Correlation of HER2 Protein Level With mRNA Level Quantified by RNAscope in Breast Cancer

Trastuzumab deruxtecan (T-DXd) has been approved by the US Food and Drug Administration (FDA) to treat patients with metastatic HER2-positive and HER2-low breast cancer, and clinical trials are examining its efficacy against early-stage breast cancer. Current HER2 immunohistochemical (IHC) assays are suboptimal in evaluating HER2-low breast cancers and identifying which patients would benefit from T-DXd. HER2 expression in 526 breast cancer tissue microarray (TMA) cores was measured using the FDA-approved PATHWAY and HercepTest IHC assays, and the corresponding RNA levels were evaluated by RNAscope. HER2 protein levels by regression analysis using a quantitative immunofluorescence score against cell line arrays with known HER2 protein levels determined by mass spectrometry were available in 48 of the cores. RNAscope was also performed in 32 metastatic biopsies from 23 patients who were subsequently treated with T-DXd, and the results were correlated with response rate. HER2 RNA levels by RNAscope strongly correlated with HER2 protein levels (P < .0001) and with HER2 IHC H-scores from the PATHWAY and HercepTest assays (P < .0001). However, neither protein levels nor RNA levels significantly differed between cases scored 0, ultralow, and 1+ by PATHWAY and HercepTest. The RNA levels were significantly higher (P = .030) in responders (6.4 ± 8.2 dots/cell, n = 12) than those in nonresponders (2.6 ± 2.2, n = 20) to T-DXd. RNAscope is a simple assay that can be objectively quantified and is a promising alternative to current IHC assays in evaluating HER2 expression in breast cancers, especially HER2-low cases, and may identify patients who would benefit from T-DXd.

Elevated homocysteine levels: What inborn errors of metabolism might we be missing?

Elevated total plasma homocysteine (hyperhomocysteinemia) is a marker of cardiovascular, thrombotic, and neuropsychological disease. It has multiple causes, including the common nutritional vitamin B12 or folate deficiency. However, some rare but treatable, inborn errors of metabolism (IEM) characterized by hyperhomocysteinemia can be missed due to variable presentations and the lack of awareness. The aim of this study is to identify undiagnosed IEM in adults with significantly elevated homocysteine using key existing clinical data points, then IEM specific treatment can be offered to improve outcome. We conducted a retrospective study with data mining and chart review of patients with plasma total homocysteine >30 μmol/L over a two-year period. We offer biochemical and genetic testing to patients with significant hyperhomocysteinemia without a clear explanation to diagnose IEM. We identified 22 subjects with significant hyperhomocysteinemia but no clear explanation. Subsequently, we offered genetic testing to seven patients and diagnosed one patient with classic homocystinuria due to cystathionine beta-synthase deficiency. With treatment, she lowered her plasma homocysteine and improved her health. This study stresses the importance of a thorough investigation of hyperhomocysteinemia in adults to identify rare but treatable IEM. We propose a metabolic evaluation algorithm for elevated homocysteine levels.

Revealing molecular architecture of FLT3 internal tandem duplication: Development and clinical validation of a web-based application to generate accurate nomenclature

INTRODUCTION

FLT3 internal tandem duplicate (ITD) is associated with unfavorable prognosis of acute myeloid leukemia; targeted therapy improves clinical outcome. We propose that FLT3-ITD detected by next generation sequencing (NGS) should be reported with the same nomenclature pattern as single nucleotide variants so that the mutation can be better interpreted clinically.

METHODS

A Python-based web application was developed to generate FLT3-ITD nomenclature as recommended by the Human Genome Variation Society (HGVS). Assembled FLT3-ITD sequences from 84 patients and 11 artificially created ITD sequences were used for the validation of this web-based application. Each sequence was inspected manually to confirm that the nomenclature was accurate.

RESULTS

Accurate nomenclatures were generated for 113 of 114 sequencing results and 7 artificial sequences. One assembled sequence and four artificial sequences were not named accurately; warning statements were automatically generated to alert further inspection. Of the 105 unique FLT3-ITDs, the ITD lengths range from 18 to 300 bp. Depending whether the ITD involves intron or extends into exon 15, three patterns were recognized. Only 44 (42%) ITDs were pure duplications, and three types of variants were identified at the 5' of ITD. When ITD involves intronic sequence, the protein may comprise inserted amino acids encoded by the intron, due to disrupted RNA splicing.

CONCLUSION

The web application generates accurate FLT3-ITD nomenclature from NGS results except in rare situations. The HGVS nomenclatures provide information on the molecular architecture of FLT3-ITDs and reveal details of complex insertions with partial duplications.

Concussion management application for amateur sports

Concussion management has become one of the most popular topics in sports medicine. Significant resources are being invested in developing protocols for professional sport associations such as the NFL and FIFA. These protocols are often expensive and require substantial resources to implement. The problem, however, runs much deeper than just professional sports. Currently there exists little infrastructure to effectively manage concussion in amateur settings such as high school, club and university sport. A more holistic approach is required to ensure that the same standard of concussion management is being implemented across the board, regardless of the available medical and financial resources. An application was developed that will allow for easily accessible baseline testing and access to a player's concussion history from anywhere in the world. The application will be used to monitor players from the day they start playing sport until they potentially become professional sport players.

Tensioning of Prolene reduces creep under cyclical load: relevance to a simple pre-operative manoeuvre

A longitudinal stretch or 'pre-tensioning' is a method employed by some surgeons to improve the handling characteristics of a suture. We used a tensile tester to assess the effect of pre-tensioning on the mechanical properties of two suture materials (3-0 Prolene and 3-0 Ethibond) commonly used for flexor tendon repair. A cyclical loading programme was used to simulate an early rehabilitation regime. All sutures were subsequently tested to failure (for ultimate tensile strength). The pre-tensioned Prolene sutures showed significantly less creep after cyclical loading in comparison to controls. Conversely pre-tensioning had no measurable effect on the deformation of Ethibond by creep. There was no effect on ultimate tensile strength for either material. The propensity of Prolene to creep (and thereby form a 'gap' in tendon repairs) can be reduced by pre-tensioning.

The hidden blood loss after hip fracture

INTRODUCTION

Despite advances in surgical and anaesthetic techniques the mortality after hip fracture has not significantly changed in the last 40 years. Pre-operative anaemia is a risk factor for peri-operative death. We speculate that a significant proportion of the blood loss related to hip fractures has occurred prior to surgery. Identifying patients at risk of pre-operative anaemia can facilitate appropriate medical optimisation. This study is unique in its attempt to quantify the blood loss associated with the initial hip injury.

