Biology: Development

Early in gestation, the human embryo is without gender. As development progresses, female sex may result from: I. absence of genetically encoded hormonal influences. II. lack of inhibition of default pathways. III. absence of genetically encoded inhibitory hormones.

I, II, III Item I is true: testosterone is genetically encoded and required to form the male system; in its absence the female system develops. Item II is true: the female system is considered to be the "default" system, since no specific trigger is required to initiate its development, only the absence of the triggers for the male system. If the "default" pathway is not inhibited, it will develop. Item III is true: one of the hormones produced during male development is Müllerian Inhibiting Factor (MIF) which acts to suppress female development. In its absence, the female system develops.

Which of the following is from mesoderm? A. Cardiac muscle B. Neurons C. Epidermis D. Epithelial cells of the intestine

A. Cardiac muscle Cardiac muscle is from mesoderm. Mesoderm is the "middle" layer of the three primary germ layers, and as such forms "middle level" structures in the body, such as bones, muscles, blood vessels, the heart, the lungs, non-glandular organs in general. The epithelial cells of the intestine come from endoderm ("inner level"), and the epidermis and neurons come from ectoderm ("outer level"). Note that the neurons are not on the outer edge of the body, but early in development the ectoderm folds inward to form the nervous system.

At which stage of development are the three germ layers (the ectoderm, the mesoderm, and the endoderm) first found? A. Gastrula B. Neurula C. Morula D. Blastula

A. Gastrula The three germ layers are first formed during gastrulation. The morula and the blastula occur prior to gastrulation (making them incorrect answer choices), and the neurula is derived from ectodermal cells formed during gastrulation (neurula is wrong and gastrula is correct).

Which of the following is the correct order of events in fertilization? A. Sperm penetrates oocyte--->Second polar body is released--->Syngamy (fusion of nuclei) ---> Cleavage B. First polar body is released --->Ovulation ---> Sperm penetrates oocyte ---> Syngamy (fusion of nuclei) --->Second polar body is released---> Cleavage C. First polar body is released---> Ovulation ---> Second polar body is released ---> Sperm penetrates oocyte ---> Cleavage D. Sperm penetrates oocyte ---> First polar body is released ---> Syngamy (fusion of nuclei) ---> Second polar body is released ---> Cleavage

A. Sperm penetrates oocyte--->Second polar body is released--->Syngamy (fusion of nuclei) ---> Cleavage The first polar body is released prior to ovulation. After the sperm penetrates the oocyte, the sperm and oocyte nuclei remain separate until the oocyte completes meiosis II and releases the second polar body. Until this time the fertilized oocyte is a dikaryon (one cell, two nuclei). After the second polar body is released, the two nuclei fuse, and the cell is known as a zygote; the zygote then begins a period of rapid cell division known as cleavage.

The epidermis of the skin is embryologically derived from the same germ layer that gives rise to which of the following? A. Spinal cord B. Stomach C. Liver D. Skeletal muscle

A. Spinal cord The epidermis of the skin is embryologically derived from the same germ layer that gives rise to the spinal cord. You must memorize which adult tissues are derived from each of the three primary germ layers. The endoderm give rise to the stomach, the colon, the liver, the pancreas, the urinary bladder, the lining of the urethra, and the epithelial parts of the trachea, lungs, pharynx, thyroid, parathyroid, and intestines. The mesoderm gives rise to skeletal muscle, the skeleton, the dermis of the skin, connective tissue, the urogenital system, all parts of the cardiovascular system, the blood, and the spleen. The ectoderm gives rise to the central nervous system (brain and spinal cord), the lens of the eye, ganglia and nerves, head connective tissue (pharyngeal cartilages, hyoid bone, auditory ossicles, etc.), the epidermis, hair, sweat glands, and mammary glands. This content is important to know for the MCAT.

Where would somatic stem cells be found in an adult human? A. Brain B. Bone marrow C. Gonads D. Epidermis

B. Bone marrow Somatic stem cells are multipotent so they can become several different (but not all) types of cells. They are found in the hematopoietic tissue of the bone marrow. The gonads produce gametes, the neurons of the brain are fully differentiated and so is the epidermis (the top most layer of skin).

Which of the following is derived from embryonic ectoderm? A. Muscle B. Cerebellum C. Gonads D. Liver

B. Cerebellum The ectoderm forms the skin and the nervous system, including the brain.

How is the final phenotype of a developing stem cell established? A. Differentiation employs methylation to create the genetic identity of the cell. B. Its genetic fate is determined and then it becomes visibly differentiated. C. Determination allows for phenotypic expression of alternative genomic splicing. D. Its genetic fate is differentiated and then it becomes visibly determined.

B. Its genetic fate is determined and then it becomes visibly differentiated. A developing stem cell's genetic fate is determined and then it becomes visibly differentiated to establish its final phenotype. Stem cells need to have their genetic programming fixed first via the process of determination. This genetic programming is then physically manifested through the process of differentiation.

In a frog embryo, which of the following cell groups gives rise to the muscles? A. Endoderm B. Mesoderm C. Neural tube D. Ectoderm

B. Mesoderm The mesoderm forms muscles, blood, bone, reproductive organs, and kidneys.

If a baby is born with both male and female genitalia, which of the following is a possible explanation? A. The baby is genetically male and there was a mutation in the gene for Wolffian inhibiting factor. B. The baby is genetically male and there was a mutation in the gene for Mullerian inhibiting factor. C. The baby is genetically XXY and the extra X chromosome caused a surge in estrogen leading to development of both genitalia. D. The baby is genetically female and there was a mutation in the gene for Wolffian inhibiting factor.

