and the initial stages of implantation (week 1)

Answer 15

Week 1 – Compaction (Day 4)
Blastomeres making up morula compact to form the Blastocyst

Embryoblast (Inner cell mass)
Trophoblast (Outer cell mass)
Blastocyst Cavity

Week 1 – Hatching (Day 5)

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

Week 1 – Implantation Begins (Day 6–7)

• The conceptus now has 107 cells.
• 8 will make the embryo
• 99 will begin the development of the foetal membrane

Question 16

LO 1.6 Describe the formation of the embryonic disc in the inner cell mass and initial cell differentiation within it

Answer 16

Week 2 – “The week of two’s”
Both an embryo and a placenta start to be formed. At the earliest stages the placenta has priority.

Differentiation
Embryoblast -> Epiblast, Hypoblast
Trophoblast -> Syncytiotrophoblast, Cytotrophoblast

The bilaminar disk and amniotic cavity are formed.

Implantation
Implantation is interstitial (invasive, surrounded and embedded). The uterine epithelium is breached and the conceptus implants within the uterine stroma.

Week 2, Day 9
Embryonic pole
Rapid development of Syncytiotrophoblast
Abembryonic pole
Primitive yolk sac formed by Heuser’s membrane spreading across the Blastocyst cavity. Yolk sac membrane is in contact with the cytotrophoblast layer.

Week 2, Day 11
Primitive yolk sac membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum.
Reticulum is later converted to extraembryonic mesoderm by cell migration

Week 2, Day 12
Maternal sinusoids invaded by syncytiotrophoblast.
Lacunae become continuous with sinusoids.
Uteroplacental circulation begins.
Uterine stroma prepares for support of the embryo.

Week 2, Day 13
Formation of secondary yolk sac
Pinches off from the primary yolk sac
A.k.a. Definitive yolk sac

Week 2, Day 14
Spaces within the extraembryonic mesoderm merge to form the chorionic cavity
The Connecting Stalk suspends the embryo and its cavities in the chorionic cavity.
The connecting stalk is a column of mesoderm, and is the future umbilical cord.
Bleeding around now can be confused with menstrual bleeding.

By the end of the second week

• The conceptus has implanted
• The embryo and its two cavities (Amniotic cavity and Yolk sac) are
• Suspended by a connecting stalk in a
• Supporting Sac (Chorionic Cavity)

Question 17

Describe possible implantation defects

Answer 17

Implantation Defects - Implantation at inappropriate sites can cause problems.

Ectopic Pregnancy – Implantation at site other than uterine body (most commonly fallopian tube). Can quickly become a life-threatening emergency.
Placenta Praevia – Implantation in lower uterine segment. Placenta grows across cervical opening. Requires C-section delivery

Question 18

Bilaminar Disk

Answer 18

The Epiblast and hypoblast. Derived from the embryoblast. This will later become the trilaminar disk with the formation of the primitive streak.

Question 19

Germ Layer

Answer 19

A layer from which other developed tissues originated from

Question 20

Ectoderm

Answer 20

The outermost layer of cells giving rise to the epidermis and nerve tissue

Question 21

Endoderm

Answer 21

The innermost layer of cells giving rise to the gut lining

Question 22

Doral Surface

Answer 22

The outermost surface during early development

Question 23

Ventral Surface

Answer 23

The innermost surface during early development

Question 24

Mesoderm

Answer 24

The middle germ layer giving rise to muscle, bone and more

Question 25

Primitive Streak

Answer 25

A narrow groove that develops on the dorsal surface of the Epiblast. It has an important role in the orientation of the embryo, determining the front and the back.

Question 26

Gastrulation

Answer 26

The reorganisation of the germ layers into the ectoderm, mesoderm and endoderm and hence establishing the origin of all tissues.

Question 27

Trilaminar disc

Answer 27

The three germ layers. Ectoderm, mesoderm and endoderm.

Question 28

What are the Ectodermal Derivatives

Answer 28

``` Skin Skin derivates (Hair, nails, lens and cornea of eye, mouth and anus lining) Brain and spinal cord Peripheral nerves Retina and iris of eye ```

Question 29

What are the Mesodermal Derivatives

Answer 29

``` Smooth, skeletal and cardiac muscle Connective tissue Bone Cartilage Blood and blood vessels Urinary system ```

Question 30

What are the Endodermal Derivatives

Answer 30

``` Lining of the digestive tract Glands associates with digestion (E.g. liver, pancreas) Other gut derivates (E.g. lungs) ```

Question 31

When is the Embryonic Period

Answer 31

Weeks 3 to 8 - Period of greatest change - All major structures and systems are formed - The most perilous for the developing child

Question 32

Describe week 3 of develoment

Answer 32

Week 3 – “Week of threes” Three cavities (with the establishment of the 1. Amniotic cavity chorionic cavity by end of week 2) 2. Yolk sac 3. Chorionic cavity Three “germ” layers – rudimentary lineages from which all others will arise Gastrulation This is the process establishing the three “germ” layers and hence the origin of all tissues. Trilaminar disc is formed and the axes observed in an adult are set. - Anterior/posterior - Dorsal/ ventral - Right/left This starts with the appearance of the primitive streak (primitive node with primitive pit) Cellular rearrangement occurs - Migration - Invagination Primitive Streak, node and pit - At the end of the second week the epiblast is a uniform disk. - In the third week the primitive streak appears on the dorsal surface of the epiblast. - The streak is a narrow groove with bulging edges - Primitive node is located at the cranial end of the streak. - Primitive pit is located at the centre of the node - As gastrulation proceed and the three germ layers are established, the primitive streak regresses - Development proceeds in a cranial/rostral to caudal direction.

Question 33

LO E2.4 Describe the migration of mesoderm through the primitive streak

Answer 33

Migration & Invagination o Cells on the epiblast migrate towards the primitive streak. o They then invaginate into the epiblast and displace the hypoblast creating a 3rd layer – the mesoderm. o Now a trilaminar disc the layers are referred to as ectoderm, mesoderm and endoderm o As more cells migrate through the streak they spread laterally and cephalad (towards the head) o Mesoderm spreads out between the ectoderm and endoderm like a sandwich filling. Leaving TWO GAPS/HOLES The Notochord The fate of invaginating epiblast depends on where in the streak or node they invaginate. - The Notochord define the phylum Chordata - It’s the basis for the axial skeleton - It drives the formation of the nervous system (neurulation) - Prenotochordal cells of epiblast migrate through the cranial part of the primitive pit - Forms a solid rod of cells running in the midline with an important signalling role. - Defines the midline - Axial skeleton forms around it - It regresses. The vestigial remnant in the adult is the nucleus pulposus of intervertebral discs. Axes (The Primitive Strek and Node) We are not amorphous – we have clearly delineated regions. Medico-anatomical terminology relies on the universal use of the anatomical position. Axes are set in the early embryo. The primitive streak appears at one end of the bilaminar disk. It defines the “front” and “back” i.e. anterior/rostral and posterior/caudal Molecular signals emanating from the primitive node ensure correct dorsal-ventral and left-right development. ``` Embryonic Disk at the end of Week 3 Three germ layers in place anteriorly Because development proceeds cephalocaudally (head to tail) gastrulation is not complete in the caudal region until week 4. Axes are established Primitive node has major role in this ```