Embryology

Before the spermatozoa can penetrate the zona pellucida of the egg it has to undergo capacitation (maturation) and acrosome reaction to bind to the zona pellucida.
Prenatal phases=
1. Gametogenesis
2. Fertilization
3. Cleavage
4. Gastrulation
5. Organogenesis
6. Foetal growth and cell differentiation
7. Gametogenesis

In fertilization the male and the female germ cells  ( forming the new indicidual) fuse.  Ovulation of oocytes occur in metaphase II & the sperm loses its tail.

The male and female pronucleus form and fuse and this fertilized oocyte is now called an zygote.
This first cleavage process takes 22-24 hours.
3. Cleavage of the early embryo is when a single fertilized egg (zygote) undergoes mitotic division and forms blastomeres (become multicellular). The cells become smaller and when it reaches 8-16 blastomere stage, it forms the morula, and the blastocyst develops.
The individual cell of the very early embryo is called Blastomeres.  The morula will enter the uterus around the 4th day after fertilization.

The blastocyst develops after the morula stage, when uterine fluid is drawn in. There is now cell differentiation: the trophectoderm cells surrounding the blastocoele, and the inner cell mass cell, which will continue on the develop into the embryo proper.
Maternal recognition of pregnancy is caused by the embryo when the blastocyst hatches from the zona pellucida, which breaks. The further developing embryo, specifically the trophoblast cells when attaching to the uterine lining, then secrete factors which are absorbed by the uterus and prevent the return to cycling= signal pregnancy.
Gestrulation if when the inner cell mass cells rearrange to form the 3 germ layer of the embryo.  First there is a formation of the bilaminar embryo:
– Epiblast
– Hypoblast
Then there is a formation of the trilaminar embryo
– Mesoderm (is the only one added to the two). The epiblast will turn into the ectoderm and the hypoblast will turn into the endoderm cells.

The embryonic disc is formed from the inner cell mass cells, when the embryo forms into 2 layers of cells. The hypoblast forms on the underside of the inner cell mass and then continues beyond like a sheeth lining the whole interior of the blastocyst. The outer trophoblast covering this inner cell mass area degenerates and exposes the surface inner cell mass which flatten out to turn into epiblast cells, the whole flat bilaminar area is now called the embryonic disc (hypoblast and epiblast cell layers).

The extra embryonic structures which form the placenta are the yolk sac, amnion, allantois and chorion.  The yolk sav is the earliest form of the placenta, and thus is extra embryonic. It is the hypoblast, or later called endoderm surrounding the original blastocoele of the blastocyst.
In the gastrulation the rearrangement of the epiblast /hypoblast cells form 3 germ layers:
– Ectoderm (outer): skin – epithelial surface
– Mesoderm (middle) muscle, connective tissue
– Endoderm (inner) gut lining.
The  primitive streak forms the mesoderm and is formed by the epiblast migrating and proliferating along the diameter of the embryonic disc, giving it a longitudinal axis.
The Hensen’s node form the notochord and mesoderm of the head.
There are two areas where the embryo remains bilaminar: the prechordal plate and cloacal membrane. They are the location of the future mouth and anus.
Notochord is an aggregation of mesoderm cells which is rod- shaped and formed in front of the primitive streak (hensen’s node) which marks the cranial caudal axis of the embryo. The notochord then develops /induces the future nervous system.

Monozygotic  twins:
The primitive streaks can sometimes not completely separate, if this happens when Siamese twins develop. Other (more common) ways is that a two cell embryo splits and each blastomere develops into a full embryo each of which has its own placenta. The other way is that a blastocyst has two inner cell masses, from which two embryos continue to develop.

Paraxial mesoderm = somites which lie to the side of the neural tube and notochord. Split into dermatome, sclerotome, myotome giving rise to subcutis, connective tissue and muscle.

Intermediate mesoderm =urogenital system( kidney, bladder & duct system, gonads).
Lateral mesoderm = intra & extra embryonic coelom.
The Body cavity derived from intraembryonic coelum (lateral mesoderm).

One abnormality of the body cavity formation can be Pericardial peritoneal hernia- which is malformation of diaphragm in cattle.
The disc shaped embryo folds into a tube = tubulation of the initially disc shaped embryo. The outer tube = body wall.
Inner tube = digestive tube
inbetween = coelomic cavity.

