Embryology Made Ridiculously Simple:

The Intra-uterine Life:

It is the time that elapses between fertilisation and the birth of a new individual. It is

about 9 calendar months.

The intra-uterine life of the individual is divided into 3 stages

Period Germinal Period Embryonic Period Foetal Period

Duration The 1st 2 weeks From 3rd week to 3rd month

3rd month to end of pregnancy

Characteristics Formation of 3 germ layers: ectoderm, endoderm and mesoderm

Differentiation of the 3 germ layers into organs and systems

Growth of the various organs and systems

Congenital anomalies

Congenital anomalies are liable to occur

Congenital anomalies are liable to occur

Congenital anomalies are less liable to occur

The 1st week of Pregnancy:

FERTILIZATION:

Fusion of the male and female gametes happens in the Fallopian tube

Egg complex is surrounded by cells called the Corona Radiata and a Zona Pellucida

Radiata is penetrated by sperm and reaches the ZP

Three glycoproteins make up the ZP, 1, 2 and 3

Initially, the sperm binds to ZP3

Liberation of enzymes found in sperm sac called acrosome

In the acrosome are acid proteinase, collagenase, acrosin amongst others

Zona Pellucida binding stimulates this release of enzymes from the sac

Acrosomal reation is the name given to this process and it helps the sperm to get through

the ZP.

Tail of the sperm also propels it forward

Influx of Calcium into the sperm head assist this process

One plasma membrane is formed by fusing the sperm and egg membranes

Nuclei (haploid/pronuceli) of both join up to form diploid egg.

So in summary Fertilisation:

Definition: The fusion of male and female gametes

Site: Lateral 1/3 of the Fallopian tube

Mechanism: The fertilising sperm pierces the corona radiata and Zona Pellucida. The head

detaches from the rest of the sperm and forms the male pronucleus which contains half the

number of chromosomes. The nucleus of the ovum is also haploid and both fuse to form a

dilpoid zygote.

Analogy:

There is a ball covered in velcro and a needle and syringe which is to be inserted into the

balloon. The ball is like the ovum and the needle is the sperm and the velcro is the Zona

Pellucida.

The ball has been on the floor for a long time and stuck to the velcro is a lot of fluff from the

rug (fluff is the corona radiata). We need to insert the needle through the velcro and into the

balloon. So we need to remove the fluff first (corona radiata). The needle needs to then get

through the velcro (the ZP) before it gets to the balloon. It attaches to the velcro first (to ZP3)

and then penetrates it. In the syringe attached to the needle is a potent acid. We're going to

use the syringe and needle to release some acid to dissolve the velcro (the acid in the

syringe is like the acrosomal sac contents), so we can penetrate the ball.

How is Polyspermy prevented?

1. The fusion of the sperm and ovum causes a change in the action potential of the plasma

membrane. This prevents other sperm attaching.

2. The influx of calcium causes the ZP to separate from the egg, thus preventing any sperm

attaching.

Past Paper Question (Medicine):

Describe the essential steps in fertilisation.

Describe cellular events in fertilisation.

Buzz Words:

Zona Pellucida, Corona Radiata, ZP3, Acrosomal Reaction, Fusion, Pro-nuceli

Cleavage:

Definition: a series of mitotic divisions occurring in the zygote. Each cell that results is

called a blastomere.

Site: these divisions occur in the zygote as it passes in the fallopian tube to reach the

uterine

cavity.

Steps:

The 2 cell stage appears at 30 hours after fertilisation

The 4 cell stage appears at around 45 hours after fertilisation

The Morula is the 12-16 cell stage and appears about 3rd day

The cells become arranged into an inner cell mass in the centre and an outer cell

mass in the periphery

The Blastocyst develops on the 4th day

As the cells of the morula continue to divide, fluid from the uterine cavity enters the

spaces between the cells.

These fluid filled spaces join together to form one large cavity called a blastocele

and the morula is now called a blastocyst.(the Zona Pellucida disappears completely

at day 4)

Blastocyst

Differentiation of the Blastocyst:

The cells of the outer cell mass form the trophoblast- will form foetal membranes later on

The cells of the inner cell mass become located at one pole- called the embryonic pole (the

opposite pole is called the abembryonic pole)

The blast attaches to the endometrium at 5-6 days after fertilisation.

Implantation: Embedding:

Definition: It is the penetration of the blastocyst into the superficial compact layer of the

endometrium.

Time: Begins day6 or 7 and ends by day 11 or 12.

Site: endometrium of the posterior wall of the fundus of the uterus

Steps:

The blastocyst becomes attached to the endometrium by its embryonic pole

The trophoblast cells covering the embryonic pole erode the epithelium of the

endometrium (possibly by enzymatic action)to allow the blastocyst to penetrate

through the defect.

After complete embedding of the blastocyst, the penetration defect is closed by a

fibrin clot.

Implantation is completed by growth of the epithelium to cover the defect.

