Developmental Physiology - MCAT Biological and Biochemical Foundations of Living Systems

The failure of the lens to form in the absence of the optic cup indicates that the optic cup is necessary for lens development, and may in fact induce it.

This fact says nothing in regards to the timing of neurulation, nor the timing of eye development. Finally, cell differentiation is a gradual process, not all-or-nothing, and the statement is irrelevant to the development of the optic cup and lens.

Anti-Mullerian hormone is present only in male fetuses. It is responsible for the degeneration of the Mullerian ducts, which form the oviducts, uterus, and upper vagina in females.

Osteoporosis is a disease that causes a decrease in bone mass and density and can be induced by malnutrition or calcium deficiency. Even if the disease appears unfamiliar, the prefix "osteo-" indicates a relationship to bone, which is closely linked to calcium levels int he body.

Calcium is also an important ion involved in neural action potentials; it is responsible for triggering the release of neurotransmitters into the synaptic cleft. A calcium deficiency may cause improper action potentials that result in spasms.

Oxygen diffuses from maternal blood to fetal blood through the placenta. Within the womb, the lungs are not able to work properly and so blood is rerouted through the foramen ovale and the ductus arteriosus. The liver is also underdeveloped, and not ready to perform the functions it will do in adulthood. The placenta takes over for the liver during gestation, and so blood is shunted away from the liver by the ductus venosus. This prevents oxygen that could be useful in other places from being wasted in the liver.

The ductus arteriosus shunts blood from the pulmonary artery to the aorta in order to bypass the lungs, since the fetal lungs are not active.

The foramen ovale shunts blood from the right atrium to the left atrium, serving as another method to bypass the lungs. The ductus venosus shunts blood away from the fetal liver. There is no shunt from the aorta to the pulmonary artery, and there is no shunt from the left atrium to the right atrium.

A pregnancy consists of three trimesters, each of which is approximately 12 weeks. The fetus's heart, brain, spinal cord, and other organs begin to form in approximately the fifth week of pregnancy—in the first trimester.

Erythroblastosis fetalis is a rare conditon in which the mother's anti-Rh antibodies enter the fetal bloodstream and attach to fetal erythrocytes. If not caught, it can result in "blue-baby syndrome." Erythroblastosis is only present in patients negative for the Rh factor, and carrying Rh positive fetuses.

The mesoderm gives rise to the muscles among other structures. The other answers are derived from the ectoderm or endoderm.

Many of the glandular structures—both exocrine and endocrine—arise from endoderm. In particular, both the thyroid and parathyroid glands bud off from the pharyngeal mucosa, which of course is endodermal in character. Of course, like all tissues, there are mesodermal components of the gland in the way of connective tissues and blood vessels. The response indicating a mesodermal component invites you to make a wrong selection because you could remember that there are mesodermal-derived tissues everywhere, but you could not specifically recall the origin of the thyroid tissue, itself.

The correct answer is cytoderm.

There are three major germ layers formed during gastrulation: the ectoderm, mesoderm, and endoderm. Major structures form from these germ layers over the course of embryogenesis and development.

Gastrulation is the formation of the three layers of the embryo: ectoderm, endoderm, and mesoderm. The endoderm gives rise to the lining of the digestive system and respiratory system. A defect in these tissues suggests a problem of that germ layer during development. The ectoderm gives rise to the nervous system and epidermis. The mesoderm gives rise to the muscle and skeletal systems.

The three embryonic germ layers are the ectoderm, mesoderm, and endoderm. The mesoderm gives rise to bone, muscle, the urinary system, and the kidneys. Ectoderm develops into the nervous system, dermis, hair, nails, eyes, and ears. The endoderm develops into the lining of internal organs, such as the lungs and the gastrointestinal tract. The notochord is not an embryonic layer and, like the kidneys, arises from the mesoderm.

The circulatory system is part of the mesoderm germ layer, and includes development of the heart. We know that three answer choices must also be part of the mesoderm, while one is not (the correct answer). The mesoderm is responsible for generating muscle, blood, and bone tissue, kidneys and gonads, as well as the heart.

This leaves us with lungs, the correct answer, which develop from the endoderm layer.

The process of gastrulation generates the three primary germ layers (ectoderm, endoderm, mesoderm). This is one of the most important steps of development.

The eight-cell stage signals the beginning of compaction, during which the cells of the blastula adhere to each other and prepare for further divisions. The epiblast and hypoblast are generated before gastrulation, but after implantation. Differentiation occurs long after the primary germ layers are formed.

The correct answer is the ectoderm. The ectoderm is responsible for generating nervous tissue and glial cells, among other features.

Ectoderm differntiates into the epidermis, nervous system, eye lens, and hair.

Mesoderm differentiates into muscle, cardiac and skeletal systems, blood, and spleen.

Endoderm differentiates into the lining of internal organs, like the digestive tract and respiratory tract.

The neural tube is a structure derived from the ectoderm that is found in chordates. The neural tube is formed by the closing of the neural folds, and runs along the dorsal-ventral axis of the developing embryo. Cells within the neural tube give rise to the neural crest. Additionally, the mature neural tube continues to develop into the central nervous system.

The ectoderm is the outer layer of the embryo, which gives rise to the external ectoderm and the neuroectoderm. The neuroectoderm can further be divided into the neural crest and the neural tube. The neural crest migrates from the neural tube early in development. While both external ectoderm and the neural tube form an epithelial layer, the neural crest does not and is actually a highly migratory cell lineage. Furthermore, the neural crest is required for innervation of endodermal organs (forming the enteric nervous system), craniofacial features, innervation of the diaphragm for breathing, and many more critical features. Due to its structural and migratory properties, and its importance in development, the neural crest is often referred to as the "fourth germ layer."

Cardiomyocytes refer to cardiac muscle cells, while hepatocytes are liver cells. Beta cells are found in the pancreas and secrete insulin.

The ectoderm gives rise to the nervous system, epidermis, lens of the eye, and the inner ear.

The lungs are derived from the endoderm. Most epithelial linings are located toward the superficial regions of the body, and arise from the ectoderm. The linings of the respiratory and digestive tracts, however, are embedded deep within the body and are derived from the endoderm.

The endoderm gives rise to most of the internal organs, including the lining of the digestive tract, the liver and pancreas, and the respiratory system.

Muscles and bones are derived from the mesoderm.

The mesoderm gives rise to muscles, bones, the circulatory system, the gonads, and the kidneys. Think of the mesoderm as the means by which your body gets around on a daily basis: muscles and bones, heart to pump blood, and kidneys to excrete waste.

The respiratory system arises from the endoderm.