Embryology Interview Preparation Guide

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The umbilical cord is a set of blood vessels that connect the fetus with the placenta. In the fetus, one extremity of the cord inserts into the center of the abdominal wall (the later scar of this insertion is the umbilicus).

The function of the umbilical cord is to allow the transport of substances, nutrients, gases, and residuals, between the fetus and the mother’s body.

The placenta has endocrine function since it secretes the hormones progesterone and estrogens that maintain the endometrium (internal covering of the uterus) and prevent menses during pregnancy. The placenta also secretes other important hormones for pregnancy regulation.

Under normal conditions, there is no passage of cells across the placenta during gestation. The placenta has a smooth mucosa separating the richly vascularized region in contact with the mother’s endometrium from the umbilical cord in contact with the fetal blood. This barrier is known as placental barrier. Although permeable to some substances (selective permeability), the placental barrier forbids the passage of cells.

Image Diversity: umbilical cord placental barrier

From the mother to the fetus the main transferred substances through the placenta are water, oxygen, nutrients, and antibodies. From the fetus to the mother, metabolic wastes including urea (nitrogen waste), and carbon dioxide are transferred.

True placenta is present in placental mammals.

The placenta is formed from the chorion of the embryo and from the mother’s endometrium. Its main function is to allow the exchange of substances between the fetus and the mother’s body.

Image Diversity: placenta placental mammals

The chorioallantois membrane is formed by juxtaposition of some regions of the chorion and the allantois. Since it is porous, the chorioallantois membrane allows the passage of gases between the embryo and the exterior thus making aerobic cellular respiration possible.

The amnion is also an adaptation to dry land since one of its functions is to prevent desiccation of the embryo.

Amnion is the membrane that covers the embryo. Chorion is the membrane that covers the amnion, the yolk sac, and the allantois. The space delimited by the chorion and the amnion is called amniotic cavity and it is filled with aminiotic fluid. The amniotic cavity has the functions of preventing desiccation of the embryo and of protecting it against mechanical shocks.

Image Diversity: amnion chorion

The allantois is an adaptation to dry land because in embryos of oviparous terrestrial beings, like reptiles and birds, the metabolic residuals cannot be immediately excreted to the aquatic surrounds (as fishes and amphibian larvae do). It was necessary then the appearing of a structure capable of storing the embryonic excretes until hatching.

The allantois is the extra embryonic membrane whose function is to store excretes of the embryo.

In placental mammals, the allantois is present but it does not exert that function since the embryonic wastes are collected by the mother’s body through the placenta.

Image Diversity: allantois

The yolk sac is formed from the covering of the vitellus by some cells originated from the primitive gut.

The yolk sac stores vitellus, the main nourishment source of nonplacental embryos.

Image Diversity: yolk sac

The presence of each extra embryonic membrane varies according to the vertebrate class.

In fishes and amphibians, only the yolk sac is present. In reptiles and aves besides the yolk sac, there are also the amnion, the chorion and the allantois. In placental mammals besides all these membranes, the placenta is present too.

The extra embryonic membranes that may be present in vertebrates are the yolk sac, the amnion, the chorion, the allantois and the placenta.

Extra embryonic membranes are membranous structures that appear paralleling the embryo and play important roles in the embryonic development. They form from the embryo but do not become part of the individual organism after its birth.

Polyembryony is the phenomenon in which a single embryo in its initial embryonic stage divides itself forming many new individuals of the same sex and 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”.

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

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.

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.

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.

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.

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

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.

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

The coeloms are originated from mesoderm.

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