Fundamentals of cell proliferation: control of the cell cycle

Nucleus pulposus cell senescence is regulated by substrate stiffness and is alleviated by LOX possibly through the integrin β1-p38 MAPK signaling pathway

Intervertebral disc degeneration (IVDD) is a main contributor to induce low back pain, and the pathogenic mechanism of IVDD remains unclear. The nucleus pulposus (NP) is a component of the intervertebral disc (IVD) that provides protection from mechanical stimuli. The matrix stiffness of NP tissue increases during the process of disc degeneration. Although several studies have found that pathological mechanical stimuli induce NP cell senescence, which is relevant for NP degeneration, however, the effect of matrix stiffness on NP cell senescence is not clear. Therefore, in the present study, we used polyvinyl alcohol (PVA) hydrogel with controllable stiffness to mimic the matrix stiffness of normal (4 kPa) and severely degenerated (20 kPa) NP tissue. Rat NP cells were isolated and cultured on substrates with different stiffness, and the cell proliferation, SA-β-gal activity, cell cycle, telomerase activity and the phenotype markers of NP cells were analyzed. Moreover, cytoskeleton staining and NP cellular Young's modulus on different substrates were also measured. To further investigate how substrate stiffness affects NP cell senescence, lysyl oxidase (LOX) was used to restore the extracellular matrix (ECM) synthesis of NP cells. The expression levels of integrin β1 and p38 MAPK were then measured. Our results showed that the 20 kPa substrate significantly induced NP cell senescence compared to the 4 kPa substrate. NP cells cultured on the 20 kPa substrate failed to maintain the expression of their phenotype markers. Furthermore, the 20 kPa substrate induced an increase of Young's modulus of NP cells, which possibly through up regulating the expressions of integrin β1 and p38 MAPK. These results indicated that the integrin β1-p38 MAPK signaling pathway may participated in substrate stiffness induced senescence of NP cells. LOX significantly increased ECM synthesis and inhibited substrate stiffness induced NP cell senescence, which indicated that matrix mechanics may be essential for maintaining the function of NP cell. Our results may provide a new perspective on the mechanism of IVDD by pathological matrix mechanics.

Mechanical Stretch Induces Annulus Fibrosus Cell Senescence through Activation of the RhoA/ROCK Pathway

Background

Intervertebral disc is responsible for absorbing and transmitting mechanical compression. Under physiological conditions, the peripheral annulus fibrosus (AF) cells are subjected to different magnitudes of transverse mechanical stretch depending on the swelling of the central nucleus pulposus tissue. However, the biological behavior of AF cells under mechanical stretch is not well studied.

Objective

This study was performed to study the effects of mechanical tension on AF cell senescence and the potential signaling transduction pathway.

Methods

Rat AF cells were made to experience different magnitudes of mechanical stretch (2% elongation and 20% elongation for 4 hours every day at 1 Hz) in a 10-day experiment period. The inhibitor RKI-1447 of the Rho-associated coiled-coil–containing protein kinases (ROCK) was added along with culture medium to investigate its role. Cell proliferation, cell cycle, telomerase activity, and expression of senescence markers (p16 and p53) were analyzed.

Results

We found that 20% elongation significantly decreased cell proliferation, promoted G0/G1 cell cycle arrest, decreased telomerase activity, and upregulated mRNA/protein expression of p16 and p53. Moreover, the inhibitor RKI-1447 partly resisted effects of 20% elongation on these parameters of cell senescence.

Conclusion

High mechanical stretch obviously induces AF cell senescence through the RhoA/ROCK pathway. This study provides us a deeper understanding on the AF cell's behavior under mechanical stretch.

Hyper-osmolarity environment-induced oxidative stress injury promotes nucleus pulposus cell senescence in vitro

