Deaths in a desert baboon troop

Over a 4-year period, three of 22 infants born in a desert-dwelling chacma baboon troop (Papio ursinus) survived to more than 6 months of age. Tick infestation and infant kidnapping by adult females were the main causes of infant death. The high neonatal mortality resulted in a rapid birth rate through a decrease in duration of interbirth intervals for mature females. There were more adult male deaths in the troop than adult female deaths. Males died from canine inflicted wounds sustained during fights.     

Geographical distribution of preconceptional radiation doses to fathers employed at the Sellafield nuclear installation,West Cumbria.

OBJECTIVE--To examine whether the geographical distribution of births associated with preconceptional exposure of fathers to radiation at the Sellafield nuclear installation is consistent with the suggestion that this exposure explains the excess of childhood lymphoid malignancy in the adjacent village of Seascale. DESIGN--Retrospective birth cohort study. SETTING--Cumbria, West Cumbria health district, and Seascale civil parish. SUBJECTS--The 10,363 children born in Cumbria during 1950-89 to fathers employed at Sellafield. MAIN OUTCOME MEASURES--The doses of external whole body ionising radiation received by fathers at Sellafield in the total time and in the six months before conception of their children; the proportions of the collective doses associated with Seascale and the rest of West Cumbria. RESULTS--9256 children were born to fathers who had been exposed to radiation before the child''s conception. Of these, 7318 had fathers who were exposed in the six months before conception. Overall 7% (38 person-Sv) of the collective total preconceptional dose and 7% (3 person-Sv) of the collective dose for the six months before conception were associated with children born in Seascale. Of all the children whose fathers worked at Sellafield, 842 (8%) were born in Seascale. The mean individual doses before conception were consistently lower in Seascale than in the rest of West Cumbria. CONCLUSIONS--The distribution of the paternal preconceptional radiation dose is statistically incompatible with this exposure providing a causal explanation for the cluster of childhood leukaemias in Seascale.     

The effect of divorce on infant mortality in a remote area of Bangladesh

The process of divorce is usually lengthy and hazardous, and can start quarrels that can lead to the abuse of women and their children. This study examines the effects of divorce on neonatal and postneonatal mortality of babies born before and after divorce in Teknaf, a remote area of Bangladesh. The longitudinal demographic surveillance system (DSS) followed 1,762 Muslim marriages in 1982-83 for 5 years to record divorce, deaths of spouse, emigration and births. It recorded 2,696 live births during the follow-up period, and their survival status during infancy. Logistic regression models were used to estimate the effect of divorce on neonatal and postneonatal mortality, controlling for maternal age at birth, parity, sex of the child and household economic status. The odds of neonatal and postneonatal deaths among babies born after divorce or less than 12 months before mothers were divorced were more than double the odds of those born to mothers of intact marriages. The odds of postneonatal deaths were two times higher among babies born more than 12 months before divorce happens than their peers. The high mortality of infants born before and after mothers were divorced may reflect how abusive marriage and divorce increase the vulnerability of women and children in rural Bangladesh. Divorce and abuse of women are difficult and intractable social and health problems that must be addressed.     

Variation of sex ratio and ABO blood group composition by month of birth of blood donors in Tokyo

Sex ratio of 17,273 blood donors born during the period between 1925 and 1935 was examined according to their month of birth and ABO blood groups in comparison with 5,810 healthy non-blood donors born in the 1900s to 1930s. The sex ratio of the blood donors and the non-blood donors varied similarly according to their month of birth with a prominent peak in summer births and a trough in winter births. This birth season with a high sex ratio was different from that of the general births during the period between 1921 and 1935, in which a maximum sex ratio was found in November. A possible explanation for the difference is the different rate of male and female infant deaths according to birth month. Variation of the sex ratio according to season of birth was not similar among the four ABO blood groups. Sex ratio of the donors with blood group B showed no elevation among the summer births. Non-blood donors with blood group B, on the contrary, showed a higher sex ratio than the others in the summer births. This difference can not be explained by infant or juvenile deaths. A possibility is that a tendency to become a blood donor is modified by the season of one's birth differently according to gender and ABO blood groups.     

Follow up study of children born elsewhere but attending schools in Seascale,West Cumbria (schools cohort).

