Embryology Made Ridiculously Simple Presentation


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GUMSA academia revision embryology lecture by Dr Aya Musbahi

Transcript of Embryology Made Ridiculously Simple Presentation

It is not birth, marriage or death but gastrulation that is the most important time in your life Lewis Walpert 1986

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. Stage1- Germinal Period- 1st 2 weeks Stage 2- Embryonic Period- 2nd week to 3rd month Stage 3- Foetal Period-3rd month to 9th month

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.

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.

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.

Describe the Essential Steps in fertilisation. Describe cellular events in fertilisation.

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

What happens next to the embryo, is very interesting and fast paced.


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.

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) On the 6th day

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

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

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.

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 Sad Forgotten Emos Try Partying But Eat Carrots Instead

1. 2.



The following things happen: Implantation: is complete (11th/12th day) Trophoblast differentiates into TWO layers (syncytio-trophoblast and cytotrophoblast) Inner cell mass becomes TWO layers (called bilaminar germ disc and composed of Ectodermal layer and Endodermal layer) 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 blastocyst is still partially embedded in the endometrium Inner cell mass forms 2 layers- an inner ENDODERM (hypoblast) (small polygonal cells) and an outer ECTODERM (epiblast) (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 trophoblastthe 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.

1. 2. 3. 4. 5. 6. 7. 8.

lacunae endometrium syncytiotrophoblast cyto-trophoblast surface epithelium of the endometrium epiblast amniotic cavity hypoblast

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 syncytiotrophoblast. 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.

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.

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

1. 2. 3. 4. 5. 6. 7. 8. 9.

syncytiotrophoblast lacunae surface epithelium of the endometrium fibrin clot epiblast amniotic cavity hypoblast primary chorionic villi/cytotrophoblast primary yolk sac

Cavities are formed inside the extraembryonic 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.

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.

1. secondary yolk sac 2. remnants of the primary yolk 3. amniotic cavity 4. extra-embryonic coelom 5. epiblast 6. connecting stalk 7. hypoblast 8.syncytiotrophoblast 9. lacunae 10. somatopleure 11. splanchnopleure

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.


2 of Pregnancy: 2 cavities 2 germ cell layers 2 trophoblast layers

Summary: 1. 2.

Changes in the embryonic disc Formation of the intra-embryonic mesodermnow a TRILAMINAR germ disc 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.

1. 2. 3. 4. 5. 6. 7.

Primitive groove Primitive pit Primitive node Oropharyngeal membrane/Prochordal plate Cardiogenic plate Sectional edge of amniotic membrane Mesoderm


EndodermFuture cloacal membrane

1. 2. 3. 4. 5.

Primitive Groove Epiblast Extraembryonic mesoderm Hypoblast Invading epiblast cells forming intra-embryonic mesoderm 6. Hypoblast again

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 groov