Nucleus pulposus (NP) cell senescence is involved in disc degeneration. The in situ osmolarity within the NP region is an important regulator of disc cell's biology. However, its effects on NP cell senescence remain unclear. The present study was aimed to investigate the effects and mechanism of hyper-osmolarity on NP cell senescence. Rat NP cells were cultured in the in situ-osmolarity medium and hyper-osmolarity medium. The reactive oxygen species (ROS) scavenger N-acetylcysteine (NAC) was added along with the medium to investigate the role of oxidative injury. Cell cycle, cell proliferation, senescence associated β-galactosidase (SA-β-Gal) activity, telomerase activity, expression of senescence markers (p16 and p53) and matrix molecules (aggrecan and collagen II) were tested to assess NP cell senescence. Compared with the in situ-osmolarity culture, hyper-osmolarity culture significantly decreased cell proliferation and telomerase activity, increased SA-β-Gal activity and cell fraction in the G0/G1 phase, up-regulated expression of senescence markers (p16 and p53) and down-regulated expression of matrix molecules (aggrecan and collagen II), and increased intracellular ROS accumulation. However, addition of NAC partly reversed these effects of hyper-osmolarity culture on cellular senescence and decreased ROS content in NP cells. In conclusion, a hyper-osmolarity culture promotes NP cell senescence through inducing oxidative stress injury. The present study provides new knowledge on NP cell senescence and helps us to better understand the mechanism of disc degeneration.

Dysbindin promotes progression of pancreatic ductal adenocarcinoma via direct activation of PI3K

Pancreatic ductal adenocarcinoma (PDAC) represents a biggest challenge in clinic oncology due to its invasiveness and lack of targeted therapeutics. Our recent study showed that schizophrenia susceptibility factor dysbindin exhibited significant higher level in serum of PDAC patients. However, the functional relevance of dysbindin in PDAC is still unclear. Here, we show that dysbindin promotes tumor growth both in vitro and in vivo by accelerating the G1/S phase transition in cell cycle via PI3K/AKT signaling pathway. Mechanistically, dysbindin interacts with PI3K and stimulates the kinase activity of PI3K. Moreover, overexpression of dysbindin in PDAC is correlated with clinicopathological characteristics significantly, such as histological differentiation (P = 0.011) and tumor size (P = 0.007). Kaplan-Meier survival curves show that patients with high dysbindin expression exhibit poorer overall survival, compared to those with low dysbindin expression (P < 0.001). Multivariate analysis reveals that dysbindin is an independent prognostic factor for pancreatic ductal adenocarcinoma (P = 0.001). Thus, our findings reveal that dysbindin is a novel PI3K activator and promotes PDAC progression via stimulation of PI3K/AKT. Dysbindin therefore represents a potential target for prognosis and therapy of PDAC.

Nucleus pulposus cell senescence is alleviated by resveratrol through regulating the ROS/NF-κB pathway under high-magnitude compression

Mechanical overloading is a risk factor of disc degeneration. Studies have demonstrated that resveratrol helps to maintain the disc cell's healthy biology. The present study aims to investigate whether resveratrol can suppress mechanical overloading-induced nucleus pulposus (NP) cell senescence in vitro and the potential mechanism. The isolated rat NP cells were seeded in the decalcified bone matrix (DBM) and cultured under non-compression (control) and compression (20% deformation, 1.0 Hz, 6 h/day) for 5 days using the mechanically active bioreactor. The resveratrol (30 and 60 μM) was added into the culture medium of the compression group to investigate its protective effects against the NP cell senescence. NP cell senescence was evaluated by cell proliferation, cell cycle, senescence-associated β-galactosidase (SA-β-Gal) activity, telomerase (TE) activity, and gene expression of the senescence markers (p16 and p53). Additionally, the reactive oxygen species (ROS) content and activity of the NF-κB pathway were also analyzed. Compared with the non-compression group, the high-magnitude compression significantly promoted NP cell senescence, increased ROS generation and activity of the NF-κB pathway. However, resveratrol partly attenuated NP cell senescence, decreased ROS generation and activity of the NF-κB pathway in a concentration-dependent manner under mechanical compression. Resveratrol can alleviate mechanical overloading-induced NP cell senescence through regulating the ROS/NF-κB pathway. The present study provides that resveratrol may be a potential drug for retarding mechanical overloading-induced NP cell senescence.

Resveratrol attenuates high glucose-induced nucleus pulposus cell apoptosis and senescence through activating the ROS-mediated PI3K/Akt pathway

BACKGROUND

Diabetes mellitus is closely correlated with disc degeneration. Nucleus pulposus (NP) cell apoptosis and senescence are typical cellular features within the degenerative disc. Resveratrol is a newly identified phytoalexin that has protective effects on cartilaginous tissue.

OBJECTIVE

To investigate the whether resveratrol can protect against high glucose-induced NP cell apoptosis and senescence, and the potential mechanism in this process.

