Systems Biology and Tissue Types - MCAT Biological and Biochemical Foundations of Living Systems

Of all the hormones listed, antidiuretic hormone (ADH) is the only one that neither acts on, nor is released by, the adrenal cortex. ADH is released from the posterior pituitary and causes the kidneys to retain more water.

Aldosterone (a mineralcorticoid) and cortisol (a glucocorticoid) are both incorrect choices because they are released by the adrenal cortex and would be greatly affected by trauma to that area. ACTH is released by the anterior pituitary and acts to stimulate the adrenal cortex; these hormones act as part of a negative feedback chain, so damage to the target area would temporarily cause more ACTH to be produced. The same goes for CRH, which is released by the hypothalamus and stimulates secretion of ACTH.

Endocrine glands secrete hormones into the blood stream, lined with endothelium, allowing them to travel through the blood and to act at a distant site. Exocrine organs, in contrast, secrete products into lumens that are lined with epithelium.

Mammary glands are used in lactation. Because the milk is not secreted into the blood, the mammary glands are not endocrine glands.

The hypothalamus releases hormones into the blood, such as corticotropin-releasing hormone (CRH). The pituitary releases hormones such as prolactin. The adrenal gland secretes hormones such as cortisol.

The adrenal medulla, derived from neural crest cells, is responsible for making and releasing epinephrine and norepinephrine. Epinephrine and norepinephrine are responsible for increasing heart rate and activating the sympathetic nervous system when released.

In contrast, the adrenal cortex is derived from mesoderm and releases steroid hormones like aldosterone and cortisol. Corticotropin-releasing hormone is made by the parvocellular neurons of the hypothalamus.

Epinephrine and norepinephrine are released by the neuroendocrine cells of the adrenal medulla. In times of stress and sympathetic nervous system activation, the adrenal medulla will release epinephrine to cause blood vessel constriction. These hormones allow for the "fight-or-flight" response.

In contrast, the adrenal cortex will secrete cortisol and other mineralcorticoids in response to long-term stress. These hormones are not involved in the fight-or-flight response, and rather serve to prepare the body to endure prolonged harsh conditions, such as dehydration, starvation, and extreme temperatures. Adrenocorticotropic hormone is released from the anterior pituitary to stimulate the adrenal cortex.

Growth hormone and thyroid-stimulating hormone do not interact with the adrenal gland.

Hormones cortisol and aldosterone are synthesized in the adrenal glands. Adrenal glands are made up of the adrenal cortex and the adrenal medulla. Adrenal medulla is involved in the synthesize of catecholamines (epinephrine and norepinephrine) whereas adrenal cortex synthesizes mineralocorticoids (aldosterone), glucocorticoids (cortisol), and androgens (testosterone, DHT, and DHEA).

Aldosterone is released upon stimulation from the renin-angiotensin system and serves to increase reabsorption of sodium in the collecting ducts of the kidney. Cortisol is released due to stress and serves to increase the metabolic rate.

Luteinizing hormone is produced and secreted by the anterior pituitary gland.

Antidiuretic hormone is produced by the hypothalamus, but secreted by the posterior pituitary gland. Epinephrine is produced by the adrenal medulla; aldosterone is produced by the adrenal cortex. Thyroxine (T4) is produced by the thyroid.

Androgens and estrogens, the sex hormones, feedback to inhibit somatotropins, the growth hormones. In humans, the sex hormones are produced in puberty, disabling the growth plates and halting growth.

Positive feedback describes an event in which a pathway generates a response that further triggers the pathway, increasing the pathway effects. In contrast, negative feedback occurs when a pathway generates a response to inhibit the pathway origin, diminishing the pathway effects. Negative feedback is a common control mechanisms in the body to maintain homeostasis, while positive feedback is inherently designed to disrupt homeostasis.

