The gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are glycoproteins produced by gonadotrope cells of the anterior pituitary gland. The two hormones act on somatic cells of the gonads in both males and females to regulate fundamental aspects of reproductive physiology, including gametogenesis and steroidogenesis. In males, LH stimulates testosterone production and sperm maturation. FSH also regulates spermatogenesis, though the importance of the hormone in this process differs across species. In females, FSH stimulates ovarian follicle maturation. Follicles are structures composed of oocytes surrounded by two types of somatic cells, granulosa and theca cells. FSH stimulates granulosa cells to proliferate and to increase their production of the aromatase enzyme. LH stimulates theca cells to make androgens, which are converted into estrogens by aromatase in granulosa cells. A surge of LH also stimulates ovulation of mature follicles. Gonadotropin-releasing hormone (GnRH) from the brain is the principal stimulator of gonadotropin synthesis and secretion from the pituitary. The sex steroids (androgens and estrogens) that are produced by the gonads in response to the gonadotropins feedback to the brain and pituitary gland. In the brain, these hormones usually slow the release of GnRH through a process called negative feedback, which in turn leads to decreases in FSH and LH. The steroids also modulate the sensitivity of the pituitary to GnRH in addition to directly regulating expression of the genes that encode the gonadotropin subunits. These effects are gene- and species-specific. In females, estrogens also have positive feedback actions in the brain and pituitary in a reproductive cycle stage-dependent manner. This positive feedback promotes GnRH and LH release, leading to the surge of LH that triggers ovulation. The gonadotropins are dimeric proteins. FSH and LH share a common α-subunit but have hormone-specific subunits, FSHβ and LHβ. The β subunits provide a means for differential regulation and action of the two hormones. In the case of FSH, there is a second gonadal feedback system that specifically regulates the FSHβ subunit. The gonads produce proteins in the transforming growth factor β (TGFβ) family called inhibins, which come in two forms (inhibin A and inhibin B). The ovary produces both inhibins whereas the testes make inhibin B alone. Inhibins selectively suppress FSH synthesis and secretion, without affecting LH. The pituitary produces additional TGFβ proteins called activins, which are structurally related to inhibins. Activins, however, stimulate FSH synthesis by promoting transcription of the FSHβ subunit gene. Inhibins act as competitive receptor antagonists, binding to activin receptors and blocking activin action, and thereby leading to decreases in FSH. Together, GnRH, sex steroids, activins, and inhibins modulate and coordinate gonadotropin production and action to promote proper gonadal function and fertility.
Daniel J. Bernard, Yining Li, Chirine Toufaily, and Gauthier Schang
Many mammals, including humans, rely primarily on vision to sense the environment. While a large proportion of the brain is devoted to vision in highly visual animals, there are not enough neurons in the visual system to support a neuron-per-object look-up table. Instead, visual animals evolved ways to rapidly and dynamically encode an enormous diversity of visual information using minimal numbers of neurons (merely hundreds of millions of neurons and billions of connections!). In the mammalian visual system, a visual image is essentially broken down into simple elements that are reconstructed through a series of processing stages, most of which occur beneath consciousness. Importantly, visual information processing is not simply a serial progression along the hierarchy of visual brain structures (e.g., retina to visual thalamus to primary visual cortex to secondary visual cortex, etc.). Instead, connections within and between visual brain structures exist in all possible directions: feedforward, feedback, and lateral. Additionally, many mammalian visual systems are organized into parallel channels, presumably to enable efficient processing of information about different and important features in the visual environment (e.g., color, motion). The overall operations of the mammalian visual system are to: (1) combine unique groups of feature detectors in order to generate object representations and (2) integrate visual sensory information with cognitive and contextual information from the rest of the brain. Together, these operations enable individuals to perceive, plan, and act within their environment.