Neurulation occurs to form the dorsal nerve cord and eventually the central nervous system.

Primary the neural plate creases inwards until the edges come  together and fuse.
Secondly the medullary cord from in the mesoderm consisting of neural ectoderm and some cells of the endoderm. Cells condense, then hollowing out occurs which leads to cavities, which merge to form a tube. It begins by signaling the ectoderm germ layer to form thick and flat neural plates. And finish by differentiating the brain and the spinal cord forming the cns. This process begin at the level of the fourth somite, then continuing cranially and caudally ending with the cranial and caudal neuropore, respectively.
the neural crest cells arises from between the non neural and the neural ectoderms. Some tissues that arise from the neural crest include: ganglia, arachnoid & pia mater, schwan cells, smooth muscle, adrenal medulla, heart connective tissue, cartilage and nerves.

During flexion of the embryo the embryo folds and forms a c shape, brining the heart beneath the head.

Summary:

  • The early embryo undergoes cleavage forming a morula in the uterine tube
  • This morula cavitates to form a blastocyst after it has entered the uterus
  • The blastocyst hatches from the zona and interacts with maternal endometrium to signal ‘pregnancy’ to the dam
  • A second layer of cells line the blastocyst forming:
  • Inner hypoblast à endoderm
  • Outer epiblast à ectoderm
  • The bilaminar embryo proper is called the embryonic disc
  • Extra-embryonic cell layers (trophectoderm and hypoblast) will contribute to placenta
  • Once bilaminar embryonic disc established primitive streak, node and groove are formed
  • This leads to formation of mesoderm
  • This is arranged into adjacent blocks of tissue – paraxial, intermediate and lateral mesoderm
  • Lateral mesoderm slits to form fluid filled spaces – intra and extra embryonic coelom
  • Embryo undergoes tubulation and flexion to form classical recognisable embryo shape
  • Embryo now has primitive nervous system, vascular system, digestive system and segmental body.

Now test yourself:

  1. What processes do spermatozoa have to undergo before they can penetrate the zona pellucida of the egg?
  1. What stage of meiosis are oocytes in at the time of fertilization?
  2. What is the fertilized oocyte called?
  3. How quickly does first cell division occur after fertilization?
  4. Define cleavage of the early embryo and its stages.
  5. What are the individual cells of the very early embryo called?
  6. At what stage does the early embryo enter the uterus in most species?
  7. What features define a blastocyst?
  8. In general, how is maternal recognition of pregnancy caused by the embryo?
  9. Define gastrulation.
  10. What are the germ layers called in the bilaminar embryo? What additional germ layer is added in the trilaminar embryo.
  11. How does the embryonic disc form and what defines it?
  12. What are the extra-embryonic structures which will form the placenta?
  13. What defines the yolk sac in embryonic development of our domestic mammal animal species?
  14. Name the three germ layers of the trilaminar embryo and one type of tissue derived from each.
  15. How does the primitive streak form and what is its role?
  16. What is the function of the Hensen’s node?
  17. There are only two areas where the embryo remains bilaminar, what are these called?
  18. Define the notochord and explain its function.
  19. What are the different ways by which monozygotic twins can arise?
  20. What are the mesoderm derivatives and what tissues/organs derive from them?
  21. What germ layer do the body cavities derive from?
  22. Name one abnormality of body cavity formation.
  23. Describe briefly tubulation of the initially disc-shaped embryo.
  24. What happens during primary and secondary neurulation, and where in the embryo does it start and finish?
  25. What is the origin of neural crest cells? Give some examples of tissues which derive from neural crest cells.
  26. Describe what happens during flexion of the embryo.

What stage of early embryonic development is shown in the image below? Label the arrows

What stage of embryo development does the schematic below show? Label the arrows, lines and bracket.

Below is the schematic of a human embryonic disc. Label the lines. What process do the arrows relate to?

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About Annettevet

I'm a Norwegian practicing as a veterinarian in Durham, England. I`ve recently qualified as a veterinarian at Glasgow University. This is a blog about my experiences as a vet student and now as a veterinarian. “The greatness of a nation and its moral progress can be judged by the way its animals are treated”
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