METHODS

In a retrospective study all patients with both a diagnosis of hip fracture and an operative delay of >48 h were assessed. The information collected included: fracture classification, serial haemoglobins and patient co-morbidities. The exclusion criteria included a pre-injury diagnosis of anaemia, anti-coagulation and gastrointestinal bleeds.

RESULTS

Between 2007/2008 sixty-eight intracapsular and fifty extracapsular hip fracture patients had serial haemoglobins and operative delays of >48 h (mean 75 h, range 48-270 h). The mean lowest recorded haemoglobin prior to surgery for both extracapsular and intracapsular fractures were 95.0 g/L (+/-SEM 2.2) and 108.5 g/L (+/-SEM 2.2) respectively. This difference was statistically significant (Student's t-test p<0.05). The mean haemoglobin drop in the extracapsular and intracapsular fracture groups was 20.2 g/L (range 0-49 g/L) and 14.9 g/L (range 0-59 g/L) respectively.

CONCLUSIONS

Hip fracture patients have a large drop in haemoglobin that is associated with the initial trauma rather than the operation. This highlights the need for anaesthetic and orthopaedic staff to be vigilant to the risk of pre-operative anaemia in this cohort of frail patients even when the initial haemoglobin is apparently normal.

The normal mammary microenvironment suppresses the tumorigenic phenotype of mouse mammary tumor virus-neu-transformed mammary tumor cells

The microenvironment of the mammary gland has been shown to exert a deterministic control over cells from different normal organs during murine mammary gland regeneration in transplantation studies. When mouse mammary tumor virus (MMTV)-neu-induced tumor cells were mixed with normal mammary epithelial cells (MECs) in a dilution series and inoculated into epithelium-free mammary fat pads, they were redirected to non-carcinogenic cell fates by interaction with untransformed MECs during regenerative growth. In the presence of non-transformed MECs (50:1), tumor cells interacted with MECs to generate functional chimeric outgrowths. When injected alone, tumor cells invariably produced tumors. Here, the normal microenvironment redirects MMTV-neu-transformed tumorigenic cells to participate in the regeneration of a normal, functional mammary gland. In addition, the redirected tumor cells show the capacity to differentiate into normal mammary cell types, including luminal, myoepithelial and secretory. The results indicate that signals emanating from a normal mammary microenvironment, comprised of stromal, epithelial and host-mediated signals, combine to suppress the cancer phenotype during glandular regeneration. Clarification of these signals offers improved therapeutic possibilities for the control of mammary cancer growth.

PRODUCTION OF PULMONARY INFARCTS BY THE INSUFFLATION OF ACID

Intrabronchial insufflation of acid causes immediate necrosis of the walls of many alveoli. Thrombosis of the alveolar vessels is an associated phenomenon. When a large number of vessels becomes affected, a clot propagates rapidly into the larger supplying vessels. The resulting lesion is indistinguishable from a hemorrhagic infarct. The infarct-like areas so frequently encountered in influenzal pneumonia, it is not unlikely, have their origin in a similar process. Infarction depends not only upon thrombosis or embolism of the large vessels, but may be initiated by extensive damage to the capillary bed. By this process infarcts may form in organs which are normally protected by collateral circulation.

EFFECT OF INTRABRONCHIAL INSUFFLATION OF ACID

Acid administered to rabbits by intrabronchial insufflation causes an immediate and extreme damage of the lung tissue. Within certain limits the degree and extent of the injury vary according to the concentration of the acid. With the greater concentrations death occurs promptly, almost immediately, and the lethal process has associated with it a decreased permeability of the pulmonary vessels. The latter fact has been confirmed by postmortem arterial injections. With weaker solutions the results, grossly and histologically, resemble those noted after influenzal pneumonia and gas poisoning. Thus, there is destruction of the epithelium of the bronchioles, the alveolar ducts, and the alveoli. The extent of the damage to the alveolar walls varies. Exudation occurs into the alveolar, interstitial, perivascular, and peribronchial tissues. Primarily this exudate is serous, but a rapid deposition of fibrin occurs, and later polymorphonuclear leucocytes and erythrocytes accumulate. In different animals, or in different portions of the same lung, there may be consolidations of different types, serofibrinous, hemorrhagic, or purulent, with or without destruction of the alveolar walls. Subsequent changes relate to the organization of the exudate, necrosis, proliferation of the epithelium in the alveoli and bronchi, and, finally, to the regeneration of the pulmonary parenchyma. Experiments now in progress indicate that similar changes can be induced by various acids, both inorganic and organic. Experiments also show that similar changes are produced in other species of animals but that species differ in their resistance according to the acid and the concentration in which it is employed.

EPITHELIAL PROLIFERATION FOLLOWING THE INTRABRONCHIAL INSUFFLATION OF ACID

The damage caused by the introduction of acid into the pulmonary parenchyma is repaired rapidly. The depth of the necrosis determines which elements will participate in the repair. If epithelium alone is injured, epithelium alone takes part in the repair. When the damage involves the deeper tissue, organization by granulation competes with and impedes the development of the epithelium. An overproduction of the epithelium occurs and may form bronchiolar polypi or extend into the peribronchial tissue.

Spontaneous compartment syndrome in association with simvastatin-induced myositis

Compartment syndrome as a result of simvastatin-induced myositis is extremely rare. This case discusses a patient with spontaneous onset compartment syndrome that necessitated four-compartment fasciotomy. A thorough investigation into its cause highlighted statin-induced myositis as the most likely aetiological agent. When investigating a patient with unexplained spontaneous compartment syndrome it is important to consider drug-induced myositis. This case reinforces the difficulty faced in diagnosing spontaneous compartment syndrome.

Interventions for primary vesicoureteric reflux

BACKGROUND

Vesicoureteric reflux (VUR) results in urine passing, in a retrograde manner, up the ureter. Urinary tract infections (UTIs) have been considered the main cause of permanent renal parenchymal damage in children with reflux. Management of these children has been directed at preventing infection by antibiotic prophylaxis and/or surgical correction of reflux. Controversy remains as to the optimum strategies.

OBJECTIVES

To evaluate the benefits and harms of different treatment options for primary VUR.