METHODS

Rat NP cells were cultured in either 10% FBS culture medium (control group) or 10% FBS with a high glucose concentration (0.2 M, experiment group) for 3 days. Resveratrol or the combination of resveratrol and LY294002 was added into the culture medium of experiment group to investigate the effects of resveratrol and the PI3K/Akt pathway.

RESULTS

High glucose significantly promoted NP cell apoptosis and NP cell senescence compared with the control group. Resveratrol exhibited protective effects against high glucose-induced NP cell apoptosis and senescence. Further analysis showed that resveratrol suppressed reactive oxygen species (ROS) generation and increased the activity of the PI3K/Akt pathway under the high glucose condition. However, the LY294002 had no significant effects on ROS content in the resveratrol-treated high glucose group.

CONCLUSION

Resveratrol can attenuate high glucose-induced NP cell apoptosis and senescence, and the activation of ROS-mediated PI3K/Akt pathway may be the potential mechanism in this process.

N‑cadherin attenuates nucleus pulposus cell senescence under high‑magnitude compression

Mechanical compression is important in disc degeneration. N-cadherin (N-CDH)-mediated signaling contributes to the maintenance of the normal nucleus pulposus (NP) cell phenotype and NP matrix biosynthesis. Our preliminary study demonstrated that a high-magnitude compression (20% deformation) promotes NP cell senescence in a three-dimensional scaffold culture system. The aim of the present study was to investigate whether N-CDH-mediated signaling alleviates NP cell senescence under the above-mentioned high-magnitude compression. NP cells were transfected with recombinant lentiviral vectors to enhance N-CDH expression. All the transfected or un-transfected NP cells were seeded into the scaffolds and subjected to 20% deformation at a frequency of 1.0 Hz for 4 h once per day for 5 days. Results indicated that N-CDH overexpressed NP cells exhibited decreased senescence-associated β-galactosidase activity and downregulated expression levels of senescence-associated markers (p16 and p53). Furthermore, the N-CDH overexpressed NP cells exhibited increased cell proliferation potency, telomerase activity and matrix biosynthesis compared with NP cells without N-CDH overexpression under high-magnitude compression. Thus, N-CDH-mediated signaling contributes to the attenuation of NP cell senescence under high-magnitude compression.

Role of p38-MAPK pathway in the effects of high-magnitude compression on nucleus pulposus cell senescence in a disc perfusion culture

Nucleus pulposus (NP) cell senescence is a typical pathological feature within the degenerative intervertebral disc. As a potential inducing and aggregating factor of disc degeneration, mechanical overloading affects disc biology in multiple ways. The present study was to investigate the NP cell senescence-associated phenotype under intermittent high compression in an ex vivo disc bioreactor culture, and the role of the p38-MAPK pathway in this regulatory process. Porcine discs were cultured in culture chambers of a self-developed mechanically active bioreactor and subjected to different magnitudes of dynamic compression (low-magnitude and high-magnitude: 0.1 and 1.3 MPa at a frequency of 1.0 Hz for 2 h per day respectively) for 7 days. Non-compressed discs were used as controls. The inhibitor SB203580 was used to study the role of the p38-MAPK pathway in this process. Results showed that intermittent high-magnitude compression clearly induced senescence-associated changes in NP cells, such as increasing β-galactosidase-positive NP cells, decreasing PCNA-positive NP cells, promoting the formation of senescence-associated heterochromatic foci (SAHF), up-regulating the expression of senescence markers (p16 and p53), and attenuating matrix production. However, inhibition of the p38-MAPK pathway partly attenuated the effects of intermittent high-magnitude (1.3 MPa) compression on those described NP cell senescence-associated parameters. In conclusion, intermittent high-magnitude compression can induce NP cell senescence-associated changes in an ex vivo disc bioreactor culture, and the p38-MAPK pathway is involved in this process.

