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Flashcards in Embryology Deck (33):

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

- 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


LO 1.3 Define a zygote

Zygote - A fertilised ovum/oocyte


LO 1.3 Define Cleavage

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


LO 1.3 Define Zona Pellucida

The glycoprotein ‘shell’ to prevent polyspermy


LO 1.3 Define Morula

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


LO 1.3 Define Ovary

The female reproductive organ producing oocytes/eggs for fertilisation


LO 1.3 Define Fallopian tube

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


LO 1.3 Define Uterus

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


LO 1.3 Define Blastocyst

Formed from the compaction of cells that make up the morula


LO 1.3 Define Trophoblast

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


LO 1.3 Define Embryoblast

(Inner cell mass) will later become the bilaminar disk


LO 1.3 Define Implantation

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


LO 1.3 Define Cytotrophoblast

(Derivative of Trophoblast) Placental membrane around the yolk sac


LO 1.3 Define Syncytiotrophoblast

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


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

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


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

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.

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

The bilaminar disk and amniotic cavity are formed.

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)


Describe possible implantation defects

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


Bilaminar Disk

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


Germ Layer

A layer from which other developed tissues originated from



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



The innermost layer of cells giving rise to the gut lining


Doral Surface

The outermost surface during early development


Ventral Surface

The innermost surface during early development



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


Primitive Streak

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.



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


Trilaminar disc

The three germ layers. Ectoderm, mesoderm and endoderm.


What are the Ectodermal Derivatives

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


What are the Mesodermal Derivatives

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


What are the Endodermal Derivatives

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


When is the Embryonic Period

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


Describe week 3 of develoment

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

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.


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

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