Records on 1546 children who were identified as having attended schools in Seascale up to November 1984 and were born since 1950 but not in the civil parish were studied. These children lived in or near Seascale for a period of time while they were attending one or more of three local schools and are an additional group to the 1068 children who were identified as born to mothers resident in Seascale in an accompanying study. Even though some of the schoolchildren apparently remained in the village for a short period only all but 7% were followed up through the National Health Service Central Register. Mortality among these children to 30 June 1986 is comparable to that expected at national rates. From all causes there were 10 observed deaths compared with 12.69 expected--a ratio of 0.79 (95% confidence interval 0.38 to 1.45)--and from cancer one observed death compared with 2.04 expected--a ratio of 0.49 (95% CI 0.01 to 2.73). No deaths from leukaemia or lymphoma were reported, but only 0.83 was expected. Since 1971 (the year when cases of cancer were first notified to the NHS Central Register) three non-fatal cases of cancer were reported, including two lymphomas, compared with 2.04 expected and two cases of carcinoma in situ of the cervix compared with 1.79 expected. In addition, there was a case of leukaemia among the schoolchildren which was known previously and had been diagnosed in 1968. There is an interesting difference between the results of this study and the results of the study of children born to mothers who were resident in Seascale. In the latter study there was an excess of leukaemia and of other cancers, but a similar finding is not apparent among children who spent some time at schools in Seascale but were born elsewhere. This raises the question of whether one or more aetiological factors in childhood cancer were acting on a locality specific basis before birth or early in life. This cannot be answered from these cohort studies, but it is hoped that the case-control study that is under way in West Cumbria will provide relevant information.     

Relative risk of early and late death in infancy among offspring of non-native-born parents residing in Washington State.

It has been suggested that geographic variation in the early neonatal (0-6 days) and postperinatal (7-364 days) mortality rates for the United States derives from positive and negative associations, respectively, with the probability of an average couple sharing one or more HLA antigens. If this theory is correct, then the ratio of 0-6-day deaths to 7-364-day deaths among the offspring of migrant parents should be similar to that seen in the area from which the mother and the father originally migrated. Information on deaths among the offspring of migrant couples by the mother's region of birth was obtained from computerized vital statistics for Washington state for the period 1968-1977. Information on infant deaths occurring in the mother's region of birth was obtained from published vital statistics for the United States for 1980-1984. Ratios of 0-6-day deaths to 7-364-day deaths in 9 geographic regions for the period 1980-1984 were placed in 4 intervals. These intervals became the standard against which the 0-6/7-364-day ratios in the offspring of migrant parents were compared by means of a test for trend in proportions. As predicted, a significant positive trend was observed when the mother and the father were born in the same region but not when they were born in different regions.     

Temporal trends in incidence of childhood cancer in Switzerland, 1985–2014

BackgroundIncidence of childhood cancer increased in most countries worldwide, but reasons are unclear. This study investigates trends of childhood cancer incidence in Switzerland from 1985 to 2014.MethodsWe extracted data on all childhood cancer cases diagnosed at ages 0–14 years in Switzerland from the Swiss Childhood Cancer Registry. We included ICCC-3 main groups I-XII and calculated age-standardised, cumulative, and age-specific incidence for different diagnostic groups. We analysed trends of annual age-standardised incidence using JoinPoint regression models.ResultsOver the study period from 1985 to 2014, 5104 of 5486 cancer diagnoses (93%) were microscopically verified. The proportion of children treated in paediatric cancer centres increased from 84% during 1985–1994 to 93% in 1995–2004 and 98% in 2005–2014 (p < 0.001). Using the World standard population, age-standardised incidence was 143 in 1985–1994, 154 in 1995–2004, and 162 per million in 2005–2014. Incidence increased by 0.7% (95% confidence interval (CI) 0.5; 1.0) per year for all cancers from 1985 to 2014, 0.8% (95% CI 0.2%–1.4%) for leukaemias over the same period, 3.0% (95% CI 0.2%–1.4%) for CNS tumours during 1985–2002, and 3.8% (95% CI 1.7%–6.0%) for epithelial neoplasms and melanomas over the period 1985–2014.ConclusionTrends in incidence were driven mostly by increases among leukaemias and CNS tumours. For CNS tumours, observed trends may be explained at least partially by diagnostic changes and improved registration. For leukaemias, rising incidence may be real and due to risk factors that experience similar increases in trends.     

Birth order and health status in a British national sample.

Researchers analyzed data from the National Child Development Study--a cohort of every child born in England, Scotland, and Wales during the 1st week of March 1953 with follow up studies in 1965, 1969, 1973, and 1980-1981 to examine the relationship between health status and birth order and whether children with low birth orders were less likely to experience illness than those with older siblings. 1st born children tended to have received the needed number of immunizations, but children of higher birth order did not tend to have received them. Further they were more likely to have attended infant welfare and toddler clinics for health care than children of higher birth order. The only childhood contagious disease which demonstrated a social class effect was pertussis. It tended to afflict children from nonmanual homes regardless of birth order. Absences from school lasting between 1 week-1 month of 1st born children were less frequent than for other children. The leading reasons for 1st, 3rd, and later born 11 year old children who experienced such long absences included infectious diseases; bronchitis; ear, nose, and throat complaints; pneumonia; tonsillitis, or viral influenza. After age 15, 1st and 2nd born children were less likely to be absent and, if absent, they tended to only miss 1 week of school. Significantly more 3rd and 4th born children were absent from school for 1 week-3 months. 1st and 2nd born children from more affluent families tended to have early childhood asthma. In conclusion, the health experiences of the later birth orders were different than those of the 1st born. This did not mean, however, that later birth order children were in poorer health than 1st born children.     

Growing up in wartime: Evidence from the era of two world wars

We document the association between war-related shocks in childhood and adult outcomes for Europeans born during the first half of the twentieth century. Using a variety of data, at both the macro- and the micro-level, we address the following questions: What are the patterns of mortality among Europeans born during this period? Do war-related shocks in childhood and adolescence help predict adult health, human capital and wellbeing of the survivors? Are there differences by sex, socio-economic status in childhood, and age when the shocks occurred? At the macro-level, we show that the secular trend towards lower mortality was interrupted by dramatic increases in mortality during World War I, the Spanish Flu, the Spanish Civil War, and World War II, and we quantify the size of these mortality shocks. Different patterns characterize these high-mortality episodes, with substantial variation by country, sex and age group. At the micro-level, we show that war-related hardship in childhood or adolescence, in particular exposure to war events and experience of hunger, is associated with worse physical and mental health, education, cognitive ability and subjective wellbeing at older ages. The strength of the association differs by sex and type of hardship, with war exposure being more important for females and experience of hunger for males. We also show that hardships matter more if experienced in childhood, and have stronger consequences if they last longer.     

Follow up of New Zealand participants in British atmospheric nuclear weapons tests in the Pacific.

OBJECTIVE--To study the health of Royal New Zealand Navy personnel who participated in atmospheric nuclear weapons tests conducted by the United Kingdom at Malden Island and Christmas Island in 1957 and 1958. DESIGN--Blinded, controlled follow up of up to 30 years. SETTING--New Zealand. SUBJECTS--528 Men known to have participated in the tests and a control group of 1504 men who were in the Royal New Zealand Navy during the same period but did not participate in the tests. MAIN OUTCOME MEASURES--Mortality and incidence of cancer. RESULTS--Follow up for the period 1957-87 was 94% complete in test participants and 91% complete in the controls. There were 70 deaths among test participants and 179 deaths among controls, yielding a relative risk of 1.08 (90% confidence interval 0.85 to 1.38, p = 0.29). The relative risk of death from causes other than cancer was 0.96 (0.71 to 1.29, p = 0.59) whereas the relative risk of death from cancer was 1.38 (0.90 to 2.10, p = 0.09) and of the incidence of cancer was 1.12 (0.78 to 1.60, p = 0.29). For cancers other than haematological malignancies the relative risk was 1.14 (0.69 to 1.83, p = 0.31) for mortality and 1.01 (0.67 to 1.50, p = 0.48) for incidence. There were seven deaths from haematological cancers among test participants (relative risk 3.25, 90% confidence interval 1.12 to 9.64, p = 0.02), including four leukaemias (5.58, 1.04 to 41.6, p = 0.03). The relative risk for incidence of haematological cancers was 1.94 (0.74 to 4.84, p = 0.10) and that for leukaemia was 5.51 (1.03 to 41.1, p = 0.03). There were no cases of multiple myeloma in the test participants during the follow up period, but the expected number was only 0.3. CONCLUSIONS--Although the numbers are small, the findings for leukaemia are similar to those for British participants in the nuclear weapons test programme. Some leukaemias, and possibly some other haematological cancers, may have resulted from participation in this programme. There is little evidence of an increased risk for non-haematological cancers, and there is no evidence of an increased risk for causes of death other than cancer.     

Paternal radiation exposure and leukaemia in offspring: the Ontario case-control study.

OBJECTIVES--To test the hypothesis that there is an association between childhood leukaemia and the occupational exposure of fathers to ionising radiation before a child''s conception. DESIGN--Case-control study with eight matched controls per case. SETTING--Regions of Ontario, Canada, with an operating nuclear facility. SUBJECTS--Cases were children (age 0-14) who died from or were diagnosed as having leukaemia from 1950 to 1988 and were born to mothers living in the vicinity of an operating nuclear facility. Controls were identified from birth certificates, matched by date of birth and residence at birth. There were 112 cases and 890 controls. MAIN OUTCOME MEASURES--Paternal radiation exposure was determined by a record linkage to the Canadian National Dose Registry. RESULTS--Six fathers of cases and 53 fathers of controls had had a total whole body dose > 0.0 mSv before the child''s conception, resulting in an odds ratio of 0.87 (95% confidence interval 0.32 to 2.34). There was no evidence of an increased leukaemia risk in relation to any exposure period (lifetime or six months or three months before conception) or exposure type (total whole body dose, external whole body dose, or tritium dose), except for radon exposure to uranium miners, which had a large odds ratio that was not significantly different from the null value. CONCLUSIONS--The findings of this study in Ontario did not support the hypothesis that childhood leukaemia is associated with the occupational exposure of fathers to ionising radiation before the child''s conception.     

Immunizations against infectious diseases and childhood cancers

Summary A study based upon an unusually large series of childhood cancers and matched controls found a significant deficit of case/control pairs in which the cancer case had fewer immunizations against infectious diseases than the matched control. All types of immunizations and cancers were affected but the case/control differences were more pronounced for older cases with late immunizations than for younger cases with early immunizations, and more pronounced for solid tumours than leukaemia. Therefore there may be immune system responses to immunizations (or simulated infections) which make it difficult for small clones of cancer cells to enlarge and are more successful in preventing localised tumours in adolescents than childhood leukaemias.     

Leukaemia and smoking habits among United States veterans.

The relation between leukaemia and smoking habits was examined in data from the veterans'' smoking study, a prospective study of mortality among 248,000 United States veterans, of whom 723 died of leukaemia during 1954-69. A significant increase in mortality from leukaemia among cigarette smokers (relative risk 1.53) was found, together with a dose-response relation with amount smoked (trend p less than 0.001). The relation was strongest (relative risk 1.72) for monocytic and chronic and unspecified myeloid leukaemias (ICD (7th revision) codes 204.1 and 204.2). For these leukaemias the increase was almost twofold (relative risk 1.93) among current smokers of over 20 cigarettes daily. Ex-cigarette smokers also showed an increase of leukaemia (relative risk 1.39; p less than 0.001). These findings are consistent with other studies and relevant to the interpretation of minor increases of leukaemia both in population and in individual based studies. If causal they also imply that smoking is responsible for many more deaths from leukaemia in adults than all other known causes combined.     

Prematurity and insulin sensitivity

Premature infants of low and extremely low birth weight represent a challenge for neonatal intensive care units and paediatricians. These neonates may be at increased risk of insulin resistance and diabetes perinatally and during childhood. During the first week of postnatal life, infants born prematurely are at risk of abnormalities in glucose homeostasis. Additionally, there are major differences in their glucose/insulin homeostasis compared with infants born at term. Preterm infants are at risk of hypoglycaemia, due to decreases in deposits of glycogen and fat that occur during the third trimester, and also to transient hyperinsulinaemia. Hyperglycaemia may also be observed in preterm infants during the perinatal period. These infants are unable to suppress glucose production within a large range of glucose and insulin concentrations, insulin secretory response is inappropriate, insulin processing is immature and there is an increased ratio of the glucose transporters Glut-1/Glut-2 in fetal tissues, which limits sensitivity and hepatocyte reaction to increments in glucose/insulin concentration during hyperglycaemia. In addition, increased concentrations of tumour necrosis factor alpha present in intrauterine growth retardation (IUGR) and induce insulin resistance. It has been proposed that the reduced insulin sensitivity may result from adaptation to an adverse in utero environment during a critical period of development. We have investigated postnatal insulin resistance in 60 children born with very low birth weight and either small for gestational age or at an appropriate size for gestational age. This study showed that IUGR, rather than low birth weight itself, was associated with increased fasting insulin levels. As poor fetal growth may be associated with the development of obesity, type 2 diabetes and the metabolic syndrome in later life, it is important that we continue to increase our understanding of the effects of IUGR on postnatal growth and metabolism.     

