摘要:

Summary(1) Luteinizing hormone (LH) is secreted as discrete pulses throughout all stages of the reproductive cycle of the ewe, including pre‐pubertal, seasonal and lactational anoestrus, and the luteal and follicular phases of the oestrous cycle. Secretion is probably also pulsatile during the preovulatory surge of LH.(2) The secretion of LH is affected by the ovarian steroids, oestradiol and progesterone, both of which act principally to reduce the frequency of the pulses. During the luteal phase the two steroids act synergistically to exert this effect, and during anoestrus oestradiol acts independently of progesterone. Androstenedione secreted by the ovary apparently has no role in the control of LH secretion.(3) The amplitude of the pulses may also be affected by the steroids but there are conflicting reports on these effects, some showing that amplitude is lowered by the presence of oestrogen and others showing increases in amplitude in the presence of oestrogen and progesterone.(4) The secretion of LH pulses is affected by photoperiod, social environment and nutrition. Under the influence of decreasing day‐length, oestradiol alone cannot reduce the frequency of pulses and the ewe experiences oestrous cycles. When day‐length is increasing, the hypothalamus becomes more responsive to oestradiol which reduces the frequency of the pulses.(5) A hypothetical pheromone secreted by rams can increase the frequency of the LH pulses in anoestrous ewes and thereby induce ovulation, possibly by inhibiting the negative feedback exerted by oestradiol.(6) The relationships between nutrition and reproduction are poorly understood, but it seems likely that the effects of nutrition are mediated partly through the hypothalamus and its control of the secretion of LH pulses.(7) The pulses of LH secreted by the anterior pituitary gland are evoked by pulses of GnRH secreted by the hypothalamus. The location of the centre controlling the GnRH pulses and the neurotransmitter involved are not

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00401.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Summary1. Atavisms emerge as evidence of localized modifications in development of an organ or of one of its parts. Different developmental processes can be triggered within the same organ rudiment, presumably in response to the same stimulus. We saw that that stimulus can have a genetic basis in a mutational event, which can be selected for. We also saw that atavism can be produced by experimental manipulation within developing systems ‐increased growth of the chick fibula, enamel production from avian ectoderm, and balancer formation in amphibians. Such atavisms arenotbased on heritable genetic changes. They indicate the developmental plasticity that exists within embryos and the relative ease with which development can be switched from one programme to another.2. Examination of mutants (wingless chicks), limbless vertebrates and experimental manipulation of embryos, shows that cell death, inductive tissue interactions and altered patterns of growth are developmental mechanisms used in the formation of atavisms.3. Differential development mechanisms can be triggered within the same organ at the same time to produce atavisms. In the guinea pig, formation of atavistic digit V involvesprolongationof growth of metatarsal V whereas formation of atavistic digit I involves development of anewmetatarsal I.4. Secondary functional modifications ensure that the atavism is integrated with the other components of the functional unit, as illustrated by extra digits in horses or guinea pigs and fibulae in birds. Atavistic 2nd and 4th digits in horses arise by continued growth of their primordia. A consequent reduction in the growth rate of digit 3, the normal single functional digit, enables all three digits to attain approximately equal lengths and so potentially to function. The altered functional load transmitted to the limbs results in secondary but correlated alterations in muscles and skeletal elements in other portions of the limbs. The fact that embryonic digit 2 normally develops to a more advanced state than digit 4 explains why digit 2 more often develops atavistically, for if variation in growth rate is the basis for the atavistic digit, digit 2 has an advantage over digit 4.5. Atavisms should not be an embarrassment to the evolutionary biologist. They are the outward and visible sign of a hidden potential for morphology change possessed by all organisms. Neither basic capacity to form the organ nor patterning information is lost. Modification of components of inductive tissue interactions helps to explain how organs are lost during evolution (also see Regal, 1977); retention of the basic mechanism explains how structures can be revived as atavisms (also see Rachootin&Thomson, 1981). Frequency of atavisms thus provides an indication of the degree of modification or loss of the underlying developmental programm

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00402.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00403.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