SEARCH STRATEGY

Randomised controlled trials (RCTs) were identified from the Cochrane Central Register of Controlled Trials, MEDLINE, EMBASE, reference lists of articles and abstracts from conference proceedings. Date of last search: June 2006

SELECTION CRITERIA

Any treatment of VUR including surgery, antibiotic prophylaxis of any duration, non-invasive techniques and any combination of therapies.

DATA COLLECTION AND ANALYSIS

Two authors independently searched the literature, determined study eligibility, assessed quality, extracted and entered data. For dichotomous outcomes, results were expressed as relative risk (RR) and 95% confidence intervals (CI). Data were pooled using the random effects model.

MAIN RESULTS

Eleven studies (1148 children) were identified. Seven compared correction of VUR (by surgery or endoscope) plus antibiotics for 1-24 months with antibiotics alone, two compared antibiotics with no treatment and two compared different materials for endoscopic correction of VUR. Risk of UTI by 2, 5 and 10 years was not significantly different between surgical and medical groups (2 years RR 1.07, 95% CI 0.32 to 2.09; 5 years RR 0.99, 95% CI 0.79 to 1.26; 10 years RR 1.06, 95% CI 0.78 to 1.44). Combined treatment resulted in a 50% reduction in febrile UTI by 10 years (RR 0.54, 95% CI 0.55 to 0.92) but no concomitant reduction in risk of new or progressive renal damage by 10 years (RR 1.03, 95% CI 0.53 to 2.00). In two small studies no significant differences in risk for UTI (RR 0.75, 95% CI 0.15 to 3.84) or renal damage (RR 1.70, 95% CI 0.36 to 8.07) were found between antibiotic prophylaxis and no treatment.

AUTHORS' CONCLUSIONS

It is uncertain whether the treatment of children with VUR confers clinically important benefit. The additional benefit of surgery over antibiotics alone is small at best. Assuming a UTI rate of 20% for children with VUR on antibiotics for five years, nine reimplantations would be required to prevent one febrile UTI, with no reduction in the number of children developing any UTI or renal damage.

Necrotising myositis: a surgical emergency that may have minimal changes in the skin

Necrotising myositis is a surgical emergency. It is underappreciated that it may present without changes in the skin. Diagnosis is therefore often delayed. We describe a case of necrotising myositis necessitating glenohumeral disarticulation. Remarkable features were the absence of skin signs and the rapidity with which the patient became extremely septic. A review of the literature has shown the importance of early diagnosis and quick decision making to minimise mortality.

Interventions for primary vesicoureteric reflux

BACKGROUND

Vesicoureteric reflux (VUR) results in urine passing, in a retrograde manner, up the ureter. Urinary tract infections (UTIs) have been considered to be the main cause of permanent renal parenchymal damage in children with reflux. Therefore management of these children has been directed at preventing infection by antibiotic prophylaxis and/or surgical correction of reflux. However controversy remains as to the optimum strategies for management of children with primary VUR.

OBJECTIVES

To evaluate the benefits and harms of the different treatment options for primary VUR.

SEARCH STRATEGY

Published and unpublished randomised controlled trials (RCTs) were identified from the Cochrane Central Register of Controlled Trials, MEDLINE, EMBASE, reference lists of articles and abstracts from conference proceedings.

SELECTION CRITERIA

RCTs were included if they compared any treatments of VUR including surgery (open and closed techniques), antibiotic prophylaxis of any duration, non-invasive techniques such as bladder training and any combination of therapies.

DATA COLLECTION AND ANALYSIS

Two reviewers independently searched the literature, determined trial eligibility, assessed quality, extracted and entered data. For dichotomous outcomes, results were expressed as relative risk (RR) and 95% confidence intervals (CI). Data were pooled using the random effects model.

MAIN RESULTS

Ten trials involving 964 evaluable children comparing long-term antibiotics and surgical correction of VUR with antibiotics (seven trials), antibiotics with no treatment (one trial) and different materials for endoscopic correction of VUR (two trials) were identified. Risk of UTI by 1-2 and 5 years was not significantly different between surgical and medical groups (by 2 years RR 1.07, 95% CI 0.55 to 2.09; by 5 years RR 0.99; 95% CI 0.79 to 1.26). Combined treatment resulted in a 60% reduction in febrile UTI by 5 years (RR 0.43, 95% CI 0.27 to 0.70) but no concomitant significant reduction in risk of new or progressive renal damage at 5 years (RR 1.05, 95% CI 0.85 to 1.29). In one small study no significant differences in risk for UTI or renal damage were found between antibiotic prophylaxis and no treatment.

REVIEWERS' CONCLUSIONS

It is uncertain whether the identification and treatment of children with VUR confers clinically important benefit. The additional benefit of surgery over antibiotics alone is small at best. Assuming a UTI rate of 20% for children with VUR on antibiotics for five years, nine reimplantations would be required to prevent one febrile UTI, with no reduction in the number of children developing any UTI or renal damage.

Cylinder gratings in conical incidence with applications to woodpile structures

We use our previous formulation for cylinder gratings in conical incidence to discuss the photonic band gap properties of woodpile structures. We study scattering matrices and Bloch modes of the woodpile, and use these to investigate the dependence of the optical properties on the number of layers. We give data on reflectance, transmittance and absorptance of metallic woodpiles as a function of wavelength and number of layers, using both the measured optical constants of tungsten and using a perfect conductivity idealization to characterize the metal. For semi-infinite metallic woodpiles, we show that polarization of the incident field is important, highlighting the role played by surface effects as opposed to lattice effects.

Plasmapheresis in the treatment of an acute pancreatitis due to protease inhibitor-induced hypertriglyceridemia

The use of protease inhibitors such as ritonavir to treat HIV-infected individuals has been associated with lipodystrophy, combined hyperlipidemias, and hypertriglyceridemia-induced pancreatitis. We report here on the treatment by plasmapheresis of a HIV-patient who presented with a rapid onset of severe ritonavir-induced hypertriglyceridemia complicated with an acute pancreatitis. A 35-year-old HIV-1 positive male following 3 weeks of ritonavir treatment presented with nausea, abdominal pain, a distended abdomen, and the following laboratory values: amylase (238 U/L), lipase (864 U/L), total cholesterol (27.1 mmol/L), and triglycerides (62.9 mmol/L). Following two plasmaphereses, the levels of total cholesterol, triglycerides, lipase, and amylase declined drastically and the patient was discharged home after 4 days with lipid and pancreatic enzyme levels within the reference range. To our knowledge, this is the first case of pancreatitis due to a PI-induced hyperlipidemia in a HIV-patient treated with plasmapheresis in an acute setting.

