Embryology Interview Questions And Answers

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

Image Diversity: germ layers

The mesoderm appears from differentiation of endodermal cells that cover the dorsal region of the archenteron.

Archenteron is the tube formed during gastrulation by means of invagination of the blastula wall inside the blatocele. It is the origin of the gastrointestinal tract. Blastopore is the opening of the archenteron to the exterior. The blastopore gives birth to one of the extremities of the digestive tube: the mouth in protostome beings, or the anus in deuterostome beings.

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

The blastula turns into gastrula in a process known as gastrulation.

Image Diversity: gastrula

After passing the morula stage in which the embryo was 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, the blastocele.

Image Diversity: blastula

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

Image Diversity: morula

The cell division in the first stage of the embryonic developments is called cleavage, or segmentation. In this stage, mitosis occurs from the zygote forming the new embryo.

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

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

Image Diversity: animal and vegetal poles

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

The embryonic growth depends directly on mitosis. Through this type of cell division, the zygote divides itself giving birth to a series of cells that by mitosis too compose differentiated tissues and organs until the formation of a complete individual.

Cnidarians are diploblastic, i.e., they present only endoderm and ectoderm. With the exception of poriferans, all remaining animals are triploblastic. Poriferans do not present differentiated tissue organization and so they do not classify regarding germ layers (although sometimes they are mentioned as diploblastic).

The neurula stage is characterized by the appearing of the neural tube along the dorsal region of the embryo. The growing of mesoderm in that region induces the differentiation of ectodermal cells just above. These cells then differentiate forming the neural tube. Therefore, the origin of the nervous system is the ectoderm (the same germ layer that gives birth to the skin).

Image Diversity: neurula

Notochord is a rodlike structure that forms the supporting axis of the embryo and gives birth to the vertebral column in vertebrates. It is formed by differentiation of mesodermal cells.

Coeloms are cavities delimited by mesoderm. Coeloms originate the cavities where the internal organs of the body are located, like the pericardial cavity, the peritoneal cavity, and the pleural cavity. Besides coelomate animals, there are acoelomate animals, like platyhelminthes, and pseudocoelomate animals, like nematodes.

Image Diversity: coelom

The coeloms are originated from mesoderm.

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

Image Diversity: pleura pericardium peritoneum

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

Somites are differentiated portions of mesodermal tissue longitudinally distributed along the embryo. The somites originate the muscle tissue and portions of the connective tissues.

Image Diversity: somites

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

Epidermis and nervous system have the same embryonic origin: the ectoderm. The epidermal appendages (like nails, hair, sweat glands, and sebaceous glands), the mammary glands, the adenohypophysys, the cornea, the crystalline lens and the retina are also derived from ectoderm.

Blood cells have mesodermal embryonic origin. Other organs made from mesoderm are covering serous membranes like the pericardium, the peritoneum and the pleura, muscles, cartilages, dermis, adipose tissue, kidneys, ureters, bladder, urethra, gonads, blood and lymph vessels, bones.

The liver and the pancreas are originated from the endoderm. Also from endodermal origin are the epithelia of the airway, the epithelia of the bladder, of the urethra and of the GI tube (excepted of the mouth and anus), the alveolar cells of the lungs and the thyroid and parathyroid glands.

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

Monozygotic twins, also known as identical twins, are those originated from one single fertilized ovum (therefore from one single zygote); monozygotic twins are genetically identical, i.e., 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 fecundated by two different sperm cells; so they are not genetically identical and they are not necessarily of the same sex.

Image Diversity: twins