B. The baby is genetically male and there was a mutation in the gene for Mullerian inhibiting factor. The Y chromosome must be present for male genitalia to develop, and Mullerian inhibiting factor (MIF) must be present to prevent female genitalia from developing. If the Y chromosome is present, and MIF is absent or mutant (non-functional), then both sets of genitalia will develop. There is no such thing as Wolffian inhibiting factor ("The baby is genetically female and there was a mutation in the gene for Wolffian inhibiting factor" and "The baby is genetically male and there was a mutation in the gene for Wolffian inhibiting factor" are wrong). An extra X chromosome would not lead to increased estrogen, and even if it did, as long as MIF was present, no female genitalia would develop ("The baby is genetically XXY and the extra X chromosome caused a surge in estrogen leading to development of both genitalia" is wrong).

How are multipotent stem cells different in their capabilities than pluripotent stem cells? A. Multipotent stem cells can become any of the three primary germ layers, while pluripotent stem cells can become several different types of cells from a single primary germ layer. B. Multipotent stem cells form all the cells of the body while pluripotent stem cells can also form the cells of the placenta. C. Multipotent stem cells can become several different types of cells from a single primary germ layer, while pluripotent stem cells can become any of the three primary germ layers. D. Multipotent stem cells form all the extraembryonic membranes while pluripotent stem cells form the embryo.

C. Multipotent stem cells can become several different types of cells from a single primary germ layer, while pluripotent stem cells can become any of the three primary germ layers. Pluripotent stem cells are the embryonic stem cells that form the embryo itself (the three primary germ layers). Totipotent stem cells precede this and form both the embryo and the placenta.

Which of the following statements about the blastula is true? A. The entire blastula undergoes differentiation into the three primary germ layers during gastrulation. B. The estrogen and progesterone in birth control pills prevent pregnancy by preventing blastulation. C. The inner cell mass develops into the embryo and the trophoblast develops into the placenta. D. The placenta forms from the trophoblast by the end of the first month of pregnancy.

C. The inner cell mass develops into the embryo and the trophoblast develops into the placenta In the blastula, the inner cell mass develops into the embryo and the trophoblast develops into the placenta. The blastula consists of an outer layer of cells called the trophoblast, a mostly-hollow core called the blastocoel, and an inner clump of cells called the inner cell mass. The blastula is the structure that implants in the wall of the uterus, with the inner cell mass developing into the embryo, and the trophoblast cells ultimately becoming the placenta. Full formation of the placenta takes approximately three months and is complete at the end of the first trimester. Only the inner cell mass undergoes differentiation into the primary germ layers, and the estrogen and progesterone in birth control pills prevent ovulation, not blastulation.

Which of the following is not derived from the inner cell mass of a blastocyst? A. Amnion B. Embryo C. Trophoblast D. Allantois

C. Trophoblast A blastocyst consists of an outer cell mass and an inner cell mass. The outer cell mass, or trophoblast, develops first into the chorion and ultimately into the placenta. The inner cell mass develops into the embryo, amnion, yolk sac and allantois (leaving these as incorrect answer choices).

Babies born prematurely (between 28-32 weeks of gestation) typically suffer from IRDS, infant respiratory distress syndrome. IRDS is characterized by difficulty in breathing, extreme exhaustion, and sometimes death, and is due to a lack of surfactant. When during the fetal stage is surfactant produced in significant quantities? A. 20 weeks B. 28 weeks C. 30 weeks D. 33 weeks

D. 33 weeks Surfactant is produced in significant quantities by about 33 weeks of gestation. Prior to then, surfactant is limited and babies born earlier than 33 weeks typically suffer from IRDS.

A researcher uses a radioactive label to trace the development of the GI glands in chicken embryos. From what primary germ layer do they come? A. Blastopore B. Ectoderm C. Mesoderm D. Endoderm

D. Endoderm The endoderm gives rise to the epithelial lining of the GI tract (except for the nasal, oral, and anal epithelium), the respiratory tract, and the urogenital organs and ducts. It also gives rise to the GI glands and the urinary bladder. The ectoderm gives rise to the nervous system, the cornea and lens, the pituitary gland, the adrenal medulla, the epidermis, and the nasal, oral, and anal epithelium. The mesoderm gives rise to all muscle, bone, connective tissue, cardiovascular system, lymphatic system, urogenital organs, and the dermis. The blastopore is not a primary germ layer and will eventually give rise to the anus.

Which of these processes happens last in embryogenesis? A. Blastulation B. Gastrulation C. Fertilization D. Neurulation

D. Neurulation Neurulation happens last in embryogenesis. Neurulation begins with the formation of the neural tube from embryonic ectoderm and follows gastrulation. Both fertilization and blastulation occur prior to gastrulation (and neurulation).

A mesodermal cell originally destined to become a heart cell gets transplanted to another location where cells become bone cells. The mesodermal cell now develops into a bone cell instead of a heart cell. The process that occurred is: A. determination. B. differentiation. C. imprinting. D. induction.

D. induction. When one type of cell is transplanted and then becomes more like its surrounding cells, that process is called induction. This typically must happen early enough in development so that specific genes are not yet turned on. Once the genetic program in a cell has been invoked, that cell's fate has been determined and the cell can no longer be induced to become a different cell (determination is wrong). Differentiation is the actual act of transforming into a more mature cell type (differentiation is wrong), and imprinting occurs when a newborn animal sees its mother for the first time and has nothing to do with cellular development (imprinting is wrong).

Which of the following structures are derivatives of the ectoderm? I. Epidermis of the skin II. Dermis of the skin III. Sweat glands

I, III The skin's epidermis and its derivatives (which include hair, nails, and sweat glands) are derived from the ectoderm (Items I and III are true). The dermis of the skin is derived from the mesoderm (Item II is false).