The inflammatory cytokine TNF-α promotes the premature senescence of rat nucleus pulposus cells via the PI3K/Akt signaling pathway

Premature senescence of nucleus pulposus (NP) cells and inflammation are two common features of degenerated discs. This study investigated the effects of the inflammatory cytokine TNF-α on the premature senescence of NP cells and the molecular mechanism behind this process. Rat NP cells were cultured with or without different concentrations of TNF-α for 1 and 3 days. The inhibitor LY294002 was used to determine the role of the PI3K/Akt pathway. NP cells that were incubated with TNF-α for 3 days followed by 3 days of recovery in the control medium were used to analyze cellular senescence. Results showed that TNF-α promoted premature senescence of NP cells, as indicated by decreased cell proliferation, decreased telomerase activity, increased SA-β-gal staining, the fraction of cells arrested in the G1 phase of the cell cycle, the attenuated ability to synthesize matrix proteins and the up-regulated expression of the senescence marker p16 and p53. Moreover, a high TNF-α concentration produced greater effects than a low TNF-α concentration on day 3 of the experiment. Further analysis indicated that the inhibition of the PI3K/Akt pathway attenuated the TNF-α-induced premature senescence of NP cells. Additionally, TNF-α-induced NP cell senescence did not recover after TNF-α was withdrawn. In conclusion, TNF-α promotes the premature senescence of NP cells, and activation of the PI3K/Akt pathway is involved in this process.

Pathophysiology of degenerative disc disease

The intervertebral disc is characterized by a tension-resisting annulus fibrosus and a compression-resisting nucleus pulposus composed largely of proteoglycan. The most important function of the annulus and nucleus is to provide mechanical stability to the disc. Degenerative disc disease in the lumbar spine is a serious health problem. Although the three joint complex model of the degenerative process is widely accepted, the etiological basis of this degeneration is poorly understood. With the recent progress in molecular biology and modern biological techniques, there has been dramatic improvement in the understanding of aging and degenerative changes of the disc. Knowledge of the pathophysiology of the disc degeneration can help in the appropriate choice of treatment and to develop tissue engineering for biological restoration of degenerated discs.

The pathophysiology of disc degeneration: a critical review

The pathophysiology of intervertebral disc degeneration has been extensively studied. Various factors have been suggested as influencing its aetiology, including mechanical factors, such as compressive loading, shear stress and vibration, as well as ageing, genetic, systemic and toxic factors, which can lead to degeneration of the disc through biochemical reactions. How are these factors linked? What is their individual importance? There is no clear evidence indicating whether ageing in the presence of repetitive injury or repetitive injury in the absence of ageing plays a greater role in the degenerative process. Mechanical factors can trigger biochemical reactions which, in turn, may promote the normal biological changes of ageing, which can also be accelerated by genetic factors. Degradation of the molecular structure of the disc during ageing renders it more susceptible to superimposed mechanical injuries. This review supports the theory that degeneration of the disc has a complex multifactorial aetiology. Which factors initiate the events in the degenerative cascade is a question that remains unanswered, but most evidence points to an age-related process influenced primarily by mechanical and genetic factors.

Senescence in human intervertebral discs

Intervertebral discs demonstrate degenerative changes relatively early in life. Disc degeneration, in turn, is associated with back pain and disc herniation, both of which cause considerable clinical problems in the western world. Cell senescence has been linked to degenerative diseases of other connective tissues such as osteoarthritis. Thus we investigated the degree of cell senescence in different regions of discs from patients with different disc disorders. Discs were obtained from 25 patients with disc herniations; from 27 patients undergoing anterior surgery for either back pain due to degenerative disc disease (n = 25) or spondylolisthesis (n = 2) and from six patients with scoliosis. In addition, four discs were obtained post-mortem. Samples were classified as annulus fibrosus or nucleus pulposus and tissue sections were assessed for the degree of cell senescence (using the marker senescence-associated-beta-galactosidase (SA-beta-Gal)) and the number of cells present in clusters. There were significantly more SA-beta-Gal positive cells in herniated discs (8.5% of cells) than those with degenerative disc disease, spondylolisthesis, scoliosis, or cadaveric discs (0.5% of cells; P < 0.001). There was more senescence of cells of the nucleus pulposus compared to those of the annulus fibrosus and in herniated discs a higher proportion of cells in cell clusters (defined as groups of three or more cells) were SA-beta-Gal positive (25.5%) compared to cells not in clusters (4.2%, P < 0.0001). This study demonstrates an increased degree of cell senescence in herniated discs, particularly in the nucleus where cell clusters occur. These clusters have been shown previously to form via cell proliferation, which is likely to explain the increased senescence. These findings could have two important clinical implications: firstly, that since senescent cells are known to behave abnormally in other locations, they may lead to deleterious effects on the disc matrix and so contribute to the pathogenesis and secondly, cells from such tissue may not be ideal for cell therapy and repair via tissue engineering.