Cylinder gratings in conical incidence with applications to modes of air-cored photonic crystal fibers

We develop a formulation for cylinder gratings in conical incidence, using a multipole method. The theory, and its numerical implementation, is applied to two-dimensional photonic crystals consisting of a stack of one-dimensional gratings, each characterized by its plane wave scattering matrix. These matrices are used in combination with Bloch's theorem to determine the band structure of the photonic crystal from the solution of an eigenvalue problem. We show that the theory is well adapted to the difficult task of locating the complete band gaps needed to support air-guided modes in microstructured optical fibers, that is, optical fibers in which the confinement of light in a central air hole is achieved by photonic band-gap effects in a periodic cladding comprising a lattice of air holes in a glass matrix.

Cellular expression of estrogen and progesterone receptors in mammary glands: regulation by hormones, development and aging

At present, there is an extensive body of literature documenting the participation of estrogen receptors (ER) and progesterone receptors (PR) in mammary gene expression. Yet, the precise roles of these receptors in regulating mammary development, carcinogenesis and the growth of a subset of tumors still remain unclear. Mammary glands are composed of various cell types with different developmental potentials. Further, ultimately, that it is their mutual interactions which dictate the behavior of mammary epithelial cells. Therefore, to resolve the roles of ER and PR in normal mammary growth, differentiation and carcinogenesis, analyses for the expression of these receptors at the level of individual cell types is of paramount importance. Accordingly, in the present studies using immunolocalization techniques, we document the ontogeny and cellular distribution of ER and PR during mammary development and in response to ovarian hormones and aging. In addition, we discuss the potential biological significances of the expression patterns of ER and PR during various physiological states. We believe that the observations reported here should provide a conceptual framework(s) for elucidating the roles of ER and PR in normal and neoplastic mammary tissues.

Signal transducer and activator of transcription (Stat) 5 controls the proliferation and differentiation of mammary alveolar epithelium

Functional development of mammary epithelium during pregnancy depends on prolactin signaling. However, the underlying molecular and cellular events are not fully understood. We examined the specific contributions of the prolactin receptor (PrlR) and the signal transducers and activators of transcription 5a and 5b (referred to as Stat5) in the formation and differentiation of mammary alveolar epithelium. PrlR- and Stat5-null mammary epithelia were transplanted into wild-type hosts, and pregnancy-mediated development was investigated at a histological and molecular level. Stat5-null mammary epithelium developed ducts but failed to form alveoli, and no milk protein gene expression was observed. In contrast, PrlR-null epithelium formed alveoli-like structures with small open lumina. Electron microscopy revealed undifferentiated features of organelles and a perturbation of cell-cell contacts in PrlR- and Stat5-null epithelia. Expression of NKCC1, an Na-K-Cl cotransporter characteristic for ductal epithelia, and ZO-1, a protein associated with tight junction, were maintained in the alveoli-like structures of PrlR- and Stat5-null epithelia. In contrast, the Na-Pi cotransporter Npt2b, and the gap junction component connexin 32, usually expressed in secretory epithelia, were undetectable in PrlR- and Stat5-null mice. These data demonstrate that signaling via the PrlR and Stat5 is critical for the proliferation and differentiation of mammary alveoli during pregnancy.

Evidence for the transforming activity of a truncated Int6 gene, in vitro

Int6/eIF3-p48 was first identified as a common integration site for MMTV in mouse mammary tumors. In all cases, the MMTV integration event resulted in an interruption of the normal Int6 transcript from one allele leaving the second allele intact and operative. We hypothesize that insertion of MMTV into Int6 results in a mutated allele that encodes a shortened Int6 mRNA and protein (Int6sh), which either modifies normal Int6 function or possesses a new independent function. To confirm the transforming potential of the mutation and its dominant function, we transfected two mammary epithelial cell lines, MCF10A (human), and HC11 (mouse), with Int6sh under the control of the elongation factor-1alpha (eEF1A) promoter. Expression of Int6sh in MCF10A and HC11 mammary epithelial cells leads to anchorage-independent growth in soft agar indicative of a transformed phenotype. Colonies selected from agar exhibited high levels of mutated Int6sh and wild type Int6 RNA transcripts by RT-PCR and Northern blot analysis. In addition, Int6sh transformed MCF10A and HC11 cells formed nodular growths, in vivo, in immune compromised hosts. NIH3T3 cells, mouse embryo fibroblasts, were also transformed to anchorage-independent growth in vitro by Int6sh expression. These observations provide direct evidence that the Int6 mutations observed in MMTV-induced tumors and hyperplasia contribute to the malignant transformation of the mammary epithelial cells.

HOSE: an objective scoring system for evaluating the results of hypospadias surgery

OBJECTIVE

To determine the accuracy and utility of a scoring system designed to allow an objective appraisal of the outcome of hypospadias repair, based on evaluating meatal location, meatal shape, urinary stream, straightness of erection, and the presence and complexity of any complicating urethral fistula.

PATIENTS AND METHODS

Twenty patients (median age 23 months) were randomly selected and reviewed at a median of 8.9 months after their hypospadias repair. Two paediatric surgeons, a nurse and one of the child's parents independently assessed each patient using the "hypospadias objective scoring evaluation" (HOSE) system. The results were collated and the level of interobserver variation assessed using the weighted kappa test.

RESULTS

The mean weighted kappa was 0.66, indicating good agreement among observers. The level of agreement was highest between surgeon and nurse at 0.70, but remained good between surgeon and parent, at 0.65.

CONCLUSIONS

Interobserver variation using the HOSE system was minimal, supporting its use as an objective outcome measure after hypospadias surgery, and facilitating an impartial evaluation of operations used in correcting hypospadias.

