Study the Stages of the Human Embryonic Development

2. What is the function of the vitellus in vertebrate eggs? How are these eggs classified according to the amount of vitellus within them?

Vitellus (yolk) is the nutritive material that accumulates in the cytoplasm of the egg (zygote), and has the function of nourishing the embryo. Depending on the amount of vitellus in them, vertebrate eggs are classified as oligolecithal (little yolk), centrolecithal, or heterolecithal (more yolk diffusely distributed) and telolecithal (more yolk concentrated at one end of the egg).

3. What are the animal pole and the vegetal pole of vertebrate eggs?

The animal pole of a telolecithal egg is the portion of the egg with little vitellus. It is opposite to the vegetal pole, which is the region where the yolk is concentrated.

• Embryonic Development Review - Image Diversity: animal and vegetal poles

4. What are the four initial stages of embryonic development?

The four initial stages of embryonic development are the morula stage, the blastula stage, the gastrula stage and the neurula stage.

5. What is cell division during the first stage of embryonic development called? How can this stage be described?

Cell division during the first stage of embryonic developments is called cleavage, or segmentation. During this stage, several mitoses occur within the zygote to form the new embryo.

6. What are the cells produced during the first stage of embryonic development called?

The cells that are produced during cleavage (the first stage of embryonic development) are called blastomeres. In this stage the embryo is called the morula (similar to a “morus”, or mulberry).

• Embryonic Development Review - Image Diversity: morula

7. After the morula stage, what is the next stage? What is the morphological feature that defines this stage?

After passing the morula stage in which the embryo is a compact mass of cells, the next stage is the blastula stage. In the blastula stage, the compactness is lost and an internal cavity filled with fluid appears inside it, called the blastocele.

• Embryonic Development Review - Image Diversity: blastula

8. After the blastula stage, what is the following stage of embryonic development? What is the passage from the blastula to the next stage called?

The blastula turns into the gastrula through a process known as gastrulation.

• Embryonic Development Review - Image Diversity: gastrula

9. What is gastrulation? How are the first two germ layers formed during gastrulation? What are these germ layers?

Gastrulation is the process through which a portion of the blastula wall invaginates inside the blastocele, forming a tube called the archenteron (a primitive intestine). The cells of the inner side of the tube form the endoderm (a germ layer) and the cells of the outer side form the ectoderm (another germ layer). This is the beginning of tissue differentiation in embryonic development.

10. What are the archenteron and the blastopore? During what stage of embryonic development are these structures formed? What happens to the archenteron and the blastopore?

The archenteron is the tube formed during gastrulation by means of the invagination of the blastula wall inside the blastocoel. It turns into the gastrointestinal tract. The blastopore is archenteron's exterior opening . The blastopore produces one of the extremities of the digestive tract: the mouth in protostome organisms, or the anus in deuterostome organisms.

11. How is the mesoderm (third germ layer) of triploblastic animals formed?

The mesoderm is formed through the differentiation of the endodermal cells that cover the dorsal region of the archenteron.

12. What are the three types of germ layers that form tissues and organs in animals?

The three germ layers are the ectoderm, the mesoderm and the endoderm.

• Embryonic Development Review - Image Diversity: germ layers

13. How are animals classified according to the germ layers present during their embryonic development?

Cnidarians are diploblastic, meaning that they only have an endoderm and ectoderm. With the exception of poriferans, all remaining animals are triploblastic. Poriferans do not have differentiated tissue organization and, as a result, have no classification regarding germ layers (although sometimes they are considered diploblastic).

14. How does the embryo turn into the neurula from gastrula? How is the neural tube formed? What is the embryonic origin of the nervous system in vertebrates?

The neurula stage is characterized by the appearance of the neural tube along the dorsal region of the embryo. The growth of the mesoderm in that region causes the differentiation of the ectodermal cells just above it. These cells then differentiate to form the neural tube. Therefore, the origin of the nervous system is the ectoderm (the same germ layer that produces the skin).

• Embryonic Development Review - Image Diversity: neurula

15. What is the notochord? How is this structure formed?

The notochord is a rodlike structure that forms the supporting axis of the embryo and which produces the spine in vertebrates. It is formed through the differentiation of mesodermal cells.

16. What is the coelom? What structures are produced by the coelom? Are all animals coelomate?

Coeloms are cavities delimited by a mesoderm. Coeloms turn into cavities where the internal organs of the body are located, such as the pericardial cavity, the peritoneal cavity and the pleural cavity.

In addition to coelomate animals, there are also acoelomate animals, such as platyhelminthes, and pseudocoelomate animals, such as nematodes.

• Embryonic Development Review - Image Diversity: the coelom

17. From which germ layer are coeloms produced?

Coeloms are produced from the mesoderm.

18. What are the pleura, the pericardium and the peritoneum?

The pleura is the membrane that covers the lungs and the inner wall of the chest; the pericardium is the membrane that covers the heart; and the peritoneum is the membrane that covers most organs of the gastrointestinal tract and part of the abdominal cavity. All these membranes surround coeloms (internal cavities).

• Embryonic Development Review - Image Diversity: pleura pericardium peritoneum

19. After the neurula stage, how can the morphology of the embryo be described, starting at its ventral portion and ending at its dorsal portion?

In a schematic longitudinal section of an embryo after the neurula stage, the outermost layer of cells is the ectoderm. In the ventral region, the archenteron tube is formed of endodermal cells. In both sides of the embryo, coeloms covered by a mesoderm are present. In the central region above the archenteron and in the middle of the coeloms, the notochord is located. In the dorsal region just above the notochord, the neural tube is located.

20. What are somites?

Somites are differentiated portions of mesodermal tissue which are longitudinally distributed along the embryo. Somites turn into muscle tissue and portions of connective tissues.

• Embryonic Development Review - Image Diversity: somites

21. What are histogenesis and organogenesis?

Histogenesis is the process of tissue formation during the embryonic development. Organogenesis is the process of organ formation. Before histogenesis and organogenesis, primitive embryonic structures have been already formed: germ layers, the neural tube, the notochord, coeloms, and somites.

22. From which germ layer are the epidermis and the nervous system produced? What other organs and tissues are made from that germ layer?

The epidermis and the nervous system have the same embryonic origin: the ectoderm. Epidermal appendages (such as nails, hair, sweat glands and sebaceous glands), the mammary glands, the adenohypophysis, the cornea, the crystalline lens and the retina are also produced from the ectoderm.

23. From which germ layer are blood cells produced? What other organs and tissues are made from that germ layer?

Blood cells have a mesodermal embryonic origin. Other organs made from the mesoderm are: serous membrane coverings such as the pericardium, the peritoneum and the pleura, muscles, cartilage, the dermis, adipose tissue, the kidneys, the ureters, the bladder, the urethra, the gonads, blood and lymph vessels, and bones.

24. From which germ layer are the liver and the pancreas produced? What other organs and tissues are made from that germ layer?

The liver and the pancreas are produced from the endoderm. Also of endodermal origin are the epithelium of the airway, the epithelium of the bladder, the epithelium of the urethra and the epithelium of the GI tract (except for the mouth and anus), the alveolar cells of the lungs and the thyroid and parathyroid glands.

25. What are twins? Genetically, what are the two types of twins that can occur?

Twins are simultaneously generated (within the mother’s uterus) offspring. Twins are classified according to zygosity as monozygotic or dizygotic twins.

Monozygotic twins, also known as identical twins, are those that originate from one single fertilized ovum (therefore from one single zygote); monozygotic twins are genetically identical, meaning that they have identical genotypes and are necessarily of the same sex. Dizygotic twins, also known as fraternal twins, are those generated from two different ova fertilized by two different sperm cells; therefore, they are not genetically identical and they are not necessarily of the same sex.

• Embryonic Development Review - Image Diversity: twins

26. What is polyembryony?

Polyembryony is the phenomenon in which a single embryo in its initial embryonic stage divides itself to form many new individuals of the same sex and who are genetically identical. This is the way, for example, in which reproduction takes place in armadillos of the genus Dasypus. Polyembryony is an example of natural “cloning”.