So in Summary Implantation:

Sixth/Seventh day

Fundus of Uterus

Embryonic pole attaches

Trophoblasts erode endometrium

Penetration defect formed

Blastocyst enters

Embeds completely

Closed by a clot

Intact epithelium again

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THE SECOND WEEK OF PREGNANCY:

• The following things happen:

1. Implantation: is complete (11th/12th day)

2. Trophoblast differentiates into TWO layers (syncytio-trophoblast and cytotrophoblast)

3. Inner cell mass becomes TWO layers (called bilaminar germ disc and composed of

Ectodermal layer and Endodermal layer)

4. TWO cavities are formed (amniotic cavity and the yolk sac)

NB) there is a rapid rate of growth in the second week compared to the first week

The Eighth Day:

The blastocyst is still partially embedded in the endometrium

Inner cell mass forms 2 layers- an inner ENDODERM (small polygonal cells) and an

outer ECTODERM (tall columnar cells).

The trophoblast starts to differentiate into 2 layers:

Outer dark zone without cell boundaries called the SYNCYTIOTROPHOBLAST and

an inner pale zone with clear boundaries called CYTOTROPHOBLAST.

An Amniotic Cavity starts to be formed : small clefts appear between the ectodermal

cells and the trophoblast, the clefts join each other to form the amniotic cavity. The

cytotrophoblast develops a layer of cells called amnioblasts which form the roof of

the amniotic cavity, while the floor is formed by the epiblast or ectoderm.

9th and 10th days:

The blastocyst becomes more deeply implanted and the fibrin clot covers the

penetration defect.

The trophoblast becomes fully differentiated into cyto and syncytio-trophoblast.

Spaces called lacunae appear in the syncytio-trophoblast.

The amniotic cavity becomes larger

A second cavity forms called the Primary Yolk Sac at the ventral aspect of the

embryonic disc. The Primary Yolk sac is the new name for the old blastocyst cavity.

Its roof is formed by the endodermal layer of the hypoblast and the rest of its lining is

called Heuser's membrane.

11th and 12th days:

The blastocyst is completely embedded by now

Lacunae in the syncytio-trophoblast formed earlier start communicating with each

other to form larger spaces. This is a primitive maternal foetal circulation.

Lots of interesting things happen to the cyto-trophoblast:

Formation of the extra-embryonic mesoderm:

cells of inner surface of the cyto-trophoblast form a loose tissue called extra-embryonic

mesoderm

Formation of extra-embryonic coelom:

Cavities are formed inside the extra-embryonic mesoderm. These cavities fuse together to

form the extra-embryonic coelom. However, the coelom doesn't replace the mesoderm

completely, rather it divides it into two, SOMATOPLEURE which lines the cyto-trophoblast

and the splanchnopleure which covers the yolk sac.

NB) The connecting stalk is a mass of mesoderm connecting the roof of the amniotic cavity

with the trophoblast.

ANALOGY:

imagine the extra-embryonic mesoderm is like the a dried up oxbow lake and the yolk sac

and amniotic cavity and bilaminar germ disc is an island in the middle. When the water flows

in the rainy season. The lake has two river banks or two lake edges on either side. The

water is like the extra-embryonic coelom and the banks on either side are the somatopleure

(on the outside- has an O) and splanchnopleure (on the inside, has an N). There is a bridge

connecting the island to the outer edge/bank. This is the connecting stalk

13th Day:

The most prominent changes are the appearance of primary chorionic villi as follows:

Parts of the cytotrophoblast at the embryonic pole project into the syncytiotrophoblast

forming primary chorionic villi, surrounded by lacunae.

The primary yolk sac gets smaller and gets pinched off and is now called the secondary yolk

sac.

video: http://www.youtube.com/watch?v=f35JpW2DqDU&feature=related

So in Summary:

Week 2 of Pregnancy:

2 cavities- yolk sac and amniotic cavity

2 germ cell layers-hypoblast and epiblast

2 trophoblast layers-cytotrophoblast and syncytiotrophoblast

The third week:

Summary:

Changes in the embryonic disc

1. Formation of the intra-embryonic mesoderm- now a TRILAMINAR germ disc

2. Formation of the notochord- which is a temporary supporting structure to the

embryonic disc

Changes to the trophoblast (chorion): 3 types of chorionic villi form and cover the

whole surface of the chorionic vesicle.

Formation of the intra-embryonic mesoderm:

Formation of the primitive streak:

At the beginning of the third week, ectodermal cells in the caudal part of the bilaminar

germ disc migrate to the midline forming a primitive streak (a narrow midline groove)-

basically the cells of the epiblast migrate downwards forming a groove.

These cells separate from the epiblast (now called ectoderm) and migrate in all

directions. This new layer of cells is called the intra-embryonic mesoderm. There is

an area in the cranial end that it doesn’t migrate to, that is the propchordal plate and

an area called the cloacal membrane, behind the primitive streak. These areas

remain BILAMINAR.