Impairment of mammary lobular development induced by expression of TGFbeta1 under the control of WAP promoter does not suppress tumorigenesis in MMTV-infected transgenic mice

It has previously been shown that transgenic female mice expressing TGFbeta1 under control of regulatory elements of the whey-acidic protein (WAP) gene were unable to lactate. This was due to the increased apoptosis of the cells committed to the lobular-lactogenic phenotype. Our goal was to determine whether the expression of WAP-TGFbeta1 transgene could inhibit MMTV (mouse mammary tumor virus) tumorigenic activity in the mammary gland. It is well known that the infection with this virus produces focal hyperplastic secretory nodules (HANs) and, some variants can also induce ductal pregnancy-dependent lesions (plaques). In either case, MMTV infection leads ultimately to the appearance of malignant mammary tumors. The results shown herein demonstrate that TGFbeta1 expression in the secretory mammary epithelium does not suppress mammary tumorigenesis in MMTV infected mice. Although MMTV infected WAP-TGFbeta1 transgenic females displayed a strong impairment of lobule-alveolar development, carcinogenesis induced by any of the four MMTV variants used herein proceeded unabated. WAP-TGFbeta1 tumors that showed a strong expression at the WAP promoter, appeared later and grew more slowly than their wild-type counterparts. Transgenic females also had a lower incidence of HANs and plaques. Our study suggests that the epithelial target cells for tumorigenic mutations are probably progenitor cells that are not susceptible to the apoptotic effect of TGFbeta1. Alternatively, their daughters cells that display the secretory phenotype and could be more involved in the formation of premalignant lesions continue to die due to the expression of the transgene.

Reducing mammary cancer risk through premature stem cell senescence

The reproductive capacity of the mammary epithelial stem cell is reduced coincident with the number of symmetric divisions it must perform. In a study of FVB/N mice with the transgene, WAP-TGFbeta1, we discovered that mammary epithelial stem cells were prematurely aged due to ectopic expression of TGF-beta1. To test whether premature aging of mammary epithelial stem cells would have an impact on susceptibility or resistance to mammary cancer, female littermates from FVB/N x WAP-TGF-beta1 mating were injected with mouse mammary tumor virus (MMTV) at 8-10 weeks of age. A total of 44 females were inoculated, maintained as breeders and observed for tumor development for up to 18 months. Only one mammary tumor appeared in 17 TGF-beta1 females while 15 were collected from 29 wild type sisters. Premalignant mammary epithelial cells in infected glands were identified by transplantation of single cell (1 x 10(5)) suspensions into nulliparous hosts and testing for hyperplastic outgrowth. Although the number of positive takes was significantly reduced with TGF-beta1 cells, both MMTV-infected TGF-beta1 and wild type cells produced hyperplastic outgrowths suggesting that premalignant transformation was achieved in each group. The results suggest a positive correlation between the procreative life-span of mammary epithelial stem cells and mammary cancer risk.

Mammary epithelial stem cells

It has recently been shown that the progeny from a single cell may comprise the epithelial population of a fully developed lactating mammary outgrowth in mice. Serial transplantation of epithelial fragments from this clonally derived gland demonstrates that the subsequently generated outgrowths are also comprised of progeny from the original antecedent. All epithelial cell types were found to be present within these clonal normal populations including luminal, myoepithelial, ductal, and lobule-committed epithelial progenitors and fully competent mammary epithelial stem cells. These observations demonstrate the presence of multipotent tissue-specific epithelial stem cells among the parenchyma of the murine mammary gland. Similarly, genetic analysis of contiguous portions of individual human mammary ducts within the same breast indicates their clonal derivation. Here, we discuss the properties, location, division-potential, senescence, and plasticity associated with mammary epithelial stem cells and present the developing evidence for their presence in human breast and their important role in the risk for breast cancer development. Further, we review the present morphologic and genetic evidence for the characterization of specific stem cell markers and lineage-limited progenitor cells in human and rodent mammary epithelium. Microsc. Res. Tech. 52:190-203, 2001. Published 2001 Wiley-Liss, Inc.

The upper range of lumbar spine bone mineral density? An examination of the current world record holder in the squat lift

Weight-bearing exercise is known to improve bone mineral density, however, excessive forces exerted on the lumbar spine can be pathologic. Cadaveric studies have calculated a hypothetical "critical compression force" at which the lumbar spine would suffer collapse. In addition, recent studies have suggested that bone density correlates with strength. Thus far studies have failed to examine elite power athletes to determine the possible upper range for bone mineral density and critical compression force. Therefore, we recruited the current world record holder in the squat lift, with a record squat lift >469 kg, for an examination of lumbar spine bone mineral density. The subject had dual energy x-ray absorptometry (DEXA) and magnetic resonance imaging (MRI) performed of the lumbar spine. The subject also had serum chemistries, cell blood count and testosterone levels performed. DEXA scan revealed the highest bone mineral density reported to date. MRI revealed normal alignment, no evidence of disc herniation or compressive disc disease. There was no frank or neural foraminal canal stenosis. The estimated compressive force generated on his lumbar spine during the squat lift of > 469 kg doubles the previously reported critical compression force. This case study supports the previously described relationship between strength and bone density and redefines the upper limits of bone density in strength athletes.

Clinical review of the 'Snodgrass' hypospadias repair

BACKGROUND

Snodgrass first described the tubularized, incised plate (TIP) urethroplasty for distal hypospadias repair in 1994 based on the concept of hingeing the incised urethral plate. The use of this technique allows a vertically orientated glanular meatus to be produced. A retrospective review of patients who had this procedure for the primary repair of distal hypospadias at New Children's Hospital was performed to objectively assess the cosmetic and functional outcomes of this technique.

METHODS

All patients having a TIP urethroplasty for the primary repair of distal hypospadias between 1996 and 1998 were requested to attend for review in an independent clinic. The parents were interviewed and the patient was examined to determine meatal location, meatal size, glanular configuration, urinary stream, straightness of erections, cosmesis, and the presence or absence of cutaneous sinus tracts or a urethral fistula.

RESULTS

Sixty patients were identified. Forty-nine were reviewed in person, 10 were reviewed by telephone and one could not be traced. The median age at surgery was 13 months (range: 6-144 months), with a median follow up of 27 months (range: 2-33 months). A glanular meatus was achieved in 57 patients (97%) and a conical glanular configuration was achieved in 58 patients (98%). The urinary stream and erections were straight in 54 (89%) and 58 patients (98%), respectively. Suture sinus tracts were present in 14 (24%) patients. Six patients (10%) developed a urethral fistula and three (5%) developed meatal stenosis. A good or satisfactory final cosmetic and functional result was achieved in 58 patients (98%).

CONCLUSIONS

The results of TIP urethroplasty are satisfactory at New Children's Hospital with the benefit of a cosmetic final outcome similar to a circumcised penis.

Effect of the depth and width of the urethral plate on tubularized incised plate urethroplasty

PURPOSE

We determined the effect of the depth and width of the urethral groove on tubularized incised plate urethroplasty for distal hypospadias.

MATERIALS AND METHODS

We retrospectively reviewed the records of 48 patients who underwent tubularized incised plate urethroplasty for distal hypospadias between September 1996 and December 1998 for whom preoperative evaluation of the depth and width of the urethral groove was available. Patients were examined by an independent clinician a median of 28 months after surgery when the neourethra was calibrated and urinary stream assessed.

RESULTS

Of the 48 patients 46 were available for clinical examination. The urinary stream was straight in 40 boys and angled in 8, while none sprayed. Urethral fistula developed in 6 patients with a urethral plate of less than 8 mm. wide (p = 0.001). The urethral groove was deep in 13 cases, moderate in 20 and shallow in 15. There were no differences among these 3 groups in regard to urinary stream direction or fistula rate. Of the boys with a shallow urethral groove 6 (40%) have a neourethral caliber of 6Fr or less versus 3 (15%) with a moderate and 0 with a deep groove. This difference was statistically significant (p = 0.028). Each patient in whom meatal stenosis developed had a shallow urethral groove.

CONCLUSIONS

Urethral groove depth appears to influence neourethral caliber after tubularized incised plate urethroplasty. A shallow groove predisposes to a narrower neourethra and meatal stenosis subsequently. We observed no evidence that incising the urethral plate increases the final urethral diameter. Urethral fistula after tubularized incised plate urethroplasty was associated with an initially narrow urethral plate.

Neutron-Capture Element Abundances in the Globular Cluster M15

High-resolution, high signal-to-noise ratio, blue-violet spectra of three red giant branch tip stars in M15 have been obtained with the Keck I High-Resolution Echelle Spectrograph. These spectra have been analyzed to determine the abundances of several neutron-capture elements, including the radioactive chronometer element thorium. There are two principal results of this study. First, the abundances of the heavier (Z>/=56) elements for each of the three stars is well matched by a scaled solar system r-process abundance distribution. Second, a weighted mean-observed Th/Eu ratio for the stars implies an age for the neutron-capture material in M15 stars of 14+/-3 Gyr, in reasonable agreement with other recent age estimates for Galactic globular clusters.

Middle cerebral artery blood flow velocity in elite power athletes during maximal weight-lifting

Cerebral blood flow velocity (CBFV) has been shown to significantly increase during dynamic exercise (running) secondary to increases in cardiac output. Static exercise (weight-lifting) induces supraphysiological arterial pressures up to 450/380 mmHg, and thus may alter CBFV. Catastrophic brain injuries such as stroke, cerebral hemorrhage, subarachnoid hemorrhage, retinal hemorrhage and retinal detachment have been associated with weight-lifting. A recent study has shown that intra-ocular pressure (IOP), which is an indirect measure of intracranial pressure, elevates to pathophysiologic levels during weight-lifting. Recent CBFV studies instituting Valsalva have demonstrated decreases in CBFV from 21%-52%. To date, no studies have examined CBFV during maximal weight-lifting to elucidate the cerebrovascular responses to extreme pressure alterations. We recruited nine elite power athletes, including a multi-world record holder in powerlifting, for a transcranial Doppler study of middle cerebral artery blood flow velocity at rest and during maximal weight-lifting. All subjects' resting blood flow velocities were within normal ranges (mean 64.4 +/- 9.5 cm sec2). Blood flow velocities were significantly (p < 0.0001) decreased in all subjects during maximal lifting (mean 48.4 +/- 10.1 cm sec2). Linear regression analysis demonstrated a significant inverse linear relationship in the net change of blood velocities from rest to maximal lift for each subject (r = 0.8585, p < 0.001). This study demonstrates that blood flow velocities are significantly decreased during heavy resistance training. The drop in CBFV during weight-lifting was significantly less than previous Valsalva studies, which likely reveals the cardiovascular, baroreflex, and cerebrovascular system adaptations occurring in these elite power athletes.

Notch signaling is essential for vascular morphogenesis in mice

The Notch gene family encodes large transmembrane receptors that are components of an evolutionarily conserved intercellular signaling mechanism. To assess the role of the Notch4 gene, we generated Notch4-deficient mice by gene targeting. Embryos homozygous for this mutation developed normally, and homozygous mutant adults were viable and fertile. However, the Notch4 mutation displayed genetic interactions with a targeted mutation of the related Notch1 gene. Embryos homozygous for mutations of both the Notch4 and Notch1 genes often displayed a more severe phenotype than Notch1 homozygous mutant embryos. Both Notch1 mutant and Notch1/Notch4 double mutant embryos displayed severe defects in angiogenic vascular remodeling. Analysis of the expression patterns of genes encoding ligands for Notch family receptors indicated that only the Dll4 gene is expressed in a pattern consistent with that expected for a gene encoding a ligand for the Notch1 and Notch4 receptors in the early embryonic vasculature. These results reveal an essential role for the Notch signaling pathway in regulating embryonic vascular morphogenesis and remodeling, and indicate that whereas the Notch4 gene is not essential during embryonic development, the Notch4 and Notch1 genes have partially overlapping roles during embryogenesis in mice.

MMTV-induced mammary tumorigenesis: gene discovery, progression to malignancy and cellular pathways

The study of the mouse mammary tumor virus (MMTV) has provided important insights into the mechanisms of gene transcription regulation by steroid hormones, the mode of action of heritable super antigens and the progressive nature of neoplastic transformation in the mammary gland. Here we describe the current situation with respect to the latter aspect of MMTV biology and the prospects for further advance in our understanding of breast cancer in humans that may be expected from a continued study of MMTV-induced mammary neoplasia. MMTV is a heritable somatic mutagen whose target range is limited. Commonly, the tumorigenic capacity of MMTV is restricted to mammary gland, whereas infection is found in a variety of cell types. In order to replicate, proviral DNA must be inserted into the cell DNA and cell division is required to fix the mutation. Yet only in the mammary epithelium does this lead to neoplastic transformation. This suggests a unique relationship between MMTV and mammary epithelium. In evaluating this relationship, we and others have discovered genes and potential gene pathways that are pertinent in mammary differentiation and neoplasia. In addition, the clonal nature of these progressive events from normal to malignant phenotype has become increasingly clear. The weight of these observations compel us to conclude that mammary neoplasms arise from multipotent mammary epithelial cells through a process of acquired mutations that are reflected in the increasingly malignant nature of the population of progeny produced by these damaged stem cells.