SummaryThe mammalian sperm acrosome reaction is a unique form of exocytosis, which includes the loss of the involved membranes. Other laboratories have suggested the involvement of hydrolytic enzymes in somatic cell exocytosis and membrane fusion, and in the invertebrate sperm acrosome reaction, but there is no general agreement on such an involvement. Although reference was made to such work in this review, the focus of the review was on the evidence (summarized below) that supports or fails to support the importance of certain hydrolytic enzymes to the mammalian sperm acrosome reaction. Because the events of capacitation, the prerequisite for the mammalian acrosome reaction, and of the acrosome reaction itself are not fully understood or identified, it is not yet always possible to determine whether the role of a particular enzyme is in a very late step of capacitation or part of the acrosome reaction.(1) The results of studies utilizing inhibitors of trypsin‐like enzymes suggest that such an enzyme has a role in the membrane events of the golden hamster sperm acrosome reaction. The enzyme involved may be acrosin, but it is possible that some as yet unidentified trypsin‐like enzyme on the sperm surface may play a role in addition to or instead of acrosin. Results obtained by others with guinea pig, ram and mouse spermatozoa suggest that a trypsin‐like enzyme is not involved in the membrane events of the acrosome reaction, but only in the loss of acrosomal matrix. Such results, which conflict with those of the hamster study, may have been due to species differences or the presence of fusion‐promoting phospholipase‐A or lipids contaminating the incubation media components, and in one case to the possibly damaging effects of the high level of calcium ionophore used. The role of the trypsin‐like enzyme in the membrane events of the hamster sperm acrosome reaction may be to activate a putative prophospholipase and/or to hydrolyse an outer acrosomal or plasma membrane protein, thus promoting fusion. A possible role of the enzyme in the vesiculation step rather than the fusion step of the acrosome reaction cannot be ruled out at present.(2) Experiments utilizing inhibitors of phospholipase‐A2, as well as the fusogenic lysophospholipid and cis‐unsaturated fatty acid hydrolysis products that would result from such enzyme activity, suggests that a sperm phospholipase‐A2is involved in the golden hamster sperm acrosome reaction. Inhibitor and LPC addition studies in guinea pig spermatozoa have led others to the same conclusion. The fact that partially purified serum albumin is important in so many capacitation media may be explained by its contamination with phospholipase‐A and/or phospholipids. Serum albumin may also play a role, at least in part, by its removal of inhibitory products released by the action of phospholipase‐A2in the membrane. The demonstration of phospholipase‐A2activity associated with the acrosome reaction vesicles and/or the soluble component of the acrosome of hamster spermatozoa, and the fact that exogenous phospholipase A2can stimulate acrosome reactions in hamster and guinea pig spermatozoa, also support a role for the sperm enzyme. The actual site or the sites of the enzyme in the sperm head are not yet known. The enzyme may be on the plasma membrane as well as, or instead of, in the acrosomal membranes or matrix. A substrate for the phospholipase may be phosphatidylcholine produced by phospholipid methylation. It is possible that more than one type of ‘fusogen’ is released by phospholipase activity (LPC and/or cis‐unsaturated fatty acids, which have different roles in membrane fusion and/or vesiculation.In addition to acting as a potential ‘fusogen’, arachidonic acid released by sperm phospholipase‐A2probably serves as precursor for cyclo‐oxygenase or lipoxygenase pathway metabolites, such as prostaglandins and HETES, which might also play a role in the acrosome reaction. Although much evidence points to a role for phospholipase‐A2, phospholipase‐C found in spermatozoa could also have a role in the acrosome reaction, perhaps by stimulating events leading to calcium gating, as suggested for this enzyme in somatic secretory cells.(3) A Mg2+‐ATPase H+‐pump is present in the acrosome of the golden hamster spermatozoon. Inhibition of this pump by certain inhibitors of ATPases (but not by those that only inhibit mitochondrial function) leads to an acrosome reaction only in capacitated spermatozoa and only in the presence of external K+. The enzyme is also inhibited by low levels of calcium, and such inhibition, combined with increased outer membrane permeability to H+and K+, and possibly plasma membrane permeability to H+(perhaps by the formation of channels), may be part of capacitation and/or the acrosome reaction. The pH of the hamster sperm acrosome has been shown to become more alkaline during capacitation, and such a change may result in the activation of hydrolytic enzymes in the acrosome or perhaps in a change in membrane permeability to Ca2+. A similar Mg2+‐ATPase has not been found in isolated boar sperm head membranes. However, that conflicting result could have been due to the use of noncapacitated boar spermatozoa for the preparation of the membranes or to protease modification of the boar sperm enzyme during assay.(4) Inhibition of Na+, K+‐ATPase inhibits the acrosome reaction of golden hamster spermatozoa, and the activity of this enzyme increases relatively early during capacitation. A late influx of K+is important for the acrosome reaction. However, this late influx may not be due to Na+, K+‐ATPase, but instead may be due to a K+permeability increase (possibly via newly formed channels) in the membranes during capacitation. It is suggested in this review that Na+, K+‐ATPase has a role early in capacitation rather than directly in the acrosome reaction (although such a role cannot yet be completely ruled out). One possible role for the enzyme in capacitation might be to stimulate glycolysis (which appears to be essential for capacitation and/or the acrosome reaction of hamster and mouse spermatozoa). The function of the influx of K+just before the acrosome reaction is probably to stimulate, directly or indirectly, the H+‐efflux required for the increase in intraacrosomal pH occurring during capacitation. Direct stimulation of the acrosome reaction by a change in membrane potential resulting directly from K+‐influx is not a likely explanation for the hamster results. However, the importance of an earlier membrane potential change, due to increased Na+, K+‐ATPase during capacitation, and/or of later membrane potential changes resulting from the pH change, cannot be ruled out. Although K+is required for the hamster acrosome reaction, other workers have reported that K+ inhibits guinea pig sperm capacitation. However, the experimental procedures used in the guinea pig sperm studies raise some questions about the interpretation of those inhibition results.(5) Ca2+‐influx is known to be required for the acrosome reaction. Others have suggested that increased Ca2+‐influx due to inhibition or stimulation of sperm membrane calcium transport ATPases are involved in the acrosome reaction. There is as yet no direct or indirect biochemical evidence that inhibition or stimulation of such enzymatic activity is involved in the acrosome reaction, and further studies are needed on those questions.(6) I suggest that the hydrolytic enzymes important to the hamster sperm acrosome reaction will also prove imp

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00404.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00405.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY