Studying Gametogenesis There are several approaches to the study of gametogenesis. Early biologists employed cytological techniques (methods of preparing cells for the study of their structure and function) and microscopy to study gamete formation. These early studies were, in fact, observations of the actual events themselves. Although these early descriptive approaches gave much information about the cells involved at each stage of gamete formation, they did not provide any information about the control mechanisms for this process. Biochemical studies have contributed to the understanding of certain regulatory substances and how they function in gametogenesis. By enhancing or inhibiting the presence of these regulatory substances in the organism, investigators have been able to elucidate many of the normal events of gametogenesis. Beginning at puberty, the hormones (substances released fromendocrine glands, generally functioning to regulate specific body activity) of the hypothalamus, the pituitary gland, and the gonads interact to establish and regulate gametogenesis in the organism. Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior portion of the pituitary gland. All three of these hormones are necessary for spermatogenesis and oogenesis. Surgical removal of the mammalian pituitary gland (hypophysectomy) in the male leads to degeneration of the testes. Testicular function can be restored in these hypophysectomized animals by administering the hormones FSH and LH. These studies suggest that FSH andLHare necessary for normal functioning of the testes. LH appears to stimulate the release of testosterone (male hormone) by certain cells (Leydig cells) of the testes. Both testosterone and FSH are necessary for spermatogenesis, but the exact role that each of these hormones plays in male sexual physiology has yet to be determined. Oogenesis in the female has been the subject of intense investigation. At the beginning of each ovarian cycle, from puberty to menopause, one primary oocyte present in the female's ovaries is activated to continue the process of gamete formation. Release of GnRH from the hypothalamus at the beginning of each cycle stimulates the anterior portion of the pituitary gland to release FSH. FSH, in turn, affects the ovaries: It stimulates a primary oocyte to mature to the point that it can be released from the ovary, as a secondary oocyte, and it causes certain cells (follicle cells) in the ovary to produce estrogens, female hormones. High estrogen levels will cause the pituitary to inhibit FSH release, a negative feedback mechanism, and stimulate LH release. These estrogen-mediated events occur at approximately the middle of the ovarian cycle. LH also affects the ovaries. LH, however, is responsible for ovulation (the release of the oocyte from the ovaries) and for the formation of a cellular structure called the corpus luteum. LH also stimulates the corpus luteum to produce progesterone, another female hormone. Eventually, high levels of progesterone will inhibit LH release from the pituitary gland, and the cycle begins anew.
Gas-Exchange Organs Animals have three basic types of gas-exchange organs: skin, invaginations (inpocketings of the epithelium), and evaginations (outpocketings of the epithelium). All three show modifications to improve the conditions of gas exchange. Skin always permits gas exchange unless it is coated with some material that limits diffusion. The skin of a snake or a turtle is so coated and permits very little gas exchange. The skin of a worm or an octopus, on the other hand, is quite thin and permits gas exchange quite freely. Invaginations of the external epithelium are basically what lungs and insect tracheas are, but in a highly modified condition. Evaginations of the skin are represented by the gills of aquatic animals; even when inside a cavity, as are fish and crab gills, they are still evaginations. There is only onewaythat animals take up oxygen from the external medium, regardless of whether that medium is air or water. Gas must passively diffuse across the membrane that separates the animal from its environment. That membrane is a type of tissue called epithelium and is similar in nature and structure to the tissue that lines other body surfaces. The different types of epithelia are classified according to their locations and functions; those lining gills, lungs, and certain other organs of gas exchange are all known as respiratory epithelia. The respiratory epithelium not only separates the internal and external fluids but also represents a barrier to the movement of materials such as gas.