Comments on the Formation of Globular Clusters from Coalesced Clouds

If a substantial fraction of the proto-Galactic halo was constituted of cloudy structures of sizes 1 kpc or larger, then collisions between these clouds would have been common during the infall of the Galaxy. Such collisions would have shaped the properties of the clouds from which globular clusters formed. If Milky Way globular clusters formed from progenitor clouds which in turn had been constructed from the coalescence of smaller cloud structures, then cluster properties that could naturally be accounted for include: (1) the low percentage of stars in globular clusters relative to the halo field, (2) the chemical homogeneity of globular clusters with respect to heavy elements, and (3) the fact that the lowest metallicity globular clusters are not as metal-poor as some halo field stars.

Intra-ocular pressure changes during maximal isometric contraction: does this reflect intra-cranial pressure or retinal venous pressure?

Recent publications have suggested that intra-ocular pressure (IOP) may be an indirect assessment of intra-cranial pressure (ICP). Both IOP and ICP have similar physiologic pressure ranges and similar responses to changes in intra-abdominal, intra-thoracic and aortic pressure. Previous studies have demonstrated the relationships between retinal arterial pressure and aortic pressure, intra-ocular pressure and retinal venous pressure, intra-cranial pressure and retinal venous pressure. Power athletes routinely utilize the Valsalva maneuver during weightlifting. In fact there are reports of stroke, cerebral hemorrhage, subarachnoid hemorrhage, conjunctival, foveal and retinal hemorrhage, retinal detachment, hiatal hernia and pneumothorax associated with weightlifting. These events are thought to occur secondary to the extreme pressure elevations that occur in the intra-abdominal, intra-thoracic, intra-cranial, intra-ocular and vascular compartments. To date no human studies have examined the IOP changes that may occur with heavy resistance exercise. Therefore, we recruited power athletes (n = 11), who had participated in prior studies, from the local metropolitan area. The athletes had blood pressure status, drug screening and medical histories performed during previous investigations. Intra-ocular pressure was measured by noncontact tonometry at rest and during maximal isometric contraction. All subjects resting IOP were within normal ranges (mean 13 +/- 2.8 mmHg). Intra-ocular pressures were significantly (p < 0.0001) elevated in each subject during maximal contraction (mean 28 +/- 9.3 mmHg). One subject's IOP reached 46 mmHg during maximal contraction. Linear regression analysis demonstrated a significant linear relationship (r = 0.62, p < 0.0001) in the net change of IOP from rest to maximal contraction for each subject. This study demonstrates that IOP elevates to pathophysiologic levels during resistance exercise. The findings of conjunctival hemorrhages in two subjects further supports IOP being reflective of retinal venous pressure. The enormous pressures generated by power athletes during weightlifting leads to elevations in ICP which obstruct venous outflow leading to hemorrhage and elevations in IOP. The question remains as to whether these intermittent bursts of elevated IOP can lead to long-term pathological sequelae.

Mammary epithelial stem cells: our current understanding

It has recently been shown that the progeny from a single cell may comprise the epithelial population of a fully developed lactating mammary outgrowth in mice. Serial transplantation of epithelial fragments from this clonally derived gland demonstrates that the subsequently generated outgrowths are also comprised of progeny from the original antecedent. Similarly, genetic analysis of contiguous portions of individual human mammary ducts within the same breast indicates their clonal derivation. These observations support the concept that multipotent tissue-specific epithelial stem cells are present among the parenchymal cells of the mammary gland. Here, we present the developing evidence for the presence of stem cells in virtually every renewing mammalian tissue as well as some classically considered to consist only of differentiated cells. Further, we review the present morphologic and biologic evidence for stem cells and lineage-limited progenitor cells in human and rodent mammary epithelium. Although a number of selective markers are known for various lineage-limited hematopoietic cells and their progeny, our understanding of the biology of the precursor cells for mammary epithelium is just beginning. Our purpose here is to develop further interest in the clarification of these issues in the biology of the mammary gland.

Functional rescue of Stat5a-null mammary tissue through the activation of compensating signals including Stat5b

Prolactin induces mammopoiesis and lactogenesis through the Janus kinase-signal transducers and activators of transcription pathway, with Stat5a being a principal and obligate cytoplasmic and nuclear signaling molecule. Mice from which the Stat5a gene has been deleted fail to develop functional mammary tissue during their first pregnancy. Lobuloalveolar outgrowth is curtailed, and epithelial cells fail to progress to functional differentiation. Here, we investigate whether the effect of Stat5a deficiency is restricted to the epithelium and whether the gland has the capacity to activate alternative signaling pathways that could restore development and function. Mammary gland transplant experiments showed that Stat5a-deficient epithelium does not differentiate in wild-type stroma, thus demonstrating a cell-autonomous role for Stat5a. The capacity of Stat5a-deficient mammary tissue to develop and secrete milk was measured after consecutive pregnancies and with postpartum suckling. Neither of these regimens could independently restore lactation. However, the combination of several pregnancies and suckling stimuli resulted in a partial establishment of lactation and an increase of Stat5b activity. These experiments demonstrate that the mammary gland has inherent plasticity that allows it to use different signals to achieve its ultimate purpose, the production of milk to nurture newborn offspring.

An entire functional mammary gland may comprise the progeny from a single cell

Any epithelial portion of a normal mouse mammary gland can reproduce an entire functional gland when transplanted into an epithelium-free mammary fat pad. Mouse mammary hyperplasias and tumors are clonal dominant populations and probably represent the progeny of a single transformed cell. Our study provides evidence that single multipotent stem cells positioned throughout the mature fully developed mammary gland have the capacity to produce sufficient differentiated progeny to recapitulate an entire functional gland. Our evidence also demonstrates that these stem cells are self-renewing and are found with undiminished capacities in the newly regenerated gland. We have taken advantage of an experimental model where mouse mammary tumor virus infects mammary epithelial cells and inserts a deoxyribonucleic acid copy(ies) of its genome during replication. The insertions occur randomly within the somatic genome. CzechII mice have no endogenous nucleic acid sequence homology with mouse mammary tumor virus; therefore all viral insertions may be detected by Southern analysis provided a sufficient number of cells contain a specific insertional event. Transplantation of random fragments of infected CzechII mammary gland produced clonal-dominant epithelial populations in epithelium-free mammary fat pads. Serial transplantation of pieces of the clonally derived outgrowths produced second generation glands possessing the same viral insertion sites providing evidence for self-renewal of the original stem cell. Limiting dilution studies with cell cultures derived from third generation clonal outgrowths demonstrated that three multipotent but distinct mammary epithelial progenitors were present in clonally derived mammary epithelial populations. Estimation of the potential number of multipotent epithelial cells that may be evolved from an individual mammary-specific stem cell by self-renewal is in the order of 10(12)-10(13). Therefore, one stem cell might easily account for the renewal of mammary epithelium over several transplant generations.