Mechanical compression modulates proliferation of transplanted chondrocytes

The presence of an appropriate number of reparative cells in an articular cartilage defect is probably necessary for consistent and successful repair. Following the transplantation of chondrocytes into a defect, cell proliferation may modulate local defect cellularity. Transplanted cells can be compressed during cartilage repair as a result of joint-loading or press-fitting a graft into a cartilage defect. The objective of this study was to characterize the proliferative response of chondrocytes after attachment to cartilage and application of static compressive stress between cartilaginous surfaces in an ex vivo model. The chondrocytes were isolated from adult bovine cartilage, cultured in high-density monolayer, resuspended, and then transplanted onto the surface of devitalized cartilage at a density of 250,000 cells/cm2. The total DNA content of transplanted cell layers increased steadily to a plateau by 5 days and represented a 4-fold increase in cell number during incubation in medium including serum and ascorbate. Over the culture period, the level of DNA synthesis ([3H]thymidine incorporation), on a per cell basis, decreased steadily (88% between days 0 and 6). The application of 24 hours of static compressive stress (0.06-0.4 MPa) to the adherent cells at 1 and 4 days after transplantation inhibited overall DNA synthesis by 70-approximately 87% compared with unloaded controls. After release from load, cell proliferation generally remained at low levels. The marked proliferation of chondrocytes when attached to cartilage without applied load and the inhibition of this proliferation by relatively low-amplitude static compressive stress may be relevant to the occasional overgrowth of tissue in some chondrocyte transplantation procedures. The dosimetry of these effects suggests that the in vivo mechanical environment may have a marked effect on proliferation of transplanted chondrocytes.

The insulin-like growth factor and epidermal growth factor families in mammary cell growth in ruminants: action and interaction with hormones

Selective breeding and improved management have had major effects in increasing peak milk yields but relatively little effect on lactation persistency. In ruminants, cell loss appears to be largely responsible for the decline in milk yield. Little is known about the longevity of individual cells, but, in lactating dairy cows, few epithelial cells are in the S phase (DNA synthesis) of the cell cycle. The IGF and epidermal growth factor families are direct mitogens, stimulating DNA synthesis in cultures of ruminant mammary epithelial cells. Receptors that mediate the effects of these growth factors, the type 1 IGF receptor and the epidermal growth factor receptor, respectively, are present at similar levels in membranes prepared from the mammary glands of nonpregnant and pregnant sheep. Binding capacity falls by parturition and remains low during lactation. These findings suggest that the drive to mammary development in pregnancy comes from control of growth factors, and, in the case of IGF, modulating binding proteins, a control exerted by hormones, which, in general, are not themselves mitogens. A paracrine or autocrine mode of action and, therefore, local growth factor synthesis, are more likely to be important than systemic concentrations of growth factor. Stimulatory growth factors produced locally by the mammary gland include IGF-I, IGF-II, transforming growth factor-alpha, and amphiregulin. More information is needed on the control of stimulatory and inhibitory growth factors and on how growth factors control the cell cycle. Knowledge of these processes could result in strategies to improve lactation persistency by increasing secretory cell renewal or reducing cell loss during lactation.

Development of the mammary gland and lactation

Many hormones, growth factors, and protooncogene products involved in mammary development and lactation have been identified; however, the mechanism of their concerted action remains to be explained. In addition to these regulatory factors, normal mammary development and lactation require the cell-cell interaction of stromal and parenchymal elements in the mammary gland. Recent studies indicate that PRL effects on target cells are likely transmitted into cells via activation of tyrosine kinase associated with a protein-designated JAK-2, while other studies have identified stimulatory and inhibitory factors that interact with the 5' promotor regions of milk-product genes.

The effect of lactogenic hormones on protein synthesis and amino acid uptake in rat mammary acinar cell culture at various physiological stages

1. The activities of protein synthesis and amino acid uptake at various physiological stages were determined by the incorporation of radioactive materials ([3H]-lysine, [14C]-cycloleucine) in rat mammary epithelial cell cultures. The activity of protein synthesis and amino acid uptake was higher in early lactation than in virgin, pregnant and late lactation stages. 2. Lactogenic hormones (prolactin, hydrocortisone and insulin) treatment related with mammary growth and differentiation increased the activities of protein synthesis and amino acid uptake. But increase of these activities was different at each physiological stage. 3. The effect of prolactin and hydrocortisone on the activities were greater in virgin, pregnant and late lactation than in early lactation. And effect of insulin was greater in pregnant and early lactation than in virgin and weanling.