Because mesoderm forms vascular tissue. These bilaminar areas do not have

mesoderm so without a blood supply they will break down. This is important as these

areas need to break down to form contact with the external environment:

Prochordal plate buccopharyngeal opening

Cloacal membrane anus

Pillow and Pillow Cover.

The two pieces of material sewn together to form the pillow cover are the epiblast (ectoderm)

and hypoblast (endoderm). When I want to stuff the pillow into the pillow cover, there needs

to be an opening, yeh? That opening is like the primitive streak. When I stuff the pillow I

make sure the whole of the pillow case is encased with pillow. The stuffing is the mesoderm,

the pillow cover represents the ectoderm and mesoderm.

Mnemonic:

Primitive streak, Midline groove of Epiblast cells, forms Mesoderm, Migrates all

directions, Trilaminar disc forms

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This whole process of mesoderm formation is called GASTRULATION

Mesoderm components MESODERM:

Mesothelium (peritoneal, pleural, pericardial)/ Muscle (striated, smooth, cardiac)

Embryologic Spleen/ Soft tissue/ Serous linings/ Sarcoma/ Somite

Osseous tissue/ Outer layer of suprarenal gland (cortex)/ Ovaries

Dura/ Ducts of genitalia

Endothelium

Renal Microglia

Mesenchyme/ Male gonad

Formation of the Notochord:

There is a thickening of the ectoderm at the cephalic end of the primitive streak and

the primitive node, remember, is where there is the central depression with the

slightly elevated area.

The cells of this primitive node proliferate and form a solid rod of cells called

notochordal process which grows in a cephalic direction between the endoderm and

ectoderm.

Inside the notochordal process Is a small central notochordal canal which passes

from the primitive pit anteriorly.

The notochord is a bit like a hollow metal pipe that passes through the pillow we

talked about earlier.

The notochord gives the embryo structure and helps it to define its axes.

Changes to the trophoblast in the 3rd week

Formation of the chorionic villi

Primary chorionic villi: begin to appear by the end of 2nd week at the embryonic

pole of the chorionic vesicle and increase in number by the beginning of the 3rd

week. Each primary villus is made up of a central core of cytotrophoblast and a

covering layer of syncytiotrophoblast.

Secondary chorionic villi: by the beginning of the 3rd week, cells from the extra-

embryonic mesoderm start to penetrate the primary chorionic villi and forms

secondary chorionic villi. Each secondary chorionic villus is made up of a central core

of extra-embryonic mesoderm, a middle zone of cytotrophoblast and an outer zone of

syncytiotrophoblast.

Tertiary chorionic villi: by the end of the 3rd week, a loop of afferent and efferent

capillaries appears in the mesodermal core of the secondary chorionic villi. The

afferent one is connected to the umbilical artery and the efferent one is connected to

the umbilical vein.

After the third week

Differentiation of the Intra-embryonic mesoderm:

The mesoderm is formed as a loose tissue between the ectoderm and endoderm by

day 17 on either end of the notochord.

The three parts:

As development proceeds, 2 longitudinal grooves appear in the mesoderm on either

side of the notochord, dividing it into 3 parts.

Paraxial mesoderm, on either side of the notochord

About 20th day, transverse grooves appear dividing the paraxial mesoderm into

somites (blocks). The first pair form day 20. 3 additional ones form each day from

21st to 30th day i.e. by the end of 1st month, 30 or 31 pairs of somites are formed. B

Day 30-40, rate has slowed and finally 42-44 pairs are formed.

S- structures of the paraxial mesoderm

O- occipital to coccygeal

M- mesodermal tissue

T-TWO parts- ventromedial and dorsolateral

E- embryo age can be estimated from number of somites

Intermediate Mesoderm:

This tissue, which temporarily connects the paraxial mesoderm with the lateral plate

In the cervical and upper thoracicregions it forms segmentally arranged cell clusters

(the future nephrotomes), whereas more caudally it forms an unsegmented dmass of

tissue known as the nephrogenic cord. From this partly segmented, partly

unsegmented intermediate mesoderm developesthe excretory units of the

urinary system.

The lateral plate mesoderm:

Forms the pleura and peritoneum.

THE PLACENTA:

Chorion frondosum (bushy or villous chorion)

Chorion laeve (smooth chorion)

Decidua basalis

Decidua capsularis

Decidua parietalis (decidua vera)

By the beginning of the 4th month, placenta has 2 components:

o Fetal portion –

o Maternal portion –

1. Chorionic & decidual plates

2. Junctional zone

3. Decidual septa & cotyledons

At full term, placenta is discoid with a diameter of 15~25 cm, is approximately 3 cm

thick, & weighs about 500~600 g. It has a:

Fetal surface

Maternal side

Function of the Placenta:

Exchange of metabolic & gaseous products between maternal & fetal

bloodstreams

Exchange of gases –

Exchange of nutrients & electrolytes –

Transmission of maternal antibodies –

Production of hormones