Niche-induced cell death and epithelial phagocytosis regulate hair follicle stem cell pool.

Tissue homeostasis is achieved through a balance of cell production (growth) and elimination (regression). In contrast to tissue growth, the cells and molecular signals required for tissue regression remain unknown. To investigate physiological tissue regression, we use the mouse hair follicle, which cycles stereotypically between phases of growth and regression while maintaining a pool of stem cells to perpetuate tissue regeneration. Here we show by intravital microscopy in live mice that the regression phase eliminates the majority of the epithelial cells by two distinct mechanisms: terminal differentiation of suprabasal cells and a spatial gradient of apoptosis of basal cells. Furthermore, we demonstrate that basal epithelial cells collectively act as phagocytes to clear dying epithelial neighbours. Through cellular and genetic ablation we show that epithelial cell death is extrinsically induced through transforming growth factor (TGF)-ß activation and mesenchymal crosstalk. Strikingly, our data show that regression acts to reduce the stem cell pool, as inhibition of regression results in excess basal epithelial cells with regenerative abilities. This study identifies the cellular behaviours and molecular mechanisms of regression that counterbalance growth to maintain tissue homeostasis.

Development and dynamics of cell polarity at a glance.

Cells exhibit morphological and molecular asymmetries that are broadly categorized as cell polarity. The cell polarity established in early embryos prefigures the macroscopic anatomical asymmetries characteristic of adult animals. For example, eggs and early embryos have polarized distributions of RNAs and proteins that generate global anterior/posterior and dorsal/ventral axes. The molecular programs that polarize embryos are subsequently reused in multiple contexts. Epithelial cells require apical/basal polarity to establish their barrier function. Migrating cells polarize in the direction of movement, creating distinct leading and trailing structures. Asymmetrically dividing stem cells partition different molecules between themselves and their daughter cells. Cell polarity can develop de novo, be maintained through rounds of cell division and be dynamically remodeled. In this Cell Science at a Glance review and poster, we describe molecular asymmetries that underlie cell polarity in several cellular contexts. We highlight multiple developmental systems that first establish cell/developmental polarity, and then maintain it. Our poster showcases repeated use of the Par, Scribble and Crumbs polarity complexes, which drive the development of cell polarity in many cell types and organisms. We then briefly discuss the diverse and dynamic changes in cell polarity that occur during cell migration, asymmetric cell division and in planar polarized tissues.

A 24-Year-Old Man Presenting with Lung Metastases from a Primary Retroperitoneal Extragonadal Choriocarcinoma.

BACKGROUND Primary retroperitoneal choriocarcinoma is a rare form of extragonadal germ cell tumor that is highly aggressive and responds poorly to chemoradiation. Extragonadal choriocarcinomas are notoriously challenging to diagnose, and have often progressed to advanced disease by the time of diagnosis. The survival rate for extragonadal choriocarcinoma is approximately 30%, which is much lower than that of extragonadal non-seminomatous germ cell tumors (GCT) in general. CASE REPORT A 24-year-old man with no significant past medical history presented with left-sided, pleuritic chest pain and back pain radiating down his left leg, of 1-year duration. Computed tomography (CT) of the chest revealed multiple bilateral pulmonary nodules and a CT of the abdomen and pelvis showed a large heterogeneous soft tissue mass measuring 9.3×8×10.5 cm. A CT-guided core needle biopsy of a lung nodule was performed and the findings were consistent with the diagnosis of metastatic choriocarcinoma. Magnetic resonance imaging (MRI) of the brain was negative for metastatic disease. Tumor markers were significant for a markedly elevated beta human chorionic gonadotropin (B-hCG) of 104 712 mIU/mL. He was diagnosed with a stage IIIC germ cell tumor, further classified as a primary retroperitoneal choriocarcinoma with lung metastasis, and was started on urgent inpatient chemotherapy. CONCLUSIONS Due to the poor outcomes associated with extragonadal choriocarcinoma, it is important to promptly and correctly identify this malignancy in order to initiate treatment in a timely manner. The following case report explores the histopathologic characterization of this malignancy and describes the clinical course and outcomes from treatment for this patient.

A Rare Presentation of Asymptomatic Spontaneous Pneumomediastinum.

Spontaneous pneumomediastinum is a rare condition characterized by interstitial air within the mediastinum without any obvious causative factors. It is most commonly found in young men, and the clinical presentation is typically associated with chest or neck pain and dyspnea. Objective findings can include subcutaneous emphysema of the neck and chest. Asymptomatic cases are exceedingly rare. In this report, we present the case of a 20-year-old woman who presented with acute psychosis and the incidental imaging finding of pneumomediastinum without any associated clinical signs or symptoms.

Spontaneous tumour regression in keratoacanthomas is driven by Wnt/retinoic acid signalling cross-talk.

A fundamental goal in cancer biology is to identify the cells and signalling pathways that are keys to induce tumour regression. Here we use a spontaneously self-regressing tumour, cutaneous keratoacanthoma (KAs), to identify physiological mechanisms that drive tumour regression. By using a mouse model system that recapitulates the behaviour of human KAs, we show that self-regressing tumours shift their balance to a differentiation programme during regression. Furthermore, we demonstrate that developmental programs utilized for skin hair follicle regeneration, such as Wnt, are hijacked to sustain tumour growth and that the retinoic acid (RA) signalling pathway promotes tumour regression by inhibiting Wnt signalling. Finally, we find that RA signalling can induce regression of malignant tumours that do not normally spontaneously regress, such as squamous cell carcinomas. These findings provide new insights into the physiological mechanisms of tumour regression and suggest therapeutic strategies to induce tumour regression.