Surgical management of the nonpalpable testis: the Children's Hospital of Philadelphia experience

PURPOSE

The management of the nonpalpable testis permits an individualized operative approach. We analyze the results of surgical management of a large series of patients with a nonpalpable testis.

MATERIALS AND METHODS

Between January 1986 and June 1994 we treated 1,866 boys with undescended testes. There were 447 testes (24%) that were not palpable at presentation. Intra-operative data on these patients were collected for age at presentation, bilateral testes position, testicular size, associated inguinal anomalies (vas, epididymis and processus vaginalis) and surgical approach. For intra-abdominal testes postoperative results of 2 surgical techniques, the Fowler-Stephens procedure and Koop orchiopexy (retroperitoneal mobilization of spermatic vessels and vas) were compared in 76 patients with at least 18 months of followup.

RESULTS

Average patient age at presentation was 34 months with 63% presenting before age 48 months. Of the impalpable testes 58% were on the left side, 35% were on the right side and 7% were bilateral. At operation 181 testes (41%) were atrophic or absent, 91 (20%) were intra-abdominal with 14 (3.1%) bilateral, 136 (30%) were in the inguinal canal and 39 (9%) were in other locations, including 22 at the pubic tubercle, 2 in the upper scrotum, 13 in the superficial inguinal pouch and 2 in the perineum. Of the intra-abdominal group associated extratesticular malformations were identified in 36 cases (39%). Attachment of the vas deferens to the testis was abnormal in 23 of 64 cases (36%), including 10 that were completely detached and 13 with head or tail attachment only. Of the 91 evaluable cases in the intra-abdominal group 38 (42%) had been treated with the Fowler-Stephens repair (5 in 2 stages), 33 (36%) with inguinal orchiopexy and intraperitoneal dissection without dividing the spermatic vessels, 5 with 2-stage procedures and vessel preservation and 14 (15%) with orchiectomy. One testis was left in situ. The inguinal approach with intraperitoneal extension was successful in defining the testis location or blind-ending vas and vessels in 100% of the cases. A single operation to perform orchiopexy was successful in 92% of the cases. Overall, results were considered excellent or acceptable in 32 of 33 cases (97%) after Koop orchiopexy and 28 of 38 (74%) after the Fowler-Stephens orchiopexy.

CONCLUSIONS

Nonpalpable testes accounted for 24% of the patients presenting with undescended testes. At surgical exploration 39% of impalpable testes were distal to the external inguinal ring, 41% were atrophic or absent and 20% were intra-abdominal. All cases were treated through a standard inguinal incision. These data provide evidence that the inguinal approach to orchiopexy with transperitoneal mobilization of the vas and vessels without transection is highly successful for the intra-abdominal cryptorchid testis and, to date, is the preferred technique for the management of the intra-abdominal undescended testis.

Cripto-1 inhibits beta-casein expression in mammary epithelial cells through a p21ras-and phosphatidylinositol 3'-kinase-dependent pathway

Cripto-1 (CR-1) is a recently discovered protein of the epidermal growth factor family that does not directly activate any of the known erbB type 1 tyrosine kinase receptors. Also, CR-1 stimulates the growth of HC-11 mouse mammary epithelial cells. We found that prior treatment of HC-11 cells with exogenous CR-1 induced a competency response to the lactogenic hormones dexamethasone, insulin, and prolactin (DIP) with respect to the induction of the milk protein beta-casein. In contrast, simultaneous treatment of mouse HC-11 cells with CR-1 in the presence of DIP inhibited beta-casein expression. The inhibitory effects of CR-1 on beta-casein expression in response to DIP were not unique to this mouse mammary epithelial cell line, because beta-casein and whey acidic protein expression in primary mouse mammary explant cultures established from midpregnant mice were also differentially inhibited by several epidermal growth factor-related peptides including CR-1. The mitogenic and differentiation effects of CR-1 are mediated by the binding of CR-1 to a cell surface receptor that is known to activate the ras/raf/mitogen-activated protein kinase (MAPK)/MAPK kinase pathway. The inhibitory response of CR-1 in HC-11 cells on beta-casein expression after treatment with DIP can be attenuated by B581, a peptidomimetic farnesyltransferase inhibitor that blocks p21ras farnesylation and activation, and by the phosphatidylinositol 3'-kinase (PI3k) inhibitor LY 294002 but not by PD 98059, a MAPK kinase inhibitor that blocks MAPK activation. These data suggest that the ability of CR-1 to block lactogenic hormone-induced expression of beta-casein is mediated through a p21ras-dependent, PI3k-mediated pathway. This is further substantiated by the observation that CR-1 is able to stimulate the tyrosine phosphorylation of the p85 PI3k regulatory subunit and to increase the activity of PI3k in HC-11 cells.

Three division-competent, structurally-distinct cell populations contribute to murine mammary epithelial renewal

Markers for the division-competent cells in mammary gland epithelium are important to the understanding of normal and neoplastic mammary epithelial growth and architectural maintenance, but have been difficult to reveal. Using the presence of condensed chromosomes as an indicator of division competence, we have described morphological markers in the nucleus and cytoplasm that reliably characterize two sets of structurally-distinct, division-competent cells in murine (i.e. mouse and rat) mammary epithelium. The markers, based, in part, on cell size, nuclear and cytoplasmic staining characteristics, and distinctive morphological features of the nucleus and cytoplasm allow classification of the division-competent cells into two categories: 'large light cells' and 'small light cells'. Based on the degree of cytoplasmic organelle differentiation, the small light cells are the least differentiated cells in the epithelium, and the large light cells appear, structurally and functionally, to be in the early stages of secretory differentiation. We demonstrate, using statistical analysis of cell counts (per unit area of epithelium) from four stages of mammary epithelial growth, that there are, in fact, three division-competent cell populations in the rat mammary epithelium, and that the large light cell is a direct precursor to terminally differentiated cells, both secretory and myoepithelial. Using our results we synthesize a morphological model of cell mitosis and the progression of epithelial differentiation in murine mammary gland from a stem cell through two progenitors.