The relation of small head circumference and thinness at birth to death from cardiovascular disease in adult life.

OBJECTIVE--To determine how fetal growth is related to death from cardiovascular disease in adult life. DESIGN--A follow up study of men born during 1907-24 whose birth weights, head circumferences, and other body measurements were recorded at birth. SETTING--Sheffield, England. SUBJECTS--1586 Men born in the Jessop Hospital. MAIN OUTCOME MEASURE--Death from cardiovascular disease. RESULTS--Standardised mortality ratios for cardiovascular disease fell from 119 in men who weighed 5.5 pounds (2495 g) or less at birth to 74 in men who weighed more than 8.5 pounds (3856 g). The fall was significant for premature cardiovascular deaths up to 65 years of age (chi 2 = 5.0, p = 0.02). Standardised mortality ratios also fell with increasing head circumference (chi 2 = 4.6, p = 0.03) and increasing ponderal index (weight/length3) (chi 2 = 3.8, p = 0.05; for premature deaths chi 2 = 6.0, p = 0.01). They were not related to the duration of gestation. Among men for whom the ratio of placental weight to birth weight was in the highest fifths the standardised mortality ratio was 137. CONCLUSION--These findings show that reduced fetal growth is followed by increased mortality from cardiovascular disease. They suggest that reduction in growth begins early in gestation. They are further evidence that cardiovascular disease originates through programming of the body''s structure, physiology, and metabolism by the environment during fetal life. Maternal nutrition may have an important influence on programming.     

Evidence for Various Types of Synthesis of Human γ Chains of Haemoglobin in Acquired Haematological Disorders

AT first, the γ chain of human haemoglobin seemed to be a constant chemical species¹, but Schroeder et al. showed that two or possibly four of the γ chain loci which control the synthesis of the two types of chains, ^Aγ and ^Bγ, are active in new born infants². The two chains differ only at position 136, where ^Aγ has alanine and ^Gγ has glycine. Thus although they are inseparable by chromatography and electrophoresis, their relative concentrations can be estimated from the ratio of these two amino-acids in the third and last peptide (γ^CB3) produced by hydrolysis of the chains by cyanogen bromide. Schroeder et al. studied the variations in the levels of these two chains during the normal ontogenic development and in several cases of hereditary persistence of foetal haemoglobin (HPHF) and β thalassaemias^3–5. We have studied the γ chains synthesized in different types of acquired anaemia with increased amounts of foetal haemoglobin, of which haemoglobin F constitutes between 1 and 15%. Such anaemias include refractory anaemias with excess myeloblasts (RAEM), acquired sideroblastic idiopathic anaemias (ASIA), idiopathic aplastic anaemias and some varieties of leukaemias, especially juvenile myeloid leukaemias (JML).     

Reproductive biology and postnatal development in the tent-making bat Artibeus watsoni (Chiroptera: Phyllostomidae)

In this study we investigated the reproductive patterns and postnatal development in the tent-making bat Artibeus watsoni . We sampled two populations in the Golfito Wildlife Refuge and Corcovado National Park, south-western Costa Rica, from June 2003 to March 2005. Most females were pregnant during the months of January and June, and most were lactating in March and July, indicating that this species exhibits seasonal bimodal polyoestry, with the first parturition peak occurring in February–March and the second in June–July. Additionally, we observed a postpartum oestrus following the first parturition, but not after the second. Females entered oestrus again in November–December and had a gestation period of c . 3 months. A female-biased sex ratio of neonates was observed during the second parturition period, and young were born at 32 and 56% of their mothers' body mass and length of forearm, respectively. Adult proportions in length of forearm were attained faster than adult proportions in body mass, and sustained flight was only possible after 35 days of age, when pups had achieved 100 and 80% of adult length of forearm and body mass proportions, respectively. Weaning and roosting independence occurred when young were c . 30–40 days old, and young females appeared to remain close to their place of birth, at least for their first mating period, whereas adult males were never recaptured near their birth site. In addition, sexual maturity was reached in as little as 3 months in females born during the first parturition period, whereas females born during the second birth period in June–July seemed to reach maturity at 6 months of age. Our results show that A. watsoni belongs to the faster lane of the slow–fast continuum of life-history variation in bats, which may be attributed primarily to its roosting and feeding ecology.     