Effects of aging on ribosomal protein L7 messenger RNA levels in cultured rat preadipocytes

Ribosomal protein L7 mRNA is a cell cycle-independent message whose levels are lower in late passage "senescent" fibroblasts than early passage cultures. To determine whether decreases in L7 mRNA levels also occur during aging in tissues in vivo and whether reduced L7 mRNA is caused by terminal differentiation, we measured L7 and adipsin (a differentiation-dependent serine protease) mRNA levels in undifferentiated and differentiated preadipocytes and glyceraldehyde-3-phosphate dehydrogenase mRNA in differentiated preadipocytes cultured from perirenal fat depots of 3-, 17-, and 24-month-old male rats. L7 mRNA levels decreased with increasing age and were not affected by differentiation. In the same cultures, adipsin mRNA levels did not increase with age but did increase with differentiation, confirming that the preadipocytes exposed to enriched medium had, in fact, differentiated. Glyceraldehyde-3-phosphate dehydrogenase mRNA levels did not change with age indicating that the decrease in L7 mRNA was not a result of a general decrease in mRNA with age. These observations are consistent with the hypotheses that decreasing L7 mRNA levels are associated with aging and that late passage fibroblasts have features in common with senescence. The observations are not consistent with the hypothesis that senescent changes in cellular function are caused by terminal differentiation.

Effect of prepartum bovine somatotropin in primigravid ewes on mammogenesis, milk production, and hormone concentrations

Twenty-five primigravid ewes were used to investigate the effect of bST, between 97 and 124 d of gestation, on mammogenesis and subsequent milk production. Five ewes (reference group) were slaughtered at 96 d of gestation, and the remaining ewes were injected daily with saline (control group: n = 10) or .1 mg/kg of BW of bST (bST group: n = 10). Following bST treatment, 5 control and 5 bST group ewes were slaughtered (slaughter group). The remaining ewes were slaughtered after lambing and being milked for 8 wk (production group). Weekly blood samples were obtained from both slaughter and production group ewes. Slaughter group ewes were also subjected to 8-h serial blood sampling at 98 d (period 1) and 123 d (period 2) of gestation. Milk production was 42% higher in ewes treated prepartum with bST than in those treated with saline. Results suggest that the increase in milk was due to an increase in mammary parenchymal cell number rather than to an increase in cellular activity. The high rate of [3H]thymidine incorporation into parenchymal tissue in reference group ewes suggests that the increase in parenchyma during the second trimester of gestation is due to cellular hyperplasia but that cellular hypertrophy may be more important during the last trimester. Plasma IGF-I concentrations were significantly higher during bST treatment and remained elevated between daily injections; the increase was greatest in period 2.

Bovine mammary myoepithelial cells. 1. Isolation, culture, and characterization

The objective of this study was to isolate, purify, culture, and characterize myoepithelial cells from bovine mammary glands. Myoepithelial cells were separated from other mammary and blood cells after collagenase digestion and centrifugation using metrizoate-ficoll gradients. Myoepithelial cells were identified by their characteristic morphology and cloned using selective detachment. They contained many densely packed myofilaments, very few cytoplasmic organelles, elongated surface projections, and a dense, irregularly shaped nuclei. Some cells were as large as 1.2 mm in culture. Myoepithelial cells contained an extensive network of cytoskeletal proteins, including alpha-smooth muscle actin, alpha-actinin, and vimentin. When cultured, they tended to repel one another and never grew as closely associated cells. The myoepithelial nature of these cells was verified by showing that they contracted in response to oxytocin, bound oxytocin, and did not produce casein. Myoepithelial cells from fetal and lactating glands grew very well in culture. Active division of myoepithelial cells could be maintained for at least 3 mo, and cells could be serially subcultured at least seven times. The successful isolation and culture of bovine mammary myoepithelial cells make utilization of these cells possible in order to study their role in mammary growth and differentiation and milk ejection.