Fever during illness is enhanced via a positive feedback system that only ends once the illness begins to alleviate. Similarly, contractions during labor will intensify via positive feedback oxytocin stimulation until the child is born. Osteoporosis is caused by an imbalance in the negative feedback system that controls blood calcium. This imbalance simply means that bone is lost more than it is gained, and is still an example of negative feedback.

Thyroid-stimulating hormone (TSH) levels would be decreased.

The thyroid hormones T3 and T4 act in a negative feedback loop to regulate thyroid activity. Release of TSH increases thyroid activity, while release of the thyroid hormones decreases thyroid activity by suppressing further release of TSH. Since the thyroid is overstimulated in Grave's Disease, excess T3 and T4 are being produced. These hormones will act on the feedback loop to suppress TSH release, lowering TSH levels in individuals with this disease. Since the antibodies bind to the TSH receptors, there are increased levels of T3 and T4 in the body, which negatively inhibit the production of TSH because the body is already flooded with the products of TSH action.

Triiodothyronine (T3) employs a negative feedback mechanism, meaning that when blood serum concentrations of T3 become too high, receptors on the thyroid gland inhibit the release of additional T3.

In contrast, a positive feedback mechanism would encourage additional release of a hormone when levels are high, resulting in an exponential increase in the hormone effects. An example of a positive feedback mechanism is the release of oxytocin during childbirth to help the uterus contract.

Oxytocin employs a positive feedback mechanism, meaning that existing levels of oxytocin encourage additional release of oxytocin. This results in an exponential increase in the hormone's effects.

In contrast, a negative feedback mechanism would prevent additional release of a hormone when levels of the existing hormone were too elevated. This results in stable homeostasis around a constant hormone concentration in the blood.

Injecting insulin into the body delivers it directly into the bloodstream, while consuming an insulin pill would require it to pass through the gastrointestinal tract. In the gut, the pill would be degraded first and then absorbed into the bloodstream. This entire process would result in a small amount of insulin circulating in the blood, and therefore be much less effective than an insulin injection. Additionally, injecting insulin directly allows more rapid administration of a more consistent dose.

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.

All blood cells, including erythrocytes and lymphocytes, come from the same stem cell precursor in the bone marrow. These hematopoietic stem cells give rise to both the myeloid and lymphoid progenitor cells, which are responsible for all blood cells and immune cells. This common precursor spawns cells that then differentiate into the various components of humoral, cell-mediated, and innate immunity.

The five types of leukocytes are: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Three of these (neutrophils, eosinophils, and basophils) contain granules, tiny sacs containing enzymes which can lyse microorganisms. The other two leukocytes (lymphocytes and monocytes) do not contain these granules.

B cells play numerous integral roles in the immune response against foreign pathogens (viruses, bacteria, and fungi), including forming transient microenvironments called germinal centers, where they produce long-lived plasma cells that are high affinity for specific antigen and memory B cells. They also serve as antigen-presenting cells and producers of cytokines and chemokines; However, B cells are not able to produce extracellular traps, which primarily are composed of DNA and work to trap pathogens. Neutrophils produce extracellular traps.

Cells of myeloid lineage include dendritic cells, monocytes, macrophages, neutrophils, basophils, and eosinophils, while cells of lymphoid lineage include NK cells, B cells and T cells.

Naive B cells (and most other immune cell subtypes) need more than one signal to become activated. They normally need B cell receptor signaling (signal 1), costimulation by other receptors (signal 2), and cytokines/chemokines (signal 3). This system is necessary in order to prevent aberrant activation of lymphocytes (safeguard against autoimmunity).

In regards to the other statements, there are numerous autoreactive B cells at any given time due to the stochastic nature of VDJ recombination and germinal center reactions. Therefore, tolerance mechanisms and checkpoints are incredibly important to keep these cells in check; central and peripheral tolerance are equally important. Self-nuclear reactive B cells and T cells are both necessary and critical in autoimmune pathogenesis. Female sex hormones are definitely believed to contribute greatly to autoimmune disease pathogenesis (e.g. estrogen). Over 75% of autoimmune patients are women.