week 2-week 3 Flashcards by Rabia Dosani

Around day 9 how do we describe the embryo

As a bilaminar disk

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Why do we describe the embryo as a bilaminar disk

As it is made up of 2 laminar layer:

A layer of epiblast cells
A layer of hypoblast cells

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What cavities can be seen within the blaminar disk

The amniotic cavity

Yolk sac

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What forms the yolk sac

Migration of hypoblast around the blastocyst cavity

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What sepcialed cells are present in the bilaminar disk

Cytotrophoblasts

Syncytiotrophoblast

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Describe Syncytiotrophoblast cells

They are multinucleate cells

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What do the Syncytiotrophoblast cells do

They anchored the developing embryo onto the uterine lining

They also secrete factors to modify the mothers white blood cells so that her body doesn’t attack the embryo

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How does an early embryo (around 9 days) receive its nutrients

Diffusion

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What fo the Syncytiotrophoblast cover slightly

The material capillary

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Which hormone is detected on a pregnancy test

Human chorionic gonadotrophin (hCG)

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Which cells secrete Human chorionic gonadotrophin (hCG)

Syncytiotrophoblast cells

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What does Human chorionic gonadotrophin (hCG) do

Helps maintain endometrium and may play a role in material immunotolerence

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In how many pregnancies does the embryo implant at an abnormal site?

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Give examples of where the embryo could abnormally implant

Implanting at the internal Os of the uterus

2. Implanting just at the start of the uterine tube

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Why can it be dangerous to have an embryo implanting at the internal os of the uterus

As it is essentially covering the birthing canal so the baby is growing on top of the placenta. if the placenta was to rupture prematurely it could lead to premature abortion of the pregnancy and haemorrhaging of the mother

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What can happen if the embryo implants at the start of the uterine tube

The uterine tube isn’t specialised to deal with a developing and expanding embryo
So the embryo will get to a certain point in development and become inviable and spontaneously abort

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What can happen if the embryo doesn’t spontaneously abort as it is growing in the uterine tube

It can lead to tubule rupture

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What are ectopic pregnancies

Pregnancies that are outside of the uterine cavity

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List some ectopic implantation sites

Implantation in the uterine tube (not just the start)
Implanting onto the ovary
Implanting near the mesentery of the small intestines

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What do we mean by spontaneous abortion

When the mother breaks down the developing tissues through necrosis usually when the embryo has implanted in the incorrect place

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What can happen if the ectopic embryo is too big for the mother to break down

The mother will calcify the embryo leading to a condone called Lithopedion stone baby

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What is Lithopedion

Essentially a stone baby where an embryo has been calcified

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Talk through the stages from fertilisation to an embryo becoming bilaminar

Sperm and egg fertilise forming a zygote
Through rapid cell devision we form a morula
Fluid filled cavity called Blastocyst forms
Blastocyst forms two layers: inner and outer cell membrane
Inner cell membrane made up of trophoblasts and outer cell membrane made up of embryo blasts
These cells further differentiate to give supporting structures and the bilaminar disk

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What does the outer cells layer of trophoblasts differentiate into

Cytotrophoblasts and Syncytiotrophoblast

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What does the inner cells layer of embryo blasts differentiate into

Hypoblasts and epiblasts this forms the bilaminar disk

After the bilaminar embryonic disk has formed what is the next stage

Transformation into a trilaminar disc (gastrulation)

When does gastrulation occur

In week 3

How does gastrulation start

With the formation of the primitive streak

What is significant about the formation the primitive streak

Establishes the body axes and aids in the formation of the trilaminar disk

Which body axes are established when the primitive streak forms

Cranial and Claudial | Right and left

What abnormalities can occur in the formation of the left/right axis

In approx 1 in 10,000 people the right and lest asymmetry of the body is reversed This is called situs inversus

What is situs inversus

When the right to left asymmetry of the body is reversed

What can complete situs inversus cause

It is often harmless but in 20% of patients they can suffer from kartagener syndrome

Name a type of partial situs inversus

Dextrocardia (isolated right sided heart)

What is kartagener syndrome

A ciliary disorder of the ciliated cells within the body

Talk through the formation of the definitive endoderm

1. Cells of epiblast move towards and through the primitive streak 2. They then replace the hypoblast cells 3. The hypoblast is completely replaced by the epiblast cells This is called the definitive endoderm

Following the formation of the definitive endoderm what do the epiblast cells do

They continue to move down through the primitive streak to form a middle layer in between the epiblast and definitive endoderm This is the mesoderm

Name the 3 germ layers

1. Endoderm 2. Mesodern 3. Ectoderm

What do the 3 germ layers form

The trilaminar embryonic disk

What are the 3 germ layers derived from

The epiblast

What does the endoderm go onto to form

Comprises the inner epithelium lining of out respiratory system, our gastrointestinal tract

What does the mesoderm go onto to form

Forms the musculoskeletal system, some of our cardiovascular system and the genital urinary system Also forms the dermis of the skin

What does the ectoderm go onto to form

Forms some of our central nervous system and some of the peripheral nervous system Also forms the epidermis of the skin

The mesoderm is highly what?

Organised

What is the mesoderm organised into

1. Paraxial mesoderm 2. Intermediate mesoderm 3. Lateral plate mesoderm

Where is the paraxial mesoderm

Next to the midline

What does the paraxial mesoderm go onto form

The majority of skeleton, skeletal muscles ad dermis of the skin

What does the intermediate mesoderm go onto form

Gonads, interval reproductive tracts and kidneys

What does the lateral plate mesoderm go onto form

Lining of body cavities

What can happen if gastrulation goes wrong

1. Can lead to sirenomelia (mermaid syndrome) | 2. Sacrococcygeal teratoma

What is sirenomelia caused by

Insufficient mesoderm being formed in the caudal region of the embryo

What can sirenomelia lead to

Abnormalities of urogenital system and lower limbs

What is Sacrococcygeal teratoma

A tumour

What is Sacrococcygeal teratoma caused by

Remnants of the primitive streak persisting

How common is Sacrococcygeal teratoma

1 in 35,000-40,000 (80% In females)

What is neurulation

It is the first event in the development of the central nervous system

What is neurulation initiated by

Initiated by signals from the notochord

What does the notochord do

They cause the overlying ectoderm to thicken and from the neural plate

What processes does the notochord initiate

1. neurulation | 2. initiates and organises the formation of the vertebrae from somites

What does the paraxial mesoderm do once it has formed

It starts dividing into blocks called somites

How does the notochord affect the somites

It induces differentiation of the somites into three parts: 1. Sclerotome 2. Myotome 3. Dermatome

What does the Sclerotome go onto form

Bone and cartilage

What does the Myotome go onto form

Skeletal muscle

What does the Dermatome go onto form

Dermis

After inducing differentiation in the Sclerotome what do the notochord do

They organise the Sclerotome

What happens to the Sclerotome and notochord

The Sclerotome cells surround the notochord to form the vertebral body and surround the neural tube to form the vertebral arch

What can happen if abnormal induction of the Sclerotome occurs

1. Spina bifida 2. Meningocele 3. Myelomeningocele