Childhood cancer and residential radon exposure – results of a population-based case-control study in Lower Saxony (Germany)

A population-based case-control study on risk factors for childhood malignancies was used to investigate a previously reported association between elevated indoor radon concentrations and childhood cancer, with special regard to leukaemia. The patients were all children suffering from leukaemia and common solid tumours (nephroblastoma, neuroblastoma, rhabdomyosarcoma, central nervous system (CNS) tumours) diagnosed between July 1988 and June 1993 in Lower Saxony (Germany) and aged less than 15 years. Two population-based control groups were matched by age and gender to the leukaemia patients. Long-term (1 year) radon measurements were performed in those homes where the children had been living for at least 1 year, with particular attention being paid to those rooms where they had stayed most of the time. Due to the sequential study design, radon measurements in these rooms could only be done for 36% (82 leukaemias, 82 solid tumours and 209 controls) of the 1038 families initially contacted. Overall mean indoor radon concentrations (27 Bq m^–3) were low compared with the measured levels in other studies. Using a prespecified cutpoint of 70 Bq m^–3, no association with indoor radon concentrations was seen for the leukaemias (odds ratio (OR): 1.30; 95% confidence interval (95% CI): 0.32–5.33); however, the risk estimates were elevated for the solid tumours (OR: 2.61; 95% CI: 0.96–7.13), mainly based on 6 CNS tumours. We did not find any evidence for an association between indoor radon and childhood leukaemia, which is in line with a recently published American case-control study. There is little support for an association with CNS tumours in the literature. Received: 14 December 1998 / Accepted in revised form: 10 June 1999     

Patterns of mortality and causes of death among Rhode Island Jews, 1979–1981

Abstract

Using information provided by institutions handling Jewish deaths, this study identified 735 deaths among Jewish residents of Rhode Island during 1979–81. Official death records then provided data on the characteristics of the deceased and on cause of death, allowing comparisons of Jewish/non‐ Jewish patterns of mortality and cause of death, as well as analysis of differentials among the Jewish decedents, taking account of birthplace and occupation. The findings indicate that relatively fewer Jewish males die at ages below 65, and more at ages 85 and over than is true of total white males. Jewish females exhibit an age‐at‐death pattern more similar to that of all white women. These sex differences characterize cause of death as well. Differences are more pronounced between Jewish and non‐Jewish males than between the female groups. Most noteworthy, Jewish male deaths from diabetes are significantly higher and deaths from respiratory disease significantly lower than among total white men. Differentials in age of death between Jewish native‐born and foreign‐born are largely a function of their differential age composition, and socioeconomic status showed no clear relation to age at death or cause of death.     

The spatial-temporal gradient of naturally occurring motoneuron death reflects the time of prior exit from the cell cycle and position within the lateral motor column

Embryonic lumbar spinal motoneurons (MNs) are characterized by a period of programmed cell death (PCD) that spans several days and occurs in a rostrocaudal gradient. The generation of these MNs also takes place in a temporal-spatial gradient, such that MNs within rostral lumbar segments exit the cell cycle earlier and MNs within progressively caudal regions are born later. In vitro studies have shown that the latest born spinal MNs, presumably through the possession of endogenous "survival properties," are also the last to acquire their trophic dependence. If the birth date and therefore spinal cord location of lumbar spinal MNs influence the spatial-temporal pattern of PCD, then earlier born MNs should die sooner and be located more rostrally than those generated later. Alternatively, if the time at which MNs die during development is unrelated to their prior exit from the cell cycle, those born at various phases should die throughout the period of PCD. We report here that lumbar MNs generated during the earliest part (embryonic day 2-3) of the proliferative period in the developing chick spinal cord tend to die during the earliest stages of the PCD period and that MNs born in successive 12-h intervals die at correspondingly later periods during PCD. Furthermore, the spatial progression of PCD of these subpopulations of MNs occurs in a rostrocaudal gradient. Finally, while MNs do appear to die in a mediolateral gradient during the period of MN PCD, this pattern is only partly accounted for by MNs born in consecutive intervals. These data support the notion that the timing and rostrocaudal location of MNs undergoing PCD reflect their time